A very important circuit in every vision chip is the front end circuitry which first receives the photocurrents. In general, the way that the input photocurrent is processed depends on the overall architecture of a specific vision chip. For example, in spatial vision chips the DC level of the inputs are important. The required photocircuits should preserve the DC level, and at the same time increase the dynamic range. For temporal vision chips the photocircuit should restore the temporal behavior of the inputs as well, while concerning the dynamic range problem.
A universal problem in image sensors is the dynamic range issue. The input light intensity varies in a large range of at least 10 decades. Human eye is capable of functioning over a 12-decade range. Obviously, adaptation mechanisms, either local or global, should come to rescue the individual circuits which at most can cope with seven decades of signal variation. The adaptation mechanisms will be covered in Section 7.7.