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Tip written by: Infraspection Institute

As the popularity of switchgear windows and ports grows, thermographers are faced with challenges unique to these devices. In this Tip we examine some of these challenges and provide advice and cautions when imaging through windows and ports.

In an effort to reduce the risk of injuries associated with arc flash, many facilities have installed IR transmissive windows or ports that permit infrared inspections without having to open panel covers. Although windows and ports can provide a measure of safety and help to reduce labor associated with infrared inspections, they pose unique challenges not associated with direct line-of-sight imaging.

Switchgear windows are typically constructed of a metal frame with a fixed IR transparent material that enables an imager to view through them. Switchgear ports consist of a metal frame with small openings through which an imager may be sighted. Depending upon type, some ports have a single hole; others have metal screens containing multiple holes.

Windows and ports will always attenuate infrared energy received by the imager. This attenuation will affect both qualitative and quantitative data; however, the greatest challenge posed by windows and ports involves temperature measurement. Due to their small opening it is not possible to accurately measure temperature through screened ports. Accurate temperature through windows and single opening ports is possible only if the following conditions are met:

• Window opening must be larger than IR lens objective
• Target must be at or beyond imager’s minimum focus distance
• Both window transmittance and target emittance values must be known and properly entered into imager’s computer
• Imager lens must be kept perpendicular to, and in contact with window

When it is not possible to meet all of the above conditions, imagery should be evaluated only for its qualitative value. As always, any inexplicable hot or cold exceptions should be investigated for cause and appropriate corrective action taken.

The Infraspection Institute Standard for Measuring and Compensating for Transmittance of an Attenuating Medium Using Infrared Imaging Radiometers provides procedures for calculating window transmittance which is critical to accurate temperature measurement. To obtain a current copy of the Standard, call 609-239-4788 or visit the Standards area of our online store.

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