The van Cittert-Zernike theorem is used to obtain in a rigorous way th
e bidimensional glitter function used in the problem of retrieving spa
tial information of surface heights from aerial images. The results sh
ow that the glitter function can be described by a circular function w
here the radius is a function of the angular extent and the position o
f the source and the viewing direction. This result comes from conside
ring the source as a circular one that is projected on the detector or
camera after reflection, taken into consideration the slopes of the s
urface and the geometrical optics.