An imager is disclosed comprising: one or more detectors, a
shutter adapted to open and close periodically to modulate the
radiation incident upon the
detector element to produce a sequence of alternating open fields and closed fields, said
detector being adapted to produce a first pair of output signals V.sub.o1 and V.sub.c1 comprising an
open field and a closed field, and a second pair of output signals V.sub.o2 and V.sub.c2 comprising an open and a closed field, a reset device adapted to reset the output of the detecting element to a predetermined
voltage after the output signals of V.sub.o1, V.sub.c1, V.sub.o2 and V.sub.c2 have all been obtained, there being no reset of the output of the detecting element between the output signals V.sub.o1, V.sub.c1, V.sub.o2 and V.sub.c2, and a processor adapted to process said first and second pairs of output signals to produce a first and a second processed
image signal from the detecting element, the first processed
image signal corresponding to the first pair of fields and the second processed
image signal corresponding to the second pair of fields. The four signals provide sufficient information to produce images,which are free from reset KTC
noise whilst removing the effects of ramp voltages produced by current leakage across the detectors.