comb filter

  
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The use of optical filters to reduce/remove light from other sources. If a fairly narrowband optical filter were used on the receiver, `off-wavelength` light could be rejected - but such filters are quite expensive, they can be difficult to implement on a very simple optical system 1, and some may not even be usable with the relatively wide spectral width of LED-based emitters. An alternative would be to
comb filter - schematic

use theatrical "gels" chosen to pass the desired wavelength with minimal attenuation while rejecting some of the dominant wavelengths of the light pollution. For red LEDs (615-650 nanometers) the Roscolux "Fire" (#19) theatrical "gel" filter (or its equivalent) has been found reduce the effects of both high-pressure sodium and mercury vapor light pollution by about 6dB while minimally impacting the desired signals. Narrower beamwidth. The use of a large-aperture lens coupled with the smallest-area photodetector practical can narrow the field-of-view of the detector allowing greater discrimination of off-axis, interfering sources. This can be accomplished with a photodetector that has an active area about the same size as the "blur circle" 2 of the lens system or by masking off a larger detector to a size that is slightly larger than that of the lens system`s blur circle. Slightly off-point the receiver to better-reject the noise source. Even if the desired signal is reduced, the net effect may be an overall improvement of the signal-noise ratio is the interfering light source is farther off-point than the desired signal. The use of a subcarrier on the link instead of baseband. This method simply shifts the audio so that it is conveyed at a frequency above where the majority of mains-induced interference dominates and this works as long as the receiver itself isn`t being overloaded by the light! Unfortunately, use of higher...



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