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Infrared windows: what are the choices?

24 October 2011

An infrared (IR) window is a data collection point for a thermal camera. It is used to separate environments from differing pressures or temperatures whilst allowing energy at a specified electromagnetic wavelength to pass between the two. Julie Reynolds explains their various forms and uses.

An IR window can 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. 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 thermal camera operator (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 in mechanical applications for machinery guards or in instances where operators wish to complete infrared and ultrasound inspections from the same access point.

Clearly, when a grill is opened it does not maintain an IP65 seal. It is, in effect, a collection of small openings intended to keep fingers and tools from breaking the plane 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 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 whether the electrical equipment is in an enclosed or guarded condition, which will ultimately dictate PPE requirements.

Certain applications may require customisation because, for example, metal cladding obstructs access to bus joins, or multiple connections are positioned just behind a dead-front panel of a power distribution unit or motor control centre. In these cases, standard IR windows may be impractical or cost prohibitive. Depending on the requirement, it is possible to design custom windows to allow inspection of applications that may have previously been impossible to inspect due to high incident energy levels.

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.

Lens materials
There are numerous types of lens materials that can be used in IR windows, so what’s the best choice? Germanium and zinc selenide, for example, are among the best broadband infrared transmitters available. Sapphire is a good transmitter in the mid-wave spectrum, being both durable and non-transmissive in the long-wave part of the spectrum.

Ultimately there is no easy answer when it comes to a question of choice because choice will depend 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 removed periodically and placed on concrete floors during routine shutdowns. Mechanical stresses – particularly those induced by shock and vibration – are likely to fracture most crystal optics or degrade the crystalline structure, increasing refraction and decreasing transmittance. Incompatibility with mechanical stress is one big reason why most crystals are not considered suitable for industrial applications.

Moreover, many crystals (such as the fluoride family) are water soluble even when coated. And while 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 similarly unsuitable for use in most industrial applications.

Polymeric lens materials
In recent years the trend has been to manufacture lenses from transmissive polymers, which are more able to cope with mechanical stress with no adverse effect on transmittance. They are stable, non-reactive to moisture, humidity, seawater and a broad spectrum of acids and alkalis.

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 withstanding 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 200oC. Even so, like all other polymeric materials used in the construction of switchgear, polymer windows must meet the same stringent flammability and impact tests.

Timed out
Time studies confirm that open panel inspections are exceptionally time- consuming and therefore costly. PPE suit-up and dress down time is another expensive task associated with this traditional method, and costs can quickly exceed thousands of pounds per inspection cycle.

Making use of IR windows eliminates virtually all preparation and PPE time, saving in excess of 90% of the time typically required to perform traditional thermographic inspections. Indeed most windows programmes see a return on investment within the first or second inspection cycle.

Julie Reynolds is global operations director, IRISS Ltd.

A guide on the subject of IR windows is available to download from the IRISS website

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