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How to Calculate Emittance
Tip written by: Infraspection Institute

Utilizing correct emittance values is imperative for accurate non-contact temperature measurements. Knowing how to accurately calculate emittance values can help to ensure the accuracy of infrared temperature measurements.

Although thermographers frequently obtain emittance values from published tables, this practice can introduce significant errors. Following the procedure listed below, it is possible to accurately calculate the E value of an object.

Equipment Required:

1. Calibrated imaging radiometer with a computer that allows thermographer to input Reflected Temperature and Emittance values
2. Natural or induced means of heating/cooling target to a stable temperature at least 10ºC above/below ambient temperature
3. Calibrated contact thermometer

Method:

1. Place imaging radiometer at desired distance from heated/cooled target. Be certain that target is larger than imager’s spot measurement area. Aim and focus imager on target
2. Measure and compensate for Reflected Temperature
3. Place imager crosshairs on target
4. Use contact thermometer to measure target temperature at location of imager crosshairs. Remove contact thermometer
5. Without moving imager, adjust E control until observed temperature matches value obtained in Step 4 above. The displayed E value is the Emittance value for this target with this imaging radiometer. For greatest accuracy, repeat above three times and average the results.

Note: This procedure requires contact with the object being measured. Be certain to observe all necessary safety precautions prior to making contact with target.

The above procedure is described in detail in the Standard for Measuring and Compensating for Emittance Using Infrared Imaging Radiometers. Copies of the Standard are available from the Infraspection Online Store or by calling 609-239-4788.

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