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For accurate radiometric readings, how many pixels must the measurement area subtend?

To get an accurate radiometric measurement, an object must subtend or cover at least a diameter of 10 pixels. With a 10 pixel-wide object, the pixel in the center of the subtended area should be absorbing the maximum amount of photon flux. The focus of a thermal lens is not perfect; pixels further from the center of the object will be affected by radiation sources enclosing the area of interest. This will lead to poor radiometry accuracy and is not recommended by FLIR.