Why Is Transmissivity Important When Using Infrared (IR) Windows?

As the head of the Maintenance and Reliability Team at your company, one of your responsibilities is to train the team on new and innovative products and technologies. Your company recently adopted a condition based maintenance program that uses Infrared (IR) Windows. For the next team training seminar, you are going to discuss Transmissivity and the important of this concept as your team performs inspections of your energized electrical assets.

Transmissivity is defined as the ability of radiation to pass through an object. Infrared inspection windows have become an industry standard in infrared electrical inspections of energized equipment increasing both the safety and efficiency of the inspection process. Commercial IR window optics can vary in material but are generally manufactured using either crystals or polymer. The inspector absolutely must know which optic is used in the windows that he will inspect. Why.?

Different optic materials will have different transmission rates. Windows with crystal optics will have different transmission rates than windows with polymer optics. Windows using the same optic will have different transmission rates if the optic is of different thicknesses from one window to the next. It’s critical that the inspector know the transmission rate of the window being inspected to insure the accuracy of the inspection data being collected.

                                                            Transmission Stability and IR Windows: The Effects of Transmissivity on Data Accuracy

What will the inspector do with the transmission rate of a window? For accurate temperature readings, the inspector must enter the correct transmission coefficient into the infrared camera’s settings to obtain the final temperature data for targets behind that window, Failure to accurately compensate for actual transmission can lead to significant errors in temperature readings.  If inaccurate data leads the inspector to a false negative conclusion on the health of an electrical asset, it could result in an expensive catastrophic failure and safety risk to the company.

An inspector may wonder why window manufacturers do not label their window with its transmission rate. A window’s transmission rate is only accurate at a specific wavelength. In the field, many different types of IR cameras are used. The detector in the infrared camera being used may use a wide band of infrared wavelengths rather than a single wavelength. Also, different cameras have variable sensitivities along the infrared spectrum meaning that your camera might be more or less sensitive if the window had a published specific wavelength label. Another consideration is that transmission values of the optic may change over time, especially if the optic is made of crystals which is known to drift due to water and other contaminant ingress into the crystal structure. By contrast, polymer optics maintain a fixed and stable transmission rate over time. These are a few of the reasons why an IR window does not have a published transmission value.

How do inspectors calculate the baseline transmittance of a window? Prior to installation of the IR window, its transmittance value should be determined. The preferred method is to calibrate the camera using an IR window and a target that has demonstrated a stable temperature reading in the range you anticipate your actual target’s temperature would be in normal operating condition. Using that transmission coefficient in that camera for future inspections with that window will provide confidence that the temperature readings of the target asset are accurate.


Transmissivity of an IR Window is a critical data point when using IR cameras to inspect energized electrical assets. The transmission coefficient value of the window must be programmed into the IR camera to determine accurate temperature readings of a target. Failure to enter the correct transmission coefficient value into the IR camera could result in accurate temperature readings that could mask a deteriorating electrical asset and could lead to catastrophic outcomes.

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