ASTM E991-21

Standard Practice for Color Measurement of Fluorescent Specimens Using the One-Monochromator Method

SIGNIFICANCE AND USE
5.1 The most general method for obtaining CIE tristimulus values or, through their transformation, other coordinates for describing the colors of fluorescent objects is by the use of spectrometric data obtained under defined and controlled conditions of illumination and viewing. This practice describes the instrumental measurement requirements, calibration procedures, and material standards needed for measuring the total spectral radiance factors of fluorescent specimens illuminated by simulated daylight approximating CIE D65 and calculating total tristimulus values and total chromaticity coordinates for either the CIE 1931 or 1964 observers.  
5.2 The precise colorimetry of fluorescent specimens requires the spectral distribution of the instrument light source illuminating the specimen closely duplicate the colorimetric illuminant used for the calculation of tristimulus values, which is CIE D65 in this practice. The fundamental basis for this requirement follows from the defining property of a fluorescent specimen: instantaneous light emission resulting from electronic excitation by absorption of radiant energy (η) where the wavelengths of emission (λ) are as a rule longer than the excitation wavelengths (1).7 For a fluorescent specimen, the total spectral radiance factors used to calculate tristimulus values are the sum of two components – an ordinary reflectance factor, β(λ)S, and a fluorescence factor, β(η,λ)F : β(λ) = β(λ)S + β(η,λ)F. Ordinary spectral reflectance factors are solely a function of the specimen's reflected radiance efficiency at the viewing wavelength (λ) and independent of the spectral distribution of the illumination. The values of the spectral fluorescent radiance factors at the viewing wavelength (λ) vary directly with the absolute spectral distribution of illumination within the excitation range (η), and consequently so will the total spectral radiance factors and derived colorimetric values. One-monochromator colorimetric spectrometers use...
SCOPE
1.1 This practice applies to the instrumental color measurement of fluorescent specimens excited by near ultraviolet and visible radiation that results in fluorescent emission within the visible range. It is not intended for other types of photoluminescent materials such as phosphorescent, chemiluminescent, or electroluminescent, nor is this practice intended for the measurement of the fluorescent properties for chemical analysis.  
1.2 This practice describes the instrumental measurement requirements, calibration procedures, and material standards needed for the color measurement of fluorescent specimens when illuminated by simulated daylight approximating CIE Standard Illuminant D65 (CIE D65).  
1.3 This practice is limited in scope to colorimetric spectrometers providing continuous broadband polychromatic illumination of the specimen and employing only a viewing monochromator for analyzing the radiation leaving the specimen.  
1.4 This practice can be used for calculating total tristimulus values and total chromaticity coordinates for fluorescent colors in the CIE Color System for either the CIE 1931 Standard Colorimetric Observer or the CIE 1964 Supplementary Standard Colorimetric Observer.  
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 31-May-2021
• ICS: 17.180.20 - Colours and measurement of light
• Technical Committee: E12 - Color and Appearance
• Drafting Committee: E12.05 - Fluorescence

Relations

• Refers: ASTM E2214-23 - Standard Practice for Specifying and Verifying the Performance of Color-Measuring Instruments
• Effective Date: 01-Dec-2023
• Refers: ASTM E1247-12(2023) - Standard Practice for Detecting Fluorescence in Object-Color Specimens by Spectrophotometry
• Effective Date: 01-Nov-2023
• Refers: ASTM E1164-23 - Standard Practice for Obtaining Spectrometric Data for Object-Color Evaluation
• Effective Date: 01-Nov-2023
• Refers: ASTM E2214-20 - Standard Practice for Specifying and Verifying the Performance of Color-Measuring Instruments
• Effective Date: 01-May-2020
• Refers: ASTM E2214-19 - Standard Practice for Specifying and Verifying the Performance of Color-Measuring Instruments
• Effective Date: 01-May-2019
• Refers: ASTM E1345-98(2019) - Standard Practice for Reducing the Effect of Variability of Color Measurement by Use of Multiple Measurements
• Effective Date: 01-May-2019
• Refers: ASTM E2214-18 - Standard Practice for Specifying and Verifying the Performance of Color-Measuring Instruments
• Effective Date: 15-Oct-2018
• Refers: ASTM E1247-12(2017) - Standard Practice for Detecting Fluorescence in Object-Color Specimens by Spectrophotometry
• Effective Date: 01-Nov-2017
• Refers: ASTM E308-17 - Standard Practice for Computing the Colors of Objects by Using the CIE System
• Effective Date: 01-May-2017
• Refers: ASTM E2214-17 - Standard Practice for Specifying and Verifying the Performance of Color-Measuring Instruments
• Effective Date: 01-May-2017
• Refers: ASTM E2214-16 - Standard Practice for Specifying and Verifying the Performance of Color-Measuring Instruments
• Effective Date: 01-Nov-2016
• Refers: ASTM D2244-15a - Standard Practice for Calculation of Color Tolerances and Color Differences from Instrumentally Measured Color Coordinates
• Effective Date: 01-Aug-2015
• Refers: ASTM E308-15 - Standard Practice for Computing the Colors of Objects by Using the CIE System
• Effective Date: 01-Apr-2015
• Refers: ASTM D2244-15 - Standard Practice for Calculation of Color Tolerances and Color Differences from Instrumentally Measured Color Coordinates
• Effective Date: 01-Jan-2015
• Refers: ASTM D2244-15e1 - Standard Practice for Calculation of Color Tolerances and Color Differences from Instrumentally Measured Color Coordinates
• Effective Date: 01-Jan-2015

Overview

ASTM E991-21: Standard Practice for Color Measurement of Fluorescent Specimens Using the One-Monochromator Method provides a comprehensive framework for the instrumental color measurement of fluorescent specimens. Developed by ASTM International, this standard focuses on obtaining precise color values for materials that emit visible light when excited by near-ultraviolet or visible radiation. The practice details the requirements for instrumentation, calibration, specimen preparation, and procedures necessary to calculate critical colorimetric parameters, specifically under simulated daylight conditions approximating CIE Standard Illuminant D65.

This standard is widely used for accurately quantifying the color properties of fluorescent materials in industries such as textiles, plastics, and safety marking. It emphasizes the importance of controlling measurement parameters to ensure repeatability, reproducibility, and traceability of results, enhancing quality assurance and facilitating compliance with international color measurement norms.

Key Topics

• Scope of Application: Applies to the instrumental color measurement of fluorescent specimens that emit visible fluorescence when excited by near ultraviolet or visible light. It does not address other photoluminescent materials (e.g., phosphorescent, chemiluminescent, or electroluminescent substances).
• Simulated Daylight (CIE D65 Illumination): Measurement conditions must closely replicate CIE D65, a standardized spectral power distribution for daylight used in colorimetric calculations.
• Instrumentation Requirements: Specifies the use of one-monochromator colorimetric spectrometers, typically with bidirectional (45:0 or 0:45 geometry), to ensure consistent analysis of reflected and fluorescent light.
• Calibration and Standardization:
  • Use of non-fluorescent white reflectance standards, system verification materials, and calibrated verification artifacts to ensure accuracy.
  • Regular verification of instrument calibration, including reflectance scale, wavelength accuracy, and stray light levels.
• Measurement and Reporting Procedures: Details on specimen preparation, measurement conditions, and the reporting of tristimulus values and chromaticity coordinates for both CIE 1931 and CIE 1964 observers.
• Significance: Addresses the challenges of measuring fluorescent colors, noting that results are highly sensitive to the spectral characteristics of instrument illumination and measurement geometry.

Applications

ASTM E991-21 is essential in sectors where fluorescent color measurement is critical for product performance and regulatory compliance:

• Textiles and Plastics: Quality control of goods treated with fluorescent dyes or optical brighteners.
• Safety and Traffic Signage: Evaluating the color properties of fluorescent retroreflective sheeting and safety apparel under standard lighting conditions.
• Material Science and Manufacturing: Ensuring color consistency in production batches for materials designed for high-visibility applications.
• Research and Development: Supporting studies on color behavior under various illumination conditions for new fluorescent materials.

By following ASTM E991-21, laboratories and manufacturers achieve:

• Enhanced inter-laboratory agreement and traceability.
• More reliable comparison of color measurements over time.
• Improved communication regarding material properties in supply chains.

Related Standards

For comprehensive color measurement and analysis, refer to related ASTM and international standards, including:

• ASTM E1164: Practice for Obtaining Spectrometric Data for Object-Color Evaluation
• ASTM E1247: Practice for Detecting Fluorescence in Object-Color Specimens by Spectrophotometry
• ASTM E308: Practice for Computing the Colors of Objects by Using the CIE System
• ASTM E1345: Practice for Reducing the Effect of Variability of Color Measurement by Use of Multiple Measurements
• ASTM E2152/E2153: Practices for Computing Colors of Fluorescent Objects and Obtaining Bispectral Photometric Data
• ISO 11664-2 / CIE S005/E-1998: CIE Standard Illuminants for Colorimetry

Each of these standards interfaces with ASTM E991-21 to ensure consistent, accurate, and internationally accepted color measurement of fluorescent specimens. Adherence to these practices supports global quality and color harmonization initiatives.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E991 − 21
Standard Practice for
Color Measurement of Fluorescent Specimens Using the
One-Monochromator Method
This standard is issued under the fixed designation E991; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
Thefundamentalprocedureforevaluatingthecolorofafluorescentobjectistoobtainspectrometric
data for specified illuminating and viewing conditions, and then use this data to compute tristimulus
values based on an International Commission on Illumination (CIE) standard observer and a CIE
standard illuminant. For a fluorescent object-color specimen, the spectral radiance factors used to
calculate tristimulus values are made up of two components — an ordinary reflectance factor and a
fluorescence factor (β = β + β ).The magnitude of the fluorescent radiance factors, and consequently
S F
the measured total radiance factors and derived color values, vary directly with the spectral
distribution of the instrument source illuminating the specimen. Consequently, the colorimetry of
fluorescent object-color specimens requires greater control of the measurement parameters in order to
obtainprecisespectrometricandcolorimetricdata.Inordertoobtainrepeatableandreproduciblecolor
values for fluorescent objects it is necessary that the illumination at the specimen surface closely
duplicate the standard illuminant used in the color calculations. The considerations involved and the
procedures used to obtain spectrometric data and compute colorimetric values for fluorescent
specimens using a one-monochromator spectrometer are contained in this practice.
1. Scope 1.4 Thispracticecanbeusedforcalculatingtotaltristimulus
values and total chromaticity coordinates for fluorescent colors
1.1 This practice applies to the instrumental color measure-
in the CIE Color System for either the CIE 1931 Standard
ment of fluorescent specimens excited by near ultraviolet and
Colorimetric Observer or the CIE 1964 Supplementary Stan-
visible radiation that results in fluorescent emission within the
dard Colorimetric Observer.
visible range. It is not intended for other types of photolumi-
nescent materials such as phosphorescent, chemiluminescent,
1.5 This standard does not purport to address all of the
or electroluminescent, nor is this practice intended for the
safety concerns, if any, associated with its use. It is the
measurement of the fluorescent properties for chemical analy-
responsibility of the user of this standard to establish appro-
sis.
priate safety, health, and environmental practices and deter-
mine the applicability of regulatory limitations prior to use.
1.2 This practice describes the instrumental measurement
requirements, calibration procedures, and material standards 1.6 This international standard was developed in accor-
needed for the color measurement of fluorescent specimens dance with internationally recognized principles on standard-
when illuminated by simulated daylight approximating CIE ization established in the Decision on Principles for the
Standard Illuminant D65 (CIE D65).
Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical
1.3 This practice is limited in scope to colorimetric spec-
Barriers to Trade (TBT) Committee.
trometers providing continuous broadband polychromatic illu-
mination of the specimen and employing only a viewing
2. Referenced Documents
monochromator for analyzing the radiation leaving the speci-
men.
2.1 ASTM Standards:
This practice is under the jurisdiction of ASTM Committee E12 on Color and
Appearance and is the direct responsibility of Subcommittee E12.05 on Fluores-
cence. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved June 1, 2021. Published June 2021. Originally contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
approved in 1984. Last previous edition approved in 2016 as E991 – 16. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/E0991-21. the ASTM website.
Copyright ©ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA19428-2959. United States
E991 − 21
D985 Test Method for Brightness of Pulp, Paper, and Paper- 3. Terminology
board (Directional Reflectance at 457 nm) (Withdrawn
3.1 Definitions—The definitions contained in Guide E179,
2010)
Terminology E284, Practice E1164, Practice E1767, and Prac-
D2244 Practice for Calculation of Color Tolerances and
tice E2153 are applicable to this practice.
Color Differences from Instrumentally Measured Color
3.2 Definitions of Terms Specific to This Standard:
Coordinates
E179 Guide for Selection of Geometric Conditions for 3.2.1 fluorescence, n—this standard uses the term “fluores-
Measurement of Reflection and Transmission Properties cence” as a general term, including both true fluorescence
-8
of Materials (with a luminescent decay time of less than 10 s) and
E284 Terminology of Appearance phosphorescence with a delay time short enough to be indis-
E308 PracticeforComputingtheColorsofObjectsbyUsing tinguishable from fluorescence for the purpose of colorimetry
the CIE System (see Practice E2153).
E691 Practice for Conducting an Interlaboratory Study to
3.2.2 fluorescent white, n—white and near white specimens
Determine the Precision of a Test Method
containing fluorescent whitening agents.
E1164 PracticeforObtainingSpectrometricDataforObject-
3.2.3 near ultraviolet radiation, n—optical radiation within
Color Evaluation
the wavelength range from 300 to 380 nm.
E1247 Practice for Detecting Fluorescence in Object-Color
Specimens by Spectrophotometry
3.2.4 referee procedure, n—a mutually agree upon testing
E1345 Practice for Reducing the Effect of Variability of
procedure utilized to resolve disputes over instrumentally
Color Measurement by Use of Multiple Measurements
tested material properties that are expressed numerically.
E1767 Practice for Specifying the Geometries of Observa-
tion and Measurement to Characterize the Appearance of
4. Summary of Practice
Materials
4.1 This practice applies to the instrumental color measure-
E2152 Practice for Computing the Colors of Fluorescent
ment of fluorescent specimens that are excited by near ultra-
Objects from Bispectral Photometric Data
violet and visible radiation and emit within the visible range.
E2153 Practice for Obtaining Bispectral Photometric Data
For methods to determine whether specimens exhibit fluores-
for Evaluation of Fluorescent Color
cence see Practice E1247. This practice provides procedures
E2214 Practice for Specifying and Verifying the Perfor-
for measuring the total spectral radiance factors of fluorescent
mance of Color-Measuring Instruments
object-color specimens under simulated daylight approximat-
E2301 Test Method for Daytime Colorimetric Properties of
ing CIE D65 using a one-monochromator colorimetric spec-
Fluorescent Retroreflective Sheeting and Marking Mate-
trometer and calculating total tristimulus values (XYZ) and
rials for High Visibility Traffic Control and Personal
total chromaticity coordinates (x,y) in the CIE Color System
Safety Applications Using 45°:Normal Geometry
for either the CIE 1931 Standard Colorimetric Observer or the
2.2 CIE Publications and Standards:
CIE 1964 Supplementary Standard Colorimetric Observer (see
CIE Publication CIE15:2018 Colorimetry, 4th Edition
CIE Publication 15).
CIE Publication No: 51.2 AMethod forAssessing the Qual-
ity of Daylight Simulators for Colorimetry
4.2 The instrument source should provide broadband illu-
CIE Publication No. 76 Intercomparison on Measurement of
mination of the specimen from 300 to 780 nm and the spectral
(Total) Spectral Radiance Factor of Luminescent Speci-
distribution of the illumination on the specimen should closely
mens
duplicate CIE D65 (see ISO 11664-2:2007(E) ⁄CIE S14/
2.3 TAPPI Standards:
E:2006). When highest measurement precision and reproduc-
T 571om-03 Diffuse brightness of paper and paperboard
ibility are required, the wavelength range should extend from
(d/0)
300 to 830 nm. Precise colorimetry of ultraviolet-activated
2.4 ISO Standards:
fluorescent specimens requires the instrument provide signifi-
ISO 11664-2:2007(E)⁄CIE S14/E:2006 CIE Standard Illu-
cantilluminationintensitybelow380nm.Forthemeasurement
minants for Colorimetry
of visible-activated fluorescent specimens, which have negli-
ISO 11475:2017 Paper and board — Determination of CIE
gible excitation below 380 nm, it is only required that the
whiteness, D65/10 degrees (outdoor daylight)
illuminationonthespecimenprovideaclosematchtoCIED65
ISO 2469:2014 Paper, board and pulps — Measurement of
over the wavelength range 380 to 780 nm.
diffuse reflectance factor
4.3 The colorimetric spectrometer should employ a bidirec-
tional optical measuring system with 45:0 or 0:45 illuminating
The last approved version of this historical standard is referenced on
and viewing geometry. The wavelength dispersive element
www.astm.org.
4 (monochromator) shall be positioned between the specimen
Available from the CIE Webshop (www.cie.co.at) or from the National
andthedetectorsystem(seeCIEPub. 76).Theinstrumentmay
Committees of the CIE (for the USA cie-usnc.org).
Available from TechnicalAssociation of the Pulp and Paper Industry (TAPPI),
employ annular, circumferential, or uniplanar influx or efflux
15 Technology Parkway South, Norcross, GA 30092, http://www.tappi.org.
optics. The use of Practice E1767 functional notation is
Available from International Organization for Standardization (ISO), ISO
recommended for the complete description of instrumentation
Central Secretariat, Chemin de Blandonnet 8, CP 401, 1214 Vernier, Geneva,
Switzerland, https://www.iso.org. geometry including cone angles, aperture size, etc. When the
E991 − 21
specimen exhibits directionality, and an instrument with uni- cent radiance factors at the viewing wavelength (λ) vary
planar geometry is used, information on directionality may be directly with the absolute spectral distribution of illumination
obtained by measuring the specimens at two or more rotation within the excitation range (η), and consequently so will the
angles. If information on directionality is not required, then total spectral radiance factors and derived colorimetric values.
multiple uniplanar measurements may be averaged, or an One-monochromator colorimetric spectrometers used in this
instrument with annular or circumferential geometry may be practice are generally designed for the color measurement of
used. However, even with annular or circumferential influx or ordinary (non-fluorescent) specimens and the precision with
efflux optics, some of the variability induced by specimen- which they can measure the color of fluorescent specimens is
optical system interactions may remain and the application of directly dependent on how well the instrument illumination
the methods in Practice E1345 may help to reduce measure- simulates CIE D65.
ment variability.
5.3 CIE D65 is a virtual illuminant that numerically defines
4.4 The important steps in the calibration of such a standardized spectral illumination distribution for daylight
instruments,andthematerialstandardsrequiredforthesesteps, and not a physical light source (2). There is no CIE recom-
are described. Guidelines are given for the selection of speci- mendation for a standard source corresponding to CIE D65 nor
mens to minimize the specimen’s contribution to the measure- is there a standardized method for rating the quality (or
ment imprecision. Parameters are identified that must be adequacy) of an instrument’s simulation of CIE D65 for the
specified when spectrometric measurements are required in generalinstrumentalcolorimetryoffluorescentspecimens.The
specific test methods or other documents. requirement that the instrument simulation of CIE D65 shall
have a rating not worse than BB (CIELAB) as determined by
4.5 Most modern colorimetric spectrometers have the ca-
the method of CIE Publication 51 has often been referenced.
pacity to compute the color coordinates of the specimen
However, the method of CIE 51 is only suitable for ultraviolet-
immediatelyfollowingthemeasurement.Whenthisisthecase,
excited specimens evaluated for the CIE 1964 (10°) observer.
the user shall select the CIE Color System and CIE D65, then
The methods described in CIE 51 were developed for UV
chose either the CIE 1931 (2°) Standard Observer or CIE 1964
activated fluorescent whites and have not been proven to be
(10°) Supplementary Observer (see Practice E308).
applicable to visible-activated fluorescent specimens.
5. Significance and Use
NOTE 1—Aging of the instrument lamp will occur with normal usage
resulting in changes in the spectral distribution and intensity of the
5.1 The most general method for obtaining CIE tristimulus
illumination on the specimen over time. Measurement of the spectral
values or, through their transformation, other coordinates for
distribution of the illumination at the sample port and evaluation of the
describing the colors of fluorescent objects is by the use of
adequacy of the CIE D65 simulation at regular intervals are recom-
spectrometric data obtained under defined and controlled
mended.
conditions of illumination and viewing.This practice describes
5.4 Differences in the absolute spectral irradiance distribu-
the instrumental measurement requirements, calibration
tion on the specimen between instrument models can produce
procedures, and material standards needed for measuring the
significantvariationinthemeasuredcolorvaluesoffluorescent
total spectral radiance factors of fluorescent specimens illumi-
specimens and result in poor reproducibility (3). In order to
nated by simulated daylight approximating CIE D65 and
reproduce adequately the spectral irradiance on the specimen
calculating total tristimulus values and total chromaticity
required for maximum measurement reproducibility, it may be
coordinates for either the CIE 1931 or 1964 observers.
necessary for a single model of instrument to be specified for
use by both buyer and seller.
5.2 The precise colorimetry of fluorescent specimens re-
quires the spectral distribution of the instrument light source
5.5 This practice is primarily for the instrumental color
illuminating the specimen closely duplicate the colorimetric
measurementofchromaticfluorescentspecimens.Whileuseof
illuminant used for the calculation of tristimulus values, which
this practice for the color measurement of fluorescent whites is
is CIE D65 in this practice. The fundamental basis for this
not precluded, other standards are more commonly used for
requirementfollowsfromthedefiningpropertyofafluorescent
measurement of these types of specimens (4, 5, 6) (see Test
specimen: instantaneous light emission resulting from elec-
Methods D985, ISO 11475, ISO 2469, and TAPPI T 571).
tronic excitation by absorption of radiant energy (η) where the
5.6 For geometrically sensitive fluorescent specimens angu-
wavelengths of emission (λ) are as a rule longer than the
7 lar tolerances on the axes and the angular aperture sizes must
excitation wavelengths (1). For a fluorescent specimen, the
bewelldefinedbytheusertoensureadequaterepeatabilityand
total spectral radiance factors used to calculate tristimulus
reproducibility. Significant variation in measurement results
valuesarethesumoftwocomponents–anordinaryreflectance
for engineered surfaces and optical materials, for example
factor, β(λ) , and a fluorescence factor, β(η,λ) : β(λ)= β(λ) +
S F S
retroreflective sheeting, can result from differences in the
β(η,λ) . Ordinary spectral reflectance factors are solely a
F
absolute axis angles of illumination and viewing and absolute
function of the specimen’s reflected radiance efficiency at the
size of the apertures between instruments (7). In order to
viewing wavelength (λ) and independent of the spectral distri-
replicate the measurement geometry, absolute angles and
bution of the illumination. The values of the spectral fluores-
angular tolerances between instruments that is required for
maximum measurement reproducibility, it may be necessary
for a single model of instrument to be specified for use by both
The boldface numbers in parentheses refer to the list of references at the end of
this practice. buyer and seller.
E991 − 21
NOTE 2—To ensure inter-instrument agreement in the measurement of
6.2 The spectrometer should provide continuous broadband
specimens with intermediate gloss, for formulation, or retroreflective
illumination of the specimen at a minimum from 340 to 700
specimens, tight geometric tolerances are required of the instrument axis
nm, preferably from 300 to 830 nm, and the spectral distribu-
angles and the instrument aperture angles.
tion of the illumination should closely duplicate CIE D65.
5.7 Bidirectional (45:0 or 0:45) geometry is recommended
6.2.1 For the measurement of ultraviolet-activated fluores-
for this practice.
cent specimens the instrument should provide illumination on
5.7.1 Hemisphericalgeometryusinganintegratingsphereis
the specimen at a minimum from 340 to 380 nm, preferably
notrecommendedbecauseofthespectralsphereerrorresulting
from 300 to 380 nm, and the spectral distribution of that
from radiation emitted by the fluorescent specimen reflecting
illumination should closely duplicate CIE D65.
off the sphere wall and re-illuminating the specimen, thereby
6.2.2 For the measurement of visible-activated fluorescent
changing the spectral illuminance distribution on the specimen
specimens the instrument should provide illumination on the
from that of the original instrument source (8).
specimen at a minimum from 380 to 700 nm, preferably from
380to780nm,andthespectraldistributionofthatillumination
NOTE 3—The spectral sphere error associated with hemispherical
should closely duplicate CIE D65.
geometry decreases as the ratio of the internal area of the sphere to the
measurement area increases. When the spectral sphere error is negligible,
6.3 The wavelength measurement interval should be 10 nm
results obtained using hemispherical geometry may for some specimens
or less. See Practice E308 and Practice E1164 for spectral
under specific measurement conditions approach those obtained using
bandpass recommendations.
45:0 geometry (9).
6.4 The instrument should be capable of reporting total
5.8 This practice provides procedures for selecting the
spectralradiancefactorvaluesasafunctionofwavelengthover
operating parameters of spectrometers used for providing data
the range from 400 to 700 nm in increments of 10 nm or less.
of the desired precision. It also provides for instrument
calibration by means of artifact standards and selection of The preferred range is from 380 to 780 nm.
suitable specimens for obtaining precision in the measure-
6.5 The instrument or an attached computer should have the
ments.
capacity to compute the color coordinates of the specimen
immedi
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: E991 − 16 E991 − 21
Standard Practice for
Color Measurement of Fluorescent Specimens Using the
One-Monochromator Method
This standard is issued under the fixed designation E991; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
The fundamental procedure for evaluating the color of a fluorescent object is to obtain spectrometric
data for specified illuminating and viewing conditions, and then use this data to compute tristimulus
values based on an International Commission on Illumination (CIE) standard observer and a CIE
standard illuminant. For a fluorescent object-color specimen, the spectral radiance factors used to
calculate tristimulus values are made up of two components — an ordinary reflectance factor and a
fluorescence factor (β = β + β ). The magnitude of the fluorescent radiance factors, and consequently
S F
the measured total radiance factors and derived color values, vary directly with the spectral
distribution of the instrument source illuminating the specimen. Consequently, the colorimetry of
fluorescent object-color specimens requires greater control of the measurement parameters in order to
obtain precise spectrometric and colorimetric data. In order to obtain repeatable and reproducible color
values for fluorescent objects it is necessary that the illumination at the specimen surface closely
duplicate the standard illuminant used in the color calculations. The considerations involved and the
procedures used to obtain spectrometric data and compute colorimetric values for fluorescent
specimens using a one-monochromator spectrometer are contained in this practice.
1. Scope
1.1 This practice applies to the instrumental color measurement of fluorescent specimens excited by near ultraviolet and visible
radiation that results in fluorescent emission within the visible range. It is not intended for other types of photoluminescent
materials such as phosphorescent, chemiluminescent, or electroluminescent, nor is this practice intended for the measurement of
the fluorescent properties for chemical analysis.
1.2 This practice describes the instrumental measurement requirements, calibration procedures, and material standards needed for
the color measurement of fluorescent specimens when illuminated by simulated daylight approximating CIE Standard Illuminant
D65 (CIE D65).
1.3 This practice is limited in scope to colorimetric spectrometers providing continuous broadband polychromatic illumination of
the specimen and employing only a viewing monochromator for analyzing the radiation leaving the specimen.
1.4 This practice can be used for calculating total tristimulus values and total chromaticity coordinates for fluorescent colors in
the CIE Color System for either the CIE 1931 Standard Colorimetric Observer or the CIE 1964 Supplementary Standard
Colorimetric Observer.
This practice is under the jurisdiction of ASTM Committee E12 on Color and Appearance and is the direct responsibility of Subcommittee E12.05 on Fluorescence.
Current edition approved Nov. 1, 2016June 1, 2021. Published November 2016June 2021. Originally approved in 1984. Last previous edition approved in 20112016 as
E991 – 11.E991 – 16. DOI: 10.1520/E0991-16.10.1520/E0991-21.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E991 − 21
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.6 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D985 Test Method for Brightness of Pulp, Paper, and Paperboard (Directional Reflectance at 457 nm) (Withdrawn 2010)
D2244 Practice for Calculation of Color Tolerances and Color Differences from Instrumentally Measured Color Coordinates
E179 Guide for Selection of Geometric Conditions for Measurement of Reflection and Transmission Properties of Materials
E284 Terminology of Appearance
E308 Practice for Computing the Colors of Objects by Using the CIE System
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
E1164 Practice for Obtaining Spectrometric Data for Object-Color Evaluation
E1247 Practice for Detecting Fluorescence in Object-Color Specimens by Spectrophotometry
E1345 Practice for Reducing the Effect of Variability of Color Measurement by Use of Multiple Measurements
E1767 Practice for Specifying the Geometries of Observation and Measurement to Characterize the Appearance of Materials
E2152 Practice for Computing the Colors of Fluorescent Objects from Bispectral Photometric Data
E2153 Practice for Obtaining Bispectral Photometric Data for Evaluation of Fluorescent Color
E2214 Practice for Specifying and Verifying the Performance of Color-Measuring Instruments
E2301 Test Method for Daytime Colorimetric Properties of Fluorescent Retroreflective Sheeting and Marking Materials for High
Visibility Traffic Control and Personal Safety Applications Using 45°:Normal Geometry
2.2 CIE Publications and Standards:
CIE Publication CIE15:2004CIE Publication CIE15:2018 Colorimetry, 3rd4th Edition
CIE Publication No: 51.2 A Method for Assessing the Quality of Daylight Simulators for Colorimetry
CIE Publication No. 76 Intercomparison on Measurement of (Total) Spectral Radiance Factor of Luminescent Specimens
2.3 TAPPI Standards:
T 571om-03 Diffuse brightness of paper and paperboard (d/0)
2.4 ISO Standards:
ISO 10526:1999ISO 11664-2:2007(E) ⁄CIE S005/E-1998S14/E:2006 CIE Standard Illuminants for Colorimetry
ISO 11475:2004ISO 11475:2017 Paper and board — Determination of CIE whiteness, D65/10 degrees (outdoor daylight)
ISO 2469:1994ISO 2469:2014 Paper, board and pulps — Measurement of diffuse reflectance factor
3. Terminology
3.1 Definitions—The definitions contained in Guide E179, Terminology E284, Practice E1164, Practice E1767, and Practice
E2153 are applicable to this test method.practice.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 fluorescence, n—this standard uses the term “fluorescence” as a general term, including both true fluorescence (with a
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luminescent decay time of less than 10 s) and phosphorescence with a delay time short enough to be indistinguishable from
fluorescence for the purpose of colorimetry (see Practice E2153).
3.2.2 fluorescent white, n—white and near white specimens containing fluorescent whitening agents.
3.2.3 near ultraviolet radiation, n—optical radiation within the wavelength range from 300 to 380 nm.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
The last approved version of this historical standard is referenced on www.astm.org.
Available in hard copy or on CD-ROM at CIE/USA c/o TLA, 7 Pond St., Salem, MA 01970 [e-mail] or electronically downloadable via the website from
the CIE Webshop (www.cie.co.at) or from the National Committees of the CIE Central Bureau (www.cie.co.at).(for the USA cie-usnc.org).
Available from Technical Association of the Pulp and Paper Industry (TAPPI), 15 Technology Parkway South, Norcross, GA 30092, http://www.tappi.org.
Available from International Organization for Standardization (ISO), 1, ch. de la Voie-Creuse, Case postale 56, CH-1211, Geneva 20, Switzerland, http://www.iso.ch.ISO
Central Secretariat, Chemin de Blandonnet 8, CP 401, 1214 Vernier, Geneva, Switzerland, https://www.iso.org.
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3.2.4 referee procedure, n—a mutually agree upon testing procedure utilized to resolve disputes over instrumentally tested material
properties that are expressed numerically.
4. Summary of Practice
4.1 This practice applies to the instrumental color measurement of fluorescent specimens that are excited by near ultraviolet and
visible radiation and emit within the visible range. For methods to determine whether specimens exhibit fluorescence see Practice
E1247. This practice provides procedures for measuring the total spectral radiance factors of fluorescent object-color specimens
under simulated daylight approximating CIE D65 using a one-monochromator colorimetric spectrometer and calculating total
tristimulus values (XYZ) and total chromaticity coordinates (x,y) in the CIE Color System for either the CIE 1931 Standard
Colorimetric Observer or the CIE 1964 Supplementary Standard Colorimetric Observer (see CIE Publication 15).
4.2 The instrument source should provide broadband illumination of the specimen from 300 to 780 nm and the spectral distribution
of the illumination on the specimen should closely duplicate CIE D65 (see ISO 10526:1999ISO⁄CIE S005/E-1998). 11664-
2:2007(E) ⁄CIE S14/E:2006). When highest measurement precision and reproducibility are required, the wavelength range should
extend from 300 to 830 nm. Precise colorimetry of ultraviolet-activated fluorescent specimens requires the instrument provide
significant illumination intensity below 380 nm. For the measurement of visible-activated fluorescent specimens, which have
negligible excitation below 380 nm, it is only required that the illumination on the specimen provide a close match to CIE D65
over the wavelength range 380 to 780 nm.
4.3 The colorimetric spectrometer should employ a bidirectional optical measuring system with 45:0 or 0:45 illuminating and
viewing geometry. The wavelength dispersive element (monochromator) shall be positioned between the specimen and the detector
system (see CIE Pub. 76). The instrument may employ annular, circumferential, or uniplanar influx or efflux optics. The use of
Practice E1767 functional notation is recommended for the complete description of instrumentation geometry including cone
angles, aperture size, etc. When the specimen exhibits directionality, and an instrument with uniplanar geometry is used,
information on directionality may be obtained by measuring the specimens at two or more rotation angles. If information on
directionality is not required, then multiple uniplanar measurements may be averaged, or an instrument with annular or
circumferential geometry may be used. However, even with annular or circumferential influx or efflux optics, some of the
variability induced by specimen-optical system interactions may remain and the application of the methods in Practice E1345 may
help to reduce measurement variability.
4.4 The important steps in the calibration of such instruments, and the material standards required for these steps, are described.
Guidelines are given for the selection of specimens to minimize the specimen’s contribution to the measurement imprecision.
Parameters are identified that must be specified when spectrometric measurements are required in specific test methods or other
documents.
4.5 Most modern colorimetric spectrometers have the capacity to compute the color coordinates of the specimen immediately
following the measurement. When this is the case, the user shall select the CIE Color System and CIE D65, then chose either the
CIE 1931 (2°) Standard Observer or CIE 1964 (10°) Supplementary Observer (see Practice E308).
5. Significance and Use
5.1 The most general method for obtaining CIE tristimulus values or, through their transformation, other coordinates for
describing the colors of fluorescent objects is by the use of spectrometric data obtained under defined and controlled conditions
of illumination and viewing. This practice describes the instrumental measurement requirements, calibration procedures, and
material standards needed for measuring the total spectral radiance factors of fluorescent specimens illuminated by simulated
daylight approximating CIE D65 and calculating total tristimulus values and total chromaticity coordinates for either the CIE 1931
or 1964 observers.
5.2 The precise colorimetry of fluorescent specimens requires the spectral distribution of the instrument light source illuminating
the specimen closely duplicate the colorimetric illuminant used for the calculation of tristimulus values, which is CIE D65 in this
practice. The fundamental basis for this requirement follows from the defining property of a fluorescent specimen: instantaneous
light emission resulting from electronic excitation by absorption of radiant energy (η) where the wavelengths of emission (λ) are
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as a rule longer than the excitation wavelengths (1).). For a fluorescent specimen, the total spectral radiance factors used to
calculate tristimulus values are the sum of two components – an ordinary reflectance factor, β(λ) , and a fluorescence factor, β(η,λ)
S F
: β(λ) = β(λ) + β(η,λ) . Ordinary spectral reflectance factors are solely a function of the specimen’s reflected radiance efficiency
S F
at the viewing wavelength (λ) and independent of the spectral distribution of the illumination. The values of the spectral fluorescent
radiance factors at the viewing wavelength (λ) vary directly with the absolute spectral distribution of illumination within the
excitation range (η), and consequently so will the total spectral radiance factors and derived colorimetric values. One-
monochromator colorimetric spectrometers used in this practice are generally designed for the color measurement of ordinary
(non-fluorescent) specimens and the precision with which they can measure the color of fluorescent specimens is directly
dependent on how well the instrument illumination simulates CIE D65.
5.3 CIE D65 is a virtual illuminant that numerically defines a standardized spectral illumination distribution for daylight and not
a physical light source (2). There is no CIE recommendation for a standard source corresponding to CIE D65 nor is there a
standardized method for rating the quality (or adequacy) of an instrument’s simulation of CIE D65 for the general instrumental
colorimetry of fluorescent specimens. The requirement that the instrument simulation of CIE D65 shall have a rating not worse
than BB (CIELAB) as determined by the method of CIE Publication 51 has often been referenced. However, the method of CIE 51
is only suitable for ultraviolet-excited specimens evaluated for the CIE 1964 (10°) observer. The methods described in CIE 51 were
developed for UV activated fluorescent whites and have not been proven to be applicable to visible-activated fluorescent
specimens.
NOTE 1—Aging of the instrument lamp will occur with normal usage resulting in changes in the spectral distribution and intensity of the illumination on
the specimen over time. Measurement of the spectral distribution of the illumination at the sample port and evaluation of the adequacy of the CIE D65
simulation at regular intervals are recommended.
5.4 Differences in the absolute spectral irradiance distribution on the specimen between instrument models can produce significant
variation in the measured color values of fluorescent specimens and result in poor reproducibility (3). In order to reproduce
adequately the spectral irradiance on the specimen required for maximum measurement reproducibility, it may be necessary for
a single model of instrument to be specified for use by both buyer and seller.
5.5 This practice is primarily for the instrumental color measurement of chromatic fluorescent specimens. While use of this
practice for the color measurement of fluorescent whites is not precluded, other standards are more commonly used for
measurement of these types of specimens (4, 5, 6) (see Test Methods D985, ISO 11475, ISO 2469, and TAPPI T 571).
5.6 For geometrically sensitive fluorescent specimens angular tolerances on the axes and the angular aperture sizes must be well
defined by the user to ensure adequate repeatability and reproducibility. Significant variation in measurement results for engineered
surfaces and optical materials, for example retroreflective sheeting, can result from differences in the absolute axis angles of
illumination and viewing and absolute size of the apertures between instruments (7). In order to replicate the measurement
geometry, absolute angles and angular tolerances between instruments that is required for maximum measurement reproducibility,
it may be necessary for a single model of instrument to be specified for use by both buyer and seller.
NOTE 2—To ensure inter-instrument agreement in the measurement of specimens with intermediate gloss, for formulation, or retroreflective specimens,
tight geometric tolerances are required of the instrument axis angles and the instrument aperture angles.
5.7 Bidirectional (45:0 or 0:45) geometry is recommended for this practice.
5.7.1 Hemispherical geometry using an integrating sphere is not recommended because of the spectral sphere error resulting from
radiation emitted by the fluorescent specimen reflecting off the sphere wall and re-illuminating the specimen, thereby changing the
spectral illuminance distribution on the specimen from that of the original instrument source (8).
NOTE 3—The spectral sphere error associated with hemispherical geometry decreases as the ratio of the internal area of the sphere to the measurement
area increases. When the spectral sphere error is negligible, results obtained using hemispherical geometry may for some specimens under specific
measurement conditions approach those obtained using 45:0 geometry (9).
The boldface numbers in parentheses refer to the list of references at the end of this practice.
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5.8 This practice provides procedures for selecting the operating parameters of spectrometers used for providing data of the
desired precision. It also provides for instrument calibration by means of artifact standards and selection of suitable specimens for
obtaining precision in the measurements.
5.9 Bispectral colorimetry using a bidirectional optical measuring system with a 45:0 or 0:45 illuminating and viewing geometry
should be used when a high level of repeatability and reproducibility are required. The bispectral, or two-monochromator, method
is the definitive method for the determination of the general radiation-transfer properties of fluorescent specimens. The bispectral
method is accepted as the referee procedure for obtaining illuminant-independent photometric data on a fluorescent specimen that
can be used to calculate its color for any desired illuminant and observer. The advantage of the bispectral method is that it avoids
the inaccuracies associated with source simulation and various methods of approximation (10, 11)) (see Practices E2152, E2153,
and Test Method E2301).
6. Apparatus
6.1 One-monochromator colorimetric spectrometer providing continuous broadband polychromatic illumination of the specimen
intended to simulate CIE D65 and having the monochromator positioned
...