Atomic line filter
An atomic line filter (ALF) is a more effective optical band-pass filter used in the physical sciences for filtering electromagnetic radiation with precision, accuracy, and minimal signal strength loss. Atomic line filters work via the absorption or resonance lines of atomic vapors and so may also be designated an atomic resonance filter (ARF). The three major types of atomic line filters are absorption-re-emission ALFs, Faraday filters and Voigt filters. Absorption-re-emission filters were the first type developed, and so are commonly called simply "atomic line filters"; the other two types are usually referred to specifically as "Faraday filters" or "Voigt filters". Atomic line filters use different mechanisms and designs for different applications, but the same basic strategy is always employed: by taking advantage of the narrow lines of absorption or resonance in a metallic vapor, a specific frequency of light bypasses a series of filters that block all other light. Atomic line filters can be considered the optical equivalent of lock-in amplifiers; they are used in scientific applications requiring the effective detection of a narrowband signal (almost always laser light) that would otherwise be obscured by broadband sources, such as daylight. They are used regularly in Laser Imaging Detection and Ranging