What are Hot mirrors and Cold mirrors?

Hot Mirror and Cold Mirror exploit the properties of dielectric coatings to selectively reflect and transmit light of specified wavelengths. Hot mirrors reflect near infrared (NIR) and infrared (IR) and allow the transmission of ultraviolet (UV) and visible light. Cold mirrors, on the other hand, reflect UV and visible light and allow NIR and IR transmission. Each provides a way to manage and filter light and heat in optical systems, so comparing hot and cold mirrors is important to highlight the applications of each.

Hot mirrors are so named because up to 90% of NIR and IR wavelengths are reflected. They deposit multiple layers of thin-film dielectric coatings on the surface. The material and thickness are chosen to selectively reflect infrared light from the surface while allowing up to 80% of UV and visible light to pass through. Highly transmissive thermal mirrors prevent heat from damaging heat-sensitive components and materials or direct heat to other purposes. The figure below shows a typical transmission curve for a thermal mirror

Hot mirror

Heat can be directed to the heat sink or away from other components or used for different needs. At the same time, ultraviolet light and visible light pass through the mirror without changing the spectral characteristics. Typically thermal mirrors are designed for 0° or 45° angle of incidence, but can be customized for any angle of incidence between 0° and 45°.
Although the name "hot mirror" means reflection, hot mirrors can also be used as effective beam splitters and bandpass filters. Heat mirrors are often found in project systems, where the heat generated by high-intensity lamps can be redirected and managed to protect components and prevent the system from overheating.
Cold mirrors are very similar to hot mirrors, except that their dielectric coating is designed to reflect 90% of UV and visible light while allowing 80% of NIR and IR light transmission. Some chilled mirrors are also designed to remove unwanted UV and visible light illumination.
Like hot mirrors, cold mirrors are typically designed for 0° or 45° angle of incidence, and they can also be used as effective beam splitters and broadband filters. Hot and cold mirrors can be combined in the system as visible bandpass filters to remove UV, NIR and IR from the imaging system.