In situ fluorometer using a synchronous detector

Claim 1

In an apparatus for measuring the light generated by fluorescing materials suspended in a liquid medium and including a dual light path optical bridge, a source of light for illuminating both paths, a transparent sample chamber disposed in one path for carrying the liquid medium, means for restricting the light entering the sample chamber to a selected wavelength, means for filtering the light leaving the sample chamber to a selected wavelength, servo means disposed in the other path for varying the optical transmissivity thereof, a photodetector for sensing the light leaving both paths, a light chopper disposed between the ends of both paths and the photodetector to alternately direct light from both paths to said photodetector, and electrical circuit means responsive to the photodetector output for controlling the servo means, the improvement characterized by: a. the light chopper comprising a motoR driven rotating shutter, b. the electrical circuit means comprising a synchronous detector, and c. a reference signal photodetector positioned such that received light is chopped by said rotating shutter for supplying a synchronizing signal for providing the phase reference to the synchronous detector.