Infrared thermometers measure temperature using blackbody radiation (generally infrared) emitted from objects. They are sometimes called laser infrared thermometers if a laser is used to help aim the thermometer (such as the popular CEM DT-8818H) or non-contact infrared thermometers to describe the device’s ability to measure temperature from a distance. By knowing the amount of infrared energy emitted by the object and its emissivity, the object's temperature can be determined by the infrared thermometer.

The distance-to-spot ratio (D:S) is the ratio of the distance to the object and the diameter of the temperature measurement area. For instance if the D:S ratio is 12:1, measurement of an object 12 inches away will average the temperature over a 1-inch diameter area.

The infrared thermometers sensor may have an adjustable emissivity setting, which can be set to measure the temperature of reflective (shiny) and non-reflective surfaces (the Tecpel DIT-512 is an example) A non-adjustable thermometer can be used to measure the temperature of a shiny surface by applying a non-shiny paint or tape to the surface.