White LED lights are everywhere, now—but how do they work? Without a doubt, you could get a red, green, and blue LED and blend them together to make white light. However, that is not how most of the work. Instead, a white LED is a violet or ultraviolet LED with a fluorescence material. The LED creates a high-frequency light (violet or UV) that makes the material fluoresce produce other colors (lower frequency).
Since this fluorescent material is not 100 percent effective, some of the UV light could pass through and be mixed in with the white light. If you want to take a white LED and create a handheld UV light, you need to block the visible colors while leaving the UV light to pass through. Some materials such as fused quartz or fluorite do precisely that and can be used to make some UV photographs.
Could there be other materials that can do the job? In the MacGyver episode mentioned above, he uses the floppy disk part from inside a 3.5-inch disk. Different floppy disks use various materials, and this round disk could possibly allow UV to pass while blocking visible light.
Start with a smartphone that has an LED light, mainly the one used for the camera flash. You only require two things from this light. First, it should be a UV LED with fluorescent material, and secondly, it should not be 100 percent effective.
Next, get some material that blocks visible light but not UV. That may require some trial and error.
Turn off the lights. If you do not, the UV light will still fluoresce some stuff that you want to look at. However, you will not be able to tell since there will be all this other visible light reflecting off stuff.
Clearly, this may not work with any old light or material, but it is at least plausible.