An image sensor is formed by a pixel array including a plurality of pixels. Certain ones of the pixels include, above their active areas, a first optical grating formed of periodically spaced apart parallel strips separated from the active area by a first insulator. Those pixels further include, in another metal level, a second optical grating formed of periodically spaced apart parallel strips separated from the first grating by a second insulator. The second optical grating is laterally shifted with respect to the first grating in a direction orthogonal to a longitudinal direction of the parallel strips.