August 2005 - Library

Figure 2 - Thermography Software Screenshot
When this rule is not observed there is a risk of reading lower a temperature than it actually is. Such risk increases if an operator
can fit only 2 x 2 pixels and it grows substantially if only one pixel reads the temperature.
Optical resolution is measured in “mrad” and it depends on the number of pixels in the detector and camera lens. Good maintenance
cameras are based on uncooled microbolometric detectors of 320 x 240 pixel size. They should be able to achieve about 1.6 to 1.4
mrad, which means that a projection of one calibrated pixel at 1m is a square with 1.6mm side. (note: figures such as 2.8 mrad mean
lower resolution, not higher).
Smaller detectors produce lower quality images even if the camera interpolates the output to the standard 320 x 240 size. (note:
look out for the word “interpolated” in the camera specifications).
Accuracy. Today, credible manufacturers offer accuracy of +-2 degrees or +- 2 % for maintenance applications. Typically, the high
quality cameras may be more accurate but the specifications usually do not reflect that for many reasons. Claims of +- 1 degree
accuracy should be met with caution.
Thermal resolution or sensitivity. This parameter together with the optical resolution is responsible for image quality. Most good
cameras based on 320 x 240 detector offer thermal resolution (not accuracy!) better than 0.08˚C which is enough to create a nice
and sharp thermal image.
Camera and software user-friendliness. This is a difficult question since people have their own “like” and “dislike” perceptions.
However, for many people a rotating display (camcorder-like) is much more convenient than a built-in one and certainly better than
no display at all. A larger display is generally more convenient than a smaller one.
Camera controls and shortcut buttons for common functions are more convenient when they are on the display. Many hardware
buttons on the camera may be confusing, on the other hand, double or triple button functioning may be annoying.
Te m p e r a t u re ranges. Specifications rarely show how many sub-ranges a camera has in, say, - 20 to 300˚C or - 20 to 500˚C
measurement range. Having less sub-ranges is more convenient, having more sub-ranges would require more frequent switching
between them and sometimes results in difficult to understand images showing a hot spot on black background. Or a good
background shot with white hot area with unreadable temperature.
Manual focusing or automatic focusing. Manual generally works better in the industrial environment and lens ring focusing is
quicker and easier than joystick focusing. Interesting to note that now top of the range camcorders also come with lens ring manual
focusing.
S o f t w a re should be stable, have enough measurement functions and, most important, provide a very good automated re p o rt writing
ability. This part is often neglected when choosing a system package but the report writing generally may take longer time than the
inspection.
When you make your shortlist, remember to check the reputation and the history of the manufacturer. Most good manufacturers
produce very reliable thermal imaging cameras, virtually maintenance free (lens cleaning doesn’t count) and not requiring regular
calibration. Most local agents are generally capable or very simple support but all serious repairs must be done overseas as having
proper repair, testing and calibration facility is too costly to maintain on a local level.
Dmitri Ishchenko works for Applied Infrared Sensing specializing in supplying thermal imaging and infrared equipment. For more
information please visit www.applied-infrared.com.au and choose Thermography section.
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