Why is a Pin Sharp Thermal Image Important?
Thermal imaging cameras help to make production processes quicker and safer, and to improve the quality of the end products. But how does the camera capture the image and how many pixels does it need to do so?
A lens is used to focus the infrared radiation of an object onto a sensor and this generates an electric signal which is proportional to its radiation. The signal is amplified and, via the following digital signal processing, is converted to an output size which corresponds to the object temperature. The measurement value can be shown on a display screen or represented as an analogue signal.
The heart of a thermal imaging camera is the Focal Plane Array (FPA) image sensor, which has anywhere between 6.400 and 1 million pixels. Each pixel itself consists of a microbolometer ranging from 12 x 12 to 35 x 35 µm² in size, which is around 0.15 µm in thickness. Its resistance changes due to its intrinsic heating, which is caused by the absorption of thermal radiation. In turn, the change in resistance causes a change in the signal voltage, which is subsequently analysed.
In principle, more pixels means more detail. Because the laws of physics also apply to thermal imaging cameras, sensors with a high pixel count are subject to certain limitations. Due to advances in semiconductor technology – much like with digital cameras for photography – it is now possible to fit even more pixels onto the same detector surface. Whilst earlier pixel sizes or pitches were typically 35 µm, they are now 12 µm. That means that the same radiation power is divided over nine pixels instead of over one. In order to retain the same thermal resolution of around one tenth of a degree Celsius, the smaller pixel needs to have three times the detectivity. This places extremely high demands on thermal isolation, temperature coefficients as well as effective utilisation of the sensor surface of the evacuated bolometer, which is not always given. In these cases, a lower image frequency would have to be chosen – e.g. 9 instead of 80 Hz – in order to integrate the image signals over a longer time period.
The lenses of these types of IR cameras are traditionally very fast with f-stops in zone 1. In order to utilise the maximum amount of thermal radiation while at the same time remaining free of atmospheric influences, all work is carried out in the spectral range of 8 to 14 µm. However, the more the pixel pitches correspond to this working wavelength, the higher the demands on a diffraction-limited imaging quality. As a result, small objects – even if they are 3 x 3 pixels in size – are often represented with considerably lower temperatures than in reality.
If the resolution is limited to a lower number of small pixels, physically small lenses with a short focal length can be utilised for standard field of view. Whilst this is a considerable cost-saver, it has the disadvantage that the smaller opening in the camera allows less light in, and this in turn needs to be compensated for with much more sensitive sensors. So-called digital structures designed to improve the resolution of the lens often influence the measured temperatures in the overall image field.
Various approaches have been used to improve the geometric resolution in the sub-pixel range via the optical or mechanical movement of the image sensor or of the entire camera. Due to the high fill factor of the pixel accompanied by the limited resolution of the lenses and the reduction in image frequency associated with said techniques, improvements are rather slight.
If, for a thermal imaging camera, you want information not only on where an object is hot, but also how hot it is, the measurement accuracy of the system must also be given for small objects. Otherwise a high resolution is not of much use with regard to the pure number of pixels. The optics and the chip must correspond to each other in terms of quality to get a thermally and geometrically well-defined thermal image. As well as ascertaining which is the smallest discernible structure, there is also the important question of the minimum size that an object needs to have in order to reliably determine its temperature. When carrying out risk assessment, precise information on current excess temperatures is extremely useful.
Correct usage is a similarly important topic. Thermal imaging cameras, just like normal digital cameras, are using a field of view (FOV) which can cover angles of 6° for a tele lens, 26° for a standard lens and up to 90° for a wide-angle lens. The further you get from the object, the larger the captured image region, and with it, the image detail that an individual pixel can capture.
The optical resolution of the measuring device must be selected depending on the size of the measurement object and the distance between it and the sensor. In the top image above, because the measuring spot is too large the thermal radiation of the considerably cooler circuit board has been included, which results in a considerably distorted temperature measurement. For this reason, the measuring spot of the camera must not be bigger than the size of the measurement object.
Because of this, for very small measurement objects or for large distances between the thermal imaging camera and the measurement object, high resolutions are vital. In a trial conducted by Optris, two different resolutions were used to measure the temperature of a wire at an identical distance and with identical environmental conditions. While a hotspot with an overheat of over 70 °C was accurately detected at 640 x 480 pixels, the measurement at a resolution of 80 x 80 pixels gave a reading less than 10 °C above ambient temperature.
CAPTURED: MEGA RESOLUTION … OR IS LESS ACTUALLY MORE?
On the one hand there are cameras with 80 x 80 pixels which are used as thermographic devices, while on the other hand there are cameras on the market with a resolution of over 3 million pixels. These, however, are often overbuilt for the industry. After all, it is the application which determines the choice of thermal imaging camera: the correct distance from the measurement object, the proper choice of lens and the resolution of the camera to the particular measurement process are far more decisive factors than getting bogged down in ever increasing numbers of pixels. A higher resolution does not necessarily result in a sharper image, not least because due to the thermal conduction temperature changes are depicted in a fluent way without sharp limitations. Even more important is the pixel size and, as a result, a related to the object detail reasonable number of pixels. For almost all applications in the industry, resolutions between 160 x 120 and 640 x 480 pixels (pin sharp VGA resolution) are totally sufficient. Today, compact infrared cameras are best suited to quick online applications in the analysis of dynamic thermal processes. On our website we offer an optics calculator for thermal imaging cameras.
This article is intended as a guide and does not replace our own free technical assistance which is always readily available.
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