Patent ID: 8115842

Claim:
A method of measuring and converting to a digital value the lighting of the photosensitive pixels of an matrix-array of pixels arranged in rows and columns, all the pixels of one and the same column being able to be selectively linked to one respective column conductor to apply to this conductor in succession two potential values, of which one is a reset potential value and the other is a useful potential value linked to the lighting of a pixel, this method using for each column a voltage ramp generator and a comparator, said method comprising the following steps: a)—the reset potential present on the column conductor is sampled in a first and a second sampling capacitors, and this potential is applied during the following steps b and c to a first input of the comparator by means of the first sampling capacitor; b)—a linear voltage ramp starting from a reference value and varying in a first direction during a first duration is applied to a second input of the comparator, through the second capacitor; c)—still through the second capacitor, a linear voltage ramp in the reverse direction is applied to this second input of the comparator; a digital value of the time that elapses between the start of this second ramp and the switching over of the comparator is counted at a fixed frequency in a counter; and the voltage of the ramp is returned to its reference value; d)—the useful potential value present on the column conductor and representing the lighting of the pixel is sampled again, but in the first capacitor only, and this potential is applied, during the following steps e and f, to the first input of the comparator by means of the first capacitor; e)—a linear voltage ramp varying in the first direction, starting from the same reference value, and identical in slope and in duration to that of the step b, is applied to the second input of the comparator, through the second capacitor; f)—still through the second capacitor, a second linear voltage ramp in the reverse direction, identical in slope to that of the step c, is applied to the second input of the comparator; a second digital value of the time that elapses between the start of this second ramp and the switching over of the comparator is counted at the same frequency in the counter, the difference between the second and the first digital values representing a measurement of the lighting of the pixel.