In order to detect the azimuth, elevation and intensity of the sun, Venier et al. have designed an analog chip [Venier et al. 96]. Pixels in this chip are located on a linear-polar coordinate (See Figure 2.28). At each pixel the photocurrent is compared with the global average and if the intensity at the pixel is high (exposed to the sun), the pixel will output currents at the radial and angular directions, and a copy of the input current to determine the intensity, as shown in Figure 2.29. The angular currents from pixels on the same angle, and the radial currents of the pixels on the same polar coordinate are summed and input to the ``center of gravity'' circuit, which is a linear diffusive network(See section 2.6) [Andreou et al. 91a, Tartagni and Perona 93, Vittoz and Arreguit 93]. The currents at both ends of the network are read, and the relative center of gravity of the radial and angular currents is found by
The chip has been fabricated in a 2 
m CMOS process in an area of
5.5 
5.5 
. There are 1365 cells each occuping an area of
.
Figure 2.28: Pixel interconnection in Venier's chip.
Figure 2.29: Pixel circuit in Venier's chip.