Guide to infrared windows

Thermal imaging is now an industry standard method of establishing the health of an electrical system. In the hands of a trained operator, an infrared camera does a great job of detecting thermal anomalies caused by conditions such as loose connections, overhead circuits and unbalanced loads. To allow this method to obtain valuable quantitative and qualitative data, however, infrared windows are increasingly being used.

Essentially, an infrared window is a data collection point for a thermal camera. It can also be described as a viewport, viewing pane, sight glass, port or grill, with each type having its own individual characteristics and meeting a different need.

Various types

Viewing panes are IR windows that have a lens secured in the housing. Because the lens forms a seal between the internal and external environments, the thermographer is not directly exposed to energised components. This usually means that high levels of personal protective equipment (PPE) are not required.

As its name suggests, an inspection grill is an infrared window containing a grill or mesh in place of a solid optic. Grills are primarily used to conduct both infrared and ultrasound inspection from the same access point. Clearly when the grill is opened it does not maintain an IP65 seal. It is, in effect, a mechanical guard intended to keep fingers and tools out of the cabinet.

It is important to understand that a grill will not maintain an enclosed state for electrical equipment since it allows the environment of the energised components to change. As such higher levels of PPE are required.

Inspection ports are usually no more than 15mm in diameter and can contain speciality lenses or adaptors. They can be open, as with grills, or be sealed with a lens like a viewing pane. It is essential, however, that the thermographer knows whether the port is open or sealed as this determines the level of PPE required.

Some IR windows are simply a housing with an open centre and a cover that secures the opening. Typically the housing will contain a grill or an optic and the design, size and material used are driven by considerations such as the required field-of-view, camera lens compatibility, intended environment and safety consideration.

Certain applications may require a custom solution: perhaps metal cladding obstructs access to bus joints or multiple connections are positioned just behind a dead-front panel of a power distribution unit or motor control centre. Thanks to the development of new materials in infrared window design, it is now possible to specify custom products at a reasonable cost. This is particularly important as it allows systems to be included in routine maintenance that were once considered ‘uninspectable’ due to high incident energy levels.

Materials

There are numerous types of lens materials that can be used in IR windows, so which should you choose? Germanium and zinc selenide, for example, are among the best broadband infrared transmitters. Sapphire, on the other hand is good in the medium wave spectrum but non-transmissive in long wave. But these performance credentials must be considered alongside the window’s intended use and operating environment.

Installing infrared windows that are not compatible with the intended environment can prove extremely costly and time consuming should they fail mechanically or functionally.

Durability

Most IR windows are designed to be installed in electrical panels that will be removed periodically and placed on solid floors during routine shutdowns. However, the mechanical stresses involved can fracture most crystal optics or degrade the crystalline structure, increasing refraction and decreasing transmittance.

The stresses can result from jarring or dropping the panel, exposure to high frequency noise or harmonics, or even exposure to environment vibration. Indeed, incompatibility with mechanical stress is the main reason why most crystals are not considered suitable for industrial applications and uncontrolled environments.

Many crystals, such as the fluoride family, are water soluble even when coated. Although the coating does slow the degradation of the crystal, there is no coating that can completely seal it.  And because they cannot maintain a stable transmission rate when exposed to humidity or moisture, these crystals are not suitable for 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 not lose their transmittance. They are stable, non-reactive to moisture, humidity, seawater and a broad spectrum of acids and alkalis.

Polymers are also extremely resilient and carry an unconditional lifetime warranty. Because they are malleable, they will absorb impact rather than shatter. And 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 that has passed standard impact tests is a reinforced polymer optic; long wave is the principal wavelength used for predictive maintenance applications.

A reinforced polymer optic can maintain a consistent thickness regardless of window diameter because the cells of the reinforcing material remain a constant diameter. This means consistent transmission rates and temperature readings, irrespective of window size. So polymer windows can be any size or shape or even curved to suit the application. A polymer window can also be teamed with an ultrasound port too so both technologies can be used to best effect.

As well opaque, clear polymer windows are now available too, making these products suitable for visual inspection as well as for inspection in UV and all three infrared spectrums.

Further advances

The very nature of polymer windows means they are able to conform to an exceptionally wide range of design and performance standards. For example, in addition to Lloyds and ABS (American Bureau of Shipping) credentials, they also have DNV accreditation. For anyone seeking engineering products that are suited to the rigors of the marine and offshore sectors, DNV is a very important attribute.

Building on the suitability of polymer windows for use in hostile working environments, another new product has just been released onto the market. Described as the ‘toughest and most reliable infrared window in the world’ it teams a clear, reinforced polymer lens with a stainless steel housing and IP67 environmentally sealed door.

This new introduction provides the largest visually clear infrared transmissive viewing area on the market today and an unparalleled field-of-view when compared to traditional, round IR windows. The cover can also be locked which protects the IR viewing pane from impact, flying debris and dust in areas of high traffic.

A standard range of sizes is available but, as with all polymer-based windows, custom designs are also available.