Two-color thermometers can be divided into monochromatic and two-color thermometers (radiation colorimetric thermometers) according to the principle. For a monochromatic thermometer, when measuring, the target area to be measured should meet the field of view of the thermometer. The measured target size needs to exceed 50% of the field of view size. When the target size is smaller than the field of view, the background radiation energy enters the optic nerve support of the thermometer, which interferes with the temperature measurement reading and causes errors.

Conversely, if the target is larger than the thermometer's field of view, the thermometer is not affected by the background outside the measurement area. The two-color thermometer has an incomplete field of view, smoke, dust, and blockage in the measurement path, and the attenuation of radiation energy has little effect on the measurement results. Colorimetric thermometers are small, vibrating targets. This is because the diameter of the light is small and flexible, and can be uploaded to the path that bends, stops, and folds the light radiant energy.