Patent ID: 7719589

Claim:
An imaging device comprising an array of pixels in which, for each pixel, there is provided: a sensor which is sensitive to a variable quantity to be imaged and for outputting a signal representative of the variation in said quantity during a monitoring period; a comparator having a first input and a second input; a connection for connecting the output of said sensor to said first input in such a way that said comparator continuously monitors said output signal; and a connection for applying a reference voltage to said second input; said comparator being such as to change its output state at the point in time at which the amplitude of said output signal crosses said reference voltage level, wherein each pixel further comprises a hit flag generator which is operable to generate a flag signal indicative of whether, during the monitoring period, the output state of said comparator changes, and readout circuitry which is operable to read the contents of a memory unit during a read period, following the monitoring period, only if the hit flag generator indicates that the output state of the comparator changed during the monitoring period.