“What’s hot?” is one of the most crucial pieces of information for debugging electronics. Poking around with a finger works fine when the parts are at least a few fractions of an inch in size, and spaced reasonably far apart. Despite inevitable burns, this is an effective technique.
But things got small! goTenna uses incredibly tiny surface-mount parts, some as small as one-hundredth of an inch. Attempts to feel for the hot ones are confused by feeling dozens of parts simultaneously. Non-contact thermometers are no good, as the focus area is several times larger than the parts we’re looking for. Probes are also too big and too slow – our only option is to use thermal cameras.
Look at that! We can see what’s happening on the board now. The trouble parts are glowing like hotel carpet under ultraviolet light; we now have total visibility into a bad situation.
But I find myself somewhat dissatisfied. Even this amazing power might not be enough. What if, instead of whole parts, we want to look at which pin on a given part is heating up? Or just something crazy small? We have a problem. Most low-end thermal cameras are fixed-focus, and the closest you can hope to get is about three feet away. Unless you want to spend a ton of money for a variable-focus model, you’re out of luck. But there’s a cheap way around this limitation: lenses! (Not normal glass ones, those are completely opaque to infrared.)
Special lenses! Ebay has them for about $20. Just look for “laser cutter lens” on the internet and you’ll find them. They’re made of Zinc Selenide, and are a deep yellow color. I use a two-inch focal length, but that’s just because it’s what I have on hand most of the time.
Mount one to a piece of cardboard and stick that on the front of your thermal camera and…
You now have a microscope that sees heat. Welcome to the future!
I would love to have my thermal camera macro mode without modification, and this article is me asking the industry to do that. How about it, industry? Can we have a nice, cheap thermal camera with close focus? There’s a real need for it, especially in the booming Maker community.
There’s just one weird wrinkle to this story: Zinc Selenide, the material that these lenses are made of, is toxic. Not drop-dead-on-the-spot toxic, but you know, wear gloves, don’t lick it, the usual precautions apply. OK! Have fun!