Hot Mirrors

Particularly for image projectors which contain some kind of incandescent lamp (e.g. a halogen lamp) as a 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. (They are sometimes called heat reflecting mirrors, although strictly speaking they reflect infrared radiation.) 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. Such a mirror is used as a folding mirror in the optical beam path.

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

Hot mirrors are realized as dielectric mirrors. Such a mirror should ideally reflect all light in the near and mid infrared. However, such a large reflection bandwidth is hard to achieve, since the refractive index contrast of the usual coating materials is relatively small. Also, the common materials eventually get absorbing for long enough wavelengths. Therefore, part of the heat protection may be accomplished by absorbing infrared light; a high reflectivity is achieved mostly in the near infrared, e.g. only up to wavelengths around 1.3 μm or 1.5 μm.

The infrared absorption in the mirror substrate may lead to some thermal effects in the mirror, which may to some extent 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.

When installing hot mirrors, the correct orientation is crucial. The dielectric coating side should typically face the light source. This ensures that the infrared radiation is reflected before it passes through the substrate glass, thereby minimizing thermal absorption within the mirror itself. Additionally, the reflected infrared radiation must be managed; usually, the housing design must account for this heat, often requiring active air cooling or heat sinks to dissipate the energy directed away from the optical axis.