Infrared windows are specialised viewing ports fitted into electrical switchgear, distribution boards, and motor control centres that allow infrared radiation to pass through a sealed panel. They let a thermographer capture accurate thermal images of live conductors, bus bars, and connections without opening the enclosure. This eliminates the need for personnel to stand in front of energised equipment with covers removed, dramatically reducing exposure to arc flash and electric shock. This guide covers the types, materials, standards, and selection criteria you need to make a sound specification.
What Is an Infrared Window and Why Does It Matter?
An infrared window is a permanently installed port, typically 50 mm to 100 mm in diameter, that replaces a blanking plate or section of panel cover on electrical equipment. It is built from a material that transmits infrared wavelengths (roughly 2 to 14 µm), letting a thermal imaging camera "see" the heat signatures inside the enclosure while the panel stays closed and the circuit stays energised.
Why not just use ordinary glass? Standard silicate glass is essentially opaque to the long-wave infrared band that most electrical thermography cameras operate in (typically 8 to 14 µm). You would get nothing useful through a normal observation window. The whole point of an IR window is that its lens, optic, or mesh is specifically engineered to transmit the wavelengths your camera needs.
The safety argument is straightforward. Every time a technician removes a switchgear cover to perform a thermal scan, they are potentially exposed to energised conductors and the risk of an arc-flash incident. An arc flash at a distribution board can release enough thermal energy to cause severe burns in a fraction of a second. Infrared windows let you keep the panel closed, keeping the operator behind a rated barrier. This is fully consistent with the hierarchy of controls: eliminating the hazard at source rather than relying solely on PPE.
How Infrared Windows Work
Every object above absolute zero emits infrared radiation proportional to its temperature. A thermal camera detects this radiation and converts it into a visible image, with hotter components appearing brighter (or in a different colour, depending on the palette). When an IR window sits between the camera and the target, some of the infrared energy is absorbed or reflected by the window material. The proportion that passes through is called the transmittance.
Transmittance matters because it affects the temperature readings. If a window transmits 75% of incident infrared energy, the camera "sees" a target that appears cooler than it actually is. Modern thermal cameras and their software can compensate for this. You enter the window's transmittance value (usually provided by the manufacturer) as a correction factor, and the camera adjusts the temperature calculation accordingly. Without this correction, you will underestimate component temperatures, potentially missing a fault. Most professional infrared thermal imaging terminology references cover transmittance and emissivity in more detail.
It is worth noting that transmittance is not uniform across all infrared wavelengths. Different window materials have different spectral responses. Calcium fluoride, for example, transmits well across a very broad band, while polymer-based windows tend to be optimised for the 8 to 14 µm long-wave range used by most uncooled cameras.
Types of Infrared Window
There are three main categories of infrared window on the market, and they differ significantly in construction, image quality, and cost. Understanding these differences is essential to making a sensible choice for your installation.
Crystal Optic Windows
These use a solid lens made from a crystalline material that is naturally transparent to infrared. Calcium fluoride (CaF₂) is the most common, though barium fluoride and germanium are also used in specialist applications. Crystal optic windows provide the highest image quality: the thermal camera receives a clear, unobstructed field of view through the optic, producing detailed thermal images with accurate temperature data. The trade-off is cost and fragility. Calcium fluoride is a relatively brittle material, and the optic must be handled carefully during installation.
Crystal Polymer Windows
Polymer-based IR windows use a proprietary polymer material that transmits infrared radiation. They are tougher than crystal optics, more resistant to impact and vibration, and generally less expensive. Image quality is good, though transmittance figures tend to be slightly lower than those of calcium fluoride. Some polymer windows are broadband (transmitting across short-wave, mid-wave, and long-wave IR), which makes them compatible with a wider range of camera types. Products like the Fluke CV Series ClirVu infrared windows fall into this category.
Mesh or Grated Windows
These are not windows in the optical sense. A mesh or grated IR port is a metal grille or perforated cover that allows infrared radiation to pass through the openings. They are the least expensive option by a wide margin and are robust. However, image quality is noticeably lower. The mesh interferes with the thermal image, creating a pattern overlay, and temperature measurement accuracy is reduced. They are best suited for qualitative screening ("is there a hot spot, yes or no?") rather than quantitative analysis where you need precise temperature values. Their IP rating contribution is also lower than a sealed crystal or polymer window.
|
Feature |
Crystal Optic (e.g. CaF₂) |
Crystal Polymer |
Mesh / Grated |
|
IR transmittance |
High (typically >90% in target band) |
Good (typically 70-80%) |
Variable, dependent on open area ratio |
|
Image quality |
Excellent, clear field of view |
Good, slight reduction vs crystal |
Lower, mesh pattern visible in image |
|
Temperature measurement |
Quantitative, accurate with compensation |
Quantitative, accurate with compensation |
Qualitative or semi-quantitative |
|
Durability / impact resistance |
Fragile, sensitive to shock |
Robust, impact resistant |
Very robust |
|
IP rating preservation |
Can maintain original IP rating |
Can maintain original IP rating |
May reduce IP rating |
|
Cost |
Highest |
Mid-range |
Lowest |
|
Arc rating availability |
Available (tested to IEEE C37.20.2) |
Available (tested to IEEE C37.20.2) |
Varies; check manufacturer data |
Arc Ratings and Safety Standards
Any infrared window installed on switchgear that could produce an arc flash needs to withstand that event without releasing the blast into the operator's face. This is where arc ratings come in. The most widely referenced test standard is IEEE C37.20.2, which subjects switchgear accessories (including IR windows) to an internal arcing fault and assesses whether they maintain their integrity.
Windows tested under this standard are sometimes classified as Type 1 (the window must not allow the arc products to escape) or Type 2 (a higher bar, requiring the window to remain intact and sealed). Not every infrared window on the market has been arc-flash tested, so always verify the manufacturer's documentation. We have written separately about the common misconceptions regarding the arc rating of infrared windows, which is worth reading before you specify.
In terms of operational standards, EN 50110-1 (Operation of electrical installations) sets out the general requirements for working on or near live equipment in Europe. NFPA 70E, while a US standard, is frequently referenced internationally for arc-flash risk assessment and is used by some multinationals operating in Ireland and the UK. Infrared windows directly support compliance with both frameworks by enabling inspection without removing covers from energised panels.
Choosing the Right Infrared Window
Selecting the correct window involves more than picking a type. Consider these practical factors:
- Aperture size: The window diameter determines the camera's field of view inside the panel. A 75 mm or 100 mm window suits most distribution board inspections. Smaller windows (50 mm) may be adequate where space is tight, but you will need to be closer to the target or use a camera with a narrower field of view.
- Number of windows per panel: One window is rarely enough. You typically need a clear line of sight to each bus bar connection, each incoming cable termination, and each outgoing way. Three to four windows per section is common on an MV switchgear panel; a simple LV board might need two.
- IP rating preservation: The installed window must maintain the enclosure's ingress protection rating. Sealed crystal and polymer windows typically achieve IP65 or IP67. Mesh ports may only reach IP20. Check the manufacturer's declared IP rating and match it to your panel's requirement.
- Indoor vs outdoor: Outdoor installations face UV exposure, temperature cycling, and moisture. Polymer windows generally handle these conditions better than uncoated calcium fluoride.
- Camera compatibility: Verify that the window's transmittance band matches your camera's detector range. Most uncooled microbolometer cameras (the type used in mainstream electrical thermography, including FLIR IRW-Series IR inspection windows pairings) operate in the 8 to 14 µm band.
Installation Considerations
Fitting infrared windows to existing panels is entirely feasible, but it does require care. The key principle is that cutting a hole in a switchgear panel must not compromise the enclosure's mechanical integrity, arc containment, or IP rating.
Position each window so that the camera has a direct, unobstructed line of sight to the components you want to inspect. Bus bar joints, cable lugs, and circuit breaker connections are the primary targets. Avoid placing windows where internal barriers, partitions, or cable bundles block the view. It sounds obvious, but it is the most common mistake on retrofit projects.
If you are working on rated switchgear (type-tested assemblies to IEC 61439 or similar), consult the panel manufacturer before cutting. Modifications to type-tested enclosures can void the type test certification if done incorrectly. Many IR window manufacturers provide detailed installation templates and guidance to ensure the panel's ratings are preserved.
Infrared Windows in a Preventive Maintenance Programme
Infrared windows deliver the most value when they are part of a structured infrared thermography maintenance programme rather than treated as a one-off installation. With windows permanently in place, a thermographer can complete a survey route quickly, scanning every panel without shutting down, without calling for an AP to isolate, and without donning full arc-flash PPE.
Typical survey frequencies range from quarterly to annually, depending on the criticality of the installation. Many insurers and facilities managers now expect evidence of regular thermographic surveys on main switchboards and distribution equipment. A baseline scan taken shortly after commissioning gives you a reference point against which future scans are compared. Any significant temperature rise relative to the baseline, or relative to an identical adjacent phase, triggers investigation.
FAQs
Does ordinary glass work for infrared thermography?
No. Standard silicate glass blocks long-wave infrared radiation in the 8 to 14 µm band, which is the range most electrical thermography cameras operate in. You need a window made from an IR-transparent material such as calcium fluoride or a specialist crystal polymer to get a usable thermal image through a sealed panel.
Can infrared windows be retrofitted to existing switchgear?
Yes. Most infrared windows are designed for retrofit installation using a hole saw and a mounting flange. However, if the switchgear is a type-tested assembly (e.g. to IEC 61439), you should consult the panel manufacturer first. Cutting into a type-tested enclosure without following the correct procedure can affect the panel's certification and arc containment rating.
How do I correct my camera readings for the window's transmittance?
Enter the manufacturer's stated transmittance value into your thermal camera's settings before scanning. Most professional cameras from Fluke and FLIR have a dedicated IR window compensation field. Without this correction, measured temperatures will read lower than actual values, and you risk missing genuine overheating faults.
Getting Started with Infrared Windows
If you are planning to install infrared windows on your switchgear, or you want to discuss which type suits your installation, our team can help with specification, supply, and survey support. Browse our range of Fluke CLKT C-Range infrared windows or get in touch to talk through your requirements.