Abstract: A device for measuring transmittance of light comprises a light source, photodetector and signal processor. The concentration of a substance in a fluid sample can be accurately determined by coating the light source with a reagent that is chemically reactive with the substance and measuring light transmitted from the coating before and after the substance is applied to the coating. The coating can contain a solid support matrix, such as a translucent nonwoven paper web of the type used in reagent test strips and similar indicator devices. The invention is useful for making clinical analyses on body fluids.