When sourcing a thermal imaging camera, it is common practice to think of pixel count or resolution as the defining specification of quality. If a thermal imaging camera has a high pixel resolution, it is generally assumed that it is a more effective piece of equipment than one with a lower count. However, pixel resolution is only one of three factors that determine how useful the image of a thermal imaging camera is. The other two are thermal sensitivity and noise. These three variables work together to generate the image quality seen by the viewer.

Pixel resolution is the maximum number of display points on the screen. Thermal sensitivity is how well the camera images as contrast increased. Noise is the failure of the camera to measure and display temperature differences smaller than is thermal sensitivity. How are these three factors interrelated?

Since what a Thermal Imaging Camera does is take radiant thermal energy and convert it into a visible image, the best we can “see” the thermal energy is limited by the number of pixels and the sensitivity of temperature differences that the camera can pick up. In many applications it is necessary to pick up on minute temperature differences. Typical thermal imaging cameras have sensitivities of 0.25 degree C to 0.05 degrees C. The visual defined by 256 colors or scales of gray, leading by division to 256 possible different temperatures that can be displayed. Often this results in the camera attempting to pick up and display temperature differences of narrower range than its thermal sensitivity. Thus a block of pixels is shown to be a blotchy, uniform mass of temperature; however, this is not reality and the image is inaccurate. This result is called noise.

Image quality improves and noise decreases proportionally, not just directly, to an IR camera’s thermal sensitivity. The smaller the thermal sensitivity unit measurable, paired with the higher the pixel resolution, the lower the noise and the better the image will be.

Other specification factors of importance include:

Spectral Range
The thermal radiation sensed by thermal imaging cameras is electromagnetic in nature and has a wavelength that falls between visible light, which we are familiar with, and terahertz radiation. Most thermal imaging cameras cover a range from 7 or 8 to 14 microns, which is the most common range of radiation emitted by objects on Earth. Cameras for research, military and industrial applications can be found with spectral ranges from0.9 to 1.7 microns, and also within the short and mid spectral ranges from 2 to 6 microns.

Temperature Range
This is an adjustable feature which the user can set. Only temperatures within the range set by the operator are displayed. Consider how decreasing the temperature range will affect noise in the image.  Cameras with a lower temperature range more easily detect residential energy leaks and moisture. Overall, some cameras are constructed with a range suitable for industrial applications, and others for home inspections.

Accuracy
For thermal imaging cameras, accuracy is a range of temperature within which the reported temperature is statistically likely to be. For instance, a camera’s accuracy could be +/- 2 degree C. Better accuracy is required for viewing smaller objects, and becomes important as the image is zoomed in on.

Field of View
The maximum area viewable at a certain distance by an IR camera is its field of view. Typically this is given in horizontal degrees by vertical degrees. Instantaneous Field of View Measurement is the smallest object the camera can obtain a temperature reading on a certain distance. As the operator, you must either move closer or zoom in to view better.

Other thermal imaging specifications exist; however, those above will help you make the best decision on which product is right for your application.

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Jason Kanigan is a technical writer for Global Test Supply, a distributor of test and measurement equipment.