Thermal Cameras – Understanding How They Work and Bolometer Technology

Like a regular camera, a thermal camera uses infrared thermal waves to affect a material (amorphous silicon or vanadium oxide), thereby changing its resistance. However, unlike a regular camera, which uses a CMOS array to detect photons that have passed through the focusing effect of a lens. On this point, even a low-budget thermal camera uses a different configuration.

The imager of a thermal camera is called a bolometer.

As bolometers often contain very large pixels compared to a visual camera, the resolution of a thermal camera can be as high as 640 x 480, which is a relatively low resolution for a visual camera. It is important to keep in mind that in thermal energy, heat is transported over a variety of electromagnetic spectrums, but only one of these spectrums – the infrared spectrum – is sampled for imaging thermal.

In this spectrum, thermal energy behaves similarly to visible light energy, in that it can travel great distances and can be focused onto the bolometer with a non-glass lens. Additionally, like visible light, this infrared radiation can reflect off mirrors.

The first thing you need to understand when using a thermal camera is that heat has a strong time dependence on the image. In the visible spectrum, light is either present or absent. In a thermal environment, the temperature of an object is constantly transmitted to other objects in the image.

Two images of the same object taken at close intervals have the effect of showing a perceptible variation. Simply placing your hand on and off the table leaves a handprint that may take several seconds to cool to room temperature.

Therefore, with a thermal camera, the timing of capture is crucial, as is controlling the overall temperature of the scene. Depending on the thickness of the heat-sealed material, the temperature can be 180F at the time of heat-sealing, 150F two seconds later, and close to room temperature in just 10 seconds, making the number of seconds between heat seals critical. Heat sealing operation and inspection.

In order to acquire valuable information, it is necessary to precisely schedule the examination of a thermal image according to the thermal event. In addition, the contents of the bag can quickly lower the heat sealing temperature.

Thermal cameras in terms of reflectivity and thermal opacity

Thermal reflectivity and opacity are the next thermal engineering problem. In Hollywood films, a thermal camera is presented as having the ability to see through barriers. Because transparent glass and the surface of a puddle are thermally opaque, a thermal camera can’t even measure through them in the real world.

Additionally, because water in the air has a temperature that transmits in the infrared spectrum, hiding the thermal signature behind the water molecule, thermal cameras cannot operate effectively in a particularly humid environment. Vapor or fog is halfway between thermal translucency and opacity, whereas ordinary dry air is thermally transparent.

This makes thermal imaging difficult in a humid or wet environment. The heat signature of objects that are in its optical path, rather than the heat signature of the metal itself, is reflected by a shiny piece of metal, which is often thermally reflective. Unexpectedly, this is why foil performs well in the oven; it divides the thermal environment in two, allowing only convection/conduction of thermal energy while rejecting IR radiation.

Therefore, thermal imaging and visual imaging are very different, but they have some similarities. Surface reflectivity, weather, humidity and resolution are all factors that affect thermal imaging. An engineer already has a lot to do, but that’s not all.

Bolometers

Which bolometer should you use – a cheaper uncooled bolometer or a more expensive cooled bolometer? What is the difference ?

A cooled bolometer produces an extremely noise-free image compared to an uncooled bolometer (from Teledyne FLIR documentation). Every pixel of your data contains noise, which is random variation. In my experience, the noise of a typical uncooled bolometer is usually less than 5 degrees.

An uncooled device is very useful and will probably work just fine if you’re making decisions in the ten degree or higher region, which is generally in the middle of the sensors’ detection range. Also, if your object is moving quickly, a cooled bolometer will be the best option. A tire is shown rotating at 20 mph with an uncooled (left) and cooled (right) bolometer (Teledyne FLIR literature).

The wheel appears to be stationary in the left image because the brief exposure of the cooled bolometer makes it difficult for the uncooled bolometer to discern the position of the rim.

The fact that thermal imaging does not require additional « lighting » or power is an advantage, but you can improve your thermal photographs by incorporating a source of black body radiation somewhere in the image. Because the thermal signature of these devices is so precise and reliable, you can account for the thermal drift of your sensor, which is crucial when trying to achieve accuracy of one degree or less using a cooled bolometer.

Remember that the image is only a grayscale image after acquisition, with warmer objects appearing whiter and cooler objects darker.

Except for the ability to quantitatively calculate the temperature in degrees for any pixel in the image, all your vision tools, including shape matching, transitions between light and dark, and even drop analysis, work exactly as in a classic vision system. Due to the low overall image resolution, processing a thermal image is usually quite fast.

It is comparable to a 640480 or 320240 camera frame, meaning that a good computer can usually get a full response to each frame at the camera’s highest frame rate. We have found that by using a camera in free frame capture mode and a vision system to determine which image to use based on the presence of a cue, we can frequently trigger the system smoothly.

This not only solves a significant problem in the food industry, where synchronization of image capture is difficult due to the requirement for cleanliness, but also improves systems used in hazardous environments. In addition, it allows you to travel in time and directly measure the thermal loss characteristics of your material. Additionally, thermal shock imaging can reveal defects in the material that are difficult to see by other means.

There are similarities and differences between standard vision cameras and thermal cameras.

In summary, a thermal camera is both similar and different from a standard vision camera. By understanding these differences and utilizing the unique capabilities of the camera and energy spectrum while mitigating limitations, functionality can be achieved that cannot be easily replicated using other, more traditional methods. Using a qualified systems integrator, who has specific experience with thermal integration, can really help you realize the benefits of thermal imaging for your process without falling into the pitfalls of implementing it yourself. even.