Infrared Windows, A Specifier’s Guide

Thermal imaging is an increasingly popular, flexible and cost effective predictive maintenance tool.  As it is non-contact technology, it allows live inspection and this is hugely beneficial for several reasons.  Productivity doesn’t have to be interrupted.  The thermal information gathered is of much greater value as the system is live.  And inspections can be conducted quickly without compromising accuracy - saving time and cost.

To harness these benefits, however, personnel safety must be assured and increasingly companies are using infrared windows to maximise the scope of thermal imaging and protect their people.

This move is driven by some alarming statistics.  Each year, electricians suffer terrible burns and even lose their lives, simply through doing their job.  Opening cabinet doors or panels to allow thermal inspection of a bus bar or transformer whilst power is applied can – and all too often does – result in life-changing and lethal arc flash incidents.

One answer of course if to isolate the power before taking measurements but this doesn’t reflect the condition of equipment or component under load.  This is why the use of infrared windows is proving to be so much the better solution.  It allows the true status of the system to been ‘seen’ under load whilst removing 99.9% of arc flash risk.


Crystal or polymer

Historically, infrared windows were only available with crystal lenses but now polymer alternatives are available too.  So which should you choose?

In order for a viewing pane, window or sight glass to be used for thermal imaging, it must allow the passage of infrared radiation.  Crystal lenses made from calcium fluoride (CaF2) were initially developed for this purpose.  The material is grown into billets, cleaved into disks and then highly polished to achieve full clarity.

Crystal is still a good choice for laboratory and high temperature applications but beyond that various factors should be considered.

First, size is an issue.  Crystal windows are normally only available in round shapes and up to four inches in diameter, allowing only a very narrow field of view.  The scope of their application is limited too. As crystals are naturally fragile, they easily fracture, especially in the presence of vibration.

Susceptibility to sunlight (UV radiation) and humidity are further factors; they both cause the crystal lens to become opaque or foggy.  In fact, even when installed in an environment whose temperature and humidity are controlled, crystal lenses begin to degrade at a non-linear rate.  And it is this degradation that inhibits transmission over time and compromises the accuracy of thermal measurements.

To counter these effects, crystal lenses must be coated.  As well as prolonging lens life in the presence of humidity and UV this protects the lens against scratching.


Benefits of polymer

Although a relative newcomer, polymer based infrared windows are rapidly gaining popularity.  The first window of this type used an opaque lens material, which allowed IR radiation to pass, but didn’t have the advantage of being see-through like crystal.  Nevertheless, it offered important benefits.

These lenses were designed to be industrial-grade, shatter- and vibration-proof and these qualities make them ideal for manufacturing and processing environments, particularly those involving food and pharmaceuticals.  The polymer material is also resistant to most acids and unaffected by UV radiation and humidity, good news for the marine and offshore sectors.

Polymer assures a fixed and stable transmission rate and this means that when once the thermal imaging camera is calibrated for a polymer window, it will always provide accurate readings.  And size is no longer an issue!  Polymer windows can be virtually any shape and even curved to fit around motors, pumps and bearing housings.  In fact they can measure up to 6 x 4 feet if that’s what the application needs.

More recently, a new generation polymer material has been developed, one that combines the durability of its predecessor yet the clear visibility of crystal.  It is a clear polymer that allows inspection in the UV, visual and all three infrared spectrums.  So inspectors can now use thermal imaging camera and UV or Corona cameras to identify arcing and tracking.

Crystal technology still has its part to play where the application involves temperatures above 270◦C but for most industrial applications, the polymer alternatives are far better suited.

The new range of polymer IR windows from RS may be specified in either opaque or clear polymer material.  Both types carry UL approval, Lloyds and American Bureau of Shipping (ABS) rating and, uniquely, CSA certification to C22.2 No. 14-10 standard; they have also been impact resistance and load tested to IEEE C27 20.2.a.3.6. Supplied with all labels and installation templates, the windows are rated at IP65/NEMA 4 once installed.


Martin Robinson