Abstract: Methods for internally and externally photostandardizing chemical assays, e.g. an ATP-bioluminescence assay. A pre-determined amount of a photosensitive derivative of the analyte of interest is used in the assay protocol which releases a known amount of the free analyte when it is exposed to a flash of visible light of pre-determined duration and intensity. By monitoring the response of a test property of the assay to the release of a known quantity of analyte, a standard value can be calculated which allows either direct determination of the amount of analyte originally present in the assayed sample or production of a photocalibration series against which results of an assayed sample can be compared.