So I realize not many people have access to the necessary equipment to directly measure the color of a beer, but fortunately, I work in a research lab and have all this stuff sitting around, waiting to be used by curious brewers such as myself. For those that do have access to a UV/Vis spectrophotometer, centrifuge, and syringe filters, I'll note my procedure below.
Wikipedia (which was the only source I could which gave the spectroscopic determination of beer color) defines the SRM of a beer as the absorbance at 430 nanometers times a constant and a dilution factor.
SRM = 12.7 * D * A, where D is the dilution factor (for an undiluted sample, D = 1, for a sample diluted 1:1 with deionized water, D = 2, etc.) and A is the absorbance at 430 nm.
When I ran my ESB through, I got an absorbance of 1.36, which resulted in an SRM value of 17.3 -- what? An SRM of 17.3 is something expected from a dark lager, not a golden ESB. However, an EBC value of 17.3 looks right on, and conversion to SRM using the formula SRM = EBC * 0.508 results in an SRM of 8.8, which is close to the color calculated by my brewing software, 9.5.
Is Wikipedia wrong, or are my measurements incorrect? Does anyone have a reference for these measurements?
For those interested, here is my procedure:
1. Dispensed 35mL of beer into a centrifuge tube
2. Centrifuged at 10,000 RPM for 10 minutes
3. Passed supernate through a 0.3 micron syringe filter
4. Decanted filtrate to a small vial
5. Pipetted 3 mL into a plastic cuvette
6. Measured the absorbance at 700 nm (This measurement will tell you if your sample is clear of turbidity - a value under 0.039 times the absorbance at 430 nm means your sample is acceptably turbidity free)
7. Measured the absorbance at 430 nm
A(700) = 0.005784
A(430) = 1.36414