Photodetectors are the doorway to vision chips. Any imperfection at this stage, with respect to desired characteristics, cannot be compensated even with a priori knowledge, or may be compensated at a high computational cost. The characteristics of the detectors, such as bandwidth, noise, linearity, and dynamic range directly affect the performance of the system. Therefore, it is highly demanding to have as perfect a photodetector as possible. Unfortunately, there is no flexibility or choice of photodetector devices in standard processes. In more than 90% of the vision chips reported photodetectors have been realized using parasitic elements found in standard processes. Fortunately, these parasitic devices have not put severe limitation on the processing capabilities of vision chips so far. The junction photodiodes, for example, have a linear behavior over a large dynamic range of more than 7 decades, with reasonable sensitivity to visible light spectrum.
The inflexibility of photodetectors may be overcome by design ideas in photocircuits. Static and dynamic characteristics of phototransduction can be improved by clever photocircuits. Active pixel detectors, are a clear example of this idea, which can be regarded as photocircuit-only vision chips, and can be a useful resource in the design of vision chips.