To extract temperature data from a thermal image or video file that does not contain radiometric metadata, you are presented with the following challenges......
1. What was the centre temperature used by the camera
2. What was the temperature span used by the camera
3. What was the Emissivity setting of the camera
4. What was the ambient temperature used by the camera in its calculations
5. Does the camera present the user with a strictly linear thermal image or an image that has been enhanced for contrast using a Histograms. Such enhancement renders linear Colour to temperature conversions inaccurate. Many cameras employ histogram based image enhancement. Their measurement systems are applied before the histogram equalisation stage so are unaffected by it.
The idea of Radiometric still images and video capture is to include all data required to calculate true pixel values in the image for later analysis. Trying to use a ‘user image’ with no additional radiometric data is challenging to say the least. Much must be known about the image source camera and how it treats the image data before storing it to memory. One technique that may be used is a form of lock-in thermography where the user places a thermal reference of known temperature in the scene. This does provide a single point of temperature reference but the cameras span and any image processing effects remain an unknown. Using two temperature references outside of the scenes ambient temperature range is a possibility. It might force the camera to set a span to include the two reference temperatures and provide some idea of the cameras span setting, though not exact ! The issue of histogram image enhancement that accentuates low Delta T areas of the scene will remain an issue however.
Much depends on the desired accuracy of any temperature information extracted from the images or video.
Finally, a thermal camera that is not radiometric and not intended for accurate temperature measurements can provide significant error in any temperature measurements that are provided to the user. If the camera is not trying to produce accurate temperature measurement as it is configured for ‘pretty pictures’, it is unlikely to be a satisfactory source of data for temperature extraction from its images. The FLIR One G2 and G3 are equipped with a Radiometric Lepton so can produce a reasonably accurate measurement but it us by no means the most reliable or accurate of temperature measurement systems.
Fraser
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