The secret to a good thermal image

Published:  28 March, 2018

The use of thermal cameras has spread to many professional environments in recent years. They are easy to handle and thermal images are quick to take. Images can also be attached to reports easily, e.g., for an inspection of an electrical installation or building as evidence of work carried out or of any faults or deviations identified. However, people often forget that an image to be used as evidence or even proof before the courts must meet certain requirements: this is not achieved with a quick snapshot. So, what characterises a really good thermal image? 

During the practical exercises in our thermography training classes we notice some participants have difficulty choosing the optimal camera settings for different tasks. Not everyone has experience in taking meaningful photographs and it is therefore hardly surprising that when armed with a thermal imaging camera, these individuals produce reports with thermal images that are devoid of meaning; some may even support the wrong conclusions. Unfortunately, such reports may be part of a critical process monitoring or maintenance programme.

Every image, digital or thermographic, needs to capture interest and tell the story. Although looking at a thermographic image isn’t an emotional experience it still has the same requirements as its digital counterpart. So, the camera user must consider focus, image detail, brightness and contrast. The object must be an appropriate size and position, heat patterns clear to facilitate temperature measurement and emissivity and reflection taken into account to ensure accuracy.

As with digital photography, there are countless possibilities in thermography for editing images provided they are saved as radiometric images. However, not all settings can be changed and not all image errors can be corrected.

The three unchangeables – the basis for a good thermal image

Focus

A professional thermal image must always be focused and sharp and the object and heat pattern must be clear and easy to recognize. A blurred image not only comes across as unprofessional and makes it harder to identify the object and any faults but can also lead to measurement errors, which are more serious the smaller the measurement object. Even if all other parameters are set correctly, the measurement values from an unfocused thermal image are highly likely to be incorrect.

Of course, the size of the detector matrix also plays a role in image quality. Images taken by cameras with fewer pixels are more blurred or “grainier” and give the impression that they are not focused. It should also be noted that not every camera can be focused so the only option is to change the distance from the object.

Temperature range

For hand-held uncooled microbolometer cameras, the “exposure” is essentially preset by the image frame rate. This means that it is not possible to choose for how long - and therefore how much - radiation hits the camera detector. For this reason, an appropriate temperature range must be selected that matches the amount of incident radiation. If a temperature range is selected that is too low, the image will be oversaturated, as objects with higher temperatures emit more infrared radiation than colder objects. If you select a temperature range that is too high, the thermal image will be “underexposed,” as can be seen in.

To take an image or temperature measurement, the lowest possible temperature range available on the camera should be selected. However, it must also include the highest temperature in the image. Depending on the camera model and configuration options, over-driven and under¬-driven areas can be displayed in a contrasting colour.

Image detail and distance

Illumination in photography corresponds in thermography to the interplay of radiation from the object and reflected radiation from the surrounding environment. The latter is unwanted because interfering reflections need to be avoided. This is achieved by choosing a suitable position from where to take images. It is also advisable to select a position from which the object of interest can be seen clearly and is not hidden. This may seem obvious but in the building sector, for example, it is common to find reports in which pipes or windows to be investigated are hidden behind furniture or blinds.

It is also important that the object under investigation, or its areas of interest, take up the whole thermal image. This is particularly true when measuring the temperature of small objects. The spot tool must be completely filled by the object to enable correct temperature measurements. Since the field of view and therefore the spot size are determined by both the distance to the object and the camera’s optics, in such situations the distance to the object must either be reduced (get closer!) or a telephoto lens must be used.

The changeables – image optimisation and temperature measurement

Level and span

After choosing the appropriate temperature range, you can adjust the contrast and brightness of the thermal image by changing the temperature intervals displayed. In manual mode, the false colours available in the palette can be assigned to the temperatures of the object of interest. This process is often referred to as “thermal tuning.” In automatic mode, the camera selects the coldest and warmest apparent temperatures in the image as the upper and lower limits of the temperature interval currently displayed. A good or problem-specific scaling of the thermal image is an important step in the interpretation of the image, and is, unfortunately, often underestimated.

Palettes and isotherms

Palettes represent intervals with the same apparent temperatures using different sets of colours. In other words, they translate specific radiation intensities into colours that are specific to a particular palette. Frequently used palettes include the grey, iron, and rainbow palettes. Grey tones are parti¬cularly suited to resolving small geometric details but are less suited to displaying small differences in temperature. The iron palette is very intuitive and also easy to understand for those without much experience in thermography. It offers a good balance between geometric and thermal resolution. The rainbow palette is more colourful and alternates between light and dark colours. This results in greater contrast but can lead to a noisy image for objects with different surfaces or many temperatures.

The isotherm is a measuring function that displays a given interval of the same apparent temperature or radiation intensity in a colour that is different from the palette. It allows you to emphasise temperature patterns in the image.

Object parameters

As we have seen, the appearance of thermal images is dependent on the thermographer’s technique and choice of settings and the look of saved radiometric images can be altered by editing. However, it is also possible to change the settings that are relevant for the calculation of temperatures. In practice, this means that the emissivity and reflected apparent tem¬perature can be altered retrospectively. If you notice that these parameters have been set incorrectly or want to add more measurement spots, the temperature measurement values will be calculated or recalculated according to the changes.

Practical tips

• Ensure that the camera is saving radiometric images.

• Choose an appropriate position from which to take images, observe the radiative situation and check the object is clearly visible and displayed at an appropriate size and position.

• If you change the emissivity, monitor the temperature range and make sure that it remains appropriate.

• Focus and use a tripod to minimise camera shake.

• Carry out thermal tuning.

Take note of the object description, object size, actual distance, environmental conditions, and operating conditions.

It is easier to edit the thermal image when it is saved or “frozen” (in “preview”). Also, since you don't have to do everything on site, you can leave dangerous zones immediately after taking the image. If possible, take a few more images than you need - including from different angles. This is preferable to taking too few! You can then choose the best image afterwards, at leisure.

Conclusion

Taking a good thermal image does not require any magic tricks - solid craft and sound work is all that is required. Many of the points mentioned may seem trivial and “old news,” particularly to amateur photographers. Of course, the equipment plays a role easier to ensure sharp images. Better, i.e. high-definition, cameras allow the fast localisation of even small anomalies, and without focusing capabilities it is always difficult to capture a sharp image. However, high-end cameras are no guarantee of good images if used incorrectly. The basis for good, professional work is education and training in thermography, exchange of knowledge with other thermographers, and of course, practical experience.

For further information please visit: www.flir.com

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