A = 2 - log10 %T
At an absorbance of 6, only one 10,000th of one percent of a particular wavelength is being transmitted through the filter (lens). Absorbance is measured with a spectrophotometer, which establishes the light transmission and calculates the absorbance. However, the spectrophotometer can only measure absorbance up to 4.5 directly. Beyond this level all values must be extrapolated. For example, if a 2 mm thick filter is measured to have an absorbance of 3, then it is assumed that a 4 mm thick filter should have an absorbance of 6.