Sensitive dual wavelength densitometer with suppression of the support introduced background optical density

An instrument for the direct photometric scanning of zones is described. The method involves the alternative flashing of two beams of light through the support. The wavelengths used are the peak of absorption and a selected reference at one of the sides of the absorption curve of the colored compound in the zone. This technique allows an inherent suppression of the background optical density of the support while maintaining full sensitivity and specificity. This results in a performance increase of about two orders of magnitude when sensitivity or precision are compared with single wavelength operating instruments.