Vision sensor with brightness constancy: towards neuromorphic color vision

A silicon retina is a VLSI (very large scale integration) chip that has been designed based on the neural architecture and functionality of a biological retina. Such devices are expected to be useful as a novel vision sensors for autonomous mobile robots due to their power efficiency, compactness, and real-time processing ability. In addition, their low noise output and wide operating range are both extremely important especially for practical applications. Kameda et al. developed a frame-based silicon retina with sustained and transient responses.1 This chip uses an active pixel sensor (APS): a conventional, sampled photosensor consisting of a photodiode and a source-follower circuit. The APS is highly light sensitive because photoelectrons are accumulated in the parasitic capacitor of the photodiode. In addition, the outputs of the chip are offset-suppressed analog voltages, with embedded sample/hold circuits to eliminate both the pattern noise due to the amplifier offset and the fixed-pattern noise of the APS. In this chip, the output voltage of the APS is linearly proportional to the input light intensity, and its sensitivity can be controlled by changing the accumulation time. However, the dynamic range for individual images is limited. We recently fabricated a wide-dynamic-range silicon retina based on the circuit design of this chip. In order to widen the dynamic range, however, we used the stepped-reset-gate-voltage technique,2 where the voltage signal fed to the gate node of the reset transistor in the APS is changed during the accumulation period. One of the advantages of this technique is that the response characteristics can be controlled by an external signal. Using this technique, we approximated a logarithmic APS response. Figure 1. The logarithmic response of the silicon retina helps it to exhibit brightness constancy. The step chart is illuminated using fluorescent lamp, and its intensity is varied from 750lx to 1500lx. The top and bottom rows of images are outputs of silicon retinas with linear and logarithmic responses, respectively. The logarithmic response is almost constant against the variation in illumination.