I have been curious about the accuracy of the CHT (cylinder head temperature) gauge. The one I have is not temperature compensated so it only reads "right" at +70°F. Today the outside air temperature was about 40°F so the CHT will be reading about 30°F high. Right after pulling into the garage, the left cylinder read 355°F (or 325°F after adjusting for ambient temperature) and the right was 375°F (or 345°F).
At a distance of a foot, the spot size is a little over an inch in diameter. The temperature reading is affected by the emissivity of the surface which is a quantitative measure of the efficiency in radiating thermal radiation. The device assumed an emissivity of 0.95. A dark, non-reflective surface would have an emissivity near 1.0 and a reflective mirror would be 0. To get an accurate reading, the emissivity needs to be near 1. I'm guessing that the cylinder head isn't a perfect 1.0 but it's not far off. Not having access to a portable IR spectrometer, I could simply paint a section of the head with flat black paint and that would raise the emissivity to get a more accurate reading. But I'm thinking that this is close enough. And I can probably just assume that the temperature compensated temperature from the CHT thermocouples is a bit high.