Rationale

From the minutes of the December 2013 meeting of ASTM E12.01:






Leland shared some observations arising from a work in progress, which suggest that certain terms introduced along with this standard, and later adopted in E284, may need to be reviewed and revised. Explanation: When we measure the color of a non-fluorescent object, we measure its spectral reflectance factor (R(?)), which is the ratio of the flux emitted by the sample into a given solid angle of collection to that which would be emitted by the perfect reflecting diffuser (PRD) under identical conditions of illumination and view. When we measure fluorescent materials, we must find a more general term for the measurand, one which is ‘agnostic’ concerning the process (e.g. reflectance or photoluminescence) by which the emitted flux arises. The members of ASTM E12, and the wider colorimetric community, have adopted spectral radiance factor (?(?)) as this more general term. ?(?) is defined as the ratio of the spectral radiance of the sample to that which would be emitted by the PRD, etc. It is generally accepted that this is the generalized equivalent of spectral reflectance factor, and that the reflectance component of spectral radiance factor (?R(?)) is essentially identical to R(?). Leland argued that it is not, and further, that ?(?) is not what we measure when we measure samples (fluorescent or not) with a spectrophotometer. The true measurand is actually something that might be called spectral flux factor, with a definition nearly identical to that of R(?), except for the fact that it allows the flux emitted from the sample to be either photoluminescent or reflectant.
Leland presented the definitions of ? and R presented in E284, and also in CIE 17.4 (the ILV). Both sets of definitions clearly indicate that ? is defined only “in a given direction.” The CIE definition of R also includes the following two notes:
NOTE 2 If the solid angle [of collection] approaches 2p sr, the reflectance factor approaches the reflectance….
NOTE 3 If the solid angle [of collection] approaches 0, the reflectance factor approaches the radiance… factor.
Leland also presented the E284 and CIE 17.4 definitions for bispectral (luminescent) radiance factor (??L(?)). Both of these omit the critical phrase “in a given direction,” and therefore, wrongly imply that it is possible to define ??L(?) for a measurement geometry with a significant solid angle of collection, such as a directional/hemispherical geometry. To correct this problem, Leland suggests that it will be necessary to review and revise the definition of bispectral radiance factor, and related terms such as spectral efficiency factor (b(?)), discrete bispectral radiance factor (B(???)), and Donaldson radiance factor (B(???)). He invited members of E12.05, and other interested parties, to join him in working on this problem, and will create a working group with the objective of drafting revisions to E2153 and E284 accordingly. Hugh Fairman, Mike Brill, and Richard Austin will participate in this work group.