There are basically two types of IR calibration sources: hot plate blackbody and cavity-type blackbody.
The hot plate style consists of a metal plate (usually aluminum) with or without concentric grooves where the temperature of the plate is set and controlled using either an inexpensive potentiometer dial or a high-end temperature controller. A thermocouple or an RTD probe senses the plate's temperature. The hot plate is usually painted dull black to improve the surface emissivity, which typically is 0.95. The calibration source with a built-in temperature controller has much better accuracy and stability compared to a potentiometer dial-type unit.
The cavity-type blackbody source consists of a blind hole in a cylinder or a sphere where cavity temperature is controlled by a temperature controller using a thermocouple probe. The cavity-type unit has a higher surface emissivity, typically 0.98, than a hot plate unit.
In order to calibrate an IR thermometer, a blackbody calibration source is required. There are three factors to consider when making the selection:
1. Type of blackbody (hot plate or cavity type) tells us about the construction and overall performance of the unit.
2. Target area (hot plate area or cavity opening) tells us how large of an area we can check our IR thermometers with. The target area should be larger than the thermometer's field of view, otherwise the thermometer will be measuring the target area plus some of the surrounding cooler areas. Normally, an IR thermometer is checked against a blackbody source at a relatively close distance of about 0.5 to 3 feet, depending on the target area.
3. Target emissivity. The higher the target emissivity, the more ideal is the calibration. At lower emissivity targets, wavelength bandwidth of the IR thermometer comes into play. Ideally, at E=1.00 the wavelength bandwidth of the DUT (device under test) is not a factor.
- Aim the infrared thermometer perpendicular to the blackbody unit's target. If aimed at an angle, reflected IR energy can impair calibration accuracy.
- The thermometer's field-of-view area at a selected distance should be smaller than the blackbody unit's target area.
- Do not bring the IR thermometer too close to the blackbody target area, especially at high temperatures. Radiated heat may impair accuracy and damage the unit under test.
- Always aim the IR thermometer to the center of the target area.
- When changing the unit's temperature setpoint, make sure the unit is fully stabilized to the new setting before testing. Usually, going up in temperature takes less time than going down.
- Do not unplug a blackbody unit while it is hot. There are usually built-in fans to remove heat even when the power is off. Let the internal fan run until the unit has cooled.
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