Rare Event Analysis of Communication Networks

An apparatus, including a light-beam-producing means, a means for splitting the bean into a measuring beam and a reference beam, a reference photocell in the path of the reference beam, a measuring photocell having a substantially flat window in the path of said measuring beam for producing a measured signal, a means for holding the semitransparent membrane in the path of the measuring beam, and means for comparing the reference signal with the measured signal for producing an output signal. The window is mounted at an angle with respect to the path of the measuring beam to deflect any reflected light away from the path of the measuring beam. A shutter may be positioned in the path of the measuring beam. In one form of the apparatus, the light source, beam splitter and reference photocell are enclosed in a dusttight enclosure having an exit window for transmitting the measuring beam. A means for producing a turbulent flow of clean gas in front of the exit window may be provided.