Abstract: The imager comprises an array 4 of detectors, and an array 6 of LEDs each detector being connected to a corresponding LED by a respective signal processing channel 5a. Common to all the channels is a ramp generator 12. Each channel has a comparator 9 which compares the ramp with the signal from its detector to produce a pulse width modulated signal for its LED. Thus the LED is always driven at the same point on its characteristic, variation of brightness being achieved by variation of pulse width. Use of a common ramp generator makes for simple change of ramp shape, temperature window, offset and .gamma. law by simply changing only the ramp generator.