When a diluted antigen solution is mixed with a corresponding antibody, the reaction between the antigen and the antibody results in the formation of immune complexes.The solution becomes turbid. Turbidimetry is based on an optical detection system that measure the turbidity, the concentration of very small particles suspended in a solution. A light beam (incoming light, yellow arrow) sent trough a solution is scattered depending on the degree of turbidity.

A photodetector mesures the reduction in the intensity of the light beam (transmitted light, blue arrows). Since the transmitted light represents a decreasing signal (the more turbid the solution, the more light will be absorbed), one normally refers to the ABSORBANCE – this being an increasing quantity in relation to the antigen concentration.