Range of Thermal Cameras - Current Market Overview from a Professional Perspective

Thermal Image Pixel Resolution, Resolution Enhancement Techniques, Variety of Optics

Thermographic devices suitable for contactless temperature measurement (infrared cameras with thermographic capabilities) have undergone rapid development in recent years. Considering that these devices appeared just 50 years ago, today they have evolved into one of the most well-known and versatile inspection tools. Therefore, it is not the lack of types that meet the customer's needs, but rather the overwhelming variety of options (manufacturers, types) in the market that poses a challenge for a customer planning to procure a thermal camera. Hence, it is time to review the development and types of these instruments from a professional perspective, and organize the current offerings based on some important technical parameters. Because: The measurement technology implemented in the cameras and the accessories available determine the device's application area, as well as the expected measurement accuracy and the achievable thermal image quality.

 

Matrix Thermal Cameras Pixel Resolution - Overview of the Current Market Selection

In addition to the geometric resolution (i.e., the size of the "pixel" on the object surface corresponding to a unique sensor), the image quality achievable with a thermal camera, or more precisely, the detail of the measurement, is determined by the number of pixels in the thermal camera. The reason for this is that for graphic (visual) recognition, a certain minimum number of pixels must fall on certain parts of the object being measured - just as we are accustomed to in digital photography. It is easy to understand that with more pixels, we can represent the object surface with greater detail or the same level of detail over a larger object surface in a single thermal image. If the number of pixels is low, many images need to be taken, and often, image montages are required for the evaluation of continuous objects and report generation (which is a very time-consuming task). This issue is not insignificant for thermal cameras. While in digital cameras we talk about resolutions of 10, 12, or even more than 20 megapixels (20 million pixels), in the case of matrix thermal cameras, the number of pixels is typically 320x240 (i.e., 76,800) or 384x288, and in the most professional thermal cameras, 640x480 (i.e., 307,200) or even 1024x768 (i.e., 786,432) pixels. There are also cameras with lower capabilities - a common type is 160x120 (i.e., only 19,200) pixels or even just 80x80 or 96x96 pixels, which consequently can only display smaller areas with acceptable detail, thus significantly limiting their area of application (or even rendering the thermal camera unusable). Thanks to the development of thermal camera sensors, cameras with more and more pixels are being produced. Interestingly, the prices per pixel for the most professional cameras - with sensor matrices of 640x480 or 1024x768 pixels and frame rates of 50 or even 240 Hz - are the most favorable (even an order of magnitude more favorable than those of low-pixel cameras).

Figure: Low-Cost thermal camera with 120x160 pixels / professional thermal camera with 2048x1536 pixels [source: PIM]

The image on the next page vividly demonstrates the impact of the number of pixels on work efficiency: The image on the right (640x480 pixels) was taken with a single on-site press of a button and - as it contains all the information of the inspected building side - can also be easily inserted into the report with just a single mouse click. In contrast, the thermal image on the left (160x120 pixels) can only capture a smaller part of the building side, and its detail leaves much to be desired. To achieve the quality of the image on the right, 16 times more images would be needed, and even for the post-processing of thermal images, overlaps are required, resulting in the need for many more - up to 20-25 on-site thermal image captures. Naturally, the acquisition time for a 640x480 pixel thermal image is several times longer.

The real inconvenience, however, awaits during report preparation, as we are faced with the time required for montaging 20-25 thermal images, which depending on our skill can take between 30 minutes and several hours. Therefore, it is worth considering whether to choose a thermal camera with fewer pixels for a smaller investment (and then pay for our savings with multiple additional work) or to acquire a thermal camera with more pixels to gain the tool necessary for efficient work.

Figure: 120 x 160 and 640 x 480 pixel thermal image [source: InfraTec]

Thermal Image Pixel Resolution Enhancement Techniques (within the thermal camera's field of view)

Software-based resolution enhancement through interpolation: Due to the relatively low number of pixels in thermal cameras, creating impressive thermal images (and consequently reports) poses great challenges, especially with thermal cameras having lower pixel counts. To alleviate this problem, some thermal camera manufacturers apply interpolation, commonly used in graphic image processing programs. This method generates an additional, mathematically interpolated pixel between each pixel pair in the captured thermal image, thus increasing the pixel count of the thermal image fourfold (by doubling horizontally and vertically). However, this method produces a thermal image that contains 75% calculated - hence not real, not measured - pixels. Therefore, the improvement of the visual appearance of the thermal image occurs at the expense of distorting the data content of the thermal image. Therefore, the application of this method is not recommended.

Increasing software resolution using hand tremor Taking into account that a sensor matrix is actually made up of individual sensors placed not seamlessly next to each other, but with a (almost half-pixel) gap around each sensor (to avoid thermal crosstalk and for the electrical connection of individual sensors), the detection of the measured object also happens only in such a "gappy" manner. In order to eliminate this, instead of interpolation, another software thermal image pixel resolution enhancement procedure has started to spread in recent years (e.g. under the names Super Resolution or UltraMax). These procedures are based on the slight horizontal and vertical field of view shifts occurring due to the hand tremor or movement of the person holding the thermal camera. The method is very simple: instead of one thermal image, the data of 16 thermal images are stored, and then with the help of software, we select the four images among them that "fit" together with a half-pixel horizontal and vertical shift due to hand tremor, and then we align the thermal images pixel by pixel next to each other or below each other. With this method, data is obtained from the empty space between the original two elementary sensors (pixels), and the number of pixels horizontally and vertically is doubled - our thermal image will have four times the resolution compared to the original detector matrix pixel count. Moreover, since the field of view detection is now seamless, the thermal camera's geometric resolution also improves (by exactly 34%). As simple (and inexpensive) as this method is, it also comes with many pitfalls. It cannot be used at all with a tripod-mounted thermal camera, and the hand tremor of a person needs to be very rarely "regular" enough for the software to find 4 images among the stored 16 thermal images that can be aligned in the manner described above. (Just think that the whole process takes nearly a second: if our hand tilts or sinks during this time, there is no way to obtain the 4 alignable thermal images.) Furthermore, the software image selection algorithm is unable to select images in cases where the thermal image does not contain sufficiently large and sharp contrasts (sufficiently steep temperature gradients) or if there is displacement within any part of the field of view. In such cases, the software - unfortunately without any warning - applies the resolution enhancement described during interpolation to achieve the desired pixel count. This results in the creation of non-existent pixel data, and the promised geometric resolution improvement by the method is not achieved! Therefore, from a measurement technology perspective, we never know which thermal image created in this way actually contains only real pixels, and thus when we can actually expect better geometric resolution. Therefore, the application of this method is not recommended. Hardware resolution enhancement with micro-scan procedure Based on the above, the fourfold pixel count resolution of the sensor matrix built into matrix thermal cameras can reliably (and guaranteed) only be achieved through hardware means. By microscanning the sensor, or by changing the position of the radiation beam projected onto the sensor matrix optically (within the thermal camera!), we alter the position of the radiation beam projected onto the sensor matrix horizontally and vertically one after the other. Thus, the radiation projected onto the empty space between the original two elementary sensors (pixels) is also detected, and thus the thermal camera's geometric resolution increases by 34% in all cases (without exception). Since this method does not rely on hand tremors, it can naturally be applied even with a tripod-mounted thermal camera. Although the micro-scan procedure cannot be considered fast (it takes 0.5 to 1 second to create a high-resolution thermal image), it is currently the only method to create real pixel extra-large thermal images with maximum geometric resolution. Examples of thermal cameras with this capability are the Jenoptik VarioCAM device families, which feature this optional function under the name Resolution Enhancement. With the VarioCAM hr 640x480 pixel detector thermal cameras in micro-scan mode, 1.23 million pixel thermal images can be created, and with the VarioCAM HD 1024x768 pixel detector thermal cameras, 3.15 million pixel - exclusively real measurement data-containing - thermal images can be produced. This provides a way to make the most detailed measurements of very large surfaces without any subsequent montages.

Figure: matrix detector and micro-scan* pixel resolution [source: PIM] * 4-fold resolution thermal image created by aligning 4 consecutive thermal images

Thermal image pixel resolution enhancement methods with post-thermal image montage

Panorama Image The thermographic representation of large objects (e.g. industrial facilities, public buildings, large machinery structures, furnaces, etc.) often requires the ability to view the entire object in a single thermographic image to recognize the relationships between the object's temperatures. This can rarely be achieved with just one thermal image - even when using the micro-scan procedure - because often even the 3.15 Mpixel is not enough for the mandatory geometric resolution, and more importantly, the on-site conditions in most cases do not allow capturing a larger object in its entirety from a single location. If the object in question is a horizontally elongated object, then the panorama image function available in several thermal camera types provides a solution. By using this, multiple (overlapping) thermal images can be taken one after the other while rotating or "moving forward" the thermal camera horizontally, and then the thermal camera software (or the associated PC software) automatically stitches these thermal images into a coherent, elongated thermal image. Of course, the software's capabilities determine whether the result of this stitching is just a coherent graphical representation (which is only a nice colorful image, therefore not correctable or further evaluable) or even a new (larger) thermal image data file (which can be corrected, processed, and evaluated with thermography evaluation software just like the original individual data files). Naturally, only the latter represents the real solution.The limitation of the method is that it can only process one horizontal image line at a time. Two-dimensional (automatic) thermal image montage Joining (montage) stored thermal images is often necessary, especially for extensive objects, but typically, horizontal (panoramic) image creation is not sufficient for this. And if multiple thermal images need to be montaged not only horizontally but also vertically, the total number of thermal images to be processed increases exponentially - along with the time required for the workflow, of course. It doesn't require much explanation how much help a software that performs automatic montage of thermal images can provide in this. Especially considering that the result of joining stored thermal images is a larger pixel-sized thermal image data file that can be evaluated without limitations (not just a graphical image). Of course, such automatic montage is subject to several conditions: the thermal images must have a sufficiently large coverage, they must be captured from the same observation angle and at the same distance. Furthermore, "mandatory" are identical measurement conditions and the same thermal camera settings. All this requires very high discipline and precise on-site work, but adhering to it pays off multiple times: for example, with the Infratec company's IRBIS3 Mosaic software, instead of hours of manual thermal image correction and montage, dozens of thermal images can be perfectly and automatically montaged in just 5 minutes into a single data file that can be evaluated without limitations. Among the special features of the mentioned software, it is worth highlighting the alignment of the thermal images' temperature scales, the correction of the thermal images' optical (perspective) deformations, and the various (selectable) mathematical methods for aligning the thermal image data with each other.

Figure: A2-sized thermal image assembled from 7 x 8 (=56!) unique 1.23 Mpixel thermal images [source: PIM] (the grid indicates the arrangement of the thermal images, the light areas show the sizes of the original thermal images / applied overlap >40%/)

Thermographic Objectives, Lenses

The most important thing: thermographic lenses cannot be made of glass, but only of materials suitable for the wavelength ranges of thermal cameras. Therefore, you cannot buy a thermal camera and then attach an optical microscope lens just because you want to measure very small objects at the moment. But even the lens of a long-wave thermal camera cannot be mounted in front of a mid-wave thermal camera (and vice versa). (In both cases, we would find that we cannot measure any radiation.) In the case of long-wave thermal cameras, the lens material is typically germanium, which is coated with a special anti-reflection layer, achieving transmission factors of over 99%. (So do not remove contaminants from the optics with chemicals or abrasive cleaners!) However, when talking about thermal camera lenses, we cannot avoid making a fundamental distinction between LowCost and professional devices. While the former is characterized by the smallest possible size (and thus the cheapest), usually with an inseparable - and perhaps not even germanium-based despite their long-wave range - built-in lens, the latter are large in size and mostly have the option to replace the lens with the one needed at the moment. (There have been LowCost devices with interchangeable lenses for a few years now!) Why is a large lens and interchangeability good? The effect of lens diameter on measurement capabilities The larger the aperture of an optical lens of a thermal camera (more precisely: its aperture), the more radiant energy reaches the thermal sensor surface. The measure of the optical system's brightness (here: the intensity of transmitted infrared radiation) is the f-number, which is the ratio of the focal length to the aperture lens diameter. Naturally, the smaller the f-number, the larger the lens diameter and the greater the energy input to the sensor, resulting in greater sensitivity and accuracy. But beware: the larger the lens diameter, the more it deviates from the ideal optical system model - the Gaussian optics. As a result, imaging errors (e.g., image distortion) increase, which can only be counteracted with increasingly sophisticated lens shapes. If we want to support the above with some numbers, let's compare the most common category of microbolometer thermal cameras. The small lenses of LowCost thermal cameras allow a sensitivity of up to 100 mK at a 50 Hz frame rate, to achieve better thermal resolution (e.g., 80 or 60 mK), the integration time needs to be increased - thus reducing the frame rate to 30, 25, or even just 9 Hz. The large lenses of professional thermal cameras, depending on the capabilities of the thermal camera manufacturer, enable thermal resolutions of 50 or even 30 mK at a 50 Hz frame rate, or even up to 240 Hz. Of course, it is not insignificant that while the lens of a LowCost thermal camera costs at most a few hundred thousand forints, the prices of thermographic optics for professional devices are in the range of over one million forints. The necessity and variety of interchangeable lenses In thermographic measurements, besides providing the geometric resolution necessary for correct temperature detection corresponding to the evaluation's observation field size, the most important thing is to ensure the necessary geometric resolution. For example, with a "standard" lens providing a 2 mrad geometric resolution, from a distance of 5 m, only objects (or object details) with a minimum size of 30 mm can be reliably detected. For measuring smaller objects, we need to choose a smaller measurement distance or a different lens. (Otherwise, the thermographic recording would not be able to detect the temperature of the small object we are interested in.) So if we replace the aforementioned "standard" lens with a telephoto lens, with a 1 mrad geometric resolution, we can measure the temperature of objects as small as 15 mm from a distance of 5 m. (Note: the ZOOM built into thermal cameras is only digital zoom, which does not solve the above problem - in fact, it even excludes a significant portion of the expensively purchased thermal image pixels from our measurement. So never use it!) Primarily, there is a wide range of interchangeable lenses for professional thermal cameras, which, for easy interchangeability, often connect to the thermal camera not with threads but with bayonets.These lenses preferably also have electronic encoding so that the thermal camera can automatically detect which lens we are using and load the calibration data file associated with the lens. The latter is necessary because the calibration of each thermal camera always takes place together with the currently installed lens to determine and correct the characteristics of both the lens and the thermal camera. Therefore, when we replace the lens, a different calibration data file is required for radiation detection correction. (Naturally, this also implies that a subsequently purchased lens entails a manufacturer's thermal camera recalibration. Also, it means that even "identical" lenses cannot be exchanged between thermal cameras without consequences.) Additional note: Unfortunately, ZOOM lenses do not exist for thermal cameras used for temperature measurement. The reason for this is partly the immeasurable cost of such lenses, but the main reason is the calibration requirement of the thermal camera: since with a ZOOM lens, the virtual aperture size varies for each magnification setting, a separate calibration would be needed for each possible (continuous ZOOM = infinitely many) setting.

Figure: Thermal image resolution achievable with telephoto lens [source: InfraTec]

  The most common lenses and their roles (or "side effects") are presented in the following list: Standard lens Depending on the pixel resolution of the thermal camera detector, with these lenses, geometric resolutions of approximately 20x15° ... 30x25° fields of view can be achieved. Telephoto lens Compared to standard lenses, typically double the geometric resolutions are achieved with halving of both dimensions of the field of view. There are also "larger" telephoto lenses that offer quartering or even tenths of the field of view sizes and geometric resolutions, thereby significantly improving the geometric resolution. Wide-angle lens Typically, compared to standard lenses, doubling of both dimensions of the field of view is achievable, but at the same time, the geometric resolution is halved. There are also so-called super-wide-angle lenses that quadruple the field of view sizes and geometric resolutions (while quartering the geometric resolution). Extension lenses, macro lenses The primary role of these lenses is to reduce the minimum measurement distance of standard lenses or telephoto lenses (minimal focal distance), allowing very small objects to be measured from distances that meet the requirements of geometric resolution. Microscope lenses Microscope lenses are used for measuring special small objects. They are usually custom-made, and their imaging capabilities are similar to optical microscopes (of course, in the thermal radiation wavelength range). Their practical disadvantages, in addition to their large size, weight, and cost, include minimal depth of field according to optical laws.

Figure: Thermal image resolution achievable with extension or microscope lens [source: InfraTec]

It is important to draw attention to the often completely erroneous use of wide-angle lenses! For example, if our concern was that while adhering to the limits of geometric resolution, from a specified maximum measurement distance, only a part of the object to be measured (e.g., a switch cabinet) could be captured in each thermal image, then the application of a wide-angle lens procured to increase the field of view not only does NOT solve our problem but worsens our situation. This is because due to the halved geometric resolution resulting from the wide-angle lens, the measurement can now only be performed from a distance of at most half of the previous distance. Consequently, the field of view of our measurement does not actually increase (as it remains the same size), but with image distortion, the viewing angle of the object surface can be quite skewed, especially towards the edges. This further negatively affects the accuracy and evaluability of our measurement. (Note: Wide-angle lenses are mainly justified in indoor building thermography. Before using them in other professions, careful consideration is required to ensure that the achieved larger field of view does not lead to other, potentially much more serious optical or metrological disadvantages.)  

Rahne Eric (PIM Ltd.) pim-kft.hu, termokamera.hu  

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