1) Introduction to Infrared Optics
Infrared Optics are used to collect, focus or collimate light in the wavelength range between 760 and 14,000 nm. This portion of IR radiation is further divided into four different spectral ranges:
| Near Infrared range (NIR) | 700 – 900 nm |
| Short-Wave Infrared range (SWIR) | 900 – 2300 nm |
| Mid-Wave Infrared range (MWIR) | 3000 – 5000 nm |
| Long-Wave Infrared range (LWIR) | 8000 – 14000 nm |
2) Short-Wave Infrared (SWIR)
SWIR applications cover the range from 900 to 2300 nm. Unlike MWIR and LWIR light that is emitted from the object itself, SWIR resembles visible light in the sense that photons are reflected or absorbed by an object, thus providing the necessary contrast for high resolution imaging. Natural light sources such as ambient start light and background radiance (aka nightglow) are such emitters of SWIR and deliver excellent illumination for outdoor imaging at night.
A number of applications that are problematic or impossible to perform using visible light are feasible using SWIR. When imaging in SWIR, water vapor, fire smoke, fog, and certain materials such as silicon are transparent. Additionally, colors that appear almost identical in the visible may be easily differentiated using SWIR.
SWIR imaging is used for multiple purposes such as electronic board and solar cell inspection, produce inspection, identifying and sorting, surveillance, anti-counterfeiting, process quality control and more.
3) Mid-Wave Infrared (MWIR)
MWIR systems operate in the 3 to 5 micron range. When deciding between MWIR and LWIR systems, one has to take several factors into account. First, the local atmospheric constituents like humidity and fog have to be considered. MWIR systems are less affected by humidity than LWIR systems, so they are superior for applications such as coastal surveillance, vessel traffic surveillance or harbor protection.
MWIR has greater atmospheric transmission than LWIR in most climates. Therefore, MWIR is generally preferable for very long-range surveillance applications exceeding 10 km distance from the object.
Moreover, MWIR is also a better option if you want to detect high-temperature objects such as vehicles, airplanes or missiles. In the image below one can see that the hot exhaust plumes are significantly more visible in the MWIR than in the LWIR.
4) Long-Wave Infrared (LWIR)
LWIR systems operate in the 8 to 14 micron range. They are preferred for applications with near room temperature objects. LWIR cameras are less affected by the sun and therefore better for outdoor operation. They are typically uncooled systems utilizing Focal Plane Array microbolometers, although cooled LWIR cameras do exist as well and they use Mercury Cadmium Tellurium (MCT) detectors. In contrast, the majority of MWIR cameras require cooling, employing either liquid nitrogen or a Stirling cycle cooler.
LWIR systems find a wide number of applications such as inspection of building and infrastructure, defect detection, gas detection and more. LWIR cameras have played an important role during the COVID-19 pandemic as they allow quick and accurate body temperature measurement.
5) IR Substrates Selection Guide
IR materials have distinct properties that allow them to perform well in the infrared spectrum. IR Fused Silica, Germanium, Silicon, Sapphire, and Zinc Sulfide/Selenide, each has strengths for infrared applications.
Zinc Selenide (ZnSe)
Zinc selenide is a light-yellow, solid compound comprising zinc and selenium. It is created by synthesis of Zinc vapour and H2 Se gas, forming as sheets on a graphite substrate. It is known for its low absorption rate and which allows for excellent uses for CO2 lasers.
| Optimum Transmission Range | Ideal Applications |
| 0.6 - 16μm | CO2 lasers and thermometry and spectroscopy, lenses, windows, and FLIR systems |
Germanium (Ge)
Germanium has a dark grey smoky appearance with a refractive index of 4.024 with low optical dispersion. It has a considerable density with a Knoop Hardness (kg/mm2): 780.00 allowing it to perform well for field optics in rugged conditions.
| Optimum Transmission Range | Ideal Applications |
| 2 - 16μm | LWIR - MWIR Thermal imaging (when AR coated), rugged optical situations |
Silicon (S)
Silicon has blue-gray appearance with a high thermal capacity that makes it ideal for laser mirrors and silicon wafers for the semiconductor industry. It has a refractive index of 3.42. Silicon components are used in electronic devices is because its electrical currents can pass via the silicon conductors much quicker compared to other conductors, it is less dense than Ge or ZnSe. AR coating is recommended for most applications.
| Optimum Transmission Range | Ideal Applications |
| 1.2 - 8μm | MWIR, NIR imaging, IR spectroscopy, MWIR detection systems |
Zinc Sulfide (ZnS)
Zinc Sulfide is an excellent choice for infrared sensors it transmits well in the IR and visible spectrum. It is typically a cost effective choice over other IR materials.
| Optimum Transmission Range | Ideal Applications |
| 0.6 - 18μm | LWIR - MWIR, visible and mid-wave or long-wave infrared sensors |
Your choice of substrate and anti-reflection coating will depend on which wavelength requires prime transmittance in your application. For instance, if you are transmitting IR light in the MWIR range, germanium may be a good choice. For NIR applications, sapphire may be ideal.
Other specifications you may want to consider in your choice of infrared optics include thermal properties and index of refraction. The thermal properties of a substrate quantify how it reacts to heat. Often, infrared optical elements will be exposed to widely varying temperatures. Some IR applications also produce a large amount of heat. To determine whether an IR substrate is suitable for your application you will want to check the index gradient and coefficient of thermal expansion (CTE). If a given substrate has a high index gradient, it may have suboptimal optical performance when used in a thermally volatile setting. If it has a high CTE, it may expand or contract at a high rate given a large change in temperature. The materials most often used in infrared optics vary widely in index of refraction. Germanium, for instance, has an index of refraction of 4.0003, compared with 1.413 for MgF. The availability of substrates with this wide range of index of refraction gives added flexibility in system design. The dispersion of an IR material measures the change in index of wavelength with respect to wavelength as well as the chromatic aberration, or the separation of wavelength. Dispersion is quantified, inversely, with the Abbe number, which is defined as ratio of the the refractive index at the d wavelength minus 1, over the difference between the index of refraction at the f and c lines. If a substrate has an Abbe number of greater than 55, it is less dispersive and we call it a crown material. More dispersive substrates with Abbe numbers of lower than 55 are called flint materials.
Infrared Optics Applications
Infrared optics have applications in a many fields, from high power CO2 lasers, which work at 10.6 μm, to night-vision thermal imaging cameras (MWIR and LWIR bands) and IR imaging. They are also important in spectroscopy, as the transitions used in identifying many trace gases are in the mid infrared region. We produce laser line optics as well as infrared components which perform well over a wide wavelength range, and our experienced team can provide full design support and consultation.
Paralight Optics is using a range of advanced processing techniques such Single Point Diamond Turning and CNC polishing to produce high-precision optical lenses from Silicon, Germanium and Zinc Sulfide that find applications in MWIR and LWIR cameras. We are able to achieve accuracies of less than 0.5 fringes PV and roughness in the range of less than 10 nm.
For more in-depth specification, please view our catalog optics or or feel free to contact us for more information.
Post time: Apr-25-2023
