L2-C

FILTERS AND BEAM SPLITTERS
Several of the applications of scientific optical filters are in photography, optical communication, radar systems, holography, and general laser and electro-optical laboratory experiments.
Filters can be classified into three categories, according to the way they’re used:
• Attenuation filters
• Wavelength-selective filters (both transmissive and reflective)
• Polarization filters
Attenuation Filters
Attenuation filters are used to reduce the intensity of a light beam. High-quality attenuation filters are said to have a "flat response." This means that they attenuate all wavelengths of light over their usable spectral range by the same amount.

Neutral-density Filters
Neutral-density filters are uniform, "grey" filters that absorb and/or reflect a fraction of the energy incident upon them. The term "neutral" is designated because the absorption and/or reflection characteristics of the filter are constant over a wide wavelength range ,It’s useful here to define "optical density" as it applies to filters. Optical density is the degree of opacity of a translucent medium