Dichroic mirror

A dichroic filter, thin-film filter, or interference filter is a very accurate color filter used to selectively pass light of a small range of colors while reflecting other colors. By comparison, dichroic mirrors and dichroic reflectors tend to be characterized by the color(s) of light that they reflect, rather than the color(s) they pass. (See dichroism for the etymology of the term.)

Dichroic filters can filter light from a white light source to produce light that is perceived by humans to be highly saturated (intense) in color. Although costly, such filters are popular in architectural and theatrical applications.

Dichroic reflectors are commonly used behind a light source to reflect visible light forward while allowing the invisible infrared light (radiated heat) to pass out of the rear of the fixture, resulting in a beam of light that is literally cooler (of lower thermal temperature). Such an arrangement allows a given light to dramatically increase its forward intensity while allowing the heat generated by the backward-facing part of the fixture to escape. Many quartz halogen bulbs have an integrated dichroic reflector for this purpose, being originally designed for use in slide projectors to avoid melting the slides, but now widely used for interior home and commercial lighting. This improves whiteness by removing excess red; however, it poses a serious fire hazard if used in recessed or enclosed luminaires by allowing infrared radiation into those luminaires. For these applications non cool beam (ALU or Silverback) lamps must be used.

...
Wikipedia