Choosing Optical Materials for Long Wave Infrared (8-12μm) Applications
Introduction
Imaging in the long-wave infrared (LWIR) spectral region, also referred to as the thermal infrared, has many useful applications in the military, security, construction, and scientific fields. Long-wave infrared imaging systems cover the 8-12 μm wavelength band, which is dominated by thermal emission from target objects. LWIR imaging systems can detect thermal signatures such as a human body in a room or a leak in a window pane even in total darkness. From the blackbody radiation curves, objects with temperatures in the -30 to 90 °C range have peak emittance in the LWIR. One major limitation optical engineers face when designing imaging systems for the LWIR is the lack of available optical materials. An engineer designing a visible system has hundreds of different materials at his/her disposal with high variability in optical, thermal, and mechanical properties. Conversely, there are relatively few materials (approximately 10 materials) available that transmit LWIR light. These materials often tend to be much more expensive and fragile than available visible spectrum materials. Even though there are relatively few materials available, there are some advantages to designing LWIR optical systems. Most materials have higher refractive indices than visible glasses, so fewer elements are needed to achieve diffraction limited performance. Also, the diffraction limited spot size and depth of focus are larger for LWIR systems, which helps a designer achieve diffraction limited performance more easily than with visible systems. Some LWIR materials have low dispersion, so color correction may not be needed. Another advantage to designing in the infrared is that materials can be diamond turned, so aspheres can be used.
Guidelines to choosing an infrared optical material
Although there are few material choices for the LWIR, a designer still needs to choose one that is optimal for a particular application. It may be the case that none of the available materials in the LWIR can fulfill every design criterion, in which case compromises or trade-offs should be made. To aide in the decision making process, there are general steps that an optical engineer should follow to further narrow down options when choosing an optical material for an LWIR application. The order in which each guideline or material property is considered depends heavily on what is most important for the application.
Transmission in spectral band of interest
Initially, a designer should narrow down the list of available materials to the ones that have the highest transmission in the spectral band of interest (in this case LWIR). Some infrared materials such as Calcium Fluoride cut off around 10 μm, making it a poor choice for an application that needs to work in the full LWIR band (8-12 μm). Figure 1 shows a plot of transmission versus wavelength of some common infrared optical materials.
Figure 1: Transmission versus wavelength for common infrared optical materials1
Optical properties
Another guideline to determining the optimal material is to consider other optical properties of interest depending on the application of the element (lens, prism, or window). For example, if the material is going to be used as a lens, optical properties such as index of refraction and dispersion are important to consider. Other important optical properties include reflection, absorption, birefringence, and temperature coefficient of refractive index (dn/dT). Refractive indices for LWIR materials are larger than standard visible glasses, with Germanium having the largest index of 4.0. Figure 2 shows indices of refraction versus wavelength for common infrared materials. Some LWIR materials such as Germanium have low dispersion, making color correction less significant. It is also important to note that dn/dT values for infrared materials are usually an order of magnitude larger than visible glasses, meaning that the power of a lens will change more significantly with a change in temperature.
Figure 2: Indices of refraction vs. wavelength for common IR materials2
Thermal properties
The thermal environment in which an optical system is going to be working in is another key property to take into account when choosing material. As previously mentioned, IR materials have significantly larger dn/dT values than visible glasses, signifying a more dramatic change in index and power for an element for a change in temperature. Other thermal properties to consider are thermal expansion coefficients, thermal conductivity, melting temperature, etc. The change in physical size of an object due to a change in temperature is proportional to the thermal expansion coefficient (CTE) of the material. Unfortunately, there are no LWIR materials that have very low CTE. Most LWIR materials have CTEs on the order of BK-7 glass.
Mechanical properties
Mechanical properties are also important for a designer to think about when choosing an optical material. Depending on the application, the material may have to withstand significant pressure, stress, etc. Mechanical properties of interest include Young’s modulus, shear modulus, elastic stiffness, hardness, yield strength, etc. As a general rule, LWIR materials are fragile, soft, and can break easily. These properties make mounting more challenging.
Other factors
Other factors that should be considered by a designer include cost of the material, weight, solubility, and how difficult the material is to machine. Infrared materials cost more than visible glasses by 2 orders of magnitude or more. Often LWIR materials are denser and thus weigh more than visible glasses. In some cases, such as sodium chloride, the material can be highly soluble in water, making it unusable in humid environments. Also, since most materials have high reflectivity, it is crucial to think about anti-reflection thin film coatings and how easily they can be added to the material.
Properties of common LWIR materials
Each LWIR material has interesting properties that make unique from other IR materials. Depending on the application, the unique properties can either make a material more or less desirable. The most common LWIR materials include germanium, zinc selenide, zinc sulfide, sodium chloride, gallium arsenide, and AMTIR-1. Each of the most common LWIR materials are described in further detail below.
Germanium (Ge)
Germanium has the largest index of refraction of all of the LWIR materials (4.0), making it much easier to achieve diffraction limited performance. Figure 3 shows spherical aberration of a standard visible BK-7 singlet versus a germanium singlet. Germanium also has extremely low dispersion (Abbe number = 942). Because of the low dispersion, color correction is often not necessary. Transmittance of germanium is constant over the LWIR band with a value of about 45%. Reflectance is very high (~ 40%), signifying a need for an anti-reflection, thin film coating on the surface. Germanium is very susceptible to temperature changes and becomes opaque at temperatures greater than 100°C.2 The change in index with temperature is about 400ppm/°C, which is 400 times greater than standard BK-7 glass.
Figure 3: Spherical aberration of a visible BK-7 singlet vs. a germanium singlet1
Zinc Selenide (ZnSe)
Zinc selenide is unique in that it transmits in the visible spectrum as well as infrared. Its transmittance is about 70% from 500nm all the way out to 20μm. It can be extremely useful for alignment of LWIR systems because visible light can be used for the alignment process. Reflectivity is high due to the large index of refraction (~2.4) so a thin-film, anti-reflection coating is needed. Zinc selenide is less sensitive to temperature change than germanium with a dn/dT value of about 70 ppm/°C. It can be matched with AMTIR-1 to make an achromatic doublet with low sensitivity to temperature changes. Figure 4 shows an achromatic, athermal doublet designed with ZnSe and AMTIR-1.
Figure 4: Achromatic, athermal doublet for LWIR using ZnSe and AMTIR-11
Zinc Sulfide (ZnS)
Multi-spectral grade zinc sulfide transmits both in the visible and infrared spectra. In the LWIR, multi-spectral grade ZnS has over 80% transmission. Non-multi-spectral grade only has 60% transmission. Because the multi-spectral grade can be used in both visible and infrared spectra, it can be helpful when aligning systems just as zinc selenide is. One form of ZnS called Irtran 2 manufactured by Eastman Kodak is resistant to thermal shock and can withstand atmospheric weathering as well as standard visible glasses.2 Zinc sulfide can be paired with other LWIR materials such as germanium, zinc selenide, and AMTIR-1 to make high quality achromats. Figure 5 shows high quality achromatic doublets using common LWIR materials.
Sodium Chloride (NaCl)
Sodium chloride is ordinary rock salt and is soluble in water. Even so, it has excellent optical properties in the LWIR. It has a continuous transmittance value of over 92% from 2 to 14μm. It can be polished easily and is commonly used for spectrometer windows, high powered laser windows, and prisms. Its crystalline structure and cleavage planes make it difficult to shape into curved surfaces. Because of its high solubility, it must be protected by evaporative coatings such as a selenium coating. Another unique property of sodium chloride is that it has a negative value of dn/dT at wavelengths longer than 220nm. Sodium chloride is also very soft.
Gallium Arsenide (GaAs)
Gallium arsenide is a hard and durable material. It has a Knoop Hardness of 731 kg/mm2 and a Young’s modulus of 83 GPa. Because of its strength and durability, it is a good material to use in field applications. It has 55% transmission out to 12 μm and reflectance of about 45%. The index of refraction in the LWIR is about 3.3 causing the large reflectance. Like germanium, the index of gallium arsenide is highly susceptible to temperature changes. The temperature coefficient of refractive index (dn/dT) is 150 ppm/°C.
Amtir-1 (GeAsSe)
AMTIR-1 is unique in that it is a glass structure material whereas most other LWIR materials are crystal structures. Transmission is 65% in the LWIR region and the refractive index is about 2.5. It is a softer material with a Young’s modulus of 22 GPa. The index of refraction is more susceptible to temperature change than visible glasses, but dn/dT is significantly less than germanium and gallium arsenide (72 ppm/°C). It can be used together with many other LWIR materials to make achromatic doublets (see Figure 5) and can also be paired with zinc selenide to make an achromatic, athermal doublet (see Figure 4).
Figure 5: high quality achromatic doublets using common LWIR materials1
Conclusion
Designing optical systems in the long-wave infrared spectral region poses some interesting challenges. There are very few optical materials to choose from, so it may be difficult to find a single material that suits all of a design’s requirements. Guidelines are given to aide in choosing acceptable LWIR materials for given applications. Optical, thermal, and mechanical properties of the materials as well as cost and weight should all be considered before a final decision is made. The most common LWIR materials such as germanium, zinc selenide, zinc sulfide, sodium chloride, gallium arsenide, and AMTIR-1 are described in detail. Unique properties of each material are outlined.
References
- Juergens, R., “Infrared Optical Systems” from Practical Optics Seminar, October 4, 2006.
- Klocek, P. Handbook of Infrared Optical Materials, Marcel Dekker, Inc., 1991.
- Rogers, P., “The selection of IR optical materials for low-mass applications,” in Design and Engineering of Optical Systems, J. Braat ed., Proc. SPIE 2774, 301-310, 1996.
- J. Burge lecture notes from OPTI 521, 2009.
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