Patent ID: 6851645

Claim:
An apparatus for determining a relative direction of radiation comprising: a window system for transmitting the radiation, the window system having a window system optical axis, the window system having Fresnel transmittance properties, the window system being configured such that if the radiation is parallel to the window system optical axis, the radiation is incident upon a surface of the window system at a boresight angle such that an instantaneous rate of change of the Fresnel transmittance as a function of angle of incidence at the boresight angle is significantly different from zero; three or more radiation detectors for detecting the radiation, the radiation detectors being positioned relative to the window system so as to receive the radiation after transmission of the radiation through the window system, each of the radiation detectors generating a respective detection signal; and a processor for processing the respective detection signals, the processor being adapted to determine the relative direction of the radiation in two orthogonal directions with respect to the window system optical axis based upon the respective detection signals.