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Tip of the Week Archive

How to Calculate Transmittance

June 03, 2024

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Tip written by: Infraspection Institute

Windows are semi-transparent materials placed between an object and an infrared instrument to separate conditioned from unconditioned spaces. When measuring temperatures through a window, it is imperative to know and enter the Transmittance value of the window into your radiometer’s computer to help ensure temperature measurement accuracy.

How to Calculate Transmittance

Because no object is 100% transmissive, infrared windows will always have Transmittance values of less than 1.0. Following the procedure listed below, it is possible to calculate the T value of any window.

Equipment Required:

  • Calibrated imaging radiometer with a computer that allows user to input Reflected Temperature and Emittance values.
  • Blackbody simulator with E ≥ 0.95 heated close to temperature of target to be measured.
  • Window that is semitransparent in the waveband of the imaging radiometer.

Method:

  1. Place imaging radiometer at desired distance from blackbody simulator.
  2. Aim and focus imager on blackbody simulator. Place crosshair on center of blackbody simulator.
  3. Set imager’s E control to 1.0
  4. Measure and compensate for Reflected Temperature.
  5. Measure and note apparent temperature of blackbody simulator.
  6. Place window directly in front of imaging radiometer’s lens.
  7. Without moving imager, adjust E control until observed temperature matches value obtained in Step 5 above. The displayed E value is the Transmittance percentage for this window with the subject imaging radiometer. For greater accuracy, repeat above steps a minimum of three times and average results.

The above procedure is described in detail in the Standard for Measuring and Compensating for Transmittance of an Attenuating Medium Using Infrared Imaging Radiometers available from Infraspection Institute. For more information or to place an order, call 609-239-4788 or visit the Infraspection Online Store.

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