A Frequent Class Question: How do I get/What is Background?

A Frequent Class Question: How do I get/What is Background?
IR Talk
The Snell Group

To be a good Thermographer, we must make a shift from the visual world to the thermal world.

In my younger years in law enforcement, it was my job to take photographs that would later be used in court as evidence. I was sent to a camera manufacture’s class and learned about the functions of a camera and how to take a photograph correctly. I had to pay attention to the right lighting and perspective. Of course, the picture had to be in focus. To get a good photo, I needed to pay attention to the background and what was included in it. A Thermographer must pay attention to all the same elements.

Like a radiometric image, if a photograph is taken out of focus, once it is saved nothing can fix it. The picture cannot be re-focused. Occasionally the Contrast (equal to Span) or Brightness (equal to Level) may not be optimized and the subject is washed out leaving the photo useless in court.

In the “picture world”, the background is characterized as what is behind the subject being photographed. In the “radiometric picture world”, the background is not only behind the subject, it is also above and below it. The background is all around us and the infrared camera will see it on reflective surfaces. Everything on earth emits energy and for that reason, this energy can be reflected onto a reflective surface. There are two types of reflections a Thermographer may need to deal with:

Specular: A perfect or mirror like reflection and one in which the source of orientation is easily determined. Most of the time, the reflection will either be the Thermographer or the camera lens.

Defused: A scattering of reflections that are much harder to decipher and the exact orientation of where the reflection is coming from is difficult to determine.

For example, the surface of a control fuse cap or buss bar located inside a high voltage cabinet have a reflective surface. When viewed with the infrared camera, a “hot spot” may appear. To prove the reflection is a reflection, the camera must be moved in a swaying motion. Once a determination is made that the “hot spot” is a reflection, it must be blocked out. This can be done by changing positions or by holding up a clipboard to block it. These reflections that appear on the surface will also have a temperature. They could either be hot or cold reflections, but first…

How do we know the camera is operating properly? When an electrician checks a meter, it is done by checking a known source, then, what’s being checked and then the known source again. If the reading is the same, the meter is operating properly. The infrared camera is the same, it’s a metering tool as well and needs to be checked each time it is used for accuracy.

To check, we normally use a human tear duct or something of a known temperature such as a cup with ice and filled to the top with water.  When the emissivity (skin or water) and background temperatures are added, the camera can give reliable data. A reading of 93°F to 97°F should be seen with the tear duct and 32°F should be seen from the water.

Background is critical because whenever an infrared camera is being used, the background setting in the menu system must be adjusted. This could change from room to room and can also change when the angle of view is changed. While inspecting windows of a multi-floor building, each window will have a different background setting.

The camera is using a formula that is called Stefan Boltzmann’s constant. We cover this topic in the Level 1 - Thermographic Applications course. The operator learns that they must enter the emissivity and background temperature. Most of the time (but not always), background temperatures will be the ambient temperature of the object’s surroundings. Different camera’s give different names for background. Some camera’s call it “RTD” or Reflective Temperature Difference and there are camera’s that will have a symbol of an arrow bouncing off a surface and call it Ambient Temperature. Others call the background “Background”. They all must be properly adjusted.

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