At what distance can the camera measure an absolute temperature?
Measuring temperature at long distance. A measurement result is not primarily a function of distance, but rather how large the area of interest is compared to the whole image. We recommend that the object size is not less than 3 x 3 pixels. For a 320×240 camera, this means roughly 1/100 of the horizontal and vertical field of view. For practical use, the spot marker on screen is designed to show the safe size.
To meet this condition, you may have to choose a narrow angle lens. Depending on camera model, lenses down to 5° FOV are available.
At distance more than around 10 meters, correction for atmospheric attenuation should be used by setting Distance.