An Engineer’s Introduction to Thermal Imaging

Thermal imaging allows users to see the unseen, but effectively adding this to any project means making sure that the right equipment and sensors are used in order to achieve the right type of capability. There are a range of factors that must be considered in order to get this right, and these are outlined below.

Today’s Thermal Technology and a New World of Applications

Thermal imaging can be a benefit to a whole array of product designs, but in order to use this effectively it is important to understand what thermal imaging really does and how it can be practically utilized in real world scenarios.

Thermal imaging works by detecting and then displaying relative differences in the intensities of the infrared energy being reflected or emitted from an object of a series of other objects. Infrared energy is around us all of the time and is completely independent of the amount of visible light that is available.

Infrared’s independence from visible light is an important factor to bear in mind as thermal imaging is not just a different kind of night vision – it sees heat and not light, so it does so 24 hours a day.

Because thermal imagers create images from differences in heat energy, anything that normally creates heat can be detected and imaged. For example, animals, people, electro-mechanical systems and industrial processes all have individual heat signatures that will be visible via thermal cameras.

Thermal imaging has a long and proven track record within law enforcement and military applications, but recent advances in technology have also made thermal imaging devices more compact, lighter and affordable for consumer and commercial applications.

Thermal cameras’ ability to detect heat is not affected by smoke, which makes them an established tool in search and rescue or firefighting applications. This factor alone has opened up a wide range of new applications as thermal imagers are well suited to detecting the differences in heat that can highlight the presence of animals or people, even in complete darkness. It can also be used to detect electro-mechanical parts that are wearing out or almost at the point of failure.

Thermal imaging offers real benefits in applications where privacy is an important concern, such as within residential video security. Thermal imaging creates an image where it is not possible to recognize or identify facial features, and additionally, thermal imaging cameras are unable to see through windows.

Thermal technology can also be used as a sensor as opposed to just a camera. It is an orthogonal technology that can complement visible imagers, reducing false alarms in security applications. Also potentially working alongside LIDAR, radar and visible cameras, in exciting new areas including self-driving cars and ADAS (advanced driver assistance systems).

Because thermal imagers and sensors do not require the presence of visible light to detect animals or people, it is possible to use them in a wide array of applications such as home security cameras, gesture recognition in games consoles, person-in-room detectors for home automation and even for driver’s vision enhancement in automotive applications.

Benefits of Thermal Sensors

What Is Thermal Imaging?

Essentially, thermal imaging detects then displays differences in infrared energy before using those differences to create color or greyscale images.

Infrared energy is a part of the electromagnetic spectrum, but it possesses wavelengths so long that they cannot be seen by the human eye.

Because of this, it is possible to use thermal imaging to create images without using any visible light. This allows them to pick up on very small temperature differences in total darkness, as well as through certain atmospheric conditions such as dust, smoke and light fog.

Absolutely everything gives off some level of infrared energy – even very cold objects like ice. In fact, theoretically, anything above Absolute Zero (the temperature at which molecular activity stops completely) will give off some level of infrared energy, meaning this can be detected by a thermal camera under the right conditions. Of course, the warmer an object gets, the more heat energy it emits.

As human beings we are most familiar with visible light, though this is actually a very narrow waveband on the electromagnetic spectrum and only encompasses the range of 0.39 to 0.75 micrometers (μm). The infrared waveband is comparatively much larger, starting near 1.0 μm and ending at 1,000 μm.

In this context, there are three particular subsets of infrared energy which are the most important. These are:

  • Short-wave Infrared (SWIR) waveband from approximately 0.9-1.7 μm
  • Mid-wave Infrared (MWIR) waveband from 3-5 μm
  • Long-wave Infrared (LWIR) waveband from 8-14 μm

Electromagnetic Spectrum

How Do Thermal Cameras Work?

Thermal cameras are able to capture infrared energy from all around us and then use that energy to create data points or images via analog or digital video outputs. Thermal cameras are generally only sensitive to one of the wavebands mentioned above: either MWIR from 3-5 μm or LWIR from 8-14 μm.

There are exceptions, but for the most part MWIR cameras need their infrared detectors to be chilled to approximately 77 kelvin in order to create an image. This chilling is achieved using an on-board cryogenic cooler, though these cooling devices add additional weight, size, complexity and cost to the camera as well as requiring extra maintenance.

Uncooled LWIR cameras such as the Lepton and Boson models are able to create images at ambient temperatures, so these are more compact, less complex and generally less expensive than their cryogenically cooled counterparts.

Thermal cameras themselves are comprised of a lens, thermal sensor and processing electronics, all contained within a mechanical housing. The lens is responsible for focusing infrared energy onto the sensor (sometimes referred to as a detector).

FLIR’s LWIR detectors are made from Vanadium Oxide (VOx). This is the most sensitive material available for detecting longwave radiation and does not produce the image artifacts often associated with more commonly used detector materials like Amorphous Silicone.

Rather than being a single device, these detectors are actually a whole array of detector elements that are available in a range of pixel configurations. The most common of these configurations are 320x256 and 640x512. The size of the array configuration is what dictates the resolution of the detector.

These resolutions may seem very low in comparison to visible light cameras, but there is a reason for this. Individual detector elements in thermal cameras are far bigger than their visible camera counterparts. In fact, visible cameras have pixels that are only 1-2 μm, while thermal camera detector elements are generally 12-17 μm each.

The increased size of these detectors is because thermal imagers must sense energy that has much larger wavelengths than visible light, so in order to do this the sensor elements themselves must be considerably bigger. As a result of this, a thermal camera will always have a much lower resolution – that is, fewer pixels – than a visible imaging camera of comparable mechanical size.

When thermal energy reaches the detector, the readout electronics then convert this into a signal that can be either passed out of the camera or, with a more advanced camera like the FLIR Boson, passed straight to system-on-a-chip circuitry for internal processing. SOC technology like this is able to provide on-board capabilities such as analytics, image processing and other advanced capabilities which had previously required complex back-end electronics to be developed by the integrator.

The lenses used in FLIR’s thermal cameras are made from Chalcogenide or Germanium, because these materials are exceptionally transparent to longwave radiation. It is also worth noting that thermal cameras cannot use the same lens materials as their visible light counterparts because those materials essentially block all infrared energy from reaching the detector. This is also why thermal cameras are unable to see through windows.

Basic Parts of a Thermal Sensor

How Do I Pick the Right Thermal Imager for My Integrated Product?

Choosing the best thermal camera for a specific application really depends on what the camera has to do within that application. There are several factors which influence this determination:

What Resolution and Lens Do I Need?

Decisions around lenses and the resolution required will be determined by three further, key, questions:

  • What needs to be imaged?
  • How far away will the objects be?
  • What level of detail is needed in the images?

A wide range of thermal imaging cameras are available. These range from lower-cost imagers designed to be used in conjunction with smartphones, right through to high-performance cameras used in critical and life-saving missions.

FLIR cameras like Boson are available in qVGA (320x256) resolution, and VGA (640x512) resolution. They have horizontal Fields of View (FOV) ranging from 4 ° to 92 °. Lower-cost solutions like Lepton are available in lower resolutions like 80x60 and 160x120.

The amount of image detail available and the camera’s potential detection range are directly related to its resolution. If, for example, the goal is to detect the presence of a single object then in fact, a single pixel may be sufficient.

If the application is more complex and the goal is to be able to recognize what the object is (for example a person, animal or vehicle) then a larger group of pixels will be required. If even more detail is required (for example, discerning if a person is armed or if it is a truck rather than a car) then even more pixels will be necessary.

By dividing the width of the scene in that FOV by the horizontal number of pixels, it is possible to work out what the smallest feature that can be detected at that distance will be. It is also possible to use a specification known as the Instantaneous Field of View (IFOV) to take the angular size of one pixel and then calculate its size at a given range.

What Sensitivity Do I Need?

The sensitivity of a camera is specified as the Noise Equivalent Differential Temperature (NEDT). This is a specific signal-to-noise measurement that shows the temperature difference needed to produce a signal that is equal to the camera’s temporal noise and, therefore, the minimum temperature difference that the camera is able to resolve.

NEDT is usually shown in milliKelvin (mK) with lower numbers representing better performance than higher numbers. For example, FLIR’s Boson camera has industry-leading sensitivity of less than 50 mK.

Resolution Comparison

Do I Need Radiometry or Just Imaging Capability?

Any thermal imaging camera is able to provide an image of the relative intensities of thermal energy within their fields of view, but some more advanced cameras are also able to give calibrated, non-contact measurements of the temperature of those objects. This process is known as radiometry.

In order to provide these measurements, the camera must be able to compensate for other sources of radiation – for example reflections and lens materials. It must also account for other factors such as atmospheric effects and the material properties of the objects being viewed – in particular their emissivity.

Once all these variables are taken into account, it is possible to take the amount of radiation received by the camera and convert this into a measured temperature value with an accuracy of around +/-2 °C.

Cameras from FLIR are available with a range of radiometric capabilities and levels of accuracy. These can vary from imaging only, to basic center spot meter functionality, right through to advanced radiometry with high levels of accuracy. Many FLIR camera are available in either imaging or radiometric variants.

What Should I Look for in an IR Supplier?

When integrating thermal imaging into a product design, there is a degree of commitment to the technology and it’s potential. As such, it is important to work with a supplier who shares that commitment, especially in these key areas:

  • Quality and reliability – it is important to ensure that cameras ‘just work’ throughout the product’s designed life cycle.
  • History – choose a supplier with a proven track record of success for their OEM partners.
  • Service standards – a supplier should offer dedicated product technical support and a local after-sales service.
  • Information – a supplier should offer quick and easy access to product specs, drawings, and interface documents.
  • Stability – choose a supplier that will be available for a long-term working relationship.
  • Forward-thinking – a good supplier will have a commitment to continuous product development and improvement.
  • Specialized knowledge – the best suppliers possess industry-specific domain knowledge in rapidly-developing areas like security, IoT, and automotive with their specific interfaces, regulations, and controls.

What Is the Total Cost of Ownership?

There are a number of considerations which must be accounted for in order to decide whether or not an OEM thermal camera is a good value prospect for a particular project. The unit cost is important, but this is not simply a case of looking at the acquisition cost – it is the total build cost which must be considered, alongside the following factors:

  • Hidden costs – unplanned costs such as difficult integration, additional engineering development time and expanded time-to-market can hinder even the best plans to introduce a new product to the market.
  • Competitive pricing – choosing a supplier that offers this is essential in order to get the product to market at a price point that enables it to be successful.
  • Support and documentation – it is important to choose a supplier that provides professional technical support to ease integration, therefore reducing time-to-market
  • Warranties – a good supplier should back their products with industry-leading warranties.

Do Thermal Cameras Require Export Licenses?

Some thermal cameras require export licences while some don’t, and it is important to ensure that export compliance requirements are understood and adhered to.

For example, FLIR employs a team of trade compliance professionals that understand these complex rules and are able to assist customers in this area.

Export requirements can also be dictated by the end product rather than the camera itself. It is important to note that where, for example, FLIR delivers a camera to a customer in the United States who then intends to integrate this into another product before exporting it, it is then the customer’s responsibility to ensure that appropriate export licensing is in place as they are the ones exporting the final product.

This information has been sourced, reviewed and adapted from materials provided by FLIR Cores and Components Group.

For more information on this source, please visit FLIR Cores and Components Group.

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