A wide dynamic range CMOS pixel with Steven's power law response

Abstract

CCD as well as CMOS pixels suffer from the drawback of limited dynamic range and hence are unable to capture the wide dynamic range available in nature. Attempts to make wide dynamic range pixel have concentrated on either multiple capture techniques or the weak inversion region of a MOS device thereby producing a logarithmic response. The later is preferred on account of its similarity to the response of human eye. However, the response to brightness in the human eye more closely follows the Steven's power law. In this paper, a CMOS pixel is presented which is capable of producing a wide dynamic range response similar to that of the Steven's power law of brightness. In addition to this response, this 4-transistor pixel designed and manufactured in standard CMOS process has the ability to change the exponent as well as the proportionality constant in its response during its operation (run time). The pixel incorporates a single transistor used as compare and cutoff switch in addition to the standard active pixel sensor circuitry. The integrated response of the pixel is constantly compared to an inverted Steven's law response curve thereby leading to the required response. On account of small transistor count, the pixel has similar fill-factor and quantum efficiency as that of a standard CMOS active pixel sensor.