This article provides an overall review of different types of infrared calibration sources (Blackbody sources) and how they differ, and how to choose and use them properly in the field.

Understanding your infrared calibration source doesn't have to be a topic too hot to handle. It is important to select the proper source for your intended purpose, and to be sure you are using your source properly to ensure the most accurate readings.

There are basically two types of infrared calibration sources: Hot Plate Blackbody source and Cavity Type Blackbody source. 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. The temperature of the plate is sensed using either a thermocouple or an RTD probe. The Hot Plate is usually painted dull black to improve the surface emissivity. The surface emissivity of a Hot Plate calibration source is typically 0.95.

Figure 1, above, shows a very basic Hot Plate Blackbody source with a potentiometer dial (Omega's Model BB-2A). Figure 2, at right, shows a high-end Hot Plate Blackbody source with a built-in temperature controller (Omega's Model BB704). 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 the temperature of the cavity is controlled by a temperature controller, using a thermocouple probe. The Cavity Type Blackbody source has a higher surface emissivity than a Hot Plate Blackbody unit. The emissivity of a Cavity Type Blackbody source is typically 0.98 or higher.

Figure 3, at left, shows a Cavity Type Blackbody source with a built-in temperature controller (Omega's Model BB705). The Cavity Type Blackbodies typically go to higher temperatures (over 1000ºF) than Hot Plate Blackbodies. In addition, having a higher emissivity value makes them ideal for precise calibration tasks.

In order to calibrate an infrared thermometer, a Blackbody Calibration Source is required. There are 3 factors to consider when selecting a Blackbody Calibration source:

• Type of Blackbody (Hot Plate or Cavity Type): tells us about the construction and overall performance of the unit.


• Target area (Hot Plate area or Cavity opening): tells us how large an area we can check with our infrared thermometers. The target area should be larger than the field of view of the thermometer; otherwise the infrared thermometer will be measuring the target area plus some of the surrounding cooler areas. Normally, an infrared thermometer is checked against a Blackbody source at a relatively close distance (about 0.5-3 ft depending on the target area).


• Target Emissivity: The higher the target emissivity, the more ideal is the calibration. At lower emissivity targets, wavelength bandwidth of the infrared thermometer comes into play. Ideally at E=1.00, wavelength bandwidth of the DUT (Device Under Test) is not a factor

When using a blackbody calibration source, certain ground rules need to be followed:

• The infrared thermometer should be aimed perpendicular to the target area of the Blackbody unit. If aimed at an angle, reflected IR energy can impair calibration accuracy.


• The field of view area of the thermometer at a selected distance should be smaller than the target area of the Blackbody unit.


• Do not bring the infrared thermometer too close to the target area of the Blackbody, especially at high temperatures. The radiated heat from the Blackbody can not only impair calibration accuracy but also potentially damage the unit under test.


• Always aim the infrared thermometer to the center of the target area.


• When changing the temperature setpoint on the Blackbody unit, make sure the unit is fully stabilized to the new temperature setting before making any calibration tests. Usually going up in temperature takes less time than going down.


• Do not unplug a Blackbody calibration source while it is hot. There are usually built-in fans to remove the heat even when the power switch is off. Let the internal fan continue to run until the unit has cooled down.