Hot mirrors and cold mirrors are multilayer dielectric mirrors that are used to separate infrared radiation (heat) from the visible part of the spectrum. These multilayer dielectric mirrors are spectrally selective for use when it is desirable to separate the energy beam into its visible and infrared components, and are typically used for heat control. The terminology of hot and cold originates in the ability of long-wavelength infrared light to heat objects much more efficiently than short-wavelength light.

Hot mirrors function as short wavepass filters that reflect long wavelength / IR. They use a short wavelength pass filter stack design at approximately 1.6(.5 H .5L)n. Because the hot mirror attempts to extend the width of the reflecting band beyond about 1.2 usually result in a loss of visible transmission caused by high order interference effects. A hot mirror typically removes about 25% of the long wavelength / IR from a tungsten source operating at 3200 K, and a wide band hot mirror removes about 45%. The removal efficiency decreases for both of these coatings at the temperature of the light source is decreased. Hot mirrors are most commonly used at normal incidence, but custom angles of incidence can be designed to specification. They are very effective in applications where there are space limitations.

Cold mirrors reflect the shorter wavelengths of visible radiation and transmit infrared radiation producing a cool beam of reflected visible light. Cold mirrors are commonly used between 0 and 45 degrees. They use a long-wavelength pass filter stack design of (.5H L .5H)n. Sometimes, they are called dichroic mirrors, meaning “two colors.”

Hot mirrors or cold mirrors can be used to overlay two beams of widely different wavelengths. The first beam is transmitted through the surface of the mirror, the second is reflected off of the mirror. The mirror is tilted so that the reflected beam will travel along the same path as the transmitted beam.

Engineers use this concept in the design of surgical lasers. The hot radiation from an infrared laser provides the power to cut tissue, but the surgeon cannot see its invisible beam. A cool, visible laser beam is combined with the infrared beam to serve as a visual guide for the laser scalpel.