Standard Reference Method - Tristimulus Color

Tristimulus Color

There has been interest in tristimulus reporting in the brewing community in recent years and the ASBC has an approved Method of Analysis for tristimulus characterization. The absorption of the sample is measured in 1 cm at 81 wavelengths separated by 5 nm starting at 380 nm and extending to 780 nm. These are converted to transmission values (by taking the antilogarithm of each absorption) and inserting the results into ASTM E-308. The reported tristimulus values are in L*a*b* color space and describe what is seen under Illuminant C (daylight) by a 10° observer when the path is 1 cm. The choice of path, illuminant, observer and color space does not represent a limitation of E-308 but rather rather the ASBC's need to standardize reporting.

If we are given only the SRM value for a beer we can compute the approximate transmission spectrum if the beer has average spectral characteristics simply by taking the antilog of :

This can be used with E-308 to calculate tristimulus color in any path, for any illuminant, for either observer in any colorspace derivable from CIE XYZ space. This formula could, for example, be used to compute color patches to be printed on transparency or card stock for use in evaluating the SRM of actual beers but color swatches prepared in this way are only valid for the illuminant, observer and path used in the E-308 calculation. The BJCP color guide was prepared in this way. This illustrates that the SRM does convey full color information if the beer has average spectral characteristics. If it does not then we need more information than just the SRM provides.