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:
- Percent transmission is low
- The window is much warmer or colder than the connection we are viewing
- We are viewing a low-emissivity connection
- 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.
John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner