Safe Thermal Inspection Access Using IR Windows

Thermal inspection systems in hazardous environments can be challenging, but specialized equipment such as infrared (IR) windows, significantly mitigates the risk. Let’s look at how IR windows operate in the context of thermal imaging and how the windows are selected and applied.

First, some definitions. The IR spectrum lies between the microwave and visible regions of the electromagnetic spectrum. IR radiation is a form of visually undetectable thermal energy, commonly known as heat. IR thermography, also known as thermal imaging, is a non-contact method for detecting and measuring temperature differences by analyzing IR radiation levels and graphically displaying the results.

The graphical display, often called a thermogram, uses colors to indicate temperature, with each color representing a specific temperature range. For example, the thermogram of an operating electric motor (Figure 1) shows the temperature of each area on the motor. The scale on the side of the thermogram shows the color-to-temperature mapping.

Figure 1 : Shown is a thermogram of an operating electric motor; the colors indicate the temperature of each part of the motor. (Image source: Fluke Electronics)

An IR thermal imaging camera, also called a thermal imager, generates thermograms. Unlike a conventional camera, the thermal imaging camera responds to long-wavelength IR radiation to develop a measured device’s heat signature. The resulting IR image is often significantly different from a visible-spectrum image. Many IR cameras also include a visible-light camera to capture a standard visible-spectrum image simultaneously, pixel-by-pixel, alongside the IR image. The combination of these two types of display images adds structural detail and provides physical context for temperature data, making it easier to interpret.

Applications for thermal imaging

There are many applications for thermal imaging. For example, it can be used to detect overheating in electrical distribution panels, indicating high-resistance connections or overloaded conductors. In mechanical systems, thermal imaging can detect motor winding and bearing defects. Architectural applications include locating heat loss in structures. It is also used in security applications to detect both animal and human intruders.

Thermal imaging has an advantage over other monitoring technologies in that it does not require physical contact with the devices being measured. This is particularly important in specific applications where the measured device operates at high temperatures, carries high voltages, or must be measured while operating.

IR and multispectral windows

Some applications for thermal imaging can pose hazards to technicians while they are taking measurements. For example, an electrical breakdown in a power distribution panel can result in an arc flash generating extremely high temperatures and light levels. The fault also generates an arc blast, a supersonic shock wave that produces deafening noise and superheated shrapnel, leading to severe injury. Arc events can occur even at voltages as low as 120 volts, but the likelihood increases with increasing voltage.

While personal protective equipment (PPE) can be used to defend against arc events, a safer, less costly alternative is to use inspection windows to isolate and protect workers.

An IR or multispectral window enables more efficient thermal imaging by allowing you to view the inside of electrical panels and enclosures without opening them, thereby preventing arc flash and blast exposure and enhancing worker safety (Figure 2).

Figure 2: Shown is an example of an IR window mounted on a control panel, allowing access for a thermal imaging camera. (Image source: Fluke Electronics)

Fluke Electronics offers IR windows compatible with their IR cameras and thermal imaging systems.

The ClirVu CLKT and ClirVu CV windows offer critical protection for technicians and engineers, allowing access to inspection areas without opening panels or enclosures. The IR windows are permanent fixtures made of arc-resistant crystal optics mounted in high-strength alloy die-cast frames with high-pressure gaskets for superior protection against arc blast. The optical windows are made of calcium fluoride (CaF₂) coated with Fluke’s broadband ClirVu coating for improved IR transmission and protection against moisture degradation. Covers protect the optics against dust and accidental damage. They have a continuous operating temperature range of -40 to 232°C (-40 to 450°F).

Both series offer windows with diameters of 2, 3, and 4 inches (in.) (50, 75, and 100 millimeters (mm)) and a choice of three different door latch types (Figure 3).

Figure 3 : These examples of the CLKT and CV windows show the available door latch selections. (Image source: Fluke Electronics)

The Fluke CV301 (Figure 3, center) is a 3 in. (75 mm) IR window using a security key latching mechanism. The 4 in. (100 mm) window version is the Fluke CV400 (Figure 3, right). This window uses a hand-turn latching mechanism that does not require a security key. The Fluke FLK-050-CLKT (Figure 3, left) features a 2 in. (50 mm) window with a twist-lock latch. The CLKT series features a multispectral window that utilizes Fluke’s QUADRABAND optical technology. The optical lens handles not only short-, medium-, and long-wave IR but also ultraviolet and visible light. It enables display fusion in compatible IR imagers, combining IR and visible light views for a more intuitive representation.

The CV series undergoes 30 cycles of testing and is rated to withstand arc events up to 63 kiloamperes (kA). The CLKT series is rated at 30 cycles of a 50 kA arc event.

These windows comply with multiple safety, arc-testing, and environmental compatibility standards.

Selecting an IR window

The selection of an IR window begins by determining which inspections will be required and whether they can be performed with the IR window and IR camera combination you plan to use. The camera's field of view and its alignment with the window play a role, but the window size and the distance to the target from the window determine the field of view through it (Figure 4).

Figure 4 : The field of view through the windows depends on the size of the window and the distance to the target from the window. (Image source: Fluke Electronics)

The table in Figure 4 shows the field of view for three different-sized windows (50, 75, and 100 mm) and the distance from the window to the target. Highlighted are the fields of view for three window sizes at a distance of 12 in. (30.48 centimeters (cm)) to the target. The field of view increases with window size and distance to the target.

Conclusion

IR and multispectral windows are key to safe diagnostic thermography. They isolate test personnel from potentially harmful arc events, high temperatures, and moving machinery while allowing access for thermal imaging devices.