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Adjusting Level and Span Automatically--Part 1

Adjusting the span (thermal contrast) and level (thermal brightness) of an image is essential to an accurate reading. Most imaging systems can make these adjustments automatically and, if you understand how the processing works (and doesn’t work), that can be a great advantage. Basically the AUTO function, if you are at a loss for how to get a good image, can often “get us in the ballpark.” So use it, but don’t think it is smarter than you are! It only does what you tell it to do—automatically! Unfortunately, sometimes as you’ll see below, ‘Auto’ can get us in trouble rather than in the ballpark.

The AUTO function “sees” the thermal scene and sets the span (thermal contrast) and level (thermal brightness) of the image to include the objects with the greatest and least radiance, emitted and/or reflected. Often, but not always, these are also the hottest and coldest objects in the scene, but remember, the imagers see radiation not temperature. This process is not unlike how a “point and shoot” visual camera defines image exposure, based on the lightest and darkest areas it interprets.

Infrared Image 1

Look at the image of this house and notice how little detail there is on the wall. The AUTO function defined the lower limit of the span using the cold sky. As a result there is little contrast or thermal definition on the wall of the house. If we would have moved closer and excluded the sky, we probably could have successfully used AUTO to adjust the image and see plenty of detail.

Infrared Image 2

The second image was taken inside an older commercial building. Again the AUTO function adjusted the image to include the very warm elbow in the steam line, causing the thermal detail of the wall to be minimized. You won’t find out anything about the wall insulation from this image!

It is essential, whenever you are using the AUTO function, to exclude the extraneous hot or cold spots from the image. While this may not always be possible, when it is, it will allow you to use AUTO effectively to get an image quickly.

When you are not able to use the AUTO function, it is very simple to switch to the MANUAL adjust function so you can easily obtain the best image. There’s more to come on this topic next week, when I’ll show you the same two images, improved dramatically by using the MANUAL adjustment feature.

Thinking Thermally,

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

2 comments to Adjusting Level and Span Automatically–Part 1 of 2

  • Dan Laughbon

    Thanks I did enjoy the post , the write up was great but I did’nt get the second image you talk about in your post!??. The space for it is there but no image.

  • Fluke Thermography

    Thanks for catching that, Dan! It should be up there now 🙂