Hot Mirror vs Cold Mirror

A cold mirror is a mirror that efficiently transmits infrared wavelengths while reflecting all visible light, while a hot mirror reflects the infrared while transmitting visible light. 

Both cold and hot mirrors are used to filter unwanted heat or energy out of a system. They are typically multi-layer dielectric mirrors, and can be designed around specific wavelength ranges.   At Avantier, we produce custom cold and hot mirrors for a wide variety of applications.

Hot and Cold Mirrors
Cold Mirror
Hot and Cold Mirrors
Hot and Cold Mirrors

Hot Mirrors

Applications of Hot and Cold Mirrors

Hot and cold mirrors provide an easy, straightforward way to funnel unwanted heat out of a system. They’re used in many different applications when the heat generation or the presence of extra IR energy might be a problem. 

An alternative use of hot and cold mirrors is to overlay two beams. One beam is reflected off the mirror, the other transmitted through the surface, and the mirror can be tilted in such a way that both beams travel along the same path. One area in which this is important is surgical lasers. Mirrors are arranged so that a cool, visible laser beam travels along the same path as the invisible hot infrared laser which cuts the tissue, enabling the surgeon to see what he is doing. 

Another type of mirror is a UV cold mirror, which reflects UV light while transmitting both infrared and visible light. UV cold mirrors are used for processes where UV light is essential and  the presence of other light would be problematic; for instance, in the photo polymerization of inks, dyes, and adhesives or in semiconductor chip manufacturing.

Hot Mirror vs. Cold Mirror Setup

Hot mirrors and cold mirrors are often used for the same purpose, protecting heat-sensitive equipment from potentially damaging IR radiation.  But since hot mirrors transmit visible light and reflect IR and cold mirrors reflect visible light while transmitting IR, the placement of the mirrors will be different. 

A hot mirror will be placed in the beam path at the point where heat should be removed, usually just before the components that need protection.  The visible light will travel directly through it and to the heat-sensitive components, while infrared light will be effectively blocked.

A hot mirror will be used as a folding mirror in the beam path, as it transmits all the infrared light, effectively eliminating it from the optical system. Visible light will be returned.

Factory Standard – Cold Mirrors


Float glass





Max temperature


Surface Flatness

3λ per inch

Surface Quality





R>95% avg 420-700nm


R>20% avg 800-2500nm

Custom Cold and Hot Mirrors at Avantier

At Avantier, we produce custom cold and hot mirrors with many widely varying parameters. We can optimize these mirrors for the degree of incidence desired (0 or 45 angle of incidence are the most common options), as well as for your desired wavelengths and system configurations. If you are unsure, our designers can work with you to help you determine exactly what kind of mirror works best for your application. Call us today to set up your initial consult or place a custom order. 


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