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Focus, Focus, Focus--Why Focus is So Important

If we think about visual photography for a minute, what comes to mind when you have a picture that is not in focus? Can you pick out the details that are important to you, does it look good, and can you see the trees from the forest? Generally we toss those bad, out of focus images in favor of the good ones—and this is true with thermal images as well.

Even more important, an infrared thermal image that is out of focus also displays bad temperature data. Note the “out of focus” image in Figure 1 and note the temperature of the center point. Compare it to the image in Figure 2, an “in focus” image. Quite a difference–and that difference could lead you to an incorrect conclusion!

Figure 1

Figure 2

Focusing a scene or object using an infrared camera is more problematic than what you might be used to using on your visual light camera. In the world of “visual light” we can take advantage of reflected light which creates share distinction between objects. In thermography, we need to deal with energy that is emitted from the surface and those “pesky” three modes of heat transfer. As heat energy moves through solids, liquids and gasses, those crisp lines between objects disappear, making focus a challenge.

There are a few simple suggestions to help, such as:

  • The human eye most often can focus in black and white (gray scale) easier than other color scales
  • Look for a sharp edge that you see in the field of view
  • Hold the camera still
  • Practice!
  • And with Fluke infrared camera you can effectively use “IR-Fusion”

Using Fluke’s IR-Fusion in the picture-in-picture mode, you can see the two images (visible and IR) slide up and down independently.  When the two line up, you are perfectly in focus!

In summary, the focus of any image is important both for clarity & accuracy of data and for the ability for others to see what is the focus of the image. Practice using any and all for the available pallets, and that will—as the saying goes—“make perfect” the image!

5 comments to Focus, Focus, Focus–Why Focus is So Important

  • I know that focus is the only thing that cannot be manipulated in the software when analyzing the photos. Why don’t manufacturers have an auto focus mode? With the laser range-finder technology, an auto focus could key on a laser dot. With the price of the IR cameras, a few hundred dollars more for an auto focus would be worth it.

  • Fluke Thermography

    Really great idea. Auto focus is something that Fluke has long looked at and we are working hard to find a reliable, trustworthy solution (have you secretly attended our meetings? ;)). Auto focus in the infrared spectrum is very different from auto focus in the visual world, though. There are already some thermal imagers with auto focus, but we have been very disappointed with the reliability and repeatability of this feature as it’s been currently executed. You can’t depend on an auto focus feature that does not deliver a 100% focused image EVERY TIME, but hopefully we can come up with an answer soon!

  • In the 25+ years I’ve been teaching folks to use thermal imagers, I’ve almost always found it easiest to get them started with a manual focusing system. The trick is being patient and focusing on an area with sharp thermal contrast. Many of the new imaging systems operate at 9Hz and you must focus SLOWLY to get good results. As the tips says, work at getting good results! It makes a difference.

    Also, the example shown is one of the most challenging sorts of things to focus on because there is no sharp edge. When I’m faced with such a task, if possible I suggest putting a “sharp” object like a pen or a wooden pointer in the field of view and focus on that first.

    Thermally yours,

    John Snell
    ASNT NDT Thermal/Infrared Level III #48166
    The Snell Group
    800-636-9820
    802-229-9820
    http://www.thesnellgroup.com
    http://www.IRTalk.com
    http://www.IRWebinars.com

  • Fluke Thermography

    Awesome tip and great point about beginning with a manual focusing system. Thanks, John!

  • Anonymous

    It seems that Ti200/300/400 solved that problem… :p

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