Infrared thermometers can be divided into monochromatic thermometers and two-color thermometers (radiation colorimetric thermometers) based on their principles. For monochromatic thermometers, the area of the measured target should fill the field of view of the thermometer during temperature measurement. It is recommended that the size of the tested target exceed 50% of the field of view. If the target size is smaller than the field of view, the background radiation energy will enter the optoacoustic symbol of the thermometer to interfere with the temperature measurement reading, causing errors. On the contrary, if the target is larger than the field of view of the thermometer, the thermometer will not be affected by the background outside the measurement area. For a colorimetric thermometer, its temperature is determined by the ratio of radiation energy within two independent wavelength bands. Therefore, when the measured target is small, not full of field of view, and there is smoke, dust, or obstruction on the measurement path, which attenuates the radiation energy, it does not have a significant impact on the measurement results. For small targets that are in motion or vibration, a colorimetric thermometer is the best choice. This is because the diameter of the light is small, flexible, and can transmit radiation energy through curved, obstructed, and folded channels.

The higher the optical resolution, i.e. increasing the D: S ratio, the higher the cost of the thermometer. The range of the infrared thermometer D: S ranges from 2:1 (low distance coefficient) to above 300:1 (high distance coefficient). If the thermometer is far away from the target and the target is small, a high distance coefficient infrared thermometer should be selected.