Only one slot in the disc is required to measure rate per minute. Figure 10.3b
shows an absolute position encoder. An array of LEDs (one for each window) with
a corresponding photo detector for each window can give the position of the
wheel at any time. Only three windows are shown in the figure, for greater accuracy more slots would be used. The pattern shown on the disc is for the gray code.
Other patterns may be used on the disc such as the binary code