Selecting the right infrared optical material is critical to achieving optimal performance and efficiency of the final product for a given application. Factors such as transmission range, refractive index and reflectivity, thermal and mechanical properties, and environmental compatibility should be considered when selecting an infrared material.
Infrared optical materials are essential for a variety of applications in the military, medical, aerospace and commercial industries. In recent years, the demand for high-performance infrared materials has increased with advances in technology. Selection of the right material for a particular application is crucial as the selection process will determine the quality, performance and cost-effectiveness of the final product.
Infrared optical materials can be broadly categorized as crystals, glass, semiconductors or metals. Crystalline materials are preferred for high-resolution applications such as laser systems and medical imaging. Silicon, germanium, zinc selenide and calcium fluoride are commonly used crystalline materials. Quartz glass(fused silica (SiO2)), borosilicate crown glass, sulfur-based glass, fluorozirconate glass, and germanate glass are a few of the glasses commonly used in infrared applications. Ceramic infrared materials are favored for their mechanical strength and resistance to extreme temperatures. Sapphire (Al2O3) and silicon carbide(SiC) are widely used ceramic IR materials.
The applications for infrared materials are diverse, each with unique requirements.
For example, aerospace and defense applications such as astronomical telescopes, missile tracking, night vision systems, thermal imaging and target detection require infrared materials to be lightweight, durable and provide good imaging and sensing capabilities.
Medical applications, such as non-invasive temperature measurement, blood analysis and tissue analysis, require infrared materials to be biocompatible, water-soluble, and have excellent transmission through biological tissue.
Commercial applications, such as security cameras and remote sensing, require IR materials that provide high sensitivity and can operate at room temperature.
When selecting an infrared material, there are several key aspects to consider:
1. Transmittance range
Transmittance is a measure of luminous flux, specified as a percentage of incident light. Different applications can operate in different regions of the IR spectrum. Depending on the wavelength used, certain IR substrates perform better. Depending on the application, different IR materials may have optimal wavelength ranges. There are three main wavelength bands involved when using optical systems within the Earth's atmosphere. They are short-wave infrared (SWIR) or near-infrared (NIR), with wavelengths from 0.75 to 3 μm; mid-wave infrared (MW), with wavelengths from 3 to 5 μm; and long-wave infrared (LWIR), with wavelengths from 8 to 14 μm.
For example, germanium (Ge) is very popular for systems in the mid-wave infrared (MW) 3- 5 μm or long-wave infrared (LWIR) 8- 12 μm spectral regions, while silicon (Si) and Sapphire (Al2O3) are commonly used in infrared optics, operating in the near-infrared (NIR) and milliwatt (MW) spectral bands.
2. Refractive Index and Reflectance
Refractive index and reflectance determine the thickness and type of coating a material requires. For example, optical materials for windows require a lower refractive index in order to minimize reflection losses, while materials used to make prisms, lenses and other optical components in high magnification, wide field of view optical systems require a higher refractive index.
3. Thermal properties
different applications require different thermal properties, such as thermal conductivity, coefficient of thermal expansion and specific heat capacity. The selection should be based on the temperature range of the application. dn/dT is the thermo-optic coefficient, i.e., the change in refractive index of the material per Kelvin temperature. The refractive index of a medium will vary with temperature. For example, germanium (Ge) has a large thermo-optic coefficient (dn/dT), which causes a large change in focus with temperature, making thermalization of the optical system difficult, while sapphire (Al2O3) has a low dn/dT and is able to withstand harsh environmental conditions.
4. Mechanical properties
Mechanical strength is critical for materials used in the manufacture of robust and durable items such as lenses, prisms and windows. Different materials have different hardness, ductility, fracture toughness and coefficient of thermal expansion. For example, germanium (Ge) is a relatively soft and brittle material, while Sapphire (Al2O3) is a hard and tough material.
5. Environmental compatibility
Some applications require materials that can withstand harsh conditions such as high humidity, radiation and chemical exposure. Infrared optical materials for medical applications may need to be non-toxic and biocompatible. Different materials have varying tolerances to acids, bases, solvents and moisture. For example, silicon (Si) is susceptible to hydrofluoric acid (HF), while Sapphire (Al2O3) is chemically inert to most corrosives.
6. Optical properties
Some IR materials may have unique optical properties such as birefringence, polarization, or non-linear behavior. Depending on the application, these properties may or may not be desired. For example, magnesium fluoride (MgF2) has birefringent properties, which make it useful in polarization-sensitive applications, but may lead to distortion or signal loss in non-polarized applications.
In summary, selecting the right infrared optical material is critical to achieving optimal performance and efficiency of the final product for a given application. Factors such as transmission range, refractive index and reflectivity, thermal and mechanical properties, and environmental compatibility should be considered when selecting an IR material. The classification of crystalline materials, glasses and ceramics provides a wide range of options for high performance IR materials.