FEATURE THERMAL IMAGING & VISION SYSTEMS
THE SECRET TO A GOOD THERMAL IMAGE T
he 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. However, people often forget that an image to be used as evidence must meet certain requirements: this is not achieved with a quick snapshot. So, what characterises a really good thermal image? Every image needs to capture interest and tell the story. Focus, image detail, brightness and contrast must be considered. 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. A good thermal image is dictated by three unchangeables: focus; temperature range; and image detail and distance.
FOCUS A professional thermal image must always be focused and sharp and the object and heat pattern must be clear and easy to recognise. A blurred image not only comes across as unprofessional and makes it harder to identify the object and any faults (see figures 1a and 1b) but can also lead to measurement errors (see figures 2a and 2b), 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 likely to be incorrect. The size of the detector matrix also
plays a role in image quality. Images taken by cameras with fewer pixels are 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
16 APRIL 2018 | INSTRUMENTATION
Christiane Buchgeister, business development manager ITC Central and Northern Europe, FLIR Systems, provides practical tips and advice for achieving high quality thermal images
that is too high, the thermal image will be “underexposed” (see figures 3a-c). 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. This may seem obvious but in the building sector, for example, it is common to find reports in which pipes to be investigated are hidden behind furniture. It is also important that the object under investigation 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 or a telephoto lens must be used (see Figure 4). Changeable factors must also be
considered for a good thermal image, including image optimisation and temperature measurement.
LEVEL AND SPAN After choosing the appropriate temperature range, you can adjust the
Figure 5a Figure 1a Figure 1b
Figure 2a
Figure 2b
Figure 3a
Figure 3b
Figure 3c
Figure 4
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 often underestimated (see figure 5a and 5b).
Figure 5b
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52
