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Infrared windows: the material facts

20 March 2012

The adoption of infrared (IR) inspection windows in industrial applications grows exponentially year on year as the increased level of awareness regarding electrical safety and risk reduction drives their acceptance. Rob Miller, Technical Advisor, Iriss Group, reports.

Used in almost every industry that utilizes or produces energy, these products save money by facilitating more efficient and safer workplace practices.  They decrease or eliminate unproductive man-hours and cumbersome personal protective equipment (PPE) by reducing direct exposure to energized conductors and high risk tasks such as removing panel covers.

The result is a streamlined process which cuts up to 90% of the man hours spent on traditional inspections whilst increasing the quality of inspections and safety of per-sonnel. 

What is an IR window?
Simply put, it is a data collection point for a thermal camera. It is used to separate personnel performing the thermographic inspection from the potentially dangerous environment within the electrical cabinet whilst allowing the radiated infrared energy at a specific range of wavelengths to pass between the two.
Some IR windows are just housings with an open centre and a cover that secures it.  Typically, however, the window will contain a grill or an optic with the design, size and material used being determined by considerations such as the required field-of-view within the cabinet, camera lens compatibility, intended environment, sealing requirements and safety considerations.

Numerous types of lens materials can be used in IR windows.  So what’s the best choice? Ultimately there is no easy answer to this question because it quite simply depends on the application.  Thermographers must give serious consideration to the IR window’s intended use and operating environment.  Installing windows that are not compatible with the intended environment could prove a costly exercise should they fail mechanically or functionally.

Many IR windows are designed to be attached to electrical panels that will be re-moved periodically and placed on cement floors during routine shutdowns.  However mechanical stresses can fracture most crystal optics or degrade the crystalline structure, increasing refraction and decreasing transmittance. 

The stresses can take the form of jarring drops, exposure to high frequency noise or harmonics, or even exposure to environment vibration.  Incompatibility with mechanical stress is one big reason why most crystals are not considered suitable for industrial applications and uncontrolled environments.

Environmental considerations
All materials have an Achilles’ heel.  Many crystals, such as the fluoride family, are water soluble even when coated.  Though the coating does slow the degradation of the crystal, there is no coating that can completely seal it.
The deterioration is further hastened as thermographers brush their lens casings against the coating, exposing the crystal surface.  Because they cannot maintain a stable transmission rate when exposed to humidity, or moisture, these crystals are not suited for use in most industrial applications.

Polymeric lens materials
In recent years there has been a move towards the use of transmissive polymers as a lens material due to their inherent resilience and stability.  These materials are unaffected by mechanical stress and will suffer no effects on transmittance.  They are stable, non-reactive to moisture, humidity, seawater and a broad spectrum of acids and alkalis. A polymer lens is well suited to withstand the rigors of the industrial environment.

Polymers are also extremely resilient.  Because they are malleable, they will tend to absorb impact rather than shatter.  When reinforced, with specially engineered grills, the optic is capable of resisting a sustained load.  As a result, the only long wave compatible IR window optic capable of passing standard impact tests is a reinforced polymer optic.
A reinforced polymer optic can maintain a consistent thickness regardless of window diameter because the cells of the reinforcing material remain a consistent diameter.  Consistent optic thickness means consistent transmission rate and therefore consistent temperature readings, regardless of window size.

The only applications ill-suited to polymer optics are those in which the ambient temperature – not the target temperature – is expected to exceed 200°C.  Even so, like all other polymeric materials used on switchgear, polymer windows must meet the same stringent flammability and impact tests.

Ultimate design flexibility
Another big benefit of polymer lens material is that an infrared window no longer has to be round.  Indeed custom windows can now be manufactured in any shape or size providing industry with exceptional inspection flexibility.

Not all assets lend themselves to the traditional round window either because they don’t give complete coverage or because so many windows are required making the application cost-prohibitive.  A panel, machine guard or cowling can be used as a template to manufacture not only an infrared viewing panel, but also an optical viewing pane.  Thanks to the use of polymer, the possibilities are endless.  The window can be square, rectangular, triangular, indeed any shape or size.

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