Limitations in the use of dielectric interference filters in wide angle optical receivers

Along a clear line-of-sight optical signal path through the atmosphere, the usefulness of interference filters in rejecting unwanted background light is limited mainly by the technology of fabrication. If the signal to be observed is strongly scattered by fog or aerosols, the received signal appears to come from a wide angular subtense. In such a case it is found that the interference filter is not a very useful device for separating signal and background. The optimum photoelectron counting receiver equipped with such a filter performs not much better than a receiver without any filter. The difficulty is an interdependence between the effective bandwidth and field of view of such filters, which results in an optimum receiver performance in which neither parameter specifically appears. The results also apply to other wavelength-dependent filters, such as the liquid crystal, and filters using polarization anisotropy along a path between polarizers.