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Transmissivity of Switchgear Windows

As the popularity of infrared transmissive switchgear windows increases, many have begun to question how much windows attenuate the observed infrared data. With no standards governing the manufacture of switchgear windows, the answer can vary depending upon the make and model of the window and the selected infrared equipment.

Infrared transmissive windows have seen increasing popularity as an alternative to opening electrical cabinets for infrared inspections. These windows typically contain an IR transmissive material supported in a metal frame and are permanently installed at strategic locations in the switchgear enclosure. During the infrared inspection the camera lens is placed against the window to inspect electrical components without having to open the subject enclosure.

As no material on earth is 100% transmissive to infrared energy, all infrared windows will qualitatively and quantitatively attenuate the infrared energy passing through the window. While it is useful to know the transmittance of the window’s optical material, there are other important factors to be considered. It should be noted that there is no way to compensate for any of the common error sources listed below.

• Many IR windows have optics far smaller than IR imager lenses vastly reducing the infrared energy reaching the detector
• Switchgear windows that become dirty over time attenuate IR energy
• Some switchgear windows are wavelength specific and will react differently according to imager selected
• Depending upon cabinet depth, it may not be possible to obtain clear focus for the subject components

The topic of switchgear windows is covered in all Infraspection Institute Certified Infrared Thermographer® Level I training courses. For more information on calculating the transmittance of switchgear windows, consult the Standard for Measuring and Compensating for Transmittance of an Attenuating Medium Using Infrared Imaging Radiometers available from Infraspection Institute.