Abstract: Light, pulsed or continuous at a wavelength (e.g. 780 nm), fluoresceces a specimen. The specimens may be combinations of a dye (preferably labelled), an antigen (e.g. rubella) and an antibody reactive with the antigen, with properties of polarizing the light when fluoresced. The light polarized in a first direction (e.g. z-axis) parallel to the incident light and in a second direction (e.g. x-axis) perpendicular to the incident light are measured. A second specimen is then provided with the antigen and the antibody but without the dye. The same light as discussed above fluoresces the second specimen and polarizes the light when fluoresced. The light polarized in the first (z-axis) and second (x-axis) directions in the second specimen is measured. These measurements are processed in a microprocessor with the measurements in the z and x directions in the first specimen to identify the antigen or, when the antigen is known, to identify the concentration of the antigen in the first specimen.