1. A device comprising: photosensitive elements defining a focal plane, the photosensitive elements to capture incident light over a wavelength band of interest; and a diffraction grating defining a transverse plane, spaced from the focal plane, to modulate the incident light, the diffraction grating including pairs of adjacent segments defining between them boundaries extending away from a center in the transverse plane, the boundaries producing curtains of minimum intensity separated by foci and extending to the focal plane; wherein the adjacent segments widen away from the center in the transverse plane.