I hope I don’t put too many of you off with this, but I feel I must (again) revisit a couple of basic issues. This week I want to talk about focusing images and next week about freezing images. I know for some readers both these tasks are easy and flawless. No doubt, many of you do it well most of the time. There are, however, still way too many folks—both new and experienced—using imagers who are just not getting the full benefit because the saved images are not quite in sharp focus. It’s sort of like having a really nice sports car but you can’t drive it fast because you don’t understand how to shift. So let’s take a short ride down IR Basics Avenue!
Still not convinced? Look at the image above (Image #1). Can you say with certainty what it is? If you guessed it was an electrical connection of some sort, you’d be correct, but what kind?! Even if you knew, what you may not realize is that an out-of-focus image means temperatures will not be accurate. You can see this with your imager by measuring the temperature of a high-emissivity surface and then watch the change as you purposefully de-focus. Still need a reason to be in sharp focus? Image #1 was taken by a well-known, supposedly highly-skilled contractor who was paid lots of money by the customer. Do you think they’ll ever be hired again once the customer sees the difference between this shoddy work and good work?
I am clear that in many cases focusing simply is not easy. It takes work to get focused sharply. I often run into situations where I need a trick to help me get there and I’ll talk about those below. And modern images have a very shallow depth of field so a difference in distance of even a few inches an mean you are out of focus rather than in focus.
But the key to learning how to focus is to set up a test situation so you can see what sharp focus really is. Then practice, practice, practice and, after that, move on to some more challenging real-world situations. Once you have mastered the basic techniques of good focus—and have gained enough experience to have confidence in your abilities—you’ll be fine 98% of the time.
What Should You Look For?
Whether you are focusing a thermal imager, your eyes, or a digital camera, you need to look for a sharp edge with high contrast. It is nearly impossible to focus on something diffuse with little or no contrast! Look for these areas of sharp thermal contrast in buildings in such places as curtains against a window, trim around a door, or a wall/ceiling corner. On the plant floor look for a warm conductor against a cooler panel, overhead lines against the cold sky, or a reflective fuse cap next to the warm body of the fuse. The keys are objects with a sharp edge and thermal contrast.
You can practice right in your office or your kitchen by putting a hot cup of coffee on a surface for a minute or two until it warms it up. Now move the cup aside and try to focus on the hot spot it leaves. I suspect you’ll find it challenging because it is a rather diffuse thermal signature. Drop a couple shiny coins on the hit spot and, voila, you have both a sharp edge on the coin and thermal contrast! Practice focusing from different distances and angles while you drink your cup of coffee.
I often use a similar trick in the field when I come across a scene that has little contrast or is otherwise hard to bring into focus. Just warm up the tip of a pen or pencil and hold it near or on the surface you’d like to focus on. Focus on the sharp, contrasting edge of the pen and, again, the surface of interest should also be in focus.