u201d for more than thirty percent of the data this is taken as an overexposed condition at 204 and the integration time or gain is lowered the second threshold is investigated at 210 if five percent of the data is dark msb is 0 the data is taken as underexposed data and the integration time or gain is increased the thresholds must be selected with an amount of hysteresis which is effective to avoid oscillations when the image has many contrasts i e between black and white for example the sum of the two percentages should exceed 100 percent fig3 shows a bar graph with the overexposure underexposure parameters the point a in fig3 is at an overexposed position if more than 30 percent of the image is in this position then the image is taken to be overexposed and the gain or integration time is lowered conversely point b is in an underexposed position if more than 75 percent of the image is in this position then the image is taken to be underexposed a second embodiment which operates according to the flowchart of fig4 uses a three threshold advance scheme this takes only the two highest bits at the input to the indicator as in the first system however this scheme uses three decoders and three counters as shown in fig1 this system counts a the.