Reflectance digital matrix photometry.

We have designed and constructed a digital matrix photometer for quantitative measurement of reflected light of small chromophoric areas or colored spots. The areas are divided conceptually into small subunits in which the reflected light is measured. This is done through stepwise scanning with a photodiode array. The sum of the individual reflectance measurements is related to the total amount of an analyte in the spot. We have found that this technique makes possible accurate and precise quantitation of light-absorbing substances in small areas of different sizes, shapes, and internal irregularity measuring as little as 1 mm across.