IR Window Certification – Be careful, it’s a minefield of false information out there!

So the question you probably have is why is the certification process such a minefield.  Well that is simple, for an IR Window to be deemed acceptable to OSHA, it must be approved by a Nationally Recognized Testing Laboratory (NRTL).  But;

  • There are many NRTLs available to use for certification such as UL, FM, CSA and Intertek to name a few.
  • All of these NTRL’s can certify an IR Window to their own standard but there are issues such as any certification from any NRTL would satisfy OSHA.
  • However, the US market requires UL Certification.

So how does an IR Window obtain UL Recognition

  • First of all we must decide on standards that will be pertinent to the applications of the product being tested.
  • All IR Windows require certifying to UL50V but depending upon environmental and other requirements, additional standards may are available.
  • Submission of the product for testing.

If successful a UL Recognized mark will be issued for the product range successfully tested.  Sounds pretty simple right? Well let’s take a look of what’s really involved with an IR window becoming certified.  There are a number of basic requirements that need to be considered when obtaining certification:

For products that are to be certified for indoor use:

  • Metallic parts must be non-corroding
  • Polymer parts must be flame retardant.  I emphasize this terminology because you will see later on how that can be misconstrued.
  • Protective covers are to remain in place during testing

For products submitted for outdoor certification:

  • We have all of the above and in addition we now have that…
  • Gasket or seal material must be subjected to accelerated aging tests
  • And that the IR window must be subjected to environmental testing using the aged gaskets or seals, all dictated by the desired environmental Type designation.

Now that the above have all been satisfied and all requirements for the chosen standards have been met, the UL Certificate can be issued.  All UL recognized components are issued with this document which shows product part numbers and standards tested to.  The UL Certificate of Compliance does not always show the Type designation, and it is up to the manufacturer to inform the end user of what the product’s type designation is. On some of the IR Windows in the market including the CorDEX IW Series this can very clearly be found on the cover; TYPE 3,12.  This is very important because the first rule of thumb to maintaining the environmental rating of the electrical gear is that the component’s rating must equal or exceed the rating of the gear itself.

For example:

If you install a Type 1 rated component into a Type 3 piece of electrical equipment, you have just de-rated the entire piece of electrical gear to Type 1.

The Type designation is contained in the UL report, and that information is not typically made available to end users.  The main idea here is to make certain that a component that is going to be installed into a higher assembly has obtained the proper certification and Type rating.

So now let’s take a look at the next issue of “Markings”

There is a common misconception when it comes to the use of the terms “listed” and “recognized”. We are often asked “Is the IR window UL listed?” Naturally this is a concern for all end users because their electrical gear is UL “listed”.

The answer is no, and that is because a window alone cannot be listed. The CorDEX IR Windows are UL “recognized” as a component. The recognized component that becomes part of a higher assembly, which IS UL “listed” as a whole.

A simplified example would be to consider your car. The car as whole assembly would be considered a car. If you were to remove one of the headlights and take it into a parts store to buy a replacement you would not refer to that headlight as a car. The same goes for a component and an assembly.

Figure 1: IW Series Certification Markings

There are special markings that coincide with a UL recognized component. As you can see in figure.1, the UL Recognized Component mark is shown and resembles a reversed UR. On the CorDEX IR Windows (figure.1) you will notice that the UR also has a “C” and a “U.S.” which means it is recognized as a component for Canada as well as the US.

Additional markings that can be found CorDEX IR Windows are the E number and warnings.  The E number is the 6 digit UL file number which engineers can use to access further information on the window from the UL Online Certification Directory.

The warning shown on the CorDEX window states that the ‘Device is incomplete without the cover secured’. This is normal with crystal IR windows as the covers protects the crystal from impact, which is completely acceptable to UL as long as the cover is closed when not in use.

IR windows that do not include a crystal optic may have the same type of warning. The reason for those warnings may have to do with requiring the cover to maintain the environmental rating or to maintain the flame retardant properties of the component.

Transmission Correction through Polymer Mesh IR Windows – Mission Impossible?

When looking at the Crystal IR Windows Transmission diagrams, we have been able to assume that there is an uninterrupted path to our target.  The use of Polymer windows supported by a metal means that this is not always the case.

Due to the complex thermal radiations signals and amounts we have to show this diagram in two parts due to the combination of optic materials; the Polymer and the Mesh.

Above the dotted line we see the signals associated with the polymer portion of the 2 part optic.

  • The orange arrow represents the energy reflected by target.
  • The blue arrow represents the energy reflected by the window.
  • The green arrow represents the energy emitted by target.
  • And the red arrow represents the energy emitted by the window.

Figure 1: Polymer Mesh IR Window

Now let’s take a look at the signal properties associated with the steel mesh portion of this optic (below the dotted line).  It is clear to see that there are only 2 signals seen by the thermal imager. This is because there is NO TRANSMISSION allowed by the steel mesh, so the reflected by target and emitted by target do not reach the camera.  This leaves the reflected by mesh signal and the emitted by mesh signal.

With such a number of signals, it creates inconsistencies due to the polymer/mesh combination, focus, target and window temperature, making it nearly impossible to calibrate over the standard PDM temperature range.

After looking at the two optics;

  • Crystal (uncoated),
  • Crystal (HydroGARD coated)
  • Polymer/Mesh combination

It is possible to summarize the differences in transmission characteristics between Polymer/mesh optics and CorDEX IW series crystal optics:

  • Polymer/mesh optics mean you are dealing with multiple complex signals seen by the thermal imager.
  • Measurements can be severely affected by focus and angle due to the not transmissive mesh.
  • Repeatable transmission correction across the PDM range is not possible.  Thus, the polymer/mesh optic IS suitable for non-measurement based thermography only.
  • With CorDEX IR windows we can achieve simple, repeatable signals to the thermal imager.
  • There is minimal surface reflection from the CorDEX IW Seires Crystal optic.
  • The Crystal optic does not affect target focus.
  • Quantitative (measurement based) thermography is now possible with the transmission correction algorithm available on-line, and on the IW Series Productivity App for iPhone.

Transmission Correction through Crystal IR Windows – Not so straight forward!

When using IR Windows in your predictive maintenance program you must firstly consider the fact that they are not 100% transmissive, and that the material that is used as the viewing area will affect the thermal radiation that can be interpreted by a thermal imaging camera.  Many manufacturers don’t consider the following aspects to be relevant to you and just give generic figures which potentially could prove to be costly.  Here we will look at all of the details broken down and explained.

Lets firstly look at what infrared transmission is?

  • Transmission is the ability of a material to allow the passage of thermal radiation through the material itself. (Panels on switchgear are not transmissive)
  • Transmission is not a direct division of before and after.
  • Transmission will be expressed as a percentage and is also made up of “multiple” IR signals
  • IR Windows are generally spectral transmitters; which means the apparent transmission will change with target temperature.

For example:

If the measured temperature of a target without an IR Window is 150˚F, and that same target with the window it is now 100˚F…We cannot expect the same ratio or difference between with the window and without the window as the target temperature changes.

There are a number of factors that affect the transmission percentages of the IR Windows including the material used by the window, the windows reflectivity and other materials used to help support the viewing area.  Here we can look at the different considerations you need to take into account when choosing an IR Window based on transmission.

So let’s take a look at how a standard crystal window works as compared to the CorDEX IW Series IR Window. In the following two diagrams you can see that there is a crystal window installed between the target and the thermal imager.

Figure 1:Crystal IR Window – No HydroGARD Coating

Figure 2: Crystal IR Window with HydroGARD Coating

In the top image you see 4 different signals that a thermal imager will see when a standard crystal IR Window is being used.

  • The orange arrow represents the energy that is “reflected” by the target.
  • The blue arrow represents the energy that is reflected by the surface of the crystal   optic itself.
  • The green arrow represents the energy that is “emitted” by the target, and thus the most important of these 4 signals.
  • The red and final arrow represents the energy emitted by the IR Window itself.

The sum of all of these signals is the Total Radiance. The goal here is to isolate all of the signals as we are ONLY interested in the energy emitted by the target.  The bottom diagram represents the same configuration but with a CorDEX IW Series Window.  You will notice that there are only 2 energy signals shown in this example, energy emitted by target and energy emitted by window.

We have eliminated the energy reflected by the target because for real-time measurements we require a high emissivity target. Experienced thermographers are familiar with some of the methods we can incorporate that help us achieve a high emissivity target. Some of these methods might include taping the target connection with a high emissivity tape, or painting the connections with high temp paint, which usually have a matte type of a finish giving the target a much higher emissivity. Keep in mind that these methods are typically used even when inspections do not include an IR Window.

It will also be possible with the CorDEX IW Series Intelligent IR Window to eliminate the energy reflected by window. We can only do this because CorDEX IW Series IR Windows include our proprietary HydroGARD™ protective coating.

This coating is actually dual purpose. The first is to reduce the reflective properties of the crystal optic for the reasons shown above. The 2nd is to provide a waterproof surface to the crystal allowing them to be used outdoors as well as indoors.

This leaves us with 2 signals; Emitted by target and Emitted by window. Since thermographers are generally interested in higher temperatures the “energy emitted by window” becomes insignificant, leaving us only with energy emitted by target. Using a unique calibration process using the remaining signals the CorDEX IW Series Intelligent IR Windows are all individually calibrated, creating a calibration map automatically.  This allows thermographer to accurately correct recorded temperatures fast and with ease.  The correction feature is available online at or on the IW Series Productivity App for iPhone.