A type of color filter or reflector made by depositing fine particles onto a glass or plastic film medium. In the case of a filter, rather than absorbing the unwanted color frequencies, they are reflected. In the case of a reflector, non-visible light energy passes through the reflector and only the visible spectrum is reflected.
From The Rosco Guide to Color Filters.pdf:
Dichroic Filters offer several benefits over plastic filters. Most obviously, a dichroic filter can withstand continuous exposer to high temperature lighting instruments without fading or degrading.
An additional benefit of filters that use selective reflection of specific wavelengths of light is very pure and saturated colors. A dichroic filter in a deep saturated blue may transmit as much as 40% more light than an absorptive filter of a comparable color. This increased output may mean fewer luminaries are required in certain situations.
On the other hand, dichroic filters when used in conventional fixtures are notorious for undesirable colors off-axis and in flare. This is usually minimized with a hexagonal glare shield, top hat, spill rings, or any combination.
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