Why Crystal IR Windows Just Don’t Work for Industrial Applications

When I think about the use of fluoride based crystal infrared windows, I think about what Albert Einstein purportedly said, “The definition of insanity is doing the same thing over and over again and expecting different results.”   Why do we insist on making infrared windows from the same materials that have been failing for years and expect different results?The IRISS website clearly states that our crystal windows are made of calcium fluoride (CaF2). However, other websites state that their window material is a proprietary material and give it clever names.    You cannot determine what material their “crystal window” is actually made of.  The reason for this marketing cleverness is that if you know the lens material, then you can begin to investigate the positives and negatives of the crystal material.  The two primary crystal materials used in industrial infrared windows are Calcium Fluoride (CaF2) and Barium Fluoride (BaF2).  This paper will discuss several commonly made false claims relating to IR windows.

Arc Resistant IR Windows?

First, I would like to address the ridiculous claims out there that crystal IR windows are Arc Resistant.  Crystal IR windows range from 2 to 4 mm in thickness depending on the size and manufacturer.  These windows are then mounted in electrical enclosures that are designed to be Arc Resistant by the Original Equipment Manufacturer (OEM) in line with the standards for such equipment and are then “Arc Containment Tested” by a Nationally Recognized Testing Laboratory (NRTL) such as UL or ETL.  If the OEM electrical equipment in question passes the test, then the equipment is given an “Arc Resistant” certification but this is applicable only to the full assembly.  All of the components fitted to the assembly do not have the right to state that they are separately Arc Resistant.  You do not see an automobile seat manufacturer claiming their seats have the highest crash test ratings as this would be deemed ridiculous.

Claiming an Infrared Window is Arc resistant as a stand-alone component is both misleading and dangerous.  We must concentrate on the facts relating to the mechanical and infrared properties of the infrared transmissive lens materials and infrared window design.  There is no room for marketing spin when it comes to safety.

Non Hygroscopic IR Windows?

All Fluoride based crystals are hygroscopic, which means they will absorb moisture (water vapor) as well as any airborne suspended acids and alkalis.  As the crystal’s relatively large molecular structure absorbs these contaminants from the atmosphere, the transmission rate (transmissivity) changes.  I explain this phenomenon to my Level 1 Thermography students by comparing the IR crystal to an air filter in your motor vehicle.  A brand new out of the box air filter allows maximum air flow through to the engine.  However, as it is used the filter starts to clog with debris and the air flow starts to diminish.  The life of the air filter will depend on the type of environment in which your motor vehicle is operating.  The filter being used in a vehicle that only drives up and down paved highways in a steady environment will last a great deal longer than the filter being used in harsh dusty environments.  Fortunately, your motor vehicle has an air flow sensor that detects when the airflow is at a level where it is no longer giving the engine enough air and the filter is then changed and the cycle starts again.

Unfortunately, your Infrared camera does not have a “transmission level” sensor to indicate what the crystal window transmission rate is.  In fact, the crystal’s ability to transmit in the infrared spectrum could be degraded almost 100% due to hygroscopic contamination and your temperatures readings could be so far off making them completely useless.  In addition, the error will read lower temperatures of components than what is actually the case meaning that you may miss a critical indicator of a potential failure.

We can and do coat infrared crystals to help increase the life of the material and slow down rate of contamination and the transmission drift.  We use several coatings, some of which are insoluble are applied to the surface of the crystal.  The problem with this coating is that the edges of the crystals are not coated.  This means moisture will inevitably creep past the seals over time and first be absorbed from the outside edges of the material and progress to the center.  We have many pictures of crystal IR windows exhibiting this phenomenon.  The problem is that by the time you are able to see the contamination with the naked eye, the IR transmission of the lens has long since been compromised.

Despite the protective coatings and because of hygroscopic transmission drift, virtually all crystal IR window manufacturers recommend measuring the transmission rate of the crystal window at least every 6 months and re-calibrating the camera transmission rate accordingly.  Of course, to do this calibration it is necessary to OPEN up the equipment to which the IR window is fitted which will require the personnel to be in full Personal Protective Equipment (PPE) unless the equipment can be safety de-energized.  The necessity to measure crystal window transmission rate at this frequency completely negates the entire purpose of IR windows which is to conduct inspections without the need to open up the equipment!  As such, crystal windows are wholly unsuitable for their stated purpose.

The other issue with the coating materials is that they have a different coefficient of thermal expansion, which means that the crystal and the coating expand and contract at different rates with variations in ambient temperature.  This causes micro-cracking and again allows moisture to be absorbed under the coating and into the crystal.  Finally, crystals in harsh industrial environments are subjected to high frequency noise and vibration which also adds to the micro cracking and escalated transmission failures in these materials.

Are IR Windows Impact Resistant?

The simple answer is no.  Crystal IR windows are mechanically inferior to other available options.  The ANSI/IEEE C37.20.2 standard requires that visual viewing panes be subjected to  impact and static load tests on both sides of the viewing pane with the cover open when fitted into switchgear over 600 Volts.  To pass this test, the window must not “crack, shatter or dislodge.”  No fluoride crystal infrared window on the market can pass an impact test.  That is an irrefutable fact!   However, you hear manufacturers claiming that this test is only for visual viewing panes and does not have anything to do with infrared windows.  This is simply untrue.  Part of any thorough thermography inspection is a visual inspection and crystal infrared windows are always also used for conducting visual inspections and as such must meet this mandatory requirement.

UL, cUL and CSA standards for visual viewing panes also all require static load and impact testing, but these standards allow the tests to be done with the window cover closed.  In addition, UL and cUL do not require impact testing on crystal windows if the optic is over 1.4 mm thick.   This is a ridiculous loophole and allows for inferior materials to be used in switchgear and other electrical apparatus.  All Polymeric materials all need to be tested regardless of the optic thickness.

UL1558 is another standard which is applicable and which requires impact and static load test for windows at double the levels of the ANSI/IEEE standard.  On the face of it you would think that this is a good test.  However, unlike the ANSI/IEEE test, the UL1558 test allows for the cover to remain fitted and the infrared window is deemed to have passed if, at the end of the test, you cannot pass a ½ inch rod through the window.  So what that means is that the crystal window can be completely smashed and still pass since the rod cannot pass through the closed metal cover.

What the standards writers also fail to acknowledge is that the largest hole size that switchgear can have is ½ an inch.  If this window is fitted into a piece of switchgear and breaks through impact it may be UL1558 compliant while the cover is closed but as soon as it is opened it may have a 3 or 4-inch hole where the crystal used to be which would invalidate the IP2X requirement.  The maintenance personnel would have no way of knowing that the crystal is broken until the cover is opened.  In addition, simply allowing air interchange between the enclosure and the room can trigger an arc flash event and it is now too late for personnel to put on the necessary PPE.

The window would require changing immediately (or as soon as the plant could manage this) and until then the thermographer would require full PPE to complete the inspection which again makes the whole process of fitting the crystal infrared window useless.

The UL1558 test standard also has a pass requirement for IR windows tested with the covers removed that falls in-line with the IEEE pass/fail requirements that the IR window lens must not crack, shatter or dislodge.  We at IRISS feel that this should be the test requirement as its more in line with the interests of the user and the safety of the equipment to which it’s fitted.  There are no crystal IR windows on the market that can pass the UL1558 test with the covers removed.

Does Size Matter?

Finally, the most compelling argument against using crystals is not their mechanical and infrared inadequacies, it is their size and cost limitations.  Crystal IR windows are limited on the sizes that can be manufactured to a maximum of 4” diameter.  The larger the crystals get, the weaker and more expensive they get to see what you want to inspect.  What that means is that you have to use more infrared windows within the electrical apparatus which starts to become more expensive both from window cost and the cost of installation.

I recently asked a meeting full of switchgear engineers why are the visual viewing panes so large on switchgear?  Their unanimous reply was, “So we can see everything we need to see.” “Exactly!”, I replied.

So why should infrared windows be any different?  Using the IRISS patented reinforced polymer optic window systems, Infrared windows can now be built in any size to fit the inspection task.  We have designed windows using the latest innovations in polymer materials and product design techniques to give engineers exactly what they need in form, fit, materials and function.  We regularly custom design large format windows up to 200 square inches or more in area and even for curved surfaces.  All of these custom designs along with our standard catalog of products are fully tested and certified to all the standards.  All testing is performed with the covers open and the polymer optic material used in IRISS patented systems will never suffer from transmission drift.  In fact, we are so confident in our technology that we offer an unconditional lifetime warranty on our reinforced polymer optic IR windows.  Designs that were never thought possible are now available and we now have Electrical Maintenance Safety Devices (EMSD’s) that are built to allow us to inspect equipment previously thought to be uninspectable.

The future of Infrared Windows & Electrical Maintenance Safety:

We now see the use of EMSD’s growing in industry and coming of age.  We have the abilities to design systems that 5 years ago would have never been thought possible.  Custom infrared windows are now punched directly into covers that allow field service technicians to install complete window units in minutes without having to cut into panels.  Now all they have to do is remove the old bolted cover and bolt the new covers in place

We now have windows designed for IsoPhase and non-seg bus bar allowing us to inspect these critical assets at any time.  We also have windows with online temperature measurement capabilities that monitor the internal ambient or joint temperatures and send alarms out to technicians if temperature alarm bands are breached.  Intelligent asset tagging solutions now exist that ensure that critical data required for inspection and maintenance remains with the asset at all times and can be backed-up to a cloud based system for asset managers to see the condition of their equipment and maintenance programs.  IRISS has all this and more within it’s ever growing EMSD portfolio.

Visit IRISS.com today to see what we can do for you.

Take a look at the video below that shows how crystal IR windows are inferior to the polymer alternative.

Polymer windows are the solution to the problem that is posed by crystal IR windows. Please click the link below to download an infographic that explains the advantages polymer IR windows have over crystal.