Abstract: A window for utilization in the receiving of intelligence from a modulated beam of collimated light, with the window being configured to restrict the field of view of the optical system used therewith, and employing the phenomenon of total internal reflection. Such a window may be used to reject the entry of unwanted radiation, such as noise, and comprises first and second generally triangularly shaped wedges of optical material. The entry face of the first wedge is generally perpendicular to a base plane, and its rear face is disposed at an acute angle to the entry face. The entry face of the second wedge is disposed at the same angle to the base plane as the rear face of the first wedge, and the two wedges are separated by a small air gap.

Abstract: A digital automatic frequency control circuit for use with a scanning type superheterodyne receiver, such circuit having an input for receiving the output of a related discriminator, and an output for providing an incrementally variable tuning voltage to a related local oscillator. My novel circuit has a search mode and a lock mode, and during the search mode, when the receiver tuning is swept past a pulsed radio signal, the discriminator produces a sequence of voltage output pulses whose polarity for a desired signal will be different from that of the signal's image. My circuit recognizes the polarity relationships of such pulses, and makes use of such information in a highly advantageous manner in order to cause the receiver to enter the lock mode only when the received signal has the correct relationship with the local oscillator, with the image signal thus being ignored.