A silicon retina is an intelligent vision sensor that can execute real-time image pre-processing by using a parallel analog circuit that mimics the structure of the neuronal circuits in the vertebrate retina. In order to enhance robustness against changes of illumination in practical environment, we have designed and fabricated a silicon retina exhibiting a wide dynamic range and a constant response against changes in the intensity of the illumination on the basis of a computational model of lightness constancy. The photosensor in the present chip approximates logarithmic illumination-to-voltage transfer characteristics due to the application of a time-modulated reset voltage technique. Two types of image processings, Laplacian-Gaussian-like spatial filtering and computing the frame difference, are carried out by using resistive networks and sample/hold circuits in the chip. As a result of these processings, the chip exhibits lightness constancy in wide range of illumination. We show that the present chip not only has a wide dynamic range but also exihibits a constant response against the changes in the illumination.
