Adaptive Pixels
To achieve a high dynamic range in image acquisition, logarithmic response functions can be implemented which project the huge variations of input light intensities on a relatively small interval of output voltages. This concept is problematic for scenes with small spatial light variations giving rise to images with very low contrast. Biological systems can adapt the slope of their logarithmic response functions. Steady signals are treated with a rather flat logarithmic response curve whereas transient signals are amplified more in order to use the available range for the output signal in a most efficient way.
As an electronics implementation of biological adaptation a positive feedback circuit integrated with a photoreceptor in analog CMOS technology has been proposed by Delbrück and Mead (CALTECH CNS Memo 30, 1994). Our group has built two different cameras based on this pixel concept using the 1.2 µm CMOS technology by AMS (Austria Micro Systems). The initial project was a 20 x 20 pixel chip with analog output and external readout and control electronics. This has been followed by a 64 x 64 pixel camera (VISOR) including on-chip digitisation.
