The SRM number is defined as 10 times the absorbance of a sample at 430 nanometers measured through a .5-inch cell. The 430-nanometer wavelength corresponds to a deep blue light, and is the wavelength at which beers appear most different from each other.
The standard was adopted in 1950 by the American Society of Brewing Chemists as an objective measurement of color unburdened by the difficulties of the Lovibond system; the measurement of the color of a beer in degrees SRM and degrees Lovibond are approximately equal and in practice can be used interchangeably to evaluate the color of intensity of beer.
Degrees SRM are related to the current EBC standard by:
Note that an earlier version of EBC color was based on absorbance at 530 nanometers, which permitted no direct conversion between the two systems. However, if one assumes a linear log absorbance spectrum (the Linner hypothesis from the realm of caramel color), and knows the Linner Hue Index, , the absorbances are related by:
A formula for converting between the old EBC color value and SRM sometimes continues to appear in literature. It should not be used, as it is flawed and based on measurements which are no longer taken.