Coating Material of Infrared Reflectors

Scanning mirrors in infrared systems and optical mirrors in various reflective optical systems need to be coated with high-reflection coatings. The highly reflective film has high reflectivity in the corresponding band. Physical and chemical properties of the film are stable. It resists the erosion of substances, such as moisture in the atmosphere.

Under normal circumstances, optical mirrors are coated with high-reflective film using vacuum evaporation metal film technology. Metals such as gold, silver, copper, aluminum, and rhodium can be used as metal materials for infrared high-reflection films. Optical thin films of silver and copper are easily oxidized in the air and greatly reducing their reflectivity. Moreover, the adhesion of silver film to glass is worse than that of aluminum. The protective film is not easy to be plated firmly. Beryllium is expensive and toxic, so beryllium film is almost not used.

At present, in infrared mirrors, the most widely used metal films are usually aluminum, gold and rhodium. Rhodium has a low reflectivity. It can be plated firmly on glass. It is resistant to friction and corrosion. It is suitable for infrared mirrors that require frequently wiping or harsh application environments and low reflectance requirements. The gold film has good chemical stability and high reflectivity. In the infrared band, gold is a good material of reflective film. However, gold is not easy to be plated firmly on glass. A transition layer of chromium or nickel needs to be added to make the gold film firmly adhere to the glass. In addition, the gold film has low strength. It is not easy to wipe.

Aluminum is easy to evaporate. It has strong adhesion to glass and a certain degree of stability, so aluminum film is most used. The aluminum film is also very soft and can not be wiped. Although aluminum interacts with the oxygen in the air to form an oxide layer of aluminum oxide on its surface, the naturally formed oxide film is very thin, which can only prevent aluminum from continuing to oxidize. It is not sufficient for protection. Besides, the oxide layer will reduce the reflectivity. In actual applications, a protective film must be coated on the surface of the aluminum film. The commonly used protective films for aluminum films are aluminum oxide, silicon oxide, silicon dioxide, etc., which are generally only used in the near-infrared band.

Diamond-like carbon film or DLC film is a kind of infrared optical film with very good performance. It has a diamond polycrystalline structure, a hardness similar to that of diamond, good physical and chemical stability and infrared transmission. The DLC coating obtained by the plasma deposition method is often used as an antireflection film and protective film for infrared windows or infrared lenses. In harsh environments, the DLC coating plays a role in moisture resistance, corrosion resistance, corrosion resistance and abrasion resistance. According to experimental research results, in the 8-12 μm waveband, the DLC protective coating on the aluminum reflector does not reduce the reflectance of optical reflector.

CLZ Precision Optics Co., Ltd. has been producing and processing optical mirrors for a long time. We’re good at providing optical mirrors made of different optical materials with kinds of optical coatings. If you need detailed information, please contact talia@clzoptics.com.