The Standard Reference Method (SRM) of Beer Color Specification
as the Basis for a New Method of Beer Color Reporting
A. J.
deLange[1], Zeta Associates, Fairfax,VA
Abstract
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.