Cooled vs uncooled thermal imaging: discover the difference
For many years, thermal imaging cameras have been used by scientists, researchers and R&D specialists for a wide range of applications, including industrial R&D, academics and research, non-destructive testing (NDT) and materials testing, and defence and aerospace.
With a thermal imaging camera you can identify problems early in the design cycle, allowing them to be documented and corrected before becoming more serious and more costly to repair. However, not all thermal cameras are created equal. For some applications, dedicated, advanced thermal cameras with high-speed stop-motion capability are required in order to get accurate measurements.
Thermal imaging in an R&D environment
Thermal imaging cameras use thermal radiation, which is not detectable by the human eye but can be converted into a visual image that depicts thermal variations across an object or scene. Covering a part of the electromagnetic spectrum, thermal energy is emitted by all objects at temperatures above absolute zero, and the amount of radiation increases with temperature.
FLIR’s thermal imaging cameras are used for capturing and recording thermal distribution and variations in real time, allowing engineers and researchers to see and accurately measure heat patterns, dissipation, leakage and other temperature factors in equipment, products and processes. Some of these cameras can distinguish temperature changes as subtle as 0.02°C. They feature state-of-the-art detector technology and advanced mathematical algorithms for high performance and precise measurements from -80 to +3000°C.
The R&D camera ranges combine high imaging performance and precise temperature measurements with powerful tools and software for analysing and reporting. This combination makes them suitable for a wide range of research, thermal testing and product validation applications.
Cooled and uncooled cameras
There is plenty of choice when it comes to thermal imaging camera systems for R&D/science applications. An often-asked question is therefore: “Should I use a cooled or an uncooled thermal imaging system, and which one is the most cost effective?” In fact, there are two classes of thermal imaging camera systems available on the market today: cooled and uncooled systems. The component costs for these two classes of systems can be quite different, making it important to decide which way to go.
Cooled thermal imaging cameras
A modern cooled thermal imaging camera has an imaging sensor that is integrated with a cryocooler. This is a device that lowers the sensor temperature to cryogenic temperatures. This reduction in sensor temperature is necessary to reduce thermally induced noise to a level below that of the signal from the scene being imaged. Cryocoolers have moving parts made to extremely close mechanical tolerances that wear out over time, as well as helium gas that slowly works its way past gas seals.
Cooled thermal imaging cameras are the most sensitive type of cameras and can detect the smallest of temperature differences between objects. They can be produced to image in the mid-wave infrared (MWIR) band and the long-wave infrared (LWIR) band of the spectrum where the thermal contrast is high due to blackbody physics. Thermal contrast is the change in signal for a change in target temperature. The higher the thermal contrast, the easier it is to detect objects against a background that may not be much colder or hotter than the object.
Uncooled thermal imaging cameras
An uncooled infrared camera is one in which the imaging sensor does not require cryogenic cooling. A common detector design is based on the microbolometer, a tiny vanadium oxide resistor with a large temperature coefficient on a silicon element with large surface area, low heat capacity and good thermal isolation. Changes in scene temperature cause changes in the bolometer temperature, which are converted to electrical signals and processed into an image. Uncooled sensors are designed to work in the LWIR band, where terrestrial temperature targets emit most of their infrared energy.
Uncooled cameras are generally much less expensive than cooled infrared cameras. The sensors can be manufactured in fewer steps, with higher yields relative to cooled sensors and less expensive vacuum packaging. Uncooled cameras do not require cryocoolers, which are very costly devices. They also have fewer moving parts and tend to have much longer service lives than cooled cameras under similar operating conditions.
Cooled cameras for R&D applications
Advantages of uncooled cameras beg the question: when is it better to use cooled thermal imaging cameras for R&D/science applications? The answer is: it depends on the application requirements.
If you want to see the minute temperature differences, need the best image quality, have fast-moving or heating targets, need to see the thermal profile or measure the temperature of a very small target, want to visualise thermal objects in a very specific part of the electromagnetic spectrum or want to synchronise your thermal imaging camera with other measuring devices, then a cooled thermal imaging camera is the instrument of choice.
Speed
Cooled cameras have much higher imaging speeds than uncooled ones. High-speed thermal imaging allows microsecond exposure times that stop the apparent motion of dynamic scenes and permit capturing frame rates exceeding 62,000 fps. Applications include thermal and dynamic analysis of jet engine turbine blades, automotive tyre or airbag inspection, supersonic projectiles and explosions, to name a few.
Cooled cameras have very fast response times and they make use of a global shutter. This means that they will read out all pixels at the same time as opposed to reading them out line by line, which is the case with uncooled cameras. This allows cooled cameras to capture images and take measurements on moving objects without image blurring.
The IR images in Figure 1 compare the capture results of a tyre rotating at 30 km/h. The first image was taken with a cooled thermal imaging camera. One would think the tyre is not spinning, but this is the result of a very fast capture rate of the cooled camera that has stopped the motion of the tyre. The uncooled camera capture rate is simply too slow to capture the rotating tyre, causing the wheel spokes to appear transparent and blurred. You cannot accurately measure temperature on blurred images.
Spatial resolution
Cooled cameras typically have greater magnification capabilities than uncooled cameras, because they sense shorter infrared wavelengths. Because cooled cameras have greater sensitivity characteristics, lenses with more optical elements or thicker elements can be used without degrading the signal-to-noise ratio, allowing for better magnification performance.
The thermal images in Figure 2 compare the best close-up magnification that can be achieved with a cooled and uncooled camera system. The image on the left was taken with a 4x close-up lens and 15 μm pitch cooled camera combination, resulting in a 3.5 μm spot size. The image on the right was taken with a 1x close-up lens and 25 μm pitch uncooled sensor, resulting in a 25 μm spot size.
Sensitivity
It is often difficult to fully appreciate the value offered by the improved sensitivity of cooled thermal cameras. How do you get a feeling of the benefit from a 50 mK sensitivity uncooled thermal camera in comparison to a 20 mK sensitivity cooled thermal camera? To help illustrate this benefit, we ran a quick sensitivity experiment (see Figures 3 and 4).
We put our hand on a wall for a brief few seconds to create a thermal handprint. The first images show the handprint immediately after the hand was removed; the second set of images shows the thermal handprint’s signature after two minutes. The cooled camera can still see most of the thermal signature of the handprint, whereas the uncooled camera only shows the partial remains of the handprint. The cooled camera clearly can detect smaller temperature differences and for longer durations than the uncooled camera. This means the cooled camera will provide better detail on your target and help you detect even the faintest of thermal anomalies.
Spectral filtering
One of the great advantages of cooled thermal cameras is the ability to easily perform spectral filtering in order to uncover details and take measurements that otherwise would be unachievable with uncooled thermal cameras. In the first example shown in Figure 5, we are using a spectral filter, either placed in a filter holder behind the lens or built into the dewar detector assembly, in order to image through flame. The end user wanted to measure and characterise the combustion of coal particles within a flame.
Using a ‘see-through flame’ spectral infrared filter, we filtered the cooled camera to a spectral waveband where the flame was transmissive and therefore we were able to image the coal particle combustion. The first image is without the flame filter and all we see is the flame itself. The second is with the flame filter and we can clearly see the combustion of coal particles.
Synchronisation
Precise camera synchronisation and triggering makes the cameras suitable for high-speed, high-sensitivity applications. Working in snapshot mode, the FLIR A6750sc is able to register all pixels from a thermal event simultaneously. This is particularly important when monitoring fast-moving objects where a standard uncooled thermal infrared camera would suffer from image blur.
The images in Figure 6 are a good example. In this example, we dropped a coin and had a sensor trigger the camera to take an image. Two drops of the same coin triggered the camera at the same time, giving you the object in the same position each time as well. With an uncooled microbolometer camera, you would not catch the coin at all due to the inability to trigger that type of detector, and if you did by luck, it would be blurred.
Thermal cameras from FLIR
The higher performance A6750sc, A8300sc, SC6000, SC7000, SC8000, X6000sc and X8000sc cooled cameras offer ultrafast, ultrasensitive performance in the MWIR and LWIR spectral bands, while the FLIR A6250sc operates in the NIR spectral band. The cameras provide good measurement capabilities in challenging set-ups for fast motion and thermal events, wide temperature range, small amplitude phenomena, multispectral analysis or very small object evaluation.
FLIR also offers a wide range of uncooled cameras, from entry-level bench test kits up to higher end systems like the FLIR T650sc. Dedicated lenses and software will adapt your camera solution to your specific application. To know exactly which cooled or uncooled camera you need, please contact your FLIR representative.
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