The Standard Reference Method (SRM) of Beer Color Specification as the Basis for a New Method of Beer Color Reporting
A. J. deLange, Zeta Associates, Fairfax,VA
Ensembles of beer optical absorption spectra normalized by SRM exhibit small variance suggesting that KL (Karhunen-Loeve, Principal Component) approximations of these spectra using only a few coefficients might facilitate reconstruction of the originals with enough fidelity to permit calculation of accurate tristimulus color. To test this hypothesis the spectra of 99 beers were measured and grouped into ensembles which were used as bases for computation of KL coefficients. Approximations to spectra were reconstructed from small numbers of these coefficients. CIELAB (tristimulus) colors computed from reconstructed and original spectra were compared using CIELAB as the metric. The sensitivity of color error to the form of the spectrum (transmission or absorption, normalized or not), number of coefficients, optical path length, illuminant, observer and ensemble makeup were studied. The data and analysis indicate that accurate () tristimulus color can be obtained under various viewing conditions from the SRM value and 2 to 6 KL coefficients. SRM plus some small number of KL coefficients thus constitutes a full reporting of visible beer color.
Key Words: color, spectrum, SRM, tristimulus, Pricipal Components, CIELAB.