Often, anti-reflective coatings are used to increase
transmission of an optic. This is often a valid use of an antireflective
coating, but it should be noted that this coating does
not, by definition, increase transmission. Rather, it only reduces
reflections off the incident side of the surface. In some cases,
absorptive anti-reflective treatments can actually reduce
transmission. In the case of interference filters, an antireflective
treatment is often superfluous. An interference filter anti-reflected at the passed wavelengths, so an extra antireflective
coating usually has little effect.
is intentionally reflective at wavelengths that are not being
passed, so the total reflection off the optic will not be
effectively reduced by an anti-reflective treatment.
Furthermore, exposed interference filters are often already