In active triangulation systems, one side of the triangulation triangle is made up of a spatial light structure of well-known shape and location, causing a pattern on the surface of the target object. The other
side of the triangle is embodied by ray bundles, imaging this pattern onto a position sensor. Because nearly all surfaces show scattering characteristics between specular and diffuse reflection, the corresponding image-forming wavefronts are not of uniform amplitude, and so the irradiance of the imaging pupil is also nonuniform. If the imaging is done by an aberrated system, this can cause deviations from the image as predicted by geometric optics. An estimate of deviations resulting from defocusing by treating the imaging process in terms of scalar diffraction theory, using a linear model for nonuniform pupil irradiance is given.