*Ronald Willey, Willey Optical, Consultants, Charlevoix, MI
Freeform optical surfaces have recently become practical to produce in quantity. Such surfaces can be viewed as the same coating problem as antireflection (AR) coating a hemisphere. The hemisphere has areas that are at all possible angles of incidence (AOI) from 0° to 90° and in all azimuths. A helpful partial solution to the freeform surface AR requirement can be provided by atomic layer deposition (ALD). ALD provides a conformal coating which is the same thickness on all exposed surfaces, independent of the surface orientation. In certain classes of freeform surface AR requirements, this may be all that is needed. If the ALD stack could further have the ideal index of refraction profile versus thickness, this would solve almost all AR problems. However, obtaining very low indices approaching that of the air medium does not currently seem to be within the realm of possibility. If different areas of a freeform surface required different designs for the ALD ARs, such designs could be produced by “tiling” the surface with different ALD designs by masking some areas while depositing on others.