1. Imaging apparatus, comprising: an image sensor, comprising an array of detector elements; objective optics, which are configured to focus optical radiation and are positioned so as to form respective first and second optical images of a scene on different, respective first and second regions of the array; first and second optical filters, having different respective first and second passbands, which are positioned so as to filter the optical radiation focused by the first and second lenses onto the first and second regions, respectively; and a subtracter, which is coupled to take a difference between respective first and second input signals provided by the detector elements in the first and second regions and to generate an output signal indicative of the difference.