How to choose a blackbody or Infrared temperature calibrator

Though the principle of radiation thermometry has been used in industry for decades, its recent growth in the HVAC, factory maintenance, aerospace, and energy industries is causing many people to inquire about infrared (IR) thermometers and the calibration equipment needed to support them. This article will give you some ideas about what to look for when choosing a calibrator for your IR thermometer.

Blackbodies vs flat-plate IR temperature calibrators

One of the first things you’ll notice when you go about selecting an infrared temperature calibrator is that there are a wide variety of form factors and target designs. A blackbody calibrator has an empty cavity with a small opening that acts as the target. This type of radiation source is often a conical or tube-shaped well that is embedded in a bath, furnace, or heat pipe. Blackbody calibrators are often used at the reference level of radiation thermometry because of their higher emissivity value.

Other types of infrared temperature calibrators, known as flat-plate IR temperature calibrators, have a painted target surface that is heated or cooled to the required calibration temperatures. The flat-plate surfaces may be completely flat and smooth, or they may have bumps or ridges. These various surface types are used by product manufacturers in hopes of manipulating a parameter called emissivity, although they are probably affecting another important parameter negatively: temperature uniformity.

The type of calibrator you need depends on the type of IR thermometers you will be calibrating. Some of the factors to consider are:

• distance-to-size-of-spot ratio (D:S ratio)
• emissivity setting of the IR thermometer
• operational spectral band of the IR thermometer
• temperature range

D:S ratio (distance to size of spot)

The D:S ratio provides a guide to the size of target your calibrator needs to have. It’s a guide because the measurement spot covered by the IR thermometer is rarely given at 100% energy. In other words, the spot size covers some percentage of the total energy taken in by the thermometer, such as 80% or 90%, but not all of it.

For example, given a 12:1 distance-to-size-of-spot ratio and a calibration distance of 18 inches, the spot size would be 18 divided by 12, or 1.5 inches. However, the percent of energy for the spot size is usually between 80% and 90%, so the rule of thumb is to triple the spot size to get to the target size that you actually need for calibration. For this example that would come to 4.5 inches. Most manufacturers use a target size of 5 inches in diameter when calibrating new infrared thermometers, because that size provides an appropriate distance-to-size-of-spot ratio for many common IR thermometers.

Blackbody calibrators typically have a very small target. A blackbody’s cavity is much longer than it is wide; therefore, the size of the opening is usually very small compared to the spot size of a typical IR thermometer. This limits their use to more expensive high-end IR thermometers with large distance-to-size-of-spot ratios, because a higher ratio means a smaller spot. The reason is because there is an optimal calibration distance and if the size-of-spot is larger than the calibration target there will be an error. However, for temperatures above 500 °C, cavities may be your only option. Although they have a small target, these blackbodies also have an emissivity very close to 1.00. If the emissivity setting of the thermometer being calibrated cannot also be set to 1.00 then very complicated corrections must be made.

Emissivity setting of IR thermometers

Did you know that infrared thermometers are actually radiometers that display temperature? The parameter actually measured by the radiation thermometer is radiance, and the temperature is determined by specifying the emissivity of the source. Emissivity settings for thermometers may be fixed or variable. The most common settings are .95, .97, and 1.00. If the emissivity of the calibrator does not match the emissivity of the calibration surface, then mathematically complex corrections must be applied. Ideally, the emissivity of the calibrator would be able to match the emissivity of fixed emissivity thermometers. See 4180 Series Precision Infrared Calibrators for an example of emissivity matching capability.

Since an infrared thermometer measures radiance, it has to be calibrated radiometrically. This means the calibrator must be a radiance source. If the emissivity of the radiance source is equal to 1.00, as is the case with a blackbody calibrator, then its output is known precisely at a given temperature. However, the output of a flat-plate calibrator does not have a known traceable value unless it has been calibrated radiometrically. In a radiometric calibration, the output of the calibrator is compared to a traceable radiometric reference standard. Two methods can be used. One method determines the emissivity and temperature losses at the surface, while the other method determines the radiance measured at a given set-point temperature.

Traditionally, infrared calibrators have been calibrated thermometrically with a platinum resistance thermometer. This method may provide the true temperature inside the block, but it does not verify the calibrator’s emissivity or the temperature drop at the surface, and thus impairs the traceability of the calibration. In addition, the emissivity of the calibrator changes with time and use. If the IR calibrator is recalibrated with a platinum resistance thermometer there will be no adjustments made for this emissivity change.

A flat-plate IR calibrator typically has an emissivity in the 0.93 to 0.97 range. The emissivity value depends on:

• Type of paint
• Type of metal
• Exposure to time and temperature
• Quality of the surface (peeling, scratches, films, ice, bumps or ridges)
• Wavelength

Flat-plate IR calibrators are usually used at the secondary or lower levels of radiation thermometry. As expected, the blackbodies are often more expensive. Since they are used for reference level work, they are designed to be compatible with reference level IR thermometers which have different distance-to-size-of-spot ratios and emissivity requirements than typical IR thermometers. These differences often make it difficult to use a cavity-type blackbody calibrator with popular handheld IR thermometers.

Operational spectral band

IR thermometers have operational spectral bands, so they only see certain wavelengths of infrared radiance. However, painted surfaces have spectral emissivity, so they have a different emissivity at different wavelengths.

Consider this example: one thermometer might have an operational wavelength of 1.5 µm where the emissivity of the calibrator surface might be 0.5. Another thermometer might measure over the operational spectral band of 8 µm to 14 µm. At the surface of the same calibrator the emissivity might be .85 at 8 µm and .97 at 14 µm and still be reported as .95 ±0.02. And on average over a certain set of wavelengths (spectral band) it may be close to that.

However, unless it is calibrated radiometrically, it is impossible to know the true emissivity. For an idea of the significance of this error, a 1% error in emissivity amounts to a 7 °C error at 500 °C. For more about emissivity, watch this brief demonstration video.

Temperature range

Your IR thermometer has a temperature range and so does an IR calibrator. From below 0 °C to 500 °C you can be served very well by a flat-plate calibrator. However, above 500 °C it requires a lot of power to heat the flat-plate surface, and the temperature losses become very significant. Usually, a cavity-type blackbody is used above 500 °C. This limits the types of thermometers that can be verified above 500 °C by calibration to higher end thermometers with larger D:S ratios.

Since handheld thermometers comprise most of the typical IR thermometer calibration workload, here are a few things to consider when selecting a flat-plate IR temperature calibrator:

• Emissivity compensation
• True radiometric calibration
• Adequate size of source
• Surface temperature uniformity
  • A uniform surface temperature is required for an accurate calibration. Uniformity can be affected by the design and placement of the heaters used to control the temperature, the controller design, the size of the target, the shape of the target, the application of the paint, blemishes in the paint, condensation, and ice.
• Dry-air purge
  • Water or ice forms on the plate surface of an IR calibrator when it cools to the ambient dew point and ice point. When this happens the emissivity will change. Using a dry-air purge eliminates this issue to keep measurements intact. When measuring in this temperature range, insist that the IR calibrator has dry-air purge capability.
• Wavelength
  • When choosing an IR temperature calibrator, always consider the operational spectral band of the device. It is important to match calibrator spectral band or wavelength with the wavelength of the IR thermometer that is being calibrated. Otherwise, issues like signal loss and noise will contribute to measurement error. Fluke Calibration’s 4180/4181 precision IR calibrators are designed to be used in the 8 to 14 μm range. This corresponds to the majority of hand-held IR thermometers on the market. It also happens to be the best spectral band for the temperature range where the majority of IR temperature measurements are made, ambient to 600 ºC.
• Knowledge and experience
  • Why buy an IR calibrator from a company that doesn’t consider calibration quality a top priority? As with any Fluke instrument, you can rely on the expertise that backs up the 4180/4181 precision IR calibrators. Not only can you have confidence that your instrument is designed with the best measurement capability in industry, Fluke Calibration experts back this up by providing classroom training and helping customers find answers for the most difficult IR calibration questions. This helps make sure you and your company have the support you need to operate a successful IR temperature calibration program.
     

IR Temperature Calibrator Selection Guide