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!
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
- 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!