ASTM E991-21

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 (?) where the wavelengths of emission (?) 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, ?(?)_S, and a fluorescence factor, ?(?,?)_{5.3?CIE D65 is a virtual illuminant that numerically defines a standardized spectral illumination distribution for daylight and not a physical light source 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 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 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 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.

5.8?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 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.