What Type of Lens Materials Are Used in Infrared Inspection Windows?
Following a recent Arc Flash incident at your workplace, you joined the company’s safety team to search for solutions that would minimize or reduce the risk of another accident occurring during an energized electrical inspection of the company’s equipment. Electrical Maintenance Safety Devices, such as Infrared Inspection (IR) Windows, allow for real time, infrared data to be collected on energized electrical assets in a closed and guarded condition. The team’s research on IR Windows reveals that there are many materials used in manufacturing the lens of an IR window. The next step is determining what type of lens material is best suited for your company electrical assets. Let’s investigate!
There are numerous types of lens materials that can be used in IR Windows. The choice of lens material is driven by the application, environmental factors, light wavelength to be measured and cost. The lens material should:
1. Withstand all mechanical impact and load test requirements
2. Have a fixed and stable transmission rate for the life of the unit
3. Transmit in the infrared wavelengths as a minimum as well as visual and ultraviolet wavelengths if desired
4. Transmit infrared radiation in the mid-wave and long-wave infrared ranges (3 – 12 microns) to coincide with most IR cameras
5. Withstand prolonged exposure to vibration without cracking or degrading
6. Withstand prolonged exposure to direct sunlight without degrading
7. Not absorb moisture (non-hygroscopic)
8. Be of an IP2X fail safe design (touch proof with less than ½“ aperture size even with the lens compromised)
9. Be able to withstand temperatures up to 250°C (482°F)
10. Be resistant to light acids, alkalis and water
11. Be made of materials that are non-carcinogenic and non-toxic and must not contain arsenic, barium, bromine, cadmium, chlorine, gallium, germanium, iodine, lead, lithium, mercury, selenium, sulfur, thallium or zinc
12. Be able to work with all makes and models of IR cameras
The most common materials used in IR windows are Calcium fluoride, Sapphire, IR Polymer, Germanium, Zinc Selenide and Barium Fluoride. Historically, Calcium Fluoride and Barium Fluoride were the most commonly used infrared window optic materials. However, since Barium Fluoride (in powder form) was classified as a possible carcinogen and known muscle toxin, Calcium Fluoride has been used more frequently.
Common Lens Materials Used in IR Windows
The table below lists the common lens materials and their respective performance criteria.
As you can see, some of the common materials in the table do not meet the requirements of being non-hygroscopic (soluble in water) or the Knoop Hardness criteria to withstand mechanical impact and vibration.
Things to Consider Before Choosing IR Window Lens Material
Thought must be given to determine the actual operating environments where IR Windows will be used. Key factors that need to be considered are:
- Is the IR Window for indoor or outdoor use?
- Is the IR Window located in a noisy environment or area of high traffic?
- Will the IR Window be subjected to severe environmental conditions such as humidity, rain, snow, sea water, acids, alkalis or extreme temperatures?
- What is the transmission rate and wavelength that the IR Window will operate in?
The choice of lens material depends on the application needed. Companies must give serious consideration to the intended use and operating environment of the IR Windows so the correct lens material is chosen. Installing Windows that are not compatible with the intended environment or application could be a costly event should the Windows fail mechanically or functionally.
Does Durability of the Lens Material Matter?
Even though crystal lenses have been used historically, they have disadvantages when used in industrial settings. Crystal windows like Calcium Fluoride and Barium Fluoride are soluble in water (hygroscopic) and will, over time, absorb water vapor into the crystalline structure reducing Infrared transmission through the lens. In addition, crystals are brittle and unable to withstand impact and mechanical stresses commonly seen in industrial settings. Mechanical stress can fracture the crystal lens or change the crystalline structure causing decreasing transmittance. If transmission rates of the material change over time, the data being recorded could be compromised. Could this be why crystal lenses themselves are not guaranteed by crystal window manufacturers?
Polymer IR Windows
Polymer IR Windows are unaffected by mechanical stress and are non-reactive to moisture, humidity, sea water and a broad range of acids and alkalis. They are well suited for industrial environments. Polymer Windows are extremely resilient and tend to absorb impact rather than shatter. Polymer Windows can be reinforced with special grills insuring that it passes industry standard impact tests. A transparent polymer optic is available allowing inspections in the ultraviolet, visual, short-, mid- and long wave infrared spectrums. The only disadvantage of a Polymer Window is if the environment exceeds an ambient temperature of 200°C (392°F) which is rare in industrial applications.
The logical approach for a company considering IR Windows is to work with the supplier that offers a variety of lens materials. They can help select the best suited Window for the application and environment.
Different lens materials will react differently to moisture, humidity, chemicals and mechanical stresses. It is mission critical that a company identify the specific application for the IR window and the specific environment where the IR Window will reside so that the appropriate lens material is selected. Companies should work with reputable IR Window suppliers that offer a variety of lens materials to help identify the appropriate lens material for the application in question.