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Adjusting Level & Span Automatically

I’m all for using the AUTO adjust feature found on most thermal imagers. That said, it does not mean I can turn my brain off! Many new thermographers believe they can just put their system on AUTO and life will be good. Not true.

AUTO adjusts the span of the image to accommodate all three cups by setting the top of the SPAN at 130F and the bottom at 30F.

How does AUTO work?

The AUTO feature is brilliant. It analyzes the image and adjusts to whatever is in it. The highest and lowest radiance levels define the top and bottom of the span setting, as well as the level of mid-point.

You can easily see AUTO in action by looking at three cups of water, one hot, one cold and one at room temperature. Notice what happens when you view all three in one image. Now watch the scale change as you isolate each cup individually. I would encourage you to practice until you are clear how AUTO works and what the limitations are.

When each cup is imaged individually, AUTO changes the span setting based on which cup is in the image. For the cold cup (left) span become 29F-70F, for the room temperature cup (middle) span becomes 64F-77F and for the hot cup (right) span becomes 64F-128F. Not my warm reflection in the center cup driving the upper limit of the span up slightly.

When does AUTO work best?

• I use the AUTO feature when I have little idea of what my setting should be. AUTO will generally get me in the ballpark, hopefully at least to have a decent image and a basic understanding of what’s going on.

• I also use AUTO when I’m looking at a wide variety of new territory, whether that is a building or a manufacturing facility. Again, my intent is to have a basic image (even if the temperature range is not refined) that gets me to the next logical step.

• When I’m looking at something that is changing temperatures fairly quickly, like a radiator heating up or fluid being transferred after a valve is opened, AUTO can change settings much faster than I can manually. This allows me to see the changes and understand them.

What are the limitations of using AUTO?

If there are any extraneous hot or cold objects in the image, they will cause AUTO to adjust and accommodate these extremes. For example, if you look at a hot line connection with the clear sky behind them, the lower end of the scale will probably be set at -20F or lower and the connector will not appear hot. Or in a house, if you have an incandescent bulb and a window in the image, the insulation patterns in the wall will be lost because the span is set too wide.

How can I use AUTO successfully?

Keep extraneous hot/cold objects out of the image when possible. Use AUTO as a starting point and, as needed, switch to MANUAL (I’ll cover this next week) or drop the image into Smartview and optimize the settings there. AUTO works well as long as you use it as a tool and not simply rely on it for an answer!

Thinking Thermally,

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

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