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image of transmission correction through Crystal IR WindowsTransmission Correction through Crystal IR Windows – Not so straight forward!

Posted on: 27th, March 2014

When using Crystal 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.

What is infrared transmission?

  • 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 Infrared 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 Infrared 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.

How does the Crystal IR Window match up?

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

CorDEX-Crystal-IR-Windows-No-Coating
Figure 1: Crystal IR Window – No HydroGARD Coating

CorDEX-Crystal-IR-Windows-With-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 IR Infrared 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 IR Infrared Windows to eliminate the energy reflected by window. We can only do this because CorDEX IW Series IR Infrared 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 IR Infrared Windows are all individually calibrated, creating a calibration map automatically. This allows thermographer to accurately correct recorded temperatures fast and with ease.

For more information on CorDEX products, call us on +44 (0) 1642 454373.

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Posted In: IR Windows Products

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