Thermal Imaging

As depicted in Figure 1, thermal imaging techniques involve discriminating between various objects in a scene based on their respective differences in temperature. The black body radiation law (see Incoherent Light Physics) describes the characteristics of IR radiation emitted by an object with a temperature above absolute zero. Importantly, the magnitude of the emission (termed irradiance or exitance) increases with temperature. Therefore, provided a detector is sensitive to IR light, it is possible to visualize the environment based on temperature variation. The ability to produce an image (or thermogram) based on IR radiation is called thermography. Thermograms discriminate between warmer foreground objects and their cooler backgrounds such as the visualization of warm-blooded animals or humans.

Thermal imaging cameras allow for object distinction without the aid of an illumination source, thereby distinguishing it from LiDAR. Thermal imaging is ideal for producing images at night or in the presence of some obscuration, such as light fog, rain, or smoke. For example, forward-looking IR or FLIR cameras are used to provide night vision on military and civilian aircraft or for use in security and surveillance. FLIR cameras usually detect radiation in the mid-wavelength IR (MWIR, 3-5 µm range) and long-wavelength IR (LWIR, 7.5-14 µm range) portions of the electromagnetic spectrum. When coupled with additional cameras that can detect VIS (0.4-0.75 µm range), NIR (0.75-1.1 µm range), or short-wavelength IR (SWIR, 1.1-2.5 µm range) radiation, multispectral imaging is possible (see below). SWIR imaging is becoming increasingly popular in many industrial applications, including the inspection of solar cells, and agricultural products, and for anti-counterfeiting.

Thermal imaging has a long history in the areas of defense and aerospace but, commercial and industrial applications have led to a dramatic increase in its use over the past few decades. This emerging market is directed and led by various detector and camera manufacturers, such as FLIR, L3 Technologies, BAE Systems, Leonardo DRS, SCD, and Sofradir. These companies are continually making improvements and reducing costs for thermal imaging components and detectors that will result in further market growth. The main segments in the thermal imaging market are summarized in Figure 2 and are described below.

Thermal imaging has a wide variety of military and defense applications for operation at night, under reduced visibility, and for target acquisition. Furthermore, these applications can cover ground, air, and maritime environments. Example applications include aircraft navigation and targeting systems, thermal weapon sights, handheld and head-mounted imagers, fire control systems for tanks, and precision missile guidance. Thermal imaging systems are frequently used in unmanned aerial vehicles (UAVs) or drones.

The recent increase in terrorist activity and asymmetrical warfare has accelerated the need for IR systems in homeland security, border control, and law enforcement. These systems are utilized to identify security perimeters at various locations including airports as well as underground and above ground train stations. Law enforcement uses the technology to manage surveillance activities, locate and apprehend suspects, investigate crime scenes, and conduct search and rescue operations. Thermal imagers have helped revolutionize firefighting by enabling the detection of trapped persons as well as the location of the base of a fire.

Thermography is useful for monitoring physiological activities, including the detection of fever and disease. This can enable improvement in patient care as well as control at borders in the event of a contagious outbreak. Thermal imaging is ubiquitous for industrial applications, including gas leak detection, building inspection, predictive maintenance, and process control. "Smart buildings" leverage thermal activity sensors to bring intelligence to building systems, e.g., lighting, HVAC, alarms, that improves efficiency, enhances occupant comfort, and optimizes workspace management. "Smart cities" use thermal imagers to enable real-time monitoring of transport systems, traffic management, and power grids with the goal of improving quality of life and optimizing city functions. The automotive camera market is driven by the new trend of autonomous cars. Thermal cameras can play a key role for pedestrian recognition and obstacle warning, particularly when driving in total darkness or in tunnels.

Multispectral imaging combines two to five spectral imaging bands (VIS, NIR, SWIR, MWIR, or LWIR) of relatively large bandwidth into a single optical system. This can enable increased functionality and reveal details beyond the capability of a single-band detection system. Advances in imaging components have allowed multispectral imaging to move beyond custom-built systems for laboratory and government applications to affordable, practical, commercial systems. Applications range from deep-space imaging using space-based telescopes, airborne surveillance, satellite-enabled remote sensing, to handheld imagers.

The image quality produced by a thermal imaging system depends on its detector and its optical components. The most important parameters for IR detectors used in thermal imaging are sensitivity, pixel pitch (pixel-to-pixel distance), and format (number of pixels). In conjunction with the imaging optics, the pixel pitch and format determine the spatial resolution and area of the target to be imaged, (see description of camera-based profilers in Laser Beam Profile Measurement for details). Generally, IR detectors for thermal imaging cameras are much more expensive than their VIS spectrum counterparts. Consequently, large format detectors with pixel areas of 1024x768 are typically found on higher-end imaging cameras.

Detector sensitivity typically refers to the minimum detectable signal that gives a SNR value of one. The metric used to describe detector sensitivity is the normalized (or specific) detectivity (D*). This value accounts for the impacts of bandwidth and detector area and is intuitive in that larger values represent greater sensitivity. A description of the normalized detectivity, as well as typical values for IR detectors, are given in Radiometric Measurement. In addition to the detector sensitivity, the amount of radiation that reaches the detector ultimately determines the detection range of a thermal imaging system. Other factors that contribute to the detection range include the target parameters, e.g., size, temperature, emissivity - the ratio of an object's radiance to that emitted by a blackbody radiator, and atmospheric transmission. Figure 3 shows atmospheric transmission as a function of wavelength with the relevant IR spectral regions indicated.

Sensors capable of detecting IR radiation include thermopiles, pyroelectrics, bolometers, and low-bandgap semiconductor photodiodes (see Thermopile Physics and Pyroelectric Physics for details). While arrays of pyroelectric sensors and microbolometers are used for thermal imaging, the use of photodiode arrays are more common (see Laser Beam Profile Measurement). This is mainly due to the greater detectivity for these types of sensors as well as the maturity of the semiconductor manufacturing processes that enable them. Since these focal plane arrays (FPAs) are made of photodiodes, their IR spectral sensitivity depends on the bandgap of the semiconductor material being used. The most common types of FPAs are constructed using InSb, InGaAs, HgCdTe and quantum-well-based photodiodes. IR-sensitive FPAs are considerably more expensive than Si-based FPAs, resulting in higher-end models being used mainly in military applications. Furthermore, detection in the MWIR and LWIR often requires cryogenic cooling to achieve reasonable detectivity levels.

Optical components play a critical role in thermal imaging both in terms of performance and cost. In particular, lenses are the most essential components for imaging (see Optical Lens Physics for more information). Some widely-used terms related to thermal imaging optics include the effective focal length (EFL), the field-of-view (FOV), the entrance pupil, and the F/# (the ratio of the EFL to the entrance pupil diameter). Generally, the main performance parameters for lenses or systems of lenses are:

• Modulation transfer function (MTF) - measures the ability to transfer (or reproduce) modulation (or contrast) in an object to the image formed by the optical component; this is a function of spatial frequency (or resolution)
• Transmission - ratio of transmitted radiation relative to incident radiation
• Relative illumination - illumination at any point on an image relative to the maximum on-axis illumination
• Distortion - image deformation caused by a change in magnification across the FOV All these parameters are a function of the optical lens design.

Consequently, lens design is crucial in terms of achieving the desired image quality. The following discusses how thermal imaging optics are designed and manufactured to meet performance requirements.

There are many lens types used in thermal imaging systems. The types of lenses chosen for a particular system can depend on (among others): the spectral range of interest, e.g., NIR, SWIR, MWIR, or LWIR, the number of FOVs required (e.g., single, dual, continuous zoom), whether or not they need to be athermal (no need for refocusing following a temperature change), and their focus mechanism, e.g., fixed, manual, or motorized. Once the system requirements are established and the lens specifications are drafted, the optical designer must supply an optical design that meets specification requirements. Additionally, the designer must be cognizant of manufacturability and cost. The stages of optical design include the following:

1. Establishing the basic geometry
2. Optimizing performance, fit, and cost
3. Building a tolerance set
4. Documenting the design and performance

The principle method of arriving at a good optical design is optimization, which is varying the numerical design parameters to achieve a desired imaging outcome such as a tight focus or a flat field. Design parameters can be extensive owing to, for example, the number of lenses involved, their shapes or curvatures, the optical materials that make up the lenses, whether there is an intermediate focus, etc. 

Optimization is based on a merit function. This function can include terms that directly impact imaging performance, such as optical aberrations. It can also include terms for controlling design geometry, such as the thickness of elements or the spacing between them. The optical designer adjusts the merit function during the optimization process to achieve the design objectives, which include performance, size, and cost.

In order to complete an optimization, a designer must have a tolerance set in addition to the optical design itself. A tolerance set can include the sensitivity limits of the manufacturing process used in lens element fabrication. These tolerances might include variations in the radius of curvature, thickness, surface irregularity, and edge thickness. There are also tolerances related to the mechanical assembly of a system. These include decenter, tilt, and distance between elements.

Lenses for thermal imaging systems are made of materials with low absorption in the IR spectral region. The main materials used to make these lenses include Ge, Si, ZnS, ZnSe, CaF₂, and chalcogenide glasses. Mirrors can also be used as optical components for thermal imaging systems. These components have surfaces that are mainly made of metals, such as aluminum and copper. More information about mirrors and their surfaces can be found in Optical Mirror Physics.

There are four main types of optical surfaces used to construct optical components: plano, spherical, aspherical, and diffractive. Plano and spherical surfaces are made to accommodate paraxial wavefronts, that is, weakly focusing geometries. Aspherical surfaces are designed to correct for wavefront errors such as spherical aberrations and therefore are more appropriate for stronger focusing geometries. Diffractive surfaces are designed to correct for chromatic aberrations.

For IR optical components, surface flatness or irregularity has a standard accuracy of λ/2 at 633 nm, which can be quite demanding at certain IR wavelengths. Surface quality standards typically require scratch and dig values of 80-50 for the LWIR region and 60-40 for the SWIR region. Surface curvatures typically require accuracies of 0.1% of the radius value or better, while standard surface roughness values are in the 25 nm RMS range. In addition to high-quality optical surfaces, lens positioning is also critical in thermal imaging systems. This can include absolute positions in the optical housing and the relative positions of two lens surfaces. Measurement tools such as high accuracy calipers, micrometers, and comparators are used to meet the required specifications.

Almost all optical surfaces associated with IR optical components are coated. The coatings serve two purposes: improve spectral performance, i.e., transmittance and reflectance, and withstand environmental conditions. Details regarding general coating characteristics and application technologies are given in Optical Coatings. Optical coatings specifically for IR optics fall into one of the following categories: high efficiency, high durability, and hard carbon. High efficiency coatings have superior spectral performance with low environmental durability while high durability coatings sacrifice spectral performance for increased environmental durability. Hard carbon coatings have single layer diamond-like coatings and possess very high environmental durability, while their spectral performance can be tailored based on the target application.

Prior to assembly of the thermal imaging system, the specification requirements are verified for all the pre-assembled parts. During assembly, all elements, both mechanical and optical, must be kept free of potentially damaging debris. The main goal is that the optical elements are properly placed according to the optical and mechanical design requirements. There are several calibration and assembly methods designed to meet these requirements which vary in complexity depending on the system. For instance, systems that are made to function in extreme environmental conditions, e.g., temperature, shock, vibrations, rain, will undergo more rigorous assembly protocols.

More sophisticated assembly and mechanical design efforts are required when optical components are no longer in fixed positions relative to one another, such as in a zoom lens. In this case, internal lenses are moved to different positions with the goal of achieving good focusing for all designed FOVs. Positioning mechanisms may either be static or dynamic and can require a high degree of accuracy with smooth movement and zero backlash to function properly. For motorized imaging systems, a controller uses an encoder along with a look-up table to properly position the lenses to achieve optimal focusing according to design specifications. IR imaging lenses are made of materials that can be sensitive to the surrounding temperature, yielding slight changes in shape and/or position with changes in temperature. To compensate for this, the controller can measure the ambient temperature and reposition the lenses to maintain optimal focusing.

As UAV technology is implemented for increasingly varied and sophisticated tasks, there has been a call to improve imaging performance. Accordingly, detector manufacturers are working on improving resolution while maintaining relatively small footprints to accommodate usage in smaller drones. While detector resolution should improve imaging performance, this must be accompanied by improvements in lens quality. See UAV Imaging Systems for additional information.

Automotive night vision systems use thermal imaging technology to allow drivers to detect pedestrians and provide a clear view of the road, even when vision is obstructed by environmental conditions such as darkness, smoke, or fog. For maximum performance and minimal collision risk, thermal imaging accuracy, quality, and long-distance object detection are critical to provide the driver with enough response time. The key to meeting these requirements is the use of high-sensitivity and high-resolution optics.

MKS Ophir has earned its reputation as a world-leading designer and supplier in the field of thermal imaging optics for the automotive market. MKS Ophir's superior athermalized lenses increase pedestrian recognition software performance, allowing a greater ability to anticipate potential hazards. Crafted with years of experience and knowledge, these IR thermal imaging lenses feature the highest quality components and materials, designed especially to meet the needs of the industry. As the sole provider of IR thermal optics for the European automotive market, MKS Ophir's lenses are integrated in the night vision systems of top European cars.

A thermal weapon sight is a device that combines a compact thermal imager and an aiming reticle. These devices can be mounted on a variety of small arms and are often used by hunters. The thermal sight can be quite useful in places when vision is obstructed by environmental conditions such as darkness, smoke, or fog. The sight makes it easy for a user to locate any source of heat, such as an animal or vehicle, against its lower-temperature background.