How does a thermal camera work?
Thermo cameras work in a similar way to standard visual light cameras, but rather than using a CMOS array to detect photons that have passed through a lens and encountered a thermal sensor, the material (amorphous silicon or vanadium oxide) is affected by IR heat waves that alter its resistance.
Also, by referring to the article on the best cheap thermal imaging camera , explanations are given in the field of thermal cameras.
In order to meet the demands of fire departments, thermal imaging cameras are designed with these components in robust and waterproof housings that are heat-resistant. Infrared radiation from warm objects, such as flames, is converted into visible light in real-time by these components. The camera displays infrared output differentials, so two objects with the same temperature will appear to be the same color.
These cameras may be attached to helmets or handheld. When using a handheld camera, one hand is required to position and operate it, leaving the other for other tasks. Also, the camera can easily be transferred between firefighters. The Fire Service uses a large number of handheld thermal imaging cameras.
The imager of a thermal camera is referred to as a bolometer. The thermal camera usually has large pixels compared to bolometers, so a resolution of 640×480 is considered high for a visual camera.
As heat is transferred in many electromagnetic spectrums, it is essential to note that only one spectrum is sampled for thermal imaging, which is the infrared spectrum. The region of the spectrum where heat energy is present behaves like visible light energy in a way that it can transmit over long distances and can be focused onto a bolometer with a non-glass lens. Additionally, infrared light is capable of bouncing off mirrors, similar to visible light.
When working with a thermal camera, the first thing to understand is that heat in an image changes very rapidly over time. Light is either present or absent in the visual spectrum. Heat is constantly transferred from one item to another in a thermal environment. In short, it means that there is a noticeable difference between two pictures of the same thing taken within a short period of time. It may take several seconds for a handprint that is left on the table after placing the hand on it and then removing it to return to ambient temperature.
A thermal camera relies on timing, and in general, a scene’s temperature must be controlled in order to capture a good image. To identify a heat seal, you have to consider how many seconds pass between the heat seal operation, as it may be 180F at the time of the heat seal, but 150F just two seconds later, and close to ambient in only 10 seconds.
Consequently, it is essential to carefully time the inspection of a thermal image to the thermal event in order to gather helpful information. Moreover, the contents of the bag may quickly turn down the heat seal temperature.
Useful links : Thermal imaging services of buildings
The reflectivity and thermal opaqueness of thermal cameras
Reflectivity and thermal opaqueness are the following challenges in thermal imaging. In Hollywood movies, thermal cameras are shown seeing through walls. Thermal cameras do not even have the ability to measure through clear glass or the surface of a puddle of water in reality since they cannot ultimately see through them.
Moreover, thermal cameras do not function well in highly humid environments because water, which is a hot liquid, transmits infrared light, thus masking the thermal signature behind the water molecule. Steam and fog are somewhere between thermally translucent and thermally opaque, while normal dry air is thermally transparent. As a result, thermal imaging is difficult to perform in a humid or spraying environment. A shiny metal is usually thermally reflective, which means that it reflects the thermal signature of the objects passing through its optical path rather than its own.
As a result, tinfoil functions well in an oven – it creates a partial thermal shield that blocks most of the IR radiation, only allowing convection/conduction to transfer thermal energy while rejecting IR radiation.
However, there are many aspects that are similar between thermal imaging and visual imaging. Surface reflectivity, time, humidity, and resolution are all crucial factors in thermal imaging. Those are many things for engineers to handle, but there are others.
How does a thermal camera work : Bolometers
Should you use an uncooled bolometer or a cooled bolometer that is more expensive? How do they differ? Compared to uncooled bolometers, cooled bolometers have very little image noise (according to Teledyne FLIR literature).
Every pixel in your data has a randomized variation known as noise. A standard uncooled bolometer, based on experience, has a noise level of fewer than 5 degrees. A device that is uncooled makes a lot of sense and is likely to work well when you are making decisions in a 10 to 15 degree range. Alternatively, if the object is moving very rapidly you might want to consider a cooled bolometer.
Thermal imaging has the advantage that no additional lighting or energy is necessary, but your images could be enhanced by implementing a black body radiation source somewhere in your image. The thermal signatures of these devices are so precise and consistent, so you can compensate for any thermal drift your sensor might experience, which is important when using a cooled bolometer to achieve accuracy below and beyond single degrees.
Once the image is gathered, remember it is a grayscale image where whiter things appear more white while darker things appear darker. In this way, all the vision tools that you would typically use in a regular vision system will work, except that you will be able to determine the temperature in degrees for any pixel in the image mathematically.
Because thermal images have a low resolution, processing them is very fast. This is equivalent to either a 640*480 or a 320*240 camera image, which means that with a modern computer, you can get a complete solution at the maximum framerate.
We have found that when we use a camera in free-running mode to capture images, we can often trigger the system by detecting some fiducial that is detected and letting the vision system decide which image to choose. This solves a massive problem in food processing, where timing the capture of images is not a simple task for sanitary reasons. Still, it is also advantageous for those systems used in dangerous environments.
By using direct measurement, you can determine the thermal loss properties of your material by rewinding and forwarding in time. In addition, thermal shocks can be used in images to reveal cracks in materials that are very hard to detect in other ways.
Comparison of regular vision cameras and thermal vision cameras
Essentially, a thermal camera and working with it is similar to a standard vision camera and different from it at the same time. Utilizing these differences and utilizing the camera’s capabilities while mitigating the limitations can create functionality that is not easily duplicated using conventional methods.
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