Optical Material: Infrared Optics is the last article in this 5-part series of blog posts that aims to highlight the various optical materials used to design and manufacture optical components for the photonics industry.
It is an increasingly common sight to see infrared (IR) thermometers and thermal cameras in public places like airports, malls, and hospitals. These devices measure the surface temperature of an object by detecting emitted or reflected IR radiation from its surface. The hotter the object, the higher the amount of IR radiation it emits. Since the temperature of an object is a measure of its heat, IR radiation can be used for contactless temperature measurement.
The IR region is a particularly important band of the electromagnetic spectrum due to its utility in a large number of applications, and also due to the fact that phenomena such molecular vibrations occur in this part of the spectrum. From the humble TV remote to pulse oximeters to self-driving cars, infrared optics play an important role in virtually every industry from healthcare, food safety, communication, space and astronomy, and defense among many others. These applications are made possible by continually evolving innovations in IR sources and materials. In this article, we will focus on commonly used IR materials and their applications.
The IR spectrum is situated between the visible and microwave regions of the electromagnetic spectrum. In terms of wavelength, it spans from 0.75 micron, which is abbreviated μ, to 12 micron and can be classified into 4 sub-bands: Near Infrared (NIR), Short Wave Infrared (SWIR), Mid Wave Infrared (MWIR), and Long Wave Infrared (LWIR.)
Near Infrared // 0.75μ — 1μ
The NIR spectral band extends from wavelengths of 0.75μ to 1μ. This band includes applications in food testing, agriculture, biomedical instrumentation, and remote controls. For instance, pulse oximeters that measure oxygen saturation in blood use a combination of red (700nm) and IR LEDs (900nm) to determine the optical absorption of oxy- and deoxyhemoglobin. Self-driving cars use LIDAR to map their surroundings. LIDAR infrared optics typically operate at wavelengths of 905nm or 940nm. Night vision goggles and image intensifier tubes also work in the NIR band. Remote control devices also operate in this band, typically using 940nm LEDs for communicating with devices.
NIR Detector Materials
By far the most common NIR material used in the aforementioned applications is Silicon. Silicon has responsivity from 400nm to 1100nm, which makes it suitable for applications in NIR detection.
NIR Transparent Materials
Many of the infrared materials that work in the UV and visible part of the electromagnectic spectrum also operate in the NIR spectrum. Glasses and ceramics such as BK7, UV and IR fused silica, quartz, and sapphire are suitable for optical windows, lenses, and other components due to excellent transmission in this spectral range. Previous blog articles have covered a range of optical materials used in the visible as well as IR portions of the electromagnectic spectrum.
Short Wave Infrared // 1μ — 2.5μ
The SWIR band extends from 1μ to 2.5μ. A large part of this band is impractical for use because of intense absorption of water and carbon dioxide. However, this band does offer operational windows for optical communications at 1.3μ and 1.55μ where there is low transmission loss and low dispersion.
Additionally, SWIR spectroscopy is routinely used in food and drug testing. Since silicon is somewhat transparent in this region, SWIR cameras are used to examine printed circuit boards due to higher penetration of radiation. This band is also used for non-invasive and non-destructive testing of artwork, packaged goods, and counterfeit testing. In recent developments, SWIR cameras are increasingly used in medical research, small animal imaging, and brain imaging among others.
SWIR Detector Materials
Since Silicon does not absorb SWIR radiation, other materials such as Germanium (Ge), Indium Gallium Arsenide (InGaAs), and Indium Gallium Arsenide Phosphide (InGaAsP) are used for SWIR detection. Germanium has responsivity from 0.8μ to 1.6μ, whereas InGaAs can operate over a wider range from 0.8μ to 1.8μ.
SWIR Transparent Materials
In addition to previously mentioned NIR materials such as IR fused silica, IR transparent materials include Zinc Selenide (ZnSe), Zinc Sulfide (ZnS), Calcium Fluoride (CaF 2) and Magnesium Fluoride (MgF 2). All of these operate from the visible spectrum up to 8–10μ. Corning HPFS®7979 IR is an excellent choice for optical windows and lenses that operate in the SWIR range.
Mid Wave Infrared // 3μ — 5μ
The MWIR band extends from 3μ to 5μ. MWIR is associated with thermal radiation that is emitted from surfaces, and most of thermometry is performed in the MWIR or long wave IR regions of the IR spectrum. The MWIR band is more suitable for indoor inspection of temperature in factories and furnaces, since ambient solar radiation can contribute to noise in these systems. Thermal cameras in airports and other indoor locations use MWIR infrared optics to detect the temperature of passengers.
MWIR Detector Materials
MWIR detector materials include Lead Selenide (PbSe), Indium Antimonide (InSb) and Mercury Cadmium Telluride (HgCdTe). Most often these detectors require cooling due to noise caused by thermal fluctuations.
MWIR Transparent Materials
Silicon is an excellent material for MWIR components such as lenses and windows as it is transparent in this spectral range. Additionally, ZnSe, ZnS, CaF 2, and MgF 2 are all suitable for optical components in the MWIR range.
Long Wave Infrared // 8μ — 12μ
The LWIR band extends from 8μ to 12μ, although some classifications extend it further to 14μ. This is the choice IR band for outdoor thermal imaging, as it is less sensitive to ambient noise. LWIR is extensively used in military surveillance applications to the possibility of obtaining excellent contrast from background emitters. And for this same reason, LWIR cameras are used to look through foliage and detect human movement. With increasing usage of drones in surveillance and reconnaissance operations, the market for LWIR cameras has been steadily increasing.
LWIR Detector Materials
There are very few detector materials available for LWIR applications as traditional semiconductors that rely on a band gap offer limited detection opportunities. The band gap is very low in this part of the electromagnetic spectrum, and thermal noise can swamp any signal. Microbolometers are the best choice of detectors in the LWIR spectrum.
LWIR Transparent Materials
Potassium Bromide (KBr) is an excellent LWIR optical material and offers the broadest transmission spectrum from 0.23μ to 26μ. Zinc Selenide (ZnSe) and Sodium Chloride (NaCl) are also good candidates for optical components with high transparency up to 17μ.