Copyright: Copyright © 1993-1999 WinWare Incorporated.


About light
Part I.- Basic principles

Chapter 1.- Introduction

1.1.- About light

In order to perceive the sensation named as color, responsible receptors sensitive to light of different wavelengths must be excited by the external stimulus i.e. light coming from a given object.
It is the light as a whole coming from the object that stimulates the receptors to different extent and is responsible for originating color perception. Depending on the origin of the light from the object, the latter can be denoted as:

•	passive diffuser or reflector: light is coming from a certain source located at certain distance away from the object and is reflected by it in all directions (diffuser) or specularly (reflector) towards the eye. In general light interacts with the object and its spectral distribution is changed as result of absorption and scattering processes; light color changes after the interaction with the object. If no changes result and light is elastically reflected (i.e. no losses), the object is called perfect diffuser.

•	self-luminous: many objects have the property to emit light upon certain excitation:

1.	Incandescence: all objects emit light when heated; this is in general result of solely temperature, nature or constitution of the object is not involved in the light production process; the ideal radiator is known as "black body".

2.	Fluorescence: many molecules possess the property to emit light after they have been excited by a light ray i.e. they have absorbed light. The light emitted is called fluorescence if the emission follows within picosecond time scale. In general the energy of the emitted light is lower than the absorbed one, the rule follows that emission occurs always at higher wavelengths than absorption.

3.	Phosphorescence: with some molecules the emission process takes more time as result of internal processes within them; the process can take seconds to hours to complete.

Since color perception depends on light (and its spectral distribution) it is important to consider the origin of it when dealing with color assessment:

•	passive diffuser: the light observed originates outside the object itself, the object acts as a modulator of the incoming rays changing their spectral distribution depending on level of interaction. Color perceived is the result of interaction of incoming ray and nature of the object.

•	self-luminous: the incoming light excites molecules to emit the radiation perceived later as color. Although in general the spectral distribution of the exciting light has an influence on the spectral distribution of the emission, in most cases (remarkably fluorescent whitening agents) this influence is minimal.

In general terms however, light detected by the eyes that Ultimately fires the color perception is a mixture of reflected (and/or diffused) light and emission from the object; the eye can not discriminate between both sources.
Furthermore and due to the nature of the light detected by the eye, color can not be considered a physical property of the observed object.


Adjusting UV levels
Part II- Measuring whiteness

Chapter 6.- Instrumental assessment

6.1.- Adjusting the UV to daylight conditions

Following the statement that samples must be measured with a light source that corresponds to an illuminant, light sources of instruments for whiteness assessment should correspond to the spectral profile of daylight, down to 300 nm in the UV region.
This is strong limitation from the instrumental point of view, because standard light sources, like unfiltered QHT lamps or other incandescent sources do not produce enough UV levels to approach the level encountered in daylight illuminant.
For this reason only instruments equipped with Xenon flash lamps can be used for measuring whiteness, the flash lamp must produce a level of UV though, that is higher than the one encountered in daylight; using a movable filter and a set of calibrated fluorescent samples it is possible to set the UV level to correspond to daylight. A further advantage of this system is that the UV level can be adjusted on a periodical basis, the reason being that the amount of UV light deteriorates continuously with lamp operation.
The software provided to operate the instrument must be able to conduct the adjusting procedure. Many types of adjusting devices are found with different instruments:

•	manual adjustment: the filter can be set manually by using a micrometer device; a disadvantage is that the software has no way to control the setting or to detect an accidental loss of adjustment

•	motor-driven filters: the software controls the movements of the filter and is able in some cases to conduct the adjustment automatically

•	Numerical UV Control (NUVC): this type of instrument is equipped with two lamps: one with UV intensity (normally close to daylight) and one with no UV intensity (using a 400 nm cut-off filter); setting for the proper UV level is made mathematically by the software, no moving parts are involved


Appendix
Appendix (Books, reviews and norms)

Internet resource

Whiteness Homepage

Selected literature

Books

A.K. Sarkar
Fluorescent Whitening Agents
Merrow Publishing Co. Watford 1971
ISBN 0-900-54121-2

T. Rubel
Optical Brighteners: Technology and Applications
1972

R. Anliker and G. Müller (Eds.)
Fluorescent Whitening Agents
Georg Thieme Publishers, Stuttgart, 1975
ISBN 3-13-517101-9

R.Williamson
Fluorescent brightening agents
Textile Science and Technology Vol. 4
Elsevier, Amsterdam, 1980
ISBN 0-444-41914-4

Milos Zahradník
The Production and application of fluorescent brightening agents
Wiley, Chichester, 1982
ISBN 0-471-10125-7

Arno Cahn
Bleachers and Brighteners
Academic American Encyclopedia, Grolier Inc., Danbury, Connecticut, 1995, Volume 3

P.G. Engeldrum
Psychrometric Scaling - a Toolkit for Imaging Systems Development
Imcotek Press, Winchester 2000

Reviews

H. Gold, S. Petersen and O. Bayer
“Fluorescent Brightening Agents of the Triazole Series”
in Gore et al. (Eds) Recent Progress in Chemistry of Natural and Synthetic Colouring Matters, pp. 605 ff, Academic Press, New York, 1962

R. Zweidler and H. Häusermann
“Optical Brighteners”
in Kirk-Othmer Encyclopedia of Chemical Technology
Vol. 3, Second Edition, pp. 737-750, J.Wiley & Sons, New York, 1964

H. Gold
“Fluorescent Brightening Agents”
In K.Venkataraman (Ed.) The Chemistry of synthetic dyes, Vol. V pp- 536-680
Academic Press, New York 1971

R. Anliker and G. Müller
“Fluorescent Whitening Agents”
In F. Coulson and F. Korte (Eds.) Environmental Quality and Safety, Suppl. Vol. IV: Fluorescent Whitening Agents
Academic Press, New York 1975

E. Siegel
“Organic Dyes and Optical Brightening Agents”
Int. Rev. Sci. Org. Chem. Ser. Two 1976, vol. 2, 259-297

P. Stensby
„Fluorescent Whitening Agents“
in W.G . Cuttler (Ed.) Detergency, Part III, pp. 730-806, Marcel Dekker, New York, 1981

R. Levene and M. Lewin
“The Fluorescent Whitening of Textiles”
In M. Lewin and S.B. Sello (Eds.) Handbook of Fiber Science and Technology Vol. 1, Part B, pp.257-304
Marcel Dekker, New York 1984

A.E. Siegrist, H. Hefti, H.R. Meyer and E. Schmidt
“Fluorescent Whitening Agents 1973-1985“
Rev. Prog. Color. Relt. Top. 1987, 17, 39-55

H. Zollinger
“Fluorescent Brighteners“
in Color Chemistry, VCH Verlagges., Weinheim, 1987, pp. 203-213

A. Mercer
“Fluorescent Brightening Agents”
In J. Shore (Ed.) Colorants and Auxiliaries: Organic Chemistry and Application Properties. Vol. 2. Auxiliaries Chapter 11, pp. 470-511, Society of Dyers and Colourists, Bradford, 1990

A.E. Siegrist, C. Eckhardt, J. Kaschig and E. Schmidt
“Optical Brighteners”
in Ullmann's Encyclopedia of Industrial Chemistry, 5th Ed. Vol. A18, pp. 153-176, 1991

H.J. McElhone
Fluorescent Whitening Agents
in Kirk-Oethmer Encyclopedia of Chemical Technology, Wiley, New York, 1994

J. Leland, N. Johnson and A. Arecchi
„Principles of Bispectral Fluorescence Colorimetry“
in A.V.Arecchi(Ed.)Photometric Engineering of Sources and Systems, 29-30 July 1997, 3140, 76-87, San Diego, USA

K.J. Smith
“Evaluation of Whiteness and Yellowness”
in Color Physics for Industry, 2nd Edition, pp. 185, Society of Dyers and Colourists, 1997

M. Holmberg
“Dyes and Fluorescent Whitening Agents”
in Papermaking Science and Technology 4. Papermaking Chemistry, J. Gullichsen, H. Paulapuro and H. Neimo (Eds.), Chapter 14, pp. 303-320, Fapet Oy, Helsinki, Finnland, 1999

Reports

A.S. Richardson
Optical Bleaching Agents
FIAT Final Report 1302, 1947

EMPA
Instrumentelle Weissbewertung in der Textilindustrie
KWF-Projekt-Nr. 222.1, Eidgenössische Materialprüfungs- und Forschungsanstalt (EMPA), St.Gallen, Switzerland, 1995

E.J. van de Plassche, P.H.F. Bont and J.M. Hesse
Exploratory Report on Fluorescent Whitening Agents (FWAs)
RIVM Rapport 601503013, 1999

Related norms

ASTM Standards on Color and Appearance measurement
Publication Code Number (PCN): 03-512096-14
ASTM, 100 Barr Harbor Drive, West Conshohocken
PA 19428-2959, U.S.A.

On color

ASTM
E308-99 Standard Practice for Computing the Colors of Objects by Using the CIE System

CIE
CIE 15.2-1986: Colorimetry, 2nd Edition (ISBN 3 900 734 00 3)

ISO
ISO 105-J01:1997 Part J01: General principles for measurement of surface colour
ISO/CIE 10526:1991: CIE standard colorimetric illuminants
ISO/CIE 10527:1991: CIE standard colorimetric observers

On whiteness

ASTM
D 985-97(1997) Standard Test Method for Brightness of Pulp, Paper and Paperboard (Directional Reflectance at 457 nm)
E259-98 Standard Practice for Preparation of Pressed Powder White Reflectance Factor Transfer Standards for Hemispherical Geometry and Bi-Directional Geometries
E308-95 Standard Practice for Computing the Colors of Objects by Using the CIE Sytem
E313-98 Standard Practice for Calculating Yellowness and Whiteness Indices from Instrumentally Measured Color Coordinates
E991-98 Standard Practice for Color Measurement of Fluorescent Specimens

AATCC
Test Method 110-1995: Whiteness of Textiles

ISO
ISO 105-J02:1997 Part J02: Instrumental assessment of relative whiteness
ISO 2470:1996 Pulp and Paper -- Measurement of diffuse blue reflectance factor (ISO brightness)
ISO 3688:1996 Pulps -- Measurement of diffuse blue reflectance factor (ISO brightness)

JIS
JIS-L 1064: Identification of fluorescent brightening agent classes on textiles

Tappi
T 452 om-92: Brightness of pulp, paper, and paperboard (directional reflectance at 457 nm)
T 525 om-92: Diffuse brightness of pulp (d/0°)
T 560 pm-96: CIE whiteness and tint of paper and paperboard (using d/0°, diffuse illumination and normal viewing)
T 562 pm-96: CIE whiteness and tint of paper and paperboard (using 45°/0°, directional illumination and normal viewing)

Dr. Claudio Puebla
Copyright © 2002 Axiphos GmbH. All rights reserved.
Revised: April 4, 2002.


Bibliography (1990-2001)
Selected literature (1990-2001)

H. Uchida
„A Study on the CIE Whiteness Formula“ J. Color Sci. Jpn. 1990, 14(2), 106-113

B.D. Jordan and G.H. Snow
“The Whiteness of Paper - Colorimetry and Visual Ranking“ Tappi J. 1991, 74, 93-101

R. Griesser
“Whiteness not Brightness: A New Way of Measuring and Controlling Production of Paper” Appita 1993, 46(6), 439-444

A.C. Jackson
"The Environmental Benefits of Modern Developments in Dyestuffs and OBAs" Paper 05 at Chemistry of Papermaking Conference, 27-28 Jan. 1993, Wilmslow, UK, 12 pp. (PBA Abstract 2616, 1993)

J.A. Bristow
“The Calibration of Instruments for the Measurement of Paper Whiteness” Color Res. Appl. 1994, 19(6), 475-483

J.A. Bristow
“What is ISO Brightness?” Tappi J. 1994, 77(May), 174-178

R.M. Christie
“Pigments, Dyes and Fluorescent Brightening Agents for Plastics” Polymer Internanional 1994, 24, 351

R. Griesser
“Assessment of Whiteness and Tint of Fluorescent Substrates with Good Inter-Instrument Correlation” Color Res. Appl. 1994, 19(6), 446-460

D. Gundlach and H. Terstiege
„Problems in Measurement of Fluorescent Materials“ Color Res. Appl. 1994, 29, 260-266

P.L. Moriarty
„Optical Brighteners for Polyester“ Book Pap. – Int. Conf. Exhib. AATCC 1994, 297-300

J.A. Bristow
“Optical Properties of Pulp and Paper - New Standardization Proposals“ Tappi J. 1995, 78(December), 45-47

E. Ganz and H.K.A. Pauli
“Whiteness and Tint Formulas of the CIE: Approximations in the L*a*b* Color Space“ Appl. Optics 1995, 34, 2998-2999

D.D. Malthouse and S.J. Popson
“A New Way of Measuring the Effect of Fluorescent Whitening Agents“ Appita 1995, 48(1), 56-58

W. Erb and M.P. Kryster
„Weighting Function for the Calculation of ISO Brightness“ TAPPI Journal 1994, 79, 279-28

R. Griesser
“CIE Whiteness and Tint: Possible Improvements“ Appita 1996, 49(2), 105-112

J.B. Kramer, S. Canonica, J. Hoigné and J. Kaschig
“Degradation of Fluorescent Whitening Agents in Sunlit Natural Waters“ Environ. Sci. Technol. 1996, 30, 2227-2234

C. Puebla and R. Griesser
"Whiteness: Concepts and Instrumental Assessment“ Paper Industry China 1996, 163-164

J. Rieker, R. Griesser and C. Puebla
„Can the CIE-Whiteness Formula Be Improved for Better Correlation?“ Textilveredlung 1996, 31, 64

J. Kaschig
"The Photochemistry of Fluorescent Whitening Agents – Impact on Light Fastness and Environmental Fate" '96 International Seminar on Surfactants and Detergents, September 22 - 25, 1996, Nanjing,P.R.China

Claudio Puebla and Zhang Gui Min
"An Overview on Whiteness Measurement and Application" '96 International Seminar on Surfactants and Detergents, September 22 - 25, 1996, Nanjing, P.R. China

C. Puebla
"Predicting Whiteness from First Principles. A Further Step Towards Recipeing White Paper." 29th Pulp and Paper Annual Meeting, ABTCP, November 4 - 8, 1996,Sao Paulo, Brazil

J.C. Zwinkels and W. Erb
„Comparison of Absolute d/0 Diffuse Reflectance Factor Scales of the NRC and the PTB“ Metrologia 1997, 34, 357-363

J.C. Zwinkels, D.S. Gignac, M. Nevins, I. Powell and A. Brewsher
„Design and Tetsing of a Two-Monochromator Reference Spectrofluorimeter for High-Accuracy Total Radiance Factor Measurements“ Appl. Optics. 1997, 36(4), 892-902

A. Tindal
"Producing and Measuring Whiteness" PITA Annual Conference “Chemicals in Papermaking”,October 1997, Manchester, UK

V.R. Kanetkar, G. Shankarling and J. Malanker
“Heterocyclic Fluorescent Brightening Agents - An Overview“ Colourage 1998, September, 35-42

G. Thurman
“Color Calibration Standards (D65) Change“ TAPPI Journal 1998, 81(May), 94

J. Kaschig, M. Schaumann and B. Schultz
"The Photochemistry of Fluorescent Whitening Agents: Impact on Light Fastness" Proceedings of the 4th World Conference on Detergents, AOCS Press, 1998, pp. 323

J. Kaschig and C. Puebla
"Exhaustion and Leveling Properties of Fluorescent Whitening Agents in Typical Asian Laundry Processes" '98 International Seminar on Surfactants and Detergents, September 23 - 25, 1998, Hangzhou, P.R. China

A. Bristow
„ISO Brightness: a More Complete Definition“ TAPPI Journal 1999, 82(10), 54-56, 985

I. Katayama, M. Iiyama and K. MasumiI
„Effect of Spectral Distribution of an Illuminant on Perceived Whiteness “ J. Illum. Engng. Inst. Japan. 1999, 83(11) 847-853

T. Shakespeare and J. Shakespeare
„Problems in Colour Measurements of Fluorescent Paper Grades“ Anal. Chimica Acta 1999, 380(2-3), 227-242

T. Akatsu, E. Toriumi, M. Ayama, K. Mukai and S. Kanaya
„Effect of Color Temperature of Fluorescent Lamp on the Whiteness Perception“ 24th Session of the CIE, Warsaw June 24-30, 1999

A.V. Makarenko and I.A. Shaykevich
“Whiteness of Paper“ Color Res. Appl. 2000, 25, 170-176

B. Mensak
“Exact Whiteness Numbers with Spectralphotometers“ Maschen Industrie 2000, 3, 40-42

J. Kaschig and C. Puebla
"Whiteness Risk Management" 5th World Surfactants Congress, May 29 - June 2, 2000, Fortezza de Basio, Italy

K. Mukai, T. Takeuchi, M. Ayama and S. Kanaya
“An Objective Method for Quantifying Whiteness Perception by Applying CIECAM97s“ 2000 Annual Conference of the Illuminating Engineering Institute of Japan, August 24-25, 2000, Kanagawa Institute of Technology, Atsugi-City, Japan

R. Olmedo
"Whiteness Retention under High Levels of Solar Radiation" JOCS/AOCS World Congress, October 22 - 27, 2000, Kyoto, Japan

J.F. Suthers
"An Overview of Optical Brighteners for Copolyester Extrusion Paper Coating Applications ANTEC 2000: Color and Appearance, May 8, 2000, Orlando, USA

A.K. Gaigalas, L. Li, O. Henderson, R. Vogt, J. Barr, G. Marti, J. Weaver and A. Schwartz „The Development of Fluorescence Intensity Standards“
J.Res.Natl.Inst.Stand.Technol. 2001, 106, 381-,89

J. Kaschig and C. Puebla
"Level Brightening Effects of Fluorescent Whitening Agents in Low Temperature Laundry Processes - A Kinetic Model" 92nd AOCS Annual Meeting, May 13 - 16, 2001, Minneapolis, USA

D. Kon, M. Ayama, S. Kanaya, K. Mukai, M. Eloholma and L. Halonen "Comparison of Whitenes Perception" Annual Conference of IEI-J, September 6 - 7, 2001, Yamaguchi University, Ube, Japan

Dr. Claudio Puebla
Copyright © 2002 Axiphos GmbH. All rights reserved.
Revised: April4, 2002.


Calibration standards
Part II- Measuring whiteness

Chapter 6.- Instrumental assessment

6.2.- Calibration standards

Mandatory condition to achieve accuracy of results is the proper calibration with certified standards. Instruments must be maintained on a periodical basis (especially when working under unstable conditions) as well the standards used for calibration.

On standards
In general there are three levels of hierarchy to be observed while issuing and using standards:

•	Primary standards: these are physical standards with certified reflectance values measured on an absolute basis by a certified institution (normally a National Institute). Primary standards are used for issuing secondary standards.

•	Secondary standards: these are physical standards with reflectance values that can be traced back to a given primary standard; its values must be checked periodically against the ones of the primary standard. Secondary standards should not be used for routine work, but to produce working standards to be used in daily work

•	Working standards: these are physical standards with reflectance values traceable back to certain secondary standard, that is stored at a central place. On a periodical basis their reflectance values are checked and re-issued using the corresponding secondary standard. Working standards are physically made out of materials that are stable and easy to clean, nevertheless they must be treated with care. If cleaned they must be immediately recalibrated in order to maintain the standard chain.

The observation of this hierarchical chain of standards is a condition to establish a quality assurance system. It must be remarked that production laboratories are situated at the third level of the chain, working standards must be checked and recalibrated by a central lab.

STFI standards
The STFI, Sweden produces periodically paper samples as secondary standards to be used to calibrate instruments. Following samples are provided:

•	non-fluorescent paper sample: containing certified reflectance values along with colorimetric data. Data can be used to produce a working standard for daily use

•	fluorescent paper sample: containing certified CIE whiteness for D65/10° conditions. This sample is used to set the UV level of instruments to daylight conditions; recently samples for C/10° conditions are also offered

TITV standards
The institute TITV, Germany offers a set of five textile samples for calibration purposes, described as follows:

•	non-fluorescent textile sample: containing certified reflectance values along with colorimetric data. Data can be used to produce a working standard for daily use

•	fluorescent textile samples: containing certified reflectance values, Ganz and CIE whiteness for D65/10° conditions. Samples are used to set the UV level of instruments to daylight conditions

Recently the TITV has started offering samples based on polyester material that show improved time stability and reproducibility.

Hohensteiner Institute standards
The Hohensteiner Institute, Germany offers a set of four textile samples with certified Ganz and CIE whiteness values for setting UV levels to D65 conditions


Assessing colors
Part I.- Basic principles

Chapter 1.- Introduction

1.2.- Assessing color

Characterization of a color can be done either visually or through an instrument; in either case this must be accompanied by information about conditions under which the assessment has been performed.

Light sources vs. illuminants
While the CIE and other organization have published norms defining illuminants, it responsibility of the assessing laboratory to calibrate the light sources used for visual assessment. Illuminants are defined as a table of relative intensities by the standardizing organization; the CIE recommended illuminants are daylight D65 and incandescent type A (see Appendix).
Light sources are physical devices that produce the light used for the assessment, their spectral distribution must match the one characterizing the corresponding illuminant in order to accept the assessment. Much confusion exists however on the correct way to calibrate light sources, since this represent an economical factor.
Calibration of light sources should be done preferably examining their spectral distribution; calibration according to color temperature should be avoided, especially when dealing with whiteness.

Defining daylight
According to the CIE daylight is defined according to the tables of illuminant D65, the region of definition extents into the ultraviolet (UV) region down to 300 nm (although for strictly visual assessment only the region between 380 and 760 nm is needed). The UV region is however important when dealing with fluorescent whites, since the fluorescence originates from the absorption in the UV region.
In the practice however it is quite difficult to "produce daylight" in its whole extension to be used in the visual assessment, this however absolutely needed when dealing with whiteness, following guidelines should be followed:

•	make sure that the observation light has a "true" daylight simulator: many booths are equipped with simulators of illuminant C that are denoted as "daylight" in the sense that C is a good approximation for D65.

•	do not trust calibrations made on color temperature basis: due to costs, some booths are equipped with fluorescent lamps denoted as "daylight", "cool white", etc. According to their specifications they have a color temperature similar to D65; they are normally three-band phosphorous pigments with tri-stimulus values close to D65, but their spectral distribution differ notably from the D65 one.

•	be careful with color rendering indices (CRI): the closer the CRI is to the value 100,  the smaller is the difference of certain number of metameric pairs; however a CRI closer to 100 does not automatically mean that the spectral distribution of the lamp is closer to that of daylight.

•	be careful when turning on the UV lamp: since most of the booths do not have a daylight simulator for the whole spectral region, they are provided with an additional UV lamp to be turned on when dealing with fluorescent white samples. The lamp irradiates in general an uncontrolled amount of UV that by no means is guaranteed to match that of daylight; the situation turns worse because the lamp stabilizes normally after some minutes of operation while coming to working thermal equilibrium.

Visual assessment
In general the human eye has no memory for color such that it is not possible, even for a trained person, to judge a color on an absolute basis, for this reason assessment can be done only for pairs of samples, the assessment is limited to a comparison of a given sample to the one considered as standard. In a general way the eye is always making a comparison between the light coming from the object and that coming from the environment; this is extremely important when assessing samples with high luminosity and low saturation values as in the case of white.
Furthermore the adaptation of the eye must be taken into account along with a standardized environment in order to produce reliable assessment:

•	Environment: it is advisable to use an observation booth with an interior painted in non-reflecting grey. No colored objects (other than the pair under assessment) should be positioned in the booth, observer and other persons should also wear neutral (grey) clothes, this latter is specially important when assessing white samples.

•	Light sources: the pair should be assessed under more than just one light source in order to establish the presence of metameric effects. Illuminant metamerism is the phenomena by which two samples appear to have the same color under one light source but they depart from each other when the light source changes. Light sources must be controlled and recalibrated on a periodical basis.

Instrumental assessment
An instrument is able to assess color on an absolute basis, provided that it has been properly calibrated. It is important to notice the way the sample is illuminated and two different families of geometries can be distinguished:

•	directional illumination: under this geometry the sample is illuminated by a light ray coming in a certain angle, the sample being observed (by the detector) under a different angle; the set illumination axis, object and observation axis form a plane. Due to certain unavoidable level of anisotropy of the sample, a certain degree of dependence of results on the orientation of the sample is observed under this geometry, for this reason it is recommended to take an average of different sample orientations in order to cancel out this effect. Following configuration is currently used:

1.	geometry 45°/0°: sample is illuminated in an angle of 45° and observed under 0° from the normal, this geometry is encountered in the paper assessment area as required by many Tappi methods.

to avoid the orientation dependence some instruments have many illuminators positioned in a ring thus introducing a rotation symmetry of the illumination; this type of annular illumination is used in instruments for continuous measurements.

•	diffuse illumination: in order to eliminate variations from anisotropic effects, the sample is positioned at a port of a hollow sphere with internal walls covered with a high diffuse reflecting white pigment; with this configuration it is assured that the sample is illuminated from all directions at the same time. Twos configurations are currently used:

1.	geometry d/0°: the sample is illuminated diffusively and observed in an angle of 0° from the normal; this geometry is encountered in the paper area as required by ISO methods.

2.	geometry d/8°: the sample is illuminated diffusively and observed in an angle of 8° from the normal; the 8° observation allows a first assessment of the gloss of the sample. This geometry is encountered in textile area.

Besides geometry different types of light detection are used; one can distinguish:

•	colorimeters: in this type of instruments light from the sample is examined (consecutively or simultaneously) by three filters simulating the tri-stimulus functions for a given illuminant. The instrument produces the values (X,Y,Z) and all colorimetric data derived from it; since the illuminant is part of the filter, colorimetric data for a different illuminant is not possible, unless the filters are replaced and a new measurement is done.

•	spectrophotometers: this type of instruments measures the reflectance spectra of the sample, rather than analyze the incoming light in colorimetric terms. Since reflectance spectrum is a physical property of the sample, calculation of colorimetric data for different illuminants and observers can be easily performed through software.

Special additional conditions are needed for the instrumental assessment of whiteness, they will be discussed later (see Instrumental assessment)


On color mixing
Part I - Basic principles

Chapter 2.- The color white

2.4.- On color mixing

Any color can be decomposed as a combination of three elements given ultimately by the tri-stimulus function describing the normal observer; in general a closer examination of the stimulus origin must be given to arrive to proper tri-stimulus values.
As stated above the eye is stimulated by the light coming from the object regardless of how the light was originated. Depending if the light source is modulated by the reflectance factors of the object and/or the object is self-luminous, the final perceived color can be seen as a mixture of both sources and understood in general terms according to the following scheme:

•	additive color mixing: the desired color can be represented as the mixture of light coming from three light sources with following normed colors: blue, green and red; an example of this method is the TV screen.
The main characteristic of additive color mixing is that the luminosity value of the resulting color is always higher than those of any of its components.

•	subtractive color mixing: the desired color can be represented as the mixture of light coming from three dyed samples illuminated with certain normed light source, with following normed colors: yellow, cyan and magenta; an example of this method is photography.
The main characteristic of subtractive color mixing is that the luminosity value of the resulting color is always lower than those of any of its components.

To calculate tri-stimulus values of samples that are both reflecting and self-luminous, both effects must be separated:


Color white
Part I - Basic principles

Chapter 2.- The color white

Being the color with the highest value of luminosity and no (or small) chroma, white is used in all applications where visibility and contrast are needed.
The psychological significance of white is important as an information carrier, most of the significances attributed to white are:

•	neutrality: this may have the significance of non-aggression (white flag for truce or deliberation) but also of cowardice (white flag as surrender). It is also interpreted as void, emptiness but also as a new start (in Orient white is the color of mourning though). The absence of chroma allows good combinations with all other colors.

•	high luminosity: as a sign of cleanness and chastity, untouched, new, unused. Contrast is based on differences of luminosity and white is the ideal background for those areas where high contrast is important (printing industry). The high luminosity is also an attention-catcher to be used fashion and art.

However it must be considered that the observer (unless well trained) is not in position to distinguish between (physical) color white and whiteness; it is the last concept that opens a wide horizon of applications allowing personal and cultural values to find an expression.
Whiteness must be understood as a cotraposition of yellow-blue that can be explained with the significance of the limiting colors yellow and blue:

•	yellow is associated with (negative) properties like old, used not fresh. In certain cultures yellow is associated with cowardice, while in Orient is reserved to erudites and monks.

•	blue is associated with coolnes, freshness and by the same token with something new; in general bleu is sign of tranquility

It is this yuxtaposition between yellow and blue that explains our preference for blueish object over those showing yellowness, a preference that is emphasized if accompanied with an increase (even sligth) of luminosity.
Preferred whites are chosen on grounds of social group and geographical and culture origin, although for several application certain shades of whites are preferred. While people in the Orient prefer a red-shaded white, Europeans prefer it neutral, and Latin Americans rather blue shaded; on the other hand underwear is always preferred in red shaded whites.
The amount of luminosity is important too, higher luminosity values result in more (perceived) beautiful whites.
In general it can be stated:

•	Whiteness will always be preferred over white, unless the latter shows more luminosity making the former appear dull.

•	each cultural group will set their own standards for the "most beautiful" white.


Contents
Table of Content

On whiteness

PREFACE

Part I - Basic principles

Chapter 1 - Introduction

1.1 - About light

1.2 - Assessing colors

Chapter 2 - The color white

2.1.- White vs. whiteness

2.2 - On near-whites

2.3 - On yellowness

2.4.- On color mixing

Chapter 3.- On whiteness

3.1.- The color of substrate

3.2.- Introduction to fluorescence

3.3.- The concept of shading

Part II - Measuring whiteness

Chapter 3.- Where are objects showing whiteness located?

Chapter 4 - One-dimensional whiteness formulas

4.1 - Formula after Berger

4.2.- Formula after Stensby

4.3.- Formula after Taube

4.2.- Whiteness index after ASTM

Chapter 5.- Two-dimensional whiteness formulas

5.1.- Method of Ganz and Griesser

5.2.- Formula after CIE

Chapter 6.- Instrumental assessment

6.1.- Adjusting UV to daylight conditions

6.2.- Calibration standards

Part III - Applications

Chapter 7.- Achieving high-whiteness values

7.1.- The importance of shading

7.2.- Evaluation of fluorescence on metameric effects

7.3.- Matching whiteness of pairs

Appendix (Books, reviews and norms)

Glossary

Index

Selected literature (1990-2001)

Dr. Claudio Puebla
Copyright © 2002 Axiphos GmbH. All rights reserved.
Revised: April 4, 2002.


Fluorescence
Part I - Basic principles

Chapter 3.- On whiteness

3.2.- Introduction to fluorescence

The light needed to compensate for the substrate yellowness is produced by a family of molecules known as Fluorescent Whitening Agents (FWA). These molecules have the property to absorb light in the near UV region (below 400 nm) and emit light (called fluorescence) at about 440 nm.
Fluorescent Whitening Agents belong to dyes family and can be incorporated into the substrate through any of following methods:

1.	exhaustion: for cotton and other cellulosic fibers presenting a surface with ionic character, ionic (normally anionic) FWAs are used that can be exhausted onto the fiber in a process similar to dyeing

2.	diffusion: for plastic materials (not having a ionic surface) non-ionic FWAs are used that diffuse into the substrate either by incorporation during extrusion or diffusion from a bath under (normally high) temperature

3.	pigmentation: some FWA pigments exist that can be incorporated through a coating process

The main limitation of FWAs is given on the available UV light, this being proportional to the amount of fluorescence produced and to the whitening power of the incorporated FWA. Thus special attention must be paid to the amount of absorption of the substrate in general and in particular in the region below 400 nm. In fact the region below 400 nm (that has no influence on the color appearance of the substrate to the human eye) dictates the amount of light left to excite the FWA, absorbing molecules present will compete with the FWA for the available light and reduce its power; in this context the use of UV absorbers (notably used in plastic and now also coming in use in textiles) may annihilate the fluorescence production and the resulting whitening effect.
In certain cases this is a desired effect (specially in the paper industry), for which families of fluorescence quenchers are available.
The fluorescence amount controls the extension of the final whitening effect and the position of the maximum controls the shade of the whiteness: shifting the fluorescence towards higher wavelengths results in green-shaded tones, shifting towards shorter wavelengths results in violet (redder) tones.
While the quantity of fluorescence is related to the amount of FWA, there some limits for the amount to be incorporated that are characteristic to each FWA and can be found in their specs sheets (process known as build-up); surpassing the high limit will result in a greenish effect (so called "greening") and in many cases in a drastic drop of the fluorescence amount, following points must be taken into consideration:

•	overloading the FWA (I): fluorescence of FWA is a process at the molecular level that involves an energy transformation process; increasing the concentration of the molecules will increase the interaction between the molecules, the energy conversion may be channeled into heat instead of fluorescence resulting in an overall drop in effectivity.

•	overloading the FWA (II): concentration increase favors the formation of dimers, the fluorescence of such dimers lies normally at lower energies. The overall result can be seen as the development of a shoulder fluorescence situated at higher wavelengths than 445 nm, adding a lemon green shade to the appearance

•	depletion of UV: the amount of UV present in the light source has finite values, a high concentration of FWA may deplete it to such extent that additional FWA molecules cannot be excited. The absorption of the FWA itself starts affecting the general appearance by shifting the fluorescence maximum towards higher wavelengths.

In general FWA are extremely effective and need quite low levels of application; typical values are 0.02% for plastics to 0.2% for cellulosics.


Glossary

A

AATCC

American Association of Textile and Colorist Chemists http://www.aatcc.org

ASTM

American Society for Testing and Materials http://www.astm.org

Axiphos GMBH

http://www.axiphos.com

B

base white

contribution of the substrate to the total perceived whiteness

bleach

destruction (normally involving chemicals like Chlorine or Oxygen) of light absorbing molecules responsible for yellowness in substrates

build-up

gain of whiteness for each additional amount of FWA

C

chroma

distance of the color from the achromatic point, also known as saturation

CIE

Commission Internationale de l'Éclairage http://www.cie.co.at

color compensation

color change through the action of a different color (see additive and subtractive mixing)

color mixing

there are two ways of mixing colors: additive mixing adds light intensities from colored sources, and subtractive mixing modulates the light from a single source with the absorption of dyes on a substrate

colorimeter

color measuring instrument having three filter simulating the tri-stimulus functions for a given illuminant

CRI

Color Rendering Index: level of approximation of a given light source to defined illuminant. The closer to value 100, the less are the metameric effects seen with a set of samples when changing from the light source to illuminant

D

D65

illuminant D65 of CIE: corresponds to spectral distribution of an average daylight

daylight

CIE D65 illuminant

Dr. Claudio Puebla

claudio.puebla@axiphos.com

F

fluorescence

emission of light by molecules that have been excited by absorption of light; the process of emission follows in the picosecond range after absorption

fluorescent white

contribution of the fluorescence (from FWA) to the total perceived whiteness

Fluorescent Whitening Agents

Fluorescent Whitening Agents (FWA) are chemical substances that upon excitation in the UV region (below 400 nm) produce blue fluorescence (peak at 445 nm) that readily compensates the yellowness of the substrate

FWA

Fluorescent Whitening Agents (FWA) are chemical substances that upon excitation in the UV region (below 400 nm) produce blue fluorescence (peak at 445 nm) that readily compensates the yellowness of the substrate

G

geometry

spatial configuration of instrument for illumination and observation of a sample

greening effect

shift of the whiteness to greenish shades as result of FWA overload; normally a strong decrease of whiteness is also observed

H

Hohensteiner Institute

http://www.hohenstein.de

hue

describes the position of a color as an angle value

I

invariant shade method

method of selection  of shading dyes to find a recipe leading to a shaded substrate with minimal shade changes with different illuminants

ISO

International Standardization Organization http://www.iso.ch

J

JIS

Japan Industrial Standards Committee http://www.jisc.org

L

light fastness

resistance of the dye to degradation upon exposure to light

luminosity

relative intensity of the color as valued by Y

M

metameric effects

denotes the phenomena that certain pairs appear to match in color under a certain illuminant, but depart when the illuminant is changed

metamerism

denotes the phenomena that certain pairs appear to match in color under a certain illuminant, but depart when the illuminant is changed

monochromatic

only one wavelength

monochromator

device that filtrates one single wavelength from a light source; in general monochromators are tunable to sweep a certain wavelength range

Munsell

Munsell color system: three dimensional arrangement of colors developed by artist Albert Munsell

O

opacity

also known as hiding power; reports the amount of light that is lost during the passage through a medium (opposite of translucency)

P

perfect diffuser

non-glossy material presenting 100% reflectance at all wavelengths

phosphorescence

emission of light by molecules that have been excited by absorption of light; the process of emission folows after milliseconds and can last for hours

polychromatic

many wavelengths at once

Q

QHT

Quartz Halogen Tungsten incandescent light; the quartz bulb let more UV through that normal incandescent sources (but not high enough to match the amount of dayligth). The added halogen prolongs the working life of the Tungsten incandescent coil

quencher

chemical added to eliminate fluorescence; passive quenchers are UV absorbers that "steal" the UV weakening the fluorescence production of the FWA, active quenchers interact actively with the FWA deactivacting the fluorescence process

S

shaded white

contribution of the shading agent to the total perceived whiteness

shading agent

blue or violet dye added to the substrate to increase whiteness perception

spectrophotometer

instrument that measures the relative intensity distribution of light in certain spectral region

STFI

Swedish Pulp and Paper Institute http://www.stfi.se

T

Tappi

Technical Association of the Pulp and Paper Industry (TAPPI) http://www.tappi.org

TITV

Textilforschungsinstitut Thüringen-Vogtland http://www.titv-greiz.de

tri-stimulus

response of eye color receptors to red (X), green (Y) and blue (Z)

U

UV light

light with wavelengths below 400 nm

W

wash fastness

resistance of the dye to be washed out

whiteness

property of an object by which it appears white to the human observer

X

Xenon flash lamp

this type of lamps emit high intensity light in the region 300 to 800 nm (and beyond); they emit high intensities in the UV region needed to match the amount present in daylight

Y

yellowness

attribute by which an object color is judged to depart from a preferred white toward yellow


High whiteness values
Part III- Applications

Chapter 7.- Achieving high whiteness values

The achievement of high whiteness values is a carefully orchestrated interplay of all three whiteness components and the opacity or the volume scattering of the sample. The latter is important because it controls the level of penetration of light into the sample, a factor that has a major impact onto the appearance of the sample; in fact its level controls:

•	opacity: if the sample is layered (textile, paper, etc) it is important to control the volume scattering within the sheet. Lower opacity values mean a deeper penetration into the material and the scattered light comes from deeper regions giving a translucency appearance readily detected by the human eye (but difficult to quantify instrumentally)

•	changes of base white: a more transparent material means a lowering in the base white as a consequence of light losses through side wards channels

•	changes of shaded white: a deeper penetration means a higher thickness and the absorption of the shading will be overvalued

•	changes of fluorescent white: a deeper penetration means a higher number of FWA molecules excited and automatically a higher level of fluorescence

Doubtless the most important point is to start with the proper level of base white; the optical properties of the substrate will dictate ultimately the behavior of shading dyes and fluorescence production. It is extremely important to study and determine carefully the optical properties of the substrate material in order to initiate an exploratory work leading to high levels of whiteness.


Importance of shading
Part III- Applications

Chapter 7.- Achieving high whiteness values

7.3.- The importance of shading

In general the human eye is more sensitive to changes in the shade rather than of the chroma, in the present case changes in the shade are seen more readily than of whiteness.
Most of the objects of daily use are seen under continuously changing illumination conditions, mainly between daylight conditions (with varying amounts of UV for indoor conditions) and artificial lighting (in many cases close to illuminant A).
Taking these two illuminants as opposite extremes of illumination conditions, one must conclude that materials are probed with two different spectral distributions, one emphasizing the blue region (daylight) and the other the orange-red region (illuminant A); in other words the whole spectrum of the visible region is involved, such that small differences in spectral values of the sample against the standard may result in large metameric effects. In fact the shaded white has a relative higher importance for illuminant A than for D65, on the other hand fluorescence is more affected by daylight.
The primary goal of shading is then not (only) to increase whiteness, but to control the shade of the underlying substrate and its influence onto the final appearance of the material. Two factors should be taken into consideration:

1.	shade of the substrate: while ideal greys and whites shows a neutral shade that is not affected by changes of the illuminant, natural (yellowish) materials show in general a reddish shade that is quickly compensated by the fluorescence, thus introducing a strong dependence on the amount of UV of the illuminant. Shading agent(s) should focus on ways to reduce changes of shade upon changes in illuminant (invariant shade method).

2.	production stability: the emergence of efficient continuous control methods makes more evident than ever the need for stable, robust recipes. A stability factor can be assigned to each recipe that gives information on its robustness (as production cost indicator); shading agents should be chosen within a general cost structure concept rather than on individual dye costs.


Index

A

About light

absorption coefficient

achromatic

additive color mixing

Adjusting UV levels

Appendix

Assessing colors

B

base white

High whiteness values

Matching whiteness pairs

Metameric effects

base white (definition)

BASF AG

Bayer AG

Berger whiteness

Bibliography (1990-2001)

black body

bleach

bleaching

blue directional reflectance factor

blue dyes

C

Calibration standards

chroma

chroma of whiteness samples

chromaticity chart

Ciba AG

CIE

Assessing colors

Formula after CIE

CIE tint

CIE whiteness

coefficient fitting

color compensation

color mixing

Color of substrate

color receptors

color saturation

Color white

color white

colorimeter

Contents

CRI

cultural preferences

D

daylight

diffuse ullumination

DIN

directional illumination

dominant wavelength

F

fluorescence

About light

Fluorescence

Fluorescence

fluorescence

fluorescence standard

fluorescent white

High whiteness values

Matching whiteness pairs

Metameric effects

fluorescent white (definition)

Fluorescent Whitening Agents (FWA)

Formula after CIE

Formula after Ganz and Griesser

FWA application

G

Ganz-Griesse whiteness

Ganz-Griesser method

Ganz-Griesser tint

Glossary

greening effect

H

high whiteness

High whiteness values

Hohensteiner Institute standards

hue

hue of whiteness samples

Hunterlab coordinates

I

illuminant A

illuminant D65

illuminants

Importance of shading

inscandescence

instrument geometry

instrumental assessment

Instrumental assessment

Introduction

invariant shade method

ISO brightness

J

J.R. Geigy AG

Formula after Ganz and Griesser

Whiteness after Stensby

L

light fastness

light sources

luminosity

luminosity of whiteness samples

M

Matching whiteness pairs

metameric effect

metameric effects

Metameric effects

metameric effects

metamerism

Munsell

N

near white

Near whites

NUVC

O

off-white

On color mixing

On whiteness

one-dimensional formulas

One-dimensional formulas

opacity

High whiteness values

Matching whiteness pairs

P

paper brightness

perceived whiteness

perfect diffuser

phosphorescence

Preface

primary standard

Q

QHT lamps

R

refraction index

S

scattering

scattering coefficient

secondary standard

self-luminous

shaded white

High whiteness values

Matching whiteness pairs

Metameric effects

The concept of shading

shaded white (definition)

shading agents

Importance of shading

The concept of shading

spectrophotometer

STFI standards

substrate

subtractive color mixing

T

Tappi brightness

Taube whiteness

The concept of shading

tint value

Titanium dioxide

TITV standards

translucency

tri-stimulus values of fluorescent samples

Two-dimensional formulas

two-dimensional whiteness formulas

two-monochromator method

U

UV adjustment

UV cut-off filter

UV depletion

V

violet dyes

visual assessment

W

wash fastness

white

White vs. whiteness

whiteness

Color white

White vs. whiteness

On whiteness

Whiteness location

Whiteness after Berger

Whiteness after Stensby

Whiteness after Taube

whiteness assessment

whiteness ASTM

whiteness components

Instrumental assessment

Metameric effects

whiteness formulas

One-dimensional formulas

Two-dimensional formulas

Whiteness index after ASTM

whiteness index ASTM

Whiteness location

whiteness matching

whiteness Stensby

working standard

X

Xenon flashlamps

Y

Yellowness

yellowness index

Dr. Claudio Puebla
Copyright © 2002 Axiphos GmbH. All rights reserved.
Revised: April 4, 2002.


Instrumental assessment
Part II- Measuring whiteness

Chapter 6.- Instrumental assessment

As stated before fluorescence depends on the intensity and intensity spectral profile of the exciting light, form this fact derives the statement: "fluorescent samples must be measured with the same illuminant used during visual assessment".
This represents a strong limitation to the measurement made with a particular instrument and to universal validity of the results.
Currently two methods are normally applied:

Two-monochromator method
With this configuration the sample is illuminated with a tunable monochromatic light (coming from one monochromator), at each wavelength the reflected light is probed by a second monochromator in the whole wavelength region. The result is a two-dimensional excitation-emission matrix that contains enough information to calculate the reflectance spectrum under any illumination condition.
This method is the only one able to produce correct results for any fluorescent sample, but only recently commercially available instruments have appeared; due to the high-level training needed for the personnel and the high cost of the instruments, there are certain reluctancy to acquire this type of instrument to be used in industrial quality control or production areas.

One-monochromator instruments
From the two possible configurations:

•	monochromatic illumination-polychromatic detection

•	polychromatic illumination-monochromatic detection

the latter one is the usually used one (both configurations will not give the same result for fluorescent samples).
Basically proper results with his configuration are obtained only and only if the light source is calibrated exactly to correspond to one illuminant; this is in general not the case (especially for Xenon flash lamps), whiteness values are only conditionally correct and the inter-instrument correlation fails specially for tint values.
New-generation instruments have more than one lamp to illuminate the sample; by using specially designed conditions it is possible to separate fluorescence from reflectance providing thus with enough information to achieve following points:

•	separation of fluorescence and reflectance allows the correct calculation of tri-stimulus values

•	fluorescence intensities can be calibrated to illuminants or conditions other than daylight

•	calculation of tri-stimulus values at varying intensities of UV are possible giving information about whiteness values for indoor conditions

•	reflectance factors allow to quantify the impact of different shading agents and control metameric effects during production

•	separation of whiteness components allows the design of control algorithms for continuous measurements


Introduction
Part I.- Basic principles

Chapter 1.- Introduction

Color perception is a basic function of living (and especially human) beings necessary for appreciation of natural environment; information provided by the color perception mechanism is processed by the brain resulting in vital information for operation, behavior and ultimately survival of the species.
While vision is quite widespread in nature, color vision is limited to those species with color receptors; color perception depends on the spectral distribution of the detected light i.e. intensities at each of the wavelengths within detection range. color receptors probe the distribution and react producing signals with different intensities, that are processed by the brain originating the color perception phenomena.
In general the process can be resumed as one stimulus (light) producing some electrical signal (color receptors in the eye) that are processed in the brain to evoke a biologically sensible conscious awareness; this relationship is matter of study of a discipline called psychophysics.
The human eye has three color receptors (connected to vision of green, red and blue) called "cones" and one more sensitive to light intensity, called rods. Physically color description is based on modelling the interactions between the light source illuminating the object, the object itself and the response of the color receptors; the description of color is related to a triad of numbers representing:

•	luminosity: this is related to the relative intensity of the light coming from the object and is connected to the response of the rods; its value is also connected to the concepts of white (high value) and black (low value).

•	chroma: this is the amount of color perceived by the eye; it is also known as color saturation; colors from monochromatic sources present the highest possible saturation values.

•	hue: this number accounts for the observed color and its value is also given as an angle going through red, yellow, to green and blue; sometimes it is also given to the related monochromatic source characterized by the so called "dominant" wavelength.

Within this context each color can be assigned unique values of luminosity, chroma and hue; two colors having the same triad values are called to be equivalent, in the sense that they are perceived as being the same under the given illumination and viewing conditions.
The color white has been matter of much controversy due to its unique characteristics and difficulties of physical production.
From a classical coloristic approach, white is indeed one of the basic colors in all color systems; it is normally defined as the contrary of black and as such it is recognized having the highest luminosity of all colors.
The total absence of hue and color saturation has been seen suspiciously to the point that traditionally it has not been considered as a color and given the property of being achromatic i.e. "without color"; by the same token black has been regarded not only as no color but also no luminosity and just next to void.
Most of the color systems consider white (and also black) only in an auxiliary sense as limit when color dilutes into "no color".
White can be considered as a color with following characteristics:

•	the highest luminosity

•	no saturation at all

•	no hue


Matching whiteness pairs
Part III- Applications

Chapter 7.- Achieving high whiteness values

7.3.- Matching whiteness of pairs

The problem posed by matching a given whiteness is sometimes more formidable than it appears at first sight. Certainly matching for just one illuminant is in most cases not difficult to achieve, to reach a level where metameric effects have been reduced to acceptable levels demands a systematic approach and most certainly an exact description of the whiteness components of the sample to be matched. In general each of the whiteness components must match (within metameric limits) in order to obtain an acceptable agreement of the samples.

Base white
Starting point for the matching job is to establish exactly the properties of the base white of the goal sample; not only the bleach level must be characterized but also the level of the opacity (for layered material) or of the volume scattering (for bulk samples).
Achieving a close match for these properties build a good base to complete the recipe, but normally it is not possible to match them to a satisfactory level such that a compromise (normally with the shading agent) must be met in order to continue.

Shaded white
Remembering that the shading agent not only affect the natural metameric potential of the substrate but also has an impact onto the opacity, caution mus be exercise to select the right dye in the proper proportion. Furthermore production stability must be also taken into consideration in order to produce a robust recipe. The absorption from the FWA must be considered also in order to obtain a stable final shade.

Fluorescent white
Selecting and adding the proper amount of FWA is the easier part if both preceding steps have been completed properly. In general a feedback is required to get the final recipe that fulfills whiteness, tint, opacity and metamerism boundary conditions


Metameric effects
Part III- Applications

Chapter 7.- Achieving high whiteness values

7.2.- Evaluation of impact of fluorescence on metameric effects

The problem posed by matching a given whiteness is sometimes more formidable than it appears at first sight. Certainly matching for just one illuminant is in most cases not difficult to achieve, to reach a level where metameric effects have been reduced to acceptable levels demands a systematic approach and most certainly an exact description of the whiteness components of the sample to be matched. In general each of the whiteness components must match (within metameric limits) in order to obtain an acceptable agreement of the samples.

Base white
Starting point for the matching job is to establish exactly the properties of the base white of the goal sample; not only the bleach level must be characterized but also the level of the opacity (for layered material) or of the volume scattering (for bulk samples).
Achieving a close match for these properties build a good base to complete the recipe, but normally it is not possible to match them to a satisfactory level such that a compromise (normally with the shading agent) must be met in order to continue.

Shaded white
Remembering that the shading agent not only affect the natural metameric potential of the substrate but also has an impact onto the opacity, caution mus be exercise to select the right dye in the proper proportion. Furthermore production stability must be also taken into consideration in order to produce a robust recipe. The absorption from the FWA must be considered also in order to obtain a stable final shade.

Fluorescent white
Selecting and adding the proper amount of FWA is the easier part if both preceding steps have been completed properly. In general a feedback is required to get the final recipe that fulfills whiteness, tint, opacity and metamerism boundary conditions


Near whites
Part I - Basic principles

Chapter 2.- The color white

2.2.- On near-whites

Near whites are colors with high luminosity (>65%) and having a finite chroma, however with values so small that they are not generally considered colored. According to norm ASTM E-313 chroma of near-whites are defined in terms of Munsell chroma as being:

•	less than 0.5 for B hues

•	less than 0.8 for Y hues

•	less than 0.3 for all other hues

Notice the lower values for hues other than B or Y, in fact the axis yellow-blue is the whiteness axis and samples having hues in this region are considered either having yellowness or being white; for the former even the expression "off-white" has been coined.

Pigments
Practically most of the natural or synthetic white pigments fall in the category of near-whites, in general belonging to Y hues.
During the refining process, pigments are subjected to bleaching and extraction of colored (in general heavy metal) ions.
Bleaching reduces absorption of the light increasing the light amount to be reflected; the reflectance process depends however further on the refraction index and on the particle size of the pigment.
Refraction index is a physical property of the material and can not be changed; the higher the values the higher is the amount of light reflected. Most of the natural pigments have quite similar refraction indices (about 1.6) with the remarkable exception of Titanium dioxide (n= 2.6).
The actions of refraction index and particle size distribution are combined in the scattering coefficient that is responsible for the amount of light reflected by the pigment; in this context the scattering coefficient affects the hiding power of the pigment, that is the important property for obtaining higher whiteness levels. While generally mineral pigments show scattering coefficients of 130-150 sq/kg, the superiority of Titanium dioxide with values of 1000 sq/kg is clearly indicated.


White vs. whiteness
Part I.- Basic principles

Chapter 2.- The color white

2.1.- White vs. whiteness

Reflectance. factors of the material give information on the level of interaction of the object with the incoming light, high numbers (for example near to 100%) denote low interaction, in other words low intensity losses and high reflectance.
For samples showing no losses all the incoming light will be reflected without (noticeable) intensity losses or change in the original spectral distribution (notice that a color will be perceived as soon as the spectral distribution changes); if in addition to this property the surface shows a very low (or non-existing) gloss the sample is said to be a perfect diffuser, color coordinates of a perfect diffuser are (Y,x,y) = (100,x0,y0) where (x0,y0) are the coordinates of the achromatic point for the corresponding illuminant.
A perfect diffuser is also a perfect white i.e. the maximum value of luminosity without any saturation or hue. Samples like these are very rare in the nature with remarkable exceptions like fresh fallen snow or clouds. On the other extreme samples that absorb all the incoming light (total loss) will appear black, the coordinates of such perfect black are (Y,x,y) = (0,x0,y0); notice that both samples differ just in the value of the luminosity.
In fact such an axis coming from ideal black to perfect diffuser is a natural axis for white colors, starting with black, going up to different levels of grey and ending with a perfect white.
White samples are then characterized by showing:

•	high levels of luminosity

•	no saturation at all

•	and consequently no hue

There are samples that appear to the eye as being white, this is a quite subjective perception and depends strongly on illumination conditions and observer himself. Samples that have this property are said to be perceived as white, their color coordinates do not lie however on the black-white axis, but they show a finite saturation level with a blue hue.
Samples showing whiteness are characterized by showing:

•	high levels of luminosity

•	finite saturation

•	blue hue

The reason for the perception of high luminescent blue samples as being white can not be formulated on theoretical or scientific basis and has to be left to psychophysics.


On whiteness
Part I - Basic principles

Chapter 3.- On whiteness

Whiteness is about perceived white i.e. those objects appearing white to the human observer. As such reflectance spectra of objects with whiteness depart strongly from those showing just the color white.
As defined earlier whiteness is defined as the contrary of yellowness i.e. objects showing whiteness present a blue hue.In fact only a yellow substrate can be made white by the methods presented here, since the final goal is connected to the elimination of the inherent yellowness of the substrate.
The final whiteness perception can be decomposed in whiteness components:

•	Base white: this correspond to the amount of absorption of the substrate, this being in general yellow. The existence of a finite amount of yellowness is however not mandatory to increase whiteness, perfect greys or a perfect white can also be added whiteness; other hues are not permitted.

•	Shaded white: this corresponds to a compensation of the yellowness through addition of a blue or violet dye. In colorimetric terms, this corresponds to a compensation through a subtractive color mixing process.

•	Fluorescent white: this corresponds to a compensation of the yellowness through addition of a blue light coming from a fluorescence process. In colorimetric terms, this corresponds to a compensation through an additive color mixing process.


One-dimensional formulas
Part II- Measuring whiteness

Chapter 4.- One-dimensional whiteness formulas

First attempt to describe whiteness were extensions of yellowness formulas that were designed around measurements done with different filters modelled after the color receptors of the eye.
With the introduction of FWAs it was clearly recognized that samples showing whiteness are situated in the blue area of the chromaticity chart and linear relationships of the type:

were tried to describe whiteness. It was established from the beginning that whiteness can be represented by a linear relationship of the form W= f(X,Y,Z), the formula given above is one of many possible ones and any space could be used such as (Y,x,y), (L,a,b) or (B,G,A); the coefficients must be found empirically, for example through a regression with visual assessment values.
One-dimensional formulas attempt to describe whiteness by just one number, the reference to a preferred white is thus already built-in in the numerical parameters. This is a serious drawback of the formulas, because they do not recognize the existence of cultural dependent existence of many preferred whites.


Preface

Measured by the number of colors perceived by the eye, the color white is by far the most encountered one in the human environment. Regardless of the area examined, textiles, paper, plastics or detergents, the color white is clearly the preferred color in many utilitarian and aesthetical applications.
On the other hand no much literature is readily available on this important theme, reflecting a series of serious problems encountered in the assessment and most remarkably in the instrumental measurement of white surfaces.
The emergence of Fluorescent Whitening Agents (FWA) during the fifties and their massive application during the next decades has been without doubt responsible for the enormous impetus experienced in this area. The addition of fluorescence makes it necessary to step up from white color to whiteness, adding a stronger psychophysical accent than encountered with other colors, and posing a challenge to visual assessment.
In fact fluorescence adds a new dimension to the industrial color area and, given the importance of white objects, it creates a subspace of its own called whiteness. On the other hand, fluorescence represents a challenge to measuring instruments; as a matter of fact many of the problems encountered in the whiteness areas originate in inadequacies of instrumental assessment.
The emergence of new techniques based on the availability of cheap and fast computer power brings solutions to many of the problems that the user come across while dealing with whiteness, thus adding fresh ideas and impetus to an old area to be rediscovered.
Much has to be done also on wide spreading information about whiteness concepts and literature.
As the managing director of Axiphos GmbH I want to present you with this small electronic book containing basic concepts on whiteness, its principles and measurement; it has no ambition to be a treatise on this theme or cover every of its aspects, but it represents a first-step to expose and explain concepts to the wide industrial audience.
The body of the booklet presents concepts of white and whiteness, the appendix contains titles of selected literature and norms.
Yours,
Dr. Claudio Puebla
Managing Director
Axiphos GmbH, Germany

April 2002


The concept of shading
Part I - Basic principles

Chapter 3.- On whiteness

3.3.- The concept of shading

Yellowness of substrate can be compensated through the addition of a violet or blue dye, color compensation results thus from a subtractive color mixing process. The type of dye used depends of the type of substrate, dyes or pigments can be used since only their color is important to achieve the effect; quantities used are measured in mgr. per kilo of substrate, posing an enormous problem for homogenization of baths or distribution within a solid substrate.
In general shading is regarded as a "cheap" way to increase whiteness, it is however a topic where much attention and care must be taken in order not to introduce unpredictable results or difficulties during production. Following points are worth to mention:

1.	light fastness: type of dye should ,match characteristics of materials and of the FWA applied in order not to introduce new variables into the system. Most applications require high levels of light fastness, this property should be considered carefully

2.	wash fastness: in textile area washing is considered important because garments must be cleaned and maintained; unfortunately it is common the use of dyes with low wash fastness resulting in an unrecoverable loss of whiteness

3.	violet or blue?: the complementary color for yellowness at high luminosity values is violet, at lower luminosity values blue is more effective.

4.	metameric effects: shading will change strongly the tint perception of resulting whiteness and introduce a change in the metameric behavior of the material with respect to standard. The use a an invariant shade technique is advised in order to avoid metamerism problems during production


Color of substrate
Part I - Basic principles

Chapter 3.- On whiteness

3.1.- The color of substrate

There are only three ways to make a substrate white:

1.	covering it with a white film: the coating must be made with a material having a high opacity in order to cover the original color of the substrate. The final white color depends thus solely on the properties of the coating and not on those of substrate.

2.	substrate bleach: this process involves the (chemical) destruction of absorbing components within the substrate, eliminating the light losses. In general even a prolonged bleach process ends up with a substantial gain of luminosity, but still a certain amount of yellowness will remain

3.	color compensation: the addition of complementary color of the substrate increases the whiteness perception. Although this process works theoretically for any substrate, industrial available dyes focus on those substrates presenting yellowness; the amount of yellowness is a limiting factor for the effectivity of the method, not any substrate can be whitened by this means.


Two-dimensional formulas
Part II- Measuring whiteness

Chapter 5.- Two-dimensional whiteness formulas

The introduction of a second dimension resolves the problem posed by the existence of multiple (culture dependent) preferred whites; each white sample is characterized by a whiteness number and a tint value calculated with formulas with the following form:

and

Whiteness numbers are referred to a neutral white characterized with the dominant wavelength of 472 nm; the perfect diffuser is assigned the whiteness value of 100.
Within this context whiteness of a sample is given by:

•	Whiteness: the distance from the achromatic point into the blue region along the dominant wavelength of 472 nm

•	Tint or shade value: the distance from the axis of neutral whites defined by the dominant wavelength of 472 nm


Whiteness location
Part II- Measuring whiteness

Chapter 3.- Where are objects showing whiteness located?

Samples showing whiteness will be located in the blue region of the CIE chromaticity chart and will fulfill both following conditions:

1.	luminosity: values over 70% will be considered as white, below this value samples will be considered as more or less grey

2.	chroma and hue: definitely hue must be situated between the dominant wavelengths 465 and 475 nm; the value of the chroma will depend strongly on the luminosity value, if too dark samples will be recognized as bluish instead of white.

An interesting detail is illustrated by those samples with very high values of luminosity, provided that the amount of additive color mixing is high enough, some samples can leave the CIE color solid and occupy a point exterior to it. While the colorimetric description does not suffer (provided the tri-stimulus values have been calculated correctly), this point illustrates the need to separate additive and subtractive color mixing process.


Whiteness index after ASTM
Part II. Measuring whiteness

Chapter 4.- One-dimensional whiteness formulas

4.2.- Whiteness index (ASTM)

Norm E-313 gives an extensive review of aspects and formulas to assess whiteness and appearance of white and near-white materials.
Interesting is the definition of a "blue directional reflectance factor" B, this is set identical with the CIE tri-stimulus function z; in many cases the factor B has been used as a measure of whiteness.
Whiteness is defined as "the attribute by which an object color is judged to approach some preferred white"; crucial is the recognition of the existence of a preferred white.
The whiteness index is defined as:

where Z and Y are the tri-stimulus values.
Important to mention are following points:

•	the formula requires the measurement with a source equivalent to illuminant C

•	instrument can be a three-filter colorimeter or spectrophotometer type with geometry 45°/0°

•	if fluorescent dyes are present or their presence is suspected, the whole wavelength region from UV to NIR must be included and reported with the results.


Whiteness after Berger
Part II.- Measuring whiteness

Chapter 4.- One-dimensional whiteness formulas

4.1.- Berger whiteness formula

This formula was developed by Mrs. A. Berger (formerly employee of Bayer AG, Germany) and presented in 1959.
Whiteness values calculated after the formula of Berger was very popular during the sixties until beginning of the eighties mainly in the paper and also textile areas.
The general form is:

where the numerical parameters are defined as:

	a	b
2° observer	3.440	3.895
10° observer	3.448	3.904

In general terms, the formula has a preference for greenish whites; in other words white samples having a greenish shade (compared to neutral white ones) will possess higher values for Berger whiteness .


Formula after CIE
Part II- Measuring whiteness

Chapter 5.- Two-dimensional whiteness formulas

5.2.- Formula after CIE

The CIE approved a whiteness and tint formula of the form:

and

where (x0,y0) are the coordinates of the achromatic point for the given illuminant.
The values of the coefficients are (strictly) valid for D65/10° conditions, however the formula has been used also with other illuminants.
Values of the tint have following meaning:

•	Tint >0: white has a greenish shade

•	Tint <0: white has a reddish shade


Formula after Ganz and Griesser
Part II- Measuring whiteness

Chapter 5.- Two-dimensional whiteness formulas

5.1.- Method of Ganz and Griesser

Dr. E. Ganz (formerly employee of Ciba AG, Switzerland) belong to the pioneer group on study of whiteness, as well as Mr. R. Griesser (formerly employee of J.R. Geigy, Switzerland). Their formula is the first one that is referred to a neutral white, to take differences in preferred whites the second dimension of the tint or shade is introduced.
The formula has the general form:

and

where the (nominal) coefficients are given for D65/10° as:

•	P= -1868.322

•	Q= -3695.690

•	C=  1809.441

•	m= -1001.223

•	n=    748.366

•	k=     68.261

Additionally:

•	Tint >0: white has a greenish shade

•	Tint <0: white has a reddish shade

•	samples differing in whiteness values less than 5 Ganz units appear undistinguishable to the human eye

•	samples differing in tint values less than 0.5 Ganz-Griesser units appear undistinguishable to the human eye

Due to inadequacies in the instruments for measuring whiteness, Mr. Griesser propagated a method where the coefficients of formulas are determined through a regression procedure using a set of fluorescent standards, coefficient get thus instrument specific values and can not be generalized.


Whiteness after Stensby
Part II. Measuring whiteness

Chapter 4.- One-dimensional whiteness formulas

4.2.- Stensby whiteness formula

This formula was developed by Mr. P. Stensby (formerly employee of J.R. Geigy AG, U.S.A.) and presented in 1968.
Whiteness values calculated after the formula of Stenby found ample application in the detergent area, specially in the United States where the use of Hunterlab system is widespread; today it has been replaced by the newly two-dimensional formulas and it is rarely seen.
The general form is:

where (L,a,b) are Hunter coordinates given by:

Stensby whiteness values have a preference for red shades i.e. red-shaded whites will show higher values on the Stensby scale.


Whiteness after Taube
Part II. Measuring whiteness

Chapter 4.- One-dimensional whiteness formulas

4.3.- Taube whiteness formula

The origin of this formula is not very clear, although it appears to be developed by Hunter it is attributed to Mr. Taube (formerly employee of BASF AG, Germany). It was mentioned in the literature before 1960.It has found ample application in the plastic sector.
The general form is:


Yellowness
Part I - Basic principles

Chapter 2.- The color white

2.3.- On yellowness

Yellowness is defined as "the attribute by which an object color is judged to depart from a preferred white toward yellow".
By this definition the object is clearly yellow, however the levels of luminosity are high  Evaluation of yellowness is very important because most of the materials, even after extensive bleaching, show a certain degree of yellowness.
Yellowness is at best characterized by the yellowness index, some formulas (extensively used in plastics) are given below:

Yellowness after BASF:

Yellowness ASTM-1925 and DIN 6167:

Yellowness ASTM-E 313-73:

All formulas are for C/2°.
Positive numbers are obtained for all yellow samples, basically formulas should be applied for those samples possessing dominant wavelengths between 570 and 580 nm. Blue samples will have negative values.

Paper brightness
Yellowness can be seen as a deficiency of blue or and absorption of blue light, all formulas given above can be understood in this context
In the paper area a related expression has been coined as paper brightness (also Tappi brightness) that reflects exactly the amount of yellowness in the pulp or paper sample; its general expression is given as:

where the function B(λ) is a bell-shaped function peaking at 460 nm with a band width of about 60 nm and defined in the norms (for example Tappi). Note that the function is not normalized and it is strictly one-dimensional in other words just the reflectance values within the narrow wavelength around 460 nm will determine the value of B.
Originally paper brightness was a quick method to establish the reflectance intensity in the blue light absorbing region and strictly tailored to pulp bleaching, where the color always goes from dark brown to pale yellow; it was done visually by using a blue filter through which samples were compared against a certain standard.
In spite of its physical (and not colorimetric) character, it is still quite widespread in the paper industry.


AATCC

American Association of Textile and Colorist Chemists http://www.aatcc.org


ASTM

American Society for Testing and Materials http://www.astm.org


Axiphos GMBH

http://www.axiphos.com


base white

contribution of the substrate to the total perceived whiteness


bleach

destruction (normally involving chemicals like Chlorine or Oxygen) of light absorbing molecules responsible for yellowness in substrates


build-up

gain of whiteness for each additional amount of FWA


chroma

distance of the color from the achromatic point, also known as saturation


CIE

Commission Internationale de l'Éclairage http://www.cie.co.at


color compensation

color change through the action of a different color (see additive and subtractive mixing)


color mixing

there are two ways of mixing colors: additive mixing adds light intensities from colored sources, and subtractive mixing modulates the light from a single source with the absorption of dyes on a substrate


colorimeter

color measuring instrument having three filter simulating the tri-stimulus functions for a given illuminant


CRI

Color Rendering Index: level of approximation of a given light source to defined illuminant. The closer to value 100, the less are the metameric effects seen with a set of samples when changing from the light source to illuminant


D65

illuminant D65 of CIE: corresponds to spectral distribution of an average daylight


daylight

CIE D65 illuminant


Dr. Claudio Puebla

claudio.puebla@axiphos.com


fluorescence

emission of light by molecules that have been excited by absorption of light; the process of emission follows in the picosecond range after absorption


fluorescent white

contribution of the fluorescence (from FWA) to the total perceived whiteness


Fluorescent Whitening Agents

Fluorescent Whitening Agents (FWA) are chemical substances that upon excitation in the UV region (below 400 nm) produce blue fluorescence (peak at 445 nm) that readily compensates the yellowness of the substrate


FWA

Fluorescent Whitening Agents (FWA) are chemical substances that upon excitation in the UV region (below 400 nm) produce blue fluorescence (peak at 445 nm) that readily compensates the yellowness of the substrate


geometry

spatial configuration of instrument for illumination and observation of a sample


greening effect

shift of the whiteness to greenish shades as result of FWA overload; normally a strong decrease of whiteness is also observed


Hohensteiner Institute

http://www.hohenstein.de


hue

describes the position of a color as an angle value


invariant shade method

method of selection  of shading dyes to find a recipe leading to a shaded substrate with minimal shade changes with different illuminants


ISO

International Standardization Organization http://www.iso.ch


JIS

Japan Industrial Standards Committee http://www.jisc.org


light fastness

resistance of the dye to degradation upon exposure to light


luminosity

relative intensity of the color as valued by Y


metameric effects

denotes the phenomena that certain pairs appear to match in color under a certain illuminant, but depart when the illuminant is changed


metamerism

denotes the phenomena that certain pairs appear to match in color under a certain illuminant, but depart when the illuminant is changed


monochromatic

only one wavelength


monochromator

device that filtrates one single wavelength from a light source; in general monochromators are tunable to sweep a certain wavelength range


Munsell

Munsell color system: three dimensional arrangement of colors developed by artist Albert Munsell


opacity

also known as hiding power; reports the amount of light that is lost during the passage through a medium (opposite of translucency)


perfect diffuser

non-glossy material presenting 100% reflectance at all wavelengths


phosphorescence

emission of light by molecules that have been excited by absorption of light; the process of emission folows after milliseconds and can last for hours


polychromatic

many wavelengths at once


QHT

Quartz Halogen Tungsten incandescent light; the quartz bulb let more UV through that normal incandescent sources (but not high enough to match the amount of dayligth). The added halogen prolongs the working life of the Tungsten incandescent coil


quencher

chemical added to eliminate fluorescence; passive quenchers are UV absorbers that "steal" the UV weakening the fluorescence production of the FWA, active quenchers interact actively with the FWA deactivacting the fluorescence process


shaded white

contribution of the shading agent to the total perceived whiteness


shading agent

blue or violet dye added to the substrate to increase whiteness perception


spectrophotometer

instrument that measures the relative intensity distribution of light in certain spectral region


STFI

Swedish Pulp and Paper Institute http://www.stfi.se


Tappi

Technical Association of the Pulp and Paper Industry (TAPPI) http://www.tappi.org


TITV

Textilforschungsinstitut Thüringen-Vogtland http://www.titv-greiz.de


tri-stimulus

response of eye color receptors to red (X), green (Y) and blue (Z)


UV light

light with wavelengths below 400 nm


wash fastness

resistance of the dye to be washed out


whiteness

property of an object by which it appears white to the human observer


Xenon flash lamp

this type of lamps emit high intensity light in the region 300 to 800 nm (and beyond); they emit high intensities in the UV region needed to match the amount present in daylight


yellowness

attribute by which an object color is judged to depart from a preferred white toward yellow