Standard Reference Method

The Standard Reference Method or SRM is one of several systems modern brewers use to specify beer color. Determination of the SRM value involves measuring the attenuation of light of a particular wavelength (430 nm) in passing through 1 cm of the beer, expressing the attenuation as an absorption and scaling the absorption by a constant (12.7 for SRM; 25 for EBC). The SRM (or EBC) number represents a single point in the absorption spectrum of beer. As such it cannot convey full color information which would require 81 points, but it does remarkably well in this regard (it conveys 92% of spectral information) even when fruit beers are considered. Auxiliary "deviation coefficients" (see Augmented SRM below) can pick up the remainder and are necessary for fruit beers and when subtle color differences in malt beers are to be characterized.

The ASBC and EBC measurements are now identical (both done at the same wavelength and in the same size cuvette) but the scaling is different. A photometer or spectrophotometer is used to measure the attenuation of light at 430 nm, as it passes through 1 cm of beer contained in a standard 1 cm by 1 cm cuvette. The absorption is the log of the ratio of the intensity of the light beam entering the sample to the intensity leaving. This difference is multiplied by 12.7 in the SRM system and 25 in the EBC (see below). For example, if the light intensity leaving is one one hundredth the light intensity entering the ratio is 100, the absorption is 2 and the SRM is 25.4. The scale factor derives from the original definition of SRM discussed in the next paragraph.

The SRM number was originally, and still is, defined by "Beer color intensity on a sample free of turbidity and having the spectral characteristics of an average beer is 10 times the absorbance of the beer measured in a 1/2-inch cell with monochromatic light at 430 nanometers."^[1] Modern spectrophotometers use 1 cm cuvettes rather than 1/2 inch ones. When a 1 cm cuvette is used, application of the Bouguer-Beer-Lambert law shows that the multiplier should be 12.7 rather than 10. When the SRM value for a beer or wort is larger than about 30 the log linear limit of some instruments using 1 cm cuvettes is approached. In such cases the sample is diluted with deionized water. Using Beer-Lambert again gives the mathematical definition of SRM in the general case as:

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