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Optical Design & Engineering

Low-reflectance, durable coatings for infrared lenses

Antireflective hard carbon that is coated with diamond-like carbon drastically reduces the ‘Narcissus effect’ of reflection in a detector.
21 January 2013, SPIE Newsroom. DOI: 10.1117/2.1201212.004581

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.


Figure 1. (upper) Ray tracing of a zoom assembly in wide field of view (WFOV). The aperture is narrow, and the number of rays normal to the surface (i.e., reflected) is high. (lower) Ray tracing of a zoom assembly in medium field of view (MFOV). The aperture is wide, and only a small percentage of rays are normal to the front surface. Images courtesy of Ophir Optronics Ltd.

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.


Figure 2. New coating concept of a multilayer stack with a DLC upper layer.

Figure 3. Radio frequency (RF)-enhanced chemical vapor deposition.4One electrode is connected to an RF voltage source, and the other is grounded. On the introduction of argon, the high voltage causes plasma to form. With carbon-rich gases such as methane, butane, or acetylene, the plasma process decomposes the gas and accelerates the carbon atoms toward the substrate to form the diamondlike carbon (DLC) layer.

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.


Figure 4. Experimental reflectance results for a low-reflection hard carbon (LRHC) design on silicon (Si) for the 3–5μm region. The average reflectance achieved in this region was 0.26 percent.

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).


Figure 5. (a) Lens assembly with a single-layer DLC front lens in WFOV. The central circle and one ring are noise from the Narcissus effect. (b) Lens assembly with a low-reflection hard carbon coating that has an average reflectance of 0.5%. The Narcissus effect is eliminated.

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.


Mordechai Gilo
Ophir Optronics Ltd
Newport Corporation
Jerusalem, Israel

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.


References:
1. L. P. Andersson, S. Berg, H. Notstrom, R. Olaison, S. Towta, Properties and coatings rates of diamond-like carbon films produced by R.F. glow discharge of hydrocarbon gases, Thin Solid Films 63, p. 155-160, 1979.
2. K. Kobayashi, K. Yamamoto, N. Mutsukura, Y. Machi, Sputtering characteristics of diamond and hydrogenated carbon films by R.F. plasma, Thin Solid Films 185, p. 71-78, 1990.
3. M. Gilo, A. Azran, Low reflectance DLC coatings on various IR substrates, Proc. SPIE 8353, p. 835320, 2012. doi:10.1117/12.917299
4. G. W. Green, A.L. Lettington, Method and apparatus for depositing coatings in a glow discharge, UK patent 2069008,