Now open the file 'Imperfect Corner Cube Retro-Reflector.zmx' which is in the zip file that can be downloaded from the last page of this article. It is identical to the previous file, except a different .POB object is used. This polygon object has only one different to the perfect model: vertex 4 (the apex of the retroreflector) is shifted by just one micron:

! back vertex 
V 4 0 0 10.001

Since the wavelength is 0.55 microns, this is almost a 2-wavelength shift, The resulting change in optical performance is dramatic. Instead of the perfectly flat OPD, we saw before, we now see this strange, non-Seidel aberration:



A typical commercial retroreflector, like this one from Melles Griot, is specified to produce less than l/4 aberration, which gives you some indication of the tolerances that these devices must be made to! 

The spot diagram (remember we are in afocal mode, so this is showing angular error) shows that the beam is being split into six component beams, as does the Huygens PSF. Ignoring the polarization effects from the total internal reflection, the Huygens PSF shows:



Note that the image delta setting has been increased to 0.005 mrad/pixel in order to capture the whole beam. When polarization effects are included, the PSF alters somewhat:



Finally, note that this is just one error: a z-shift of the apex vertex, which leads to a change in apex angle. The apex vertex can be shifted in x and y as well as z, and the (x,y,z) coordinates of the other vertices can be similarly perturbed.