The most basic design consists of a lens to focus the infrared thermal radiation on to a detector, which converts the radiant power to an electrical signal that can be displayed in units of temperature after being compensated for ambient temperature. This configuration facilitates temperature measurement from a distance without contact with the object to be measured. As such, the infrared thermometer is useful for measuring temperature under circumstances where thermocouples or other probe type sensors cannot be used or do not produce accurate data for a variety of reasons.
The actual area being measured is determined by the distance-to-spot ratio of the IR thermometer you are using and this varies from model to model. As the distance from the object being measured increases, the spot size of the area being measured also increases.
For example this Sper Scientific’s IR Pen has a distance to spot ratio of 1:1 meaning that at 1" from the target it will measures the temperature of a 1" circle. At 2" it will measure a 2" diameter and so forth.
Spot Size : Distance
They can be used to serve a wide variety of temperature monitoring functions. A few examples provided include:
Checking mechanical equipment or electrical circuit breaker boxes or outlets for hot spots.
Checking heater or oven temperature, for calibration and control purposes.
Detecting hot spots / performing diagnostics in electrical circuit board manufacturing.
Monitoring materials in process of heating and cooling, for research and development or manufacturing quality control situations.