I purchased for cheap (I think it was around $6 from Amazon) an ultra-violet (UV) flashlight. It is great fun to take it out into the woods at night to see what fluoresces. We seem to have a species of millipede that glows quite brightly. Even dead sections of the skeleton glow brightly. But I digress …
The standard wood glue I, and many other luthiers use, is Titebond. This glue has the interesting property that is glows yellow under UV light. What I have found is than when one is doing something like sanding bindings flush with the sides, examining the seam area under UV light will reveal even the smallest, thinnest spot of glue still left on the wood, stuff you can not see with the naked eye. If this glue is left there it will effect the way the finish colors the wood, and you will get a light colored spot, have to sand it out, and re-do the finish (a time consuming pain). A little UV light makes things easy.
As an added ‘bonus’ one discovers how different wood reacts to UV. Black Locust, which I have used, glows bright lime green. You can see every bit of black locust sawdust on the shop floor because it glows like little stars. Working on the latest uke, I discovered that the spalting lines left by the fungus that the ambrosia beetle import to create “ambrosia maple” also glows lime green. Not all the wood, just the edge lines.
