Cold Mirrors

What are Cold Mirrors?

Particularly for image projectors which contain some kind of incandescent lamp (e.g. a halogen lamp) as light source for the illumination, there can be a substantial heat load on the optical system. This is essentially because not only the desired visible light is generated, but also the more substantial power in the infrared. That infrared light may be absorbed in optical glasses, for example, heat them up, and cause various kinds of detrimental effects, such as mechanical stress and deformations which cause optical aberrations.

In order to eliminate or at least mitigate that problem, special mirrors (heat control filters) have been developed which can work in two different ways as optical filters for removing infrared light:

• Hot mirrors reflect the infrared radiation while transmitting most of the useful visible light. Such a mirror is simply added to the beam path before the optical components which need to be protected.
• Cold mirrors reflect the visible light, while transmitting or absorbing most of the infrared light. (They are sometimes called heat transmitting mirrors, although strictly speaking they transmit or absorb infrared radiation.) Such a mirror is used as a folding mirror in the optical beam path.

Cold mirrors are realized as dielectric mirrors. They should ideally reflect all visible light, but this is challenging for a dielectric mirror. Therefore, there can be a substantial loss for the shortest and longest parts of the visible wavelength region, i.e., for blue and red light. The substrate will usually transmit the shorter wavelength components of the infrared, while absorbing the longer wavelength components.

In contrast to dichroic mirrors, which usually have specified optical properties only for two narrow wavelength regions, cold mirrors have such properties (although with less strict specifications) for much broader wavelength regions.

The infrared absorption in the mirror substrate may lead to some thermal effects in the mirror, which may somewhat degrade the system performance. Such effects can be minimized by using a substrate glass with particularly good thermal resistance, including relatively small thermal expansion. For example, borosilicate glasses and fused silica are suitable for that purpose. Soda–lime glasses are sufficient only for lower powers.

As is common for dielectric mirrors, the reflection spectrum is optimized for a certain angle of incidence — for example, for normal incidence or for 45° — and the performance can be substantially worse for other incidence angles.

There are also UV cold mirrors, which reflect in the ultraviolet region while transmitting visible and infrared light. They are used for processes like semiconductor chip manufacturing and the photopolymerization of inks, dyes or adhesives, where the UV light is needed for the process and other light would be disturbing through thermal or other effects.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains 29 suppliers for cold mirrors. Among them:

⚙ hardware

Knight Optical offers stock cold mirrors which reflect visible wavelengths whilst transmitting NIR light. These mirrors can be cut to custom dimensions to meet our customers’ requirements.

⚙ hardware

We offer on-demand fabrication of cold mirrors. They can separate visible from IR light and have a SiO₂ protective layer. We can provide all shapes and dimensions.

⚙ hardware

Shanghai Optics is a custom optics manufacturer with over 55 years of optics manufacturing experience. Our cold mirrors are perfect for applications that require high reflectance in the visible spectrum and excellent transmission in the infrared. These cold mirrors are manufactured using a special proprietary all dielectric coating technique that yields the most durable commercially available product. Our cold mirrors will pass Mil. C-675, snap tape tests and are capable of operation at temperatures as high as 600 °C. They will also operate and survive in extreme humidity environments. Cold mirrors are available from stock in one inch and two inch diameter round or one inch, two inch and four inch square configurations. Larger sheets are also available and may be cut into custom sizes.

⚙ hardware

A cold mirror is a specialized longpass filter, an optical mirror which is designed to reflect shorter wavelengths of light at a 45° angle of incidence while transmitting undesired heat (infrared radiation).

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