Abstract: Light, pulsed or continuous at a particular wavelength (e.g. 780 nm), fluoresces a specimen. The specimen may be combinations of an antigen (e.g. rubella) labelled with a fluorescent dye, unlabeled antigen or hapten and an antibody reactive with the antigen or hapten. The light polarized in a first direction (e.g. z-axis) parallel to the electric field of the incident light and in a second direction (e.g. x-axis) perpendicular to the first direction is measured. A second specimen is then provided with the antigen and the antibody but without the dye. The same light as discussed above excites the second specimen and polarizes the light. 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.