A method for measurement of multiple light spot positions on one position-sensitive detector (PSD)

The measurement method described relies on using light sources, each of which is modulated at different frequencies and then demodulated in the sensor signal-processing circuit using a pulse amplitude modulation (PAM) scheme. The position of each light spot can be determined even if there are other beams irradiating the PSD at the same time. In a very simple experiment, the light beams from two LEDs were projected onto one PSD, with one LED modulated at 10 kHz and the other at 5 kHz. The PSD was calibrated by scanning the light beams across the PSD in equidistant grid lines over a 4-mm×7.2-mm area. It was found that the resolution, linearity, and accuracy of the measurement are not affected even if multiple light beams are continuously present. The number of light beams which can be used simultaneously is limited only by the bandwidth of the PSD and the sampling time of the sample/hold devices used in the DC restoration circuit