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Radiometric Measurements---How Thermographers Can Measure them Accurately

In life we often make corrections to our first impressions or first courses of action. Whether we are following directions while driving, stepping on the scale to check our weight or adding cream and sugar to our coffee, corrections are an important part of getting it right! Without them, we don’t get to where we were going, we weigh too much (never too little!) or the coffee just isn’t right

For thermographers, making corrections to our radiometric measurements is just as essential as making sure the driving directions actually get us where we wanted to go. Why can’t these high-tech instruments get it right? Because we are not actually measuring temperatures! We are, as a matter of fact, measuring the radiation emitted from, reflected by or coming through a surface.

How do we measure radiometric temperatures accurately? We must make corrections! In many situations that will get us within ±2ºC of reality—not bad for work being done before we’ve even had our first cup of coffee in the morning.

The imager sees all the sources simply as radiation and converts that sum total into an apparent temperature. Unfortunately, only the emitted energy is related to surface temperature. The reflected and the transmitted energy tell us nothing about the actual temperature. It is not unlike walking up to a plate glass door and being confused about the reflections that overlay the scene we see through the door—unless we correct for this reality, we may bump into the door!

If all this sounds complex, it is! In fact, the whole 4-day Level II course is essentially spent on these and related issues. The reality of our work is that without corrections, most measurements would be inaccurate—nearly all just a bit off but many others far enough off the mark to make us wonder if thermography works at all!

Too many thermographers don’t grasp the basics of corrections for emissivity, reflected temperature and, in some important situations, for transmission as well. Often we hear of people simply ignoring emissivity corrections, or failing to understand background corrections or, poor souls, having no idea how to make a measurement through an infrared window!

While temperatures are not always the most important detail, they are often a useful part of our analysis and we need to know how reliable they are. Our goal is to simplify reality so we can make measurements, with appropriate corrections, that will be within ±2ºC of reality for many situations. That is a cup of coffee worth enjoying to the last drop!

We’ll spend the next several weeks discussing these corrections in detail so you can feel more confident in your work. You’ll know when you are able to get measurements that are reliable, how to get them and, most importantly, when measurements will be unreliable despite the corrections.

Thinking Thermally,

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

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