Low-reflectance, durable coatings for infrared lenses
Thermal imaging systems are used in many night vision applications, from fire-fighting and commercial safety driving to the modern battlefield. Due to conditions in the field, silicon and germanium lenses need to withstand acids, salt water and salt spray, wear and severe environmental changes, while preserving high transmittance over a wide spectral range.
In IR cameras, the effect of a detector's reflection on the detector itself (the Narcissus effect) is a major source of noise. This reflection is usually related to the front surface of the lens assembly and is greater if the detector is cooled. In a fixed-focal-length assembly, the surface can be designed to eliminate this effect, but in a zoom-lens assembly, the effect is hard to eliminate.
To demonstrate why, Figure 1 shows the ray tracings of a zoom assembly in wide field of view (WFOV) and in medium field of view (MFOV). For WFOV, the aperture used is narrow, and some rays are reflected (i.e., normal to the surface), which gives rise to the Narcissus effect. In the MFOV case, when the lens is zooming in on an object, the aperture is much wider and the relative proportion of reflected rays is lower. In other zoom lenses, the manufacturer eliminated the capability to reach the full extent of WFOV due to the Narcissus effect. However, we noticed differences in the Narcissus effect if we changed the front lens in our zoom lenses, which led us to a correlation between the coating and the extent of the internal reflections.
The Narcissus effect can be reduced by a suitable antireflection coating of the front lens. For the 3–5μm region, the preferred substrate for this lens is usually silicon, mainly because it is both hard and inexpensive. For the 8–11.5μm region, germanium is preferred. A frequent coating for durability is hard carbon, also known as diamond-like carbon (DLC). Our goal was to develop a lower-reflection DLC coating without compromising durability.
The standard DLC coating is a single layer of carbon atoms, which adheres well to silicon and germanium substrates and is also a good optical match for them: at roughly 1.9, its index of refraction is close to the square root of the silicon and germanium indices.1, 2 This coating has excellent durability, but the average reflection of about 3.3% at the 3–5μm region results in a noticeable Narcissus effect. We designed a new type of coating—low-reflection hard carbon (LRHC)—that is multilayered with a DLC overlayer designed for low reflection (see Figure 2 for the design).3First, we coated dielectric layers with either silicon or germanium top layers in a physical vapor deposition coating chamber. Then we added the DLC layer in a plasma-enhanced chemical vapor deposition chamber (see Figure 3). We chose the thickness of the DLC layer carefully so that it was thin enough for low internal absorption, but sufficiently thick for adequate durability.
As stated, for the 3–5μm region, the substrate for an external lens is usually silicon. With an LRHC coating on silicon, the average reflectance achieved in the 3.5–5μm region was 0.26% (see Figure 4). The average transmittance achieved was 98.5%. For the 8–12μm region, we designed several coatings on germanium, zinc sulfide (ZnS) and zinc selenide (ZnSe) substrates with a DLC top layer to withstand severe durability requirements. Experimental results for a germanium substrate were that the average reflectance in the 8–11.5μm range was 0.62%, with average transmittance of 94% in the same region.
We applied several antireflective coatings to the front silicon lens of a 15–300mm zoom assembly. A WFOV lens with a single-layer DLC compared well with a similar lens with an LRHC coating. The typical average reflectance of 4.8% in the 3–5μm region for the single-layer DLC coating was reduced to just 0.5% for the lens with the LRHC coating. In the first case, the effect was noticeable, while in the second case, the effect was eliminated (see Figure 5).
The results for our designs show that a multilayer coating with a DLC top layer has good potential for external surfaces. We demonstrated several designs for a silicon substrate in the 3–5μm region and for a germanium substrate in the 8–11.5μm region. Other designs show that this concept can also be applied to ZnSe and ZnS substrates.
All the coatings passed the durability tests required of a DLC coating (humidity, severe abrasion, salt immersion, salt vapor and acid corrosivity), including the ‘5000 revolutions wiper sand test.’ They can be applied on lens assemblies for high external durability and low Narcissus effect. We are still working on further reducing the average reflectance in the LRHC design for the 8–12μm region. In other cases where the reflectance of the LRHC is still not low enough, we are working on a high-durability coating (that withstands all the environmental tests except the wiper test) with average reflectance of 0.15% and lower.
Newport Corporation
Mordechai Gilo received his PhD in electrical engineering from Tel Aviv University, Israel, in 2000. He is now the process development manager of R&D and engineering. His areas of interest are optical polishing, diamond turning, and optical coatings. He holds two patents and has published more than 15 scientific papers.
