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Correcting Measurements Through Infrared Windows

Over the past several weeks, we’ve seen how essential it is to make corrections to the raw data our imaging systems detect. Without these adjustments to emissivity and reflected background temperature, we can never achieve the ±2ºC accuracy that our systems are capable of achieving in ideal circumstances.

One additional correction—for the partial transmission of radiation through a material—is much more challenging. As more and more of us are using “infrared windows” to more safely view loaded electrical components, the need for this type of correction has become more critical. While achieving a high level of measurement accuracy is not always possible, especially if we are viewing a low-emissivity connector through the window, at a minimum we should understand the physics involved.

We’ve all experienced looking through a window, often one that was dirty and slightly reflective. We are not able to see the world as clearly or accurately! When we look through an infrared window, the same thing happens due to the simple fact we are looking through a material that is not 100% transparent. As a result, we may not be able see all the detail, and measured temperatures may not be accurate without making another correction for “percent transmission.” While we can’t get into all the details of using infrared windows in this week’s blog, we can see that this correction is crucial. Fortunately, it can easily be made in some imagers (notably the Ti32) and in all cases, just as easily in the software.

The first step is to determine the transmission correction to use. This specification is typically supplied by the window manufacturer. If not, or if transmission has changed over time, you can measure it by conducting a few simple tests (more on this in another blog). All you need to do is input the correction value and the imager or software will automatically make the necessary corrections.

The truth is, this correction alone may not put us within the ±2ºC accuracy—especially if:

  1. Percent transmission is low
  2. The window is much warmer or colder than the connection we are viewing
  3. We are viewing a low-emissivity connection
  4. We are not in close proximity to the window

Regardless, this correction for transmission is an important one and, when made in the greater context of good measurement practice, it can help us achieve the high accuracies we want when using infrared windows.

If you want to practice making corrections for percent transmission, try it in the safe surroundings of your office by looking through either a sample infrared window or various thickness of thin-film plastic. You’ll quickly get an idea of what works and what doesn’t and soon be ready to make these corrections in a real, field situation. For additional, valuable information on corrections for transmission with regards to IR windows, please visit Fluke IR Windows.

Thinking Thermally,

John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner

5 comments to Correcting Measurements Through Infrared Windows

  • Thats one of that write-up which it’s worth to bookmark. Definitely wonderful article.

  • Tony Walsh

    I am looking at performing IR through IR Windows in the near future. Could you tell me how to account and/or test for the transmissity of an IR window while systems are non-energized and energized. The manufacturedo provide spec’s on this but IR Windows do degrade over time and their t value decreases. Should this t-test only be performed using the ‘mug test” or similar testing during shut-downs? Can it be done while systems are energized? Hoew often shoud the transmission test be performed on IR Windows?

  • Fluke Thermography

    Tony – Thank you for the questions! Please see the below responses, and also, please note that thermography focuses on the detection of potential issues, as opposed to actual temperature measurement–which might help to better understand these answers!

    Very generally:
    1) Get a target with high emissivity and temperature equal to or greater than the temperature of the IR Window.
    2) Set emissivity appropriately, and set transmission rate on imager to 100%.
    3) With your thermal imager properly focused on the target at a specified distance, note the temperature of the target.
    4) Place IR Window in front of target
    5) Matching focus and distance, adjust transmission setting until target temperature from 3 is reached.
    6) This should serve as your transmission rate for all but your more steep viewing angles through this window.
    7) Note the surface temperature of the window/panel as well, as the transmission rate can fluctuate based upon the temperature of the crystal optic. (You should probably record both your transmission rate and the temperature for each window installation somewhere near the window or in an inspection route document.)

    Fluke IR Windows are guaranteed never to degrade over time. All Fluke IR Windows are coated with a special coating that makes them impervious from degradation and transmission loss–which is the reason why Fluke has never had an IR Window returned :) . Hope this information helps, and feel free to ask any other questions we can help answer!

    Thanks.

  • David

    I’ve heard that the coating isn’t doing anything. Can you point me to any long-term, third-party data proving the effects of the coating? Theory suggests that it is impossible to have a coating completely seal against water while having no negative impact on transmission (afterall, the crystal is what is IR transparent, not the coating material). Why isn’t real data on this more readily provided?

  • Fluke Thermography

    While degradation and moisture absorption have been historic issues with crystal IR Windows, with the Fluke exclusive ClirVu® coating, this is no longer a concern. To date, Fluke has never had an IR Window returned for this reason. ClirVu® completely coats the optic – making it impervious to moisture. Cyclic humidity testing has been performed per International Electric Code (IEC) Standard 60068-2-3, a generally accepted industry standard testing for environmental withstand capability. Fluke IR Windows have been installed in some of the harshest environments for over 10 years without incidence. These include the hot and humid Gulf of Mexico (not to mention salt water) and the brutally cold Canadian tundra. The Fluke IR Windows are also covered by a lifetime warranty.

    The ClirVu® coating has a minute impact on transmission rate. It’s important to note that accounting for transmission rate consistently is more important than the transmission rate itself. The key with all infrared scans, especially those using IR Windows, is consistency. Use the same thermal imager, at the same settings, looking at the same inspection points and you will be able to compare results over time to effectively monitor equipment health.

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