What is Infrared Optics? An Introduction to Infrared Optics
Author: Eng. Boyan Milenov
Editor: Eng. Vera Ilieva
Published: May 17, 2021
Table of Contents:
- What is Infrared Optics?
- Short-Wave Infrared (SWIR)
- Mid-Wave Infrared (MWIR)
- Long-Wave Infrared (LWIR)
- Optics used in MWIR and LWIR systems
1) What is Infrared Optics
Infrared Optics are used to collect, focus or collimate light in the wavelength range between 700 and 16000 nm. They are further sub-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) Optics used in MWIR and LWIR systems
High-purity, optical grade Silicon is the material of choice for MWIR cameras. This is due to a combination of factors. Silicon offers excellent transmission in the MWIR range and a lack of birefringence. At the same time, it has low density and is therefore lightweight, while having high thermal stability and high durability. It is naturally abundant and therefore relatively cheaper than other IR optical materials that could be used in the MWIR such as Germanium.
Germanium on the other hand has excellent optical and mechanical properties which compensate for the fact that it is quite expensive. It can be easily ground and diamond turned, unlike Silicon. Germanium has high refractive index, which makes it very useful for LWIR systems and for elements with long radii. It is relatively heavy and has a high coefficient of thermal expansion (CTE), which has to be taken into account by optical designers.
Zinc Sulfide is another common material used both in the MWIR and LWIR. Unlike Silicon and Germanium which are typically grown using the Czochralski method (or Float Zone method). Zinc Sulfide is grown using Chemical Vapor Deposition (CVD). It can be both polished and diamond turned.
Cimcoop 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.