An integrated optical transient sensor

Abstract

The implementation of a compact continuous-time optical transient sensor with commercial CMOS technology is presented. In its basic version, this sensor consists of a photodiode, five transistors and a capacitor. The proposed circuit produces several output signals in parallel. These include a sustained, logarithmically compressed measure of the incoming irradiance, half-wave rectified and thresholded contrast-encoding measures of positive and negative irradiance transients, and a signal that shows a combination of the sustained and the bidirectional transient response. The particular implementation reported in this work responds to abrupt irradiance changes with contrasts down to less than 1% for positive transients and 25% for negative transients. Circuit modifications leading to more symmetric contrast thresholds around 5% are also described. Due to their compactness these transient sensors are suitable for implementation in monolithic one- or two-dimensional imaging arrays. Such arrays may be used to sense local brightness changes of an image projected onto the circuit plane, which typically correspond to moving contours.