Laser thermometers can be divided into monochromatic thermometers and dual-color thermometers (radiation colorimetric thermometers) according to the principle. For monochromatic thermometers, when measuring temperature, the target area to be measured should fill the field of view of the thermometer. It is recommended that the size of the object to be measured exceeds 50% of the field of view. If the size of the target is smaller than the field of view, the background radiation energy will enter the photoacoustic branch of the thermometer, interfere with the temperature measurement reading, and cause errors. Conversely, if the target is larger than the thermometer's field of view, the thermometer will not be affected by background outside the measurement area.

This is because light has a small diameter and is flexible and can transmit optical radiation energy in curved, occluded, folded channels, so targets that are difficult to access, harsh conditions, or close to electromagnetic fields can be measured. The distance factor is determined by the ratio of D:S, that is, the ratio of the distance D between the thermometer probe and the target to the diameter of the measured target. The higher the optical resolution, i.e. the increased D:S ratio, the higher the cost of the thermometer. If, due to environmental conditions, the thermometer needs to be installed away from the target, and smaller targets need to be measured, a thermometer with a higher optical resolution should be selected.