Infrared Thermometers, or IR Thermometers, are a type of non-contact thermometer that works by detecting the thermal infrared radiation coming from an object and converting it to a temperature. Since Infrared Thermometers do not need to touch the surface being measured, they are useful for determining the temperature of objects that cannot be reached or are at a distance. A laser pointer built into the thermometer helps to guide the operator to the area or object being measured.
To properly calibrate IR Thermometers, an infrared radiation source, or blackbody is required. A blackbody calibrator typically consists of an empty cavity or a flat surface that is capable of being heated, or in some cases cooled, to calibrate a number of temperature points in the range of the IR Thermometer.
Theoretically, a blackbody emits the maximum amount of infrared radiation for any given temperature and at the same intensity in all directions. Although a perfect blackbody does not exist, black bodies are very efficient at emitting radiation and since the emissivity of the blackbody is a known value, it is possible to accurately calibrate the IR Thermometer.
Many blackbody calibrators contain a temperature well behind the blackbody surface where a high accuracy temperature probe, such as a PRT, can be placed to monitor the temperature to obtain more accurate readings.
IR Thermometer Field of View
Blackbody calibrators are available in various source sizes. Larger sizes are preferred due to the infrared thermometer’s field-of-view. The measurement area of an IR Thermometer is in the shape of a cone projecting out from the face of the thermometer. The measurement area becomes wider as the distance increases. Most infrared thermometers come with a specified distance to size ratio (D:S). For example, a 10:1 distance to size ratio states that the sensing area will be 1 inch in diameter for a distance of 10 inches. However, this area does not contain all of the radiation received by the infrared thermometer. For a proper measurement, there should be at least two to three times the diameter as needed for the measurement distance.
IR Thermometer Calibration Setup
The blackbody calibrator should be placed in an area where any reflected surfaces will not affect the measurements. The blackbody calibrator should also be located in an area away from any exterior walls or windows and airflow from HVAC vents and doorways should be minimized during the calibration. Before calibrating the infrared thermometer, the thermometer should be allowed to reach the ambient temperature of the room, typically, around 30 minutes. The ambient temperature affects the reference temperature of the infrared thermometer so this is important when calibrating a thermometer brought in from a different environment.
IR Thermometer Calibration Process
To begin the calibration process, the blackbody should be set to the first calibration point and allowed to stabilize. It is more efficient to calibrate the lowest temperature point first and the higher calibration points last as the blackbody usually heats up faster than it cools down. The infrared thermometers emissivity setting should be set to the blackbody calibrator’s calibrated emissivity, typically 0.95, if it has an adjustable emissivity setting. Some infrared thermometers only have a fixed emissivity of 0.95.
The measuring distance then needs to be set. Most distances are set from the front housing of the infrared thermometer to the blackbody surface. The distance will depend on the diameter of the blackbody surface and the distance to size ratio of the thermometer.
The measurement is then made by holding the trigger on the thermometer. The trigger should be held for approximately 5 seconds or around ten times longer than the infrared thermometer’s stated response time.
The infrared thermometer measurement area should be centered on the blackbody surface. This can be done by using the laser pointer however, the laser pointer is typically offset from the measurement cone by a small distance and will not point to the exact center of the measurement cone, especially when being used at short distances. The proper method is to sweep the infrared thermometer up and down and side to side across the blackbody surface looking for the highest displayed temperature during the sweep. That temperature should be recorded as the readout temperature for the calibration. The calibration process is then repeated for each temperature point required for the calibration.