Your guide to infrared window materials
15 September 2011
In this second part of 10 Things You Should Know About Infrared Windows IRISS guides you through the various types of lens materials that can be used. The choice is usually driven by the application, environment, wavelength and cost considerations. We examine the environmental factors, explore the suitability of crystal optics and ask does durability matter. And we also explain the huge design potential of transmissive polymer lens materials.
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As a reminder, back in Chapter 1 we covered the following five (5) points:
1. “Infrared window” is a generic term – there are actually several different categories of infrared windows available, all filling a different need.
2. If installed on energized electrical equipment, does the window maintain an “enclosed” and “guarded” condition for the cabinet enclosure? Does opening the window mix the outside and inside environments, thereby negating the “enclosed” state?
3. PPE requirements may differ dramatically depending upon the type of infrared window system used.
4. Always seek, and document, appropriate approvals for any custom modifications.
5. Be aware of the environmental ratings on the switchgear. Never install an infrared widow with a rating lower than that of the original enclosure.
Now...on to Chapter 2!!
See this article as a recorded webinar go to: http://www.iriss.com/materials_used_in_infrared_windows.php
Infrared Window Lens Materials
There are numerous types of lens materials that can be used in infrared (IR) windows. The specific choice is usually driven by the application, environment, wavelength, and cost considerations. For example, a mid-wave R&D application with high ambient temperature requirements may use materials that would be unsuitable for long wave condition monitoring of industrial applications.
What is the best material for an Infrared Window?
Germanium and Zinc Selenide are among the best broadband infrared transmitters available. Sapphire is a great transmitter in the mid-wave (MWIR) spectrum (sometimes referred to as “short wave”). It is incredibly durable – yet is non-transmissive in the long wave (LWIR). Ultimately there is no easy answer to this question because it quite simply depends on the application. In the end, 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 to be a costly exercise should they fail mechanically or functionally.
Does durability matter?
Will the windows be handled by scientists who will treat the optics like fragile lenses? Or will they be attached to electric panels that will be removed periodically and placed on cement floors during “house cleaning” shutdowns?
Mechanical stresses can fracture most crystal optics or degrade the crystalline structure, increasing refraction and decreasing transmittance. The stresses can take the form of jarring drops, exposure to high frequency noise or harmonics, or even exposure to environmental vibration. Incompatibility with mechanical stress is one big reason why most crystals are not considered suitable for industrial applications and uncontrolled environments.
What are the environmental factors?
Will the windows be used in a controlled laboratory environment, or will they be installed in a factory setting or an outdoor substation?
All materials have an Achilles’ heel. Polymers would not be the answer in a kiln application. Likewise, many crystals, such as the Fluoride family, are water soluble (also called “hydroscopic”) even when coated. Because they cannot maintain a stable transmission rate when exposed to humidity, or moisture, these crystals are not suited for use in most industrial applications. Carefully consider the operating environment before choosing an infrared window lens material.
Suitability of crystal optics
Traditionally, Fluoride crystals (Calcium Fluoride: CaF2) and (Barium Fluoride: BaF2) were the most commonly used infrared window optic materials. However, when Barium Fluoride was classified as a carcinogen, CaF2 became primary option.
As shown in Table 2, both BaF2 and CaF2 are hydroscopic. It has long been a standard practice to prolong the life of these materials by coating them in an effort to slow the inevitable deterioration due to moisture absorption. Though the coating does slow the degradation of the crystal, there is no coating that can completely seal the entire crystal. The deterioration is further hastened as thermographers brush their lens casings against the coating thereby exposing the crystal surface. The greater the exposure to humidity, and moisture – the quicker the crystal’s transmission deterioration. (Transmission stability is explored in greater depth in Chapter 3.)
All crystal window manufacturers determine the minimum thickness requirement for a given window diameter by calculating its “modulus of rupture”. If the window thickness stays constant while the diameter increases. The mathematical relationship is expressed in The Modulus of Rupture - figures below.
However, you don’t need a formula to understand the concept. If you have ever broken a stick over your knee, you know it is much easier to break a longer stick than it is to break a smaller piece of the same stick. The same is true of crystals – the bigger the crystal the more fragile it becomes, unless the thickness of the crystal is increased. Be aware that increasing the thickness of the materials will decrease the transmittance of that material and will affect temperature readings (see Chapter 3 for details).
If CaF2 is capable of resisting 14.7 psi (1 atmosphere) of pressure, the minimum thickness required for a two-inch diameter crystal is 2.8mm; a three-inch diameter crystal should be 4.2 mm thick; and a four-inch diameter crystal should be 5.6 mm thick. It is worth noting here that Arc Resistant Switchgear is typically set so that plenum vents open at just 25 psi to redirect the blast forces away from panels where personnel would be interacting with the equipment. At 25 psi (the minimum force which a crystal window optic would experience during an arc incident) the two-inch crystal would have to be 3.7 mm thick; a three-inch crystal would have to be at least 5.5 mm thick; and a four-inch crystal would require a minimum thickness of 7.3 mm to maintain its integrity. Thinner Calcium Fluoride would shatter. Unfortunately, no crystal IR windows are even close to that thickness. Therefore, any claims by a crystal manufacturer to resist the effects of an arc apply only to the integrity of the window housing with the cover closed, and do not (and cannot) apply to the integrity of the fragile crystal.
Polymeric Lens Materials
The past several years have seen a move toward the use of transmissive polymers as a lens material due to their inherent resiliency and stability. These materials are unaffected by mechanical stress and will suffer no effects on transmittance. They are stable: nonreactive to moisture, humidity, seawater, and a broad spectrum of acids and alkalis – in short, they are well suited to handle the rigors of the industrial environment.
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 resisting a sustained load. As a result, the only long wave compatible IR window optic capable of passing industry standard impact tests (as will be explored in Chapter 7) 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 — regardless of window size.
The only applications ill-suited to polymer optics are those in which the ambient temperature (not target temperature) is expected to exceed 200°C (392°F). Even so, like all other polymeric materials used on switchgear, polymer windows must meet the same stringent flammability and impact tests as prescribed by UL94. The classifications within UL94 take into consideration:
• Size and thickness of the part
• Distance from un-insulated, live parts
• Hot wire ignition
• High current arc ignition
• High voltage arc tracking rate
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