A CMOS Active Pixel Sensor With Light Intensity Filtering Characteristics for Image Thresholding Application

A CMOS image sensor with light intensity filtering feature for realizing image thresholding is proposed. The simple sensing pixel circuit with less than 5 transistors is designed to provide a tunable non-linear photo-response with a peak signal at a specified intensity level. This unique intensity-filtering photo-response characteristic is achieved by the push/pull forces between the n+/p photo-diode and a parasitic p+/n-well diode on the PMOS control transistor. With proper timing control, tunable intensity filtering transfer functions can be obtained on the unique pixel. Based on the behavior model of the pixel's photo-response, image thresholding as well as edge detection are successfully demonstrated by this non-linear imager.