Infrared Calibrators / Blackbodies

Infrared Calibrators / Blackbodies

Infrared Calibrators, also called Blackbodies, are reference standards used to calibrate Infrared Thermometers and Thermal Imaging Cameras . Even those IR Thermometers that cannot be adjusted benefit from testing to verify the consistency and validity of results. Virtually any instrument with a spot size diameter less than the cavity size can be calibrated. It is important not to have the IR thermometer too close to the target. This will cause the IR thermometer’s optics to heat excessively, which will cause false readings. It is also important to be not too far away. This will cause the target to not fill the IR thermometer’s spot size and will cause a false reading.

Advice for Selecting an Infrared Calibrator

• Make sure to know the desired temperature range
• Select a model with a target area larger than the spot size diameter of the instrument to be checked
• Decide between surface type or cavity type infrared calibrator

How does an Infrared Calibrator work?

The units use a plate that is heated. The plate is painted with a black paint with emissivity of 0.95. The temperature is controlled by a digital controller. The controller uses a precision platinum RTD as a sensor and controls the surface temperature. Models that achieve temperatures below ambient use a Peltier Thermoelectric Cooling system.

The IR calibrator is calibrated with an emissivity setting of 0.95. The IR calibrator has a variable emissivity adjustment that allows the user to vary their apparent emissivity from 0.90 to 1.00. This setting should match the IR thermometer's emissivity setting. It is best to use the emissivity setting of 0.95. However, some IR thermometers do not allow for an emissivity setting of 0.95. For these instruments, the calibrator's emissivity setting should be set to the IR thermometer's emissivity setting.
                                                                 
Every object with a temperature above absolute zero (0 Kelvin) radiates energy over a wide spectral band. For example, if a significant part of this energy is within the band of 400–700 nm, we can see that energy. This is the visible light band. This is the case with an electric stove burner at a temperature of 800°C. The burner will appear red or orange to the eye (red hot). That burner is also emitting energy at other wavelengths, which we cannot see. This includes wavelengths in the infrared portion of the electromagnetic spectrum.

An example of an object emitting energy at wavelengths we can see is the sun. By the same respect, if we are measuring an object at room temperature, (23°C), the peak wavelength is 9.8μm. The temperature corresponding to a peak wavelength at 8 μm is 192°F (89°C) and the temperature corresponding to a peak wavelength at 14 μm is −86°F (-66°C). This is one of the reasons the 8 – 14 μm is widely used in handheld IR thermometers.
IR thermometers take advantage of this peak wavelength phenomenon. They measure the amount of energy radiating from an object and calculate temperature based on this measured energy. In most handheld IR thermometers, the sensor and optical system measure IR energy in the 8-14μm band.

Emissivity is defined as the ratio of the energy emitted at a temperature to the energy emitted by a perfect blackbody at that same temperature. A perfect blackbody would have an emissivity of 1.0. However, in the real world there is no such thing as a perfect blackbody.

For example, if a perfect blackbody emits 10000 W/m2 at a given temperature and a material emits 5000 W/m2 at that same temperature, then the emissivity of that material is 0.5 or 50%. If another material emits 9500 W/m2 at that same temperature, it has an emissivity of 0.95.

It is important to note that for any opaque material, the ratio of energy reflected plus the ratio of energy transmitted is equal to 1.0 (this is known as Kirchhoff’s Law). Therefore, if a material’s emissivity is 0.95, the material reflects 5% of the energy radiated by objects facing it. By contrast, if an object has an emissivity of 0.50, the material reflects 50% of the energy radiated by objects facing it. This means this reflected energy can contribute to measurement accuracy. This is especially true when measuring materials with lower emissivity, and objects at lower temperatures.
A lack of knowledge of emissivity itself can contribute greatly to inaccuracy in IR temperature measurement. For an example, say we are measuring an object at 500°C. We assume it has an emissivity of 0.95. However, its emissivity is really 0.93. This would cause our 8-14 μm IR thermometer to read the temperature 6.7 degrees low, a – 6.7°C error in temperature measurement.

Emissivity, blackbodies and graybodies

Most people associate a blackbody calibration source with calibrating infrared thermometers. Although the word blackbody specifically refers to an ideal surface that emits and absorbs electromagnetic radiation with the maximum amount of power possible at a given temperature, many calibrators with non-ideal surfaces are also referred to as "blackbody calibrators." While an ideal surface would have an emissivity equal to 1.00, many of these "blackbody calibrators" have an emissivity of approximately 0.95 (better described as a “graybody”). A true blackbody calibration source would usually be a long cavity with a narrow opening. Unfortunately, the opening is usually too narrow to be useful for calibrating common infrared thermometers, which require a large target size for an accurate calibration. The advantage of a true blackbody calibration source is that the emissivity is precisely known. Whereas traditional flat plate calibrators have emissivities with uncertainties too large for meaningful calibrations of most thermometers.