Hot Mirror vs Cold Mirror

Mirrors are well-known and widely used optical components. Just walk into virtually any bathroom—whether it is at a rest stop, a high-end hotel, or your own home—and you will find at least one mirror. And they come in a variety of shapes and configurations. Take a peek into the high ceiling corners of a convenience store and you’ll find convex mirrors that offer a wide-eye view of the store, while any flashlight uses a concave mirror to focus light.

The ubiquity of mirrors makes them seem commonplace and straightforward; however, highly specialized dielectric mirrors offer optical properties for selective filtering, heat management, laser beam steering, focusing, and more. For example, hot mirrors and cold mirrors are critical components for filtering and heat management in applications like microscopes, telescopes, space exploration, head-up displays, imaging and projection.

Hot and cold mirrors use the properties of dielectric coatings to selectively reflect and transmit light at prescribed 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 the transmission of NIR and IR. Each offer ways to manage and filter light and heat in an optical system.

Hot Mirrors

Hot mirrors are named as such because up to 90% of NIR and IR wavelengths are reflected. They have multiple layers of thin-film dielectric coating deposited on the surface. The materials and thicknesses are chosen to selectively reflect infrared light from the surface while allowing up to 80% of UV and visible light to transmit through.

High transmission hot mirrors prevent heat from damaging thermally sensitive components and materials or for directing heat for other purposes. Figure 1 shows an example of a typical transmission curve for a hot mirror.

Hot Mirror Transmission Curve
Figure 1 – Example of a Standard Hot Mirror Transmission Curve

The heat may be directed to a heat sink or away from other components or used for different needs. At the same time, the UV and visible light are transmitted through the mirror without altering the spectral characteristics.

Typically, off-the-shelf hot mirrors are designed for either 0° or 45° angle of incidence, as shown in Figure 2. These are the angles where the dielectric coatings are optimized for maximum NIR and IR reflection. Custom hot mirrors can be made for any angle of incidence between 0° and 45°.

How a Hot Mirror Works
Figure 2 – How a Hot Mirror Works

Although the name ‘hot mirror’ implies reflection, hot mirrors can also be used as effective beam splitters and bandpass filters.

Hot 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 system overheating. Figure 3 shows a simplified schematic of a projection system that uses both a hot mirror and a cold mirror to manage heat from a lamp.

Projector Schematic
Figure 3 – Simplified schematic of a DLP imaging projector with a hot mirror and cold mirror

Cold Mirrors

Cold mirrors are remarkably similar to hot mirrors except that their dielectric coatings are designed to reflect 90% of UV and visible light while allowing 80% of NIR and IR light to transmit. Some cold mirrors are also designed to remove undesirable UV and visible illumination. Figure 4 shows an example of the reflectance curve for a cold mirror. It is not coincidental that a cold mirror’s reflectance curve looks similar to the hot mirror’s transmittance curve. It’s a feature that makes them hot versus cold mirrors.

Cold Mirror Reflectance Curve
Figure 4 – Cold Mirror Reflectance Curve

Like hot mirrors, off-the-shelf cold mirrors are designed for either 0° or 45° angle of incidence, as shown in Figure 5. They can also be used as effective beam splitters and broadband filters. Hot mirrors and cold mirrors can be used in conjunction in a system as a visible light bandpass filter to remove UV, NIR and IR for imaging systems.

How a Cold Mirror Works
Figure 5 – How a Cold Mirror Works

In recent years, cold mirrors have become a critical component for heads-up displays (HUDs). Borrowing from HUD designed for aircraft, these displays are becoming more prominent in the automotive industry. A display is reflected onto the windshield or a display mirror in front of the vehicle operator. See Figure 6 for a simplified schematic of a HUD alongside the view of a HUD from a driver’s perspective. To prevent image distortion and heat buildup, a cold mirror is used to project the visible part of that image while the NIR and IR are transmitted through the mirror for heat management. This allows HUD systems to use heat generating Cathode Ray Tube (CRT) light sources while remaining compact.

Figure 6 – Simplified schematic of a heads-up-display (HUD) with a cold mirror and the view from the driver’s perspective

Summary

While mirrors seem to be omnipresent in our daily lives, they are also essential components in various science and technology-based applications. Specialized hot and cold mirrors utilize fine-tuned dielectric coatings to help directed NIR and IR light in various optical systems making them useful for heat management and filtering applications. They can often be found in the imaging and laser systems and, most notably, gaining prominence in automotive HUDs.

EMF, a Dynasil company, is a leading provider of cold mirrors to a global automotive components manufacturer based in Japan. It has shipped more than 800,000 cold mirrors without a single field failure.

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