The specialists of ASTROHN Technology Ltd have developed a series of antireflection coatings for optical partsadmin
The specialists of ASTROHN Technology Ltd have developed a series of antireflection coatings applied on the surface of optical parts by evaporation in vacuum. To develop optical devices operating in the middle infrared range (λ = 3–14 μm), materials are used with high refractive indices (for germanium nλ=4,00, for silicon nλ=3,50, for zinc selenide nλ=2,5), which causes a number of certain difficulties.
One of the most basic problems is the high reflection from one surface of the optical part (the reflection of pure germanium at λ = 8–10 μm Rλ=40%) is very high, and if the lens consists of 3-4 germanium lenses, its integral transmittance will slightly exceed 10%, which is negligible. For the proper functioning of the optical system, its integral transmittance must be at least 85%, and optimal optical systems using aspherical optics can provide about 95% transmittance.
To ensure such transmittance, a team of ASTROHN Technology Ltd developed a number of antireflective coatings deposited on the surface of optical parts by evaporation in a vacuum. One such coating is DLC (Diamond Like Carbon). The main tasks this coating solves are enlightenment of germanium optics with simultaneous high mechanical protective properties. The hardness of the coatings deposited by the plasma method is comparable to the hardness of diamond (5000–10000 Hv). The DLC coating applied to the surface of the germanium lens completely eliminates the risk of mechanical damage due to abrasion.
The enlightening ability of such a single-layer coating is significantly inferior to traditional antireflection coatings. The integrated transmittance in the operating wavelength range slightly exceeds 80%. This is given the fact that the maximum transmission can reach significant values (Тmax=99,0%). Having tested a finished optical system with a DLC applied on the first surface, we obtained a transmittance of about 81%, while a floor system with a multilayer broadband antireflective coating applied to all surfaces provided a transmission in the wavelength range from 8 to 14 μm of the order of T=95%. At first glance, an increase of 15% is not significant, but for devices operating in the infrared region it is very significant.
After a small study of the usual operating conditions of our thermal imaging devices, it was found that the effect of abrasives on the lenses is practically absent, and the use of DLC coatings is not feasible. It is much more beneficial to use a broadband antireflection coating, which gives a significant increase in transmission and has a very good resistance to environmental effects.