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Thermal imaging is a technology that allows users to view the heat emitted by objects as images without any need for visible light. All bodies naturally generate infrared radiation depending on their temperature. Thermal imaging can capture and transform that radiation into an actionable visual image, so that users can actually "see the invisible".

1.1. What exactly is thermal imaging?

Thermal imaging involves detecting the infrared radiation that is naturally emitted by all objects, according to their temperature. That radiation may be invisible to the human eye, but it can be captured by special detectors that transform the heat signature into images. These images show the temperature variations as color tones or in greyscale, thereby allowing users to view the differences in thermal radiation in the detector's field of vision.

1.2. The physical principle behind infrared radiation

Thermal imaging is based on one of the core principles of physics. In other words, any body whose temperature is greater than absolute zero (-273.15°C) emits electromagnetic radiation, mainly in the infrared band. The hotter the object, the more radiation it emits.

This radiation is invisible to the human eye, but it can be detected with sensitive sensors operating in specific infrared spectral bands. The wavelengths typically used in thermal imaging applications range from 3 to 14 microns, which corresponds to the medium-wave infrared band (MWIR) and the long-wave infrared band (LWIR).

1.3. A fully-fledged technology alongside other imaging systems

Thermal imaging differs from several other detection and vision technologies:

• Visible light imaging: relies on ambient or artificial light to reproduce a scene. It is incapable of detecting radiation in low-light conditions or complete darkness.
• Night vision (image intensifiers): amplifies the residual light (starlight or moonlight), but still requires a minimum level of ambient light.
• Radar imaging: uses radio waves to map a scene. It is extremely effective at detecting shapes and structures, but provides very few thermal or visual details.
• Lidar: uses laser pulses to measure distances. This method is highly accurate, but it tends to be restricted when atmospheric conditions are less than ideal.

Meanwhile, thermal imaging is capable of operating during the day and at night. It is unaffected by ambient light levels and can be used in dark, smoke-filled or cluttered areas. As such, thermal imaging is an invaluable ally for applications where other technologies' limitations are only too apparent, such as perimeter security systems, autonomous driving and predictive maintenance.

An infrared detector is a key part of a thermal imaging system. It is responsible for capturing the infrared radiation that objects emit naturally according to their temperature and then converting that radiation into a usable signal for generating a thermal image.

2.1. Detection principle

Any object whose temperature is higher than absolute zero emits electromagnetic radiation in the infrared band. An infrared optical lens (often made from germanium or chalcogenide) focuses this radiation towards an infrared focal plane array, which consists of a grid with thousands of heat-sensitive micro-detectors.

These detectors measure the intensity of the infrared radiation received and translate the information into an electrical signal. The hotter the object, the higher the signal's intensity. This signal is then processed and converted into a visible image.

2.2. Cooled vs uncooled: two technologies, two applications

Infrared detectors can be divided into two main categories:

Uncooled detectors, which are also known as microbolometers. The electrical resistance in these sensors changes according to the temperature of the radiation received. They work at ambient temperature, meaning that they are lighter, more compact, less expensive, and ideally suited to civil, industrial and automotive applications. They are currently in widespread use.

Cooled detectors, which require cryogenic cooling (often using a Stirling micro-cooler). These detectors are more complex and sensitive, so they are capable of identifying extremely subtle differences in temperature, while offering a greater range. They are preferred for the most demanding applications, such as defense, space, and long-range surveillance.

2.3. Thermal image processing

Once the radiation has been captured, the electrical signal from the sensor is routed through an ROIC (readout integrated circuit), which conditions the signal. The signal is then digitally processed to produce a thermal image.

The final image can subsequently be displayed in grayscale or false colors (thermal color palettes) to highlight the temperature variations. The image can also be automatically analyzed in embedded decision support systems, such as self-driving vehicles and intelligent security systems.

Infrared radiation covers a wide swathe of the electromagnetic spectrum and extends beyond visible light. It can be divided into several spectral bands, known as "wavelengths", each of which features a specific set of physical properties. When choosing the infrared band, the decision will depend on the application's environment and technical constraints, and the type of object that needs observing.

3.1. NIR – Near Infrared (0.78 to 1 µm)

The near-infrared band is the closest to the visible light spectrum and lies just outside the range that is visible to the human eye. In some conditions, sensors operating in this band are capable of delivering images featuring a higher level of precision and contrast than the images obtained through conventional visible imaging.

Just like visible light, NIR radiation is reflected by surfaces, meaning that an NIR light source is required to obtain a usable image. Unlike the MWIR and LWIR bands, NIR is unable to detect the thermal radiation that objects naturally emit at ambient or moderate temperatures. Only objects that have been heated to very high temperatures (over 1,000°C) emit enough radiation to be seen in this band without an external light source.

The NIR technology is used for surveillance applications. A CMOS sensor that has been enhanced to cover the near-infrared spectrum can offer effective night-vision capabilities when combined with an NIR LED light source. This is the very principle by which most of today's video surveillance cameras operate. However, this technology suffers from a limited range, which tends to be less than 50 meters, due to its dependence on an active light source.

Near infrared can also be found in the latest smartphones, where it is used for facial recognition.

It is also used for spectroscopy applications in the pharmaceutical, chemical and agri-food industries. The NIR technology is also ideally suited to non-destructive testing for assessing a material's moisture level, composition or purity, even through glass or plastic containers.

3.2. SWIR – Short-Wave Infrared (1 to 2.7 µm)

Short-wave infrared (SWIR) is a reflective band, just like visible light, but it offers a number of extra abilities, such as greater penetration into smoke, fog and certain materials. It offers enhanced vision, complete with shading and contrast, in challenging conditions, and is often used for :

• Inspecting organic matter, such as sorting foodstuffs and performing quality control on pharmaceutical capsules, especially with its ability to "see" through glass.
• Recycling and detecting defects that are invisible to the naked eye.
• Monitoring temperatures in the steel and glass production industries, and other sectors.
• Keeping sites under close surveillance using cameras that can "see" through fog and smoke.
• Detecting single-frequency lasers and camouflaged vehicles on the battlefield.

The SWIR band is also extremely useful for spotting camouflaged objects or distinguishing between similar color tones, meaning that this is a valuable technology for detecting counterfeit goods or identifying defects along production lines.

Finally, it can "see" hotspots, generally between 500°C and 3,000°C.

3.3. MWIR – Medium-Wave Infrared (3 to 5 µm)

MWIR is the spectral band where thermal images really start taking shape through temperature gradients. The emission peak ranges between 100°C and 500°C.

MWIR is especially suited to detecting very hot targets, such as engines, flames or in-service industrial equipment. It is less susceptible to humidity, which explains why it is a valued solution for long-range and high-resolution applications, including maritime surveillance, gas detection (methane, propane, etc.), border surveillance and eruption column measurements.

Since MWIR requires cryogenic cooling, it can achieve ultra-high levels of sensitivity, meaning that it is tailor-made for the defense industry for detecting military aircraft, as well as for scientific research, and critical infrastructure.

3.4. LWIR – Long-Wave Infrared (7 to 14 µm)

Long-wave infrared is the most widely used spectral band in thermal imaging. It corresponds to the emission peak of ambient temperature bodies, so it is ideal for viewing terrestrial objects. LWIR cameras are equally effective during the day and at night.

They are suitable for:

• Local situational awareness and anti-intrusion systems.
• In the automotive sector, LWIR is involved in the development of advanced driver-assistance systems (ADAS), particularly for detecting pedestrians.
• Wildlife observation.
• Fire protection applications (detecting fires through smoke and aerosols).
• Building thermographic surveys (poor insulation, ingress, etc.) and predictive maintenance.
• Industrial maintenance (glass, plastics and metal production, automotive sector, etc.).
• Detecting gas leaks (OGI).

LWIR detectors are often uncooled, meaning that they are more compact, robust and cost-effective. They are also prime candidates for embedded systems and consumer applications, including automobiles, thermography, and smart objects.

3.5. VLWIR – Very Long-Wave Infrared (11 to 50 µm)

This wavelength is geared towards such applications as chemical analysis, Earth observation, astrophysics and astronomy.

                                                                                        (MTG satellites - Courtesy of ESA)

3.6. Summary of the wavelengths and their applications

An infrared detector's format refers to the characteristics of its pixel grid, i.e. its resolution (number of columns x rows) and pixel pitch size (expressed in microns). These specifications have a direct influence on the optical system's dimensions, thermal image quality, detection range and field of vision.

4.1. Resolution and pixel pitch: the two key components of the format

An infrared detector comprises a two-dimensional network of heat-sensitive pixels. Two main criteria define the detector's optical performance:

• Resolution: the higher the number of pixels, the sharper the image. This means that users can detect and identify targets more effectively, especially at long distances.
• Pixel pitch: this refers to the center-to-center distance between two adjacent pixels. The smaller the pixel pitch (e.g. 12 µm, 10 µm or 7 µm), the greater the possibility of miniaturizing the optical system, while maintaining the same field of vision.

Combining a high resolution and with a small pixel pitch will produce sharp, detail-packed images, while reducing the size and weight of the thermal systems, which is a game-changer for embedded applications (drones, vehicles and handheld devices). However, a smaller pixel receives less energy, which reduces its sensitivity, so improving the signal-to-noise ratio may prove to be a challenge.

4.2. Available pixel pitch at LYNRED vs technology and format:

4.3. Choosing the right format

When it comes to choosing the right format, the decision depends on the:

• Observation distance: greater distances require higher resolutions.
• Required field of vision: small pixel pitches help retain a wide field of vision in a compact format.
• Type of application: stationary or embedded observation, confined or open environment, intended for the general public or experts, etc.

For example, a QVGA 17 µm detector would be the perfect solution for an entry-level handheld system, whereas an SXGA 10 µm will deliver exceptional performance for a long-range tactical system.

5.1. What does NETD mean?

NETD (Noise Equivalent Temperature Difference) is one of the most critical parameters when evaluating the performance of an infrared detector. It specifies the smallest temperature difference that a detector is capable of identifying, i.e. the lowest level of thermal contrast that the sensor can detect.

The terms thermal sensitivity and thermal contrast are sometimes used. NETD is expressed in Kelvin (K) or milli-Kelvin (mK). The lower the NETD, the more sensitive the detector.

A lower NETD value will also result in a higher quality thermal image, since the detector can identify minimal variations in temperature.

5.2. How is NETD measured?

Technically speaking, NETD is defined as the ratio between the RMS noise (the detector's background noise, expressed in volts) and the detector's response to a temperature variation (known as responsivity, expressed in V/K):

NETD is the ratio between RMS noise and sensitivity:

NETD (K) = RMS noise (V) / responsivity (V/K)

However, it is vitally important to point out that NETD is heavily dependent on the optical system used, especially the aperture of the lens (f-number). When comparing the performance of two detectors, it is essential that they have been measured with optical systems possessing the same f-number. An optical system with a larger aperture (lower f-number) improves responsivity and therefore the NETD value.

5.3. Why is NETD important?

NETD has a direct influence on thermal image quality and the system's ability to detect small temperature differences. This is especially critical in the following areas:

• Perimeter or night-time surveillance, where the system needs to be capable of spotting intruders in environments containing a low level of thermal contrast.
• Predictive maintenance, where systems starting to overheat must be pinpointed before the situation turns critical.
• Industrial quality control, where a subtle thermal defect could be the tell-tale sign of a nonconforming product.
• Scientific or medical imaging, where thermal precision is critically important.

5.4. How do you interpret a NETD value?

• A NETD value < 50 mK is considered to be excellent for demanding applications.
• A NETD value between 50 and 100 mK is suited to most everyday uses (industry, buildings, mobility, etc.).
• A NETD value > 100 mK is sufficient for applications with large thermal differences and where sensitivity is less critical.

NETD is a key indicator when comparing infrared detectors. However, it should never be used as the only criterion. Detectors should also be benchmarked in terms of their resolution, spectral range and type of optical system, and obviously the intended application.

To choose the right infrared detector, it is important to think of the NETD value as a fitness for purpose variable rather than an absolute quality criterion.

At LYNRED, customer support is so much more than a simple business agreement. We believe in building a long-term technical partnership driven by trust, expertise and a local service.

6.1. A dedicated technical team

You can count on a dedicated customer support team of experienced engineers (optical systems, electronics, cryogenics and R&D) who are capable of interacting with your R&D services, engineering departments or industrial platforms.

Our team also delivers proactive support throughout the project lifecycle, from defining the specifications to going live with the solution, with the ability to provide a swift response at each stage.

6.2. Training Center & courses

As a LYNRED customer, you have access to a 110 sqm training and test center on our premises in France, right next to our production facilities. This center is the ideal venue for putting sensors through their paces in real conditions, confirming the NETD, responsivity and noise values, and improving integration.

We offer a broad array of bespoke training courses, whether onsite on in our training center, supported by a selection of e-learning modules that can be accessed in your customer space (technical resources, videos, calculators, FAQ, etc.).

6.3. Documentation & customer extranet

You have access to a secure extranet portal with a comprehensive document library, including product datasheets, technical specifications, user guides, application notes and a FAQ. The library's content is regularly updated and designed to give customer teams an even greater sense of autonomy and independence.

6.4. Partnership & trust

At LYNRED, we are driven by our determination to actively coordinate with our customers and build effective collaboration to ensure a seamless knowledge transfer process, since trust forms the backbone of a successful partnership. Trust is key to dispelling any doubts that could arise during prototyping or testing.

Trust allows both sides to move forward together and look ahead to the future with less uncertainty. It allows the company to grow by continually staying ahead of the innovation curve, while defining its long-term strategic and technical roadmap.

Our dedicated team can provide you with bespoke support spanning our entire product range. Our customer care services are available to all customers. They have the power to accelerate the development cycle for your infrared solutions and reduce their time to market by boosting your teams' technical capabilities.

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