[xattt]

A shoe-fitting fluoroscope! I’m only half-joking, but one using terahertz radiation would be very handy to have while shoe-shopping with children.

Kids can’t reliably tell you the fit of their shoes, and some footwear doesn’t yield to the “thumb-at-toe” test.

(1) https://spectrum.ieee.org/amp/when-xrays-were-all-the-rage-a...

[mordechai9000]

I think you could use visible light and photogrammetry for the same purpose. You might need to take your socks off, though. Or put on a pair of the disposable thin "try on" socks the stores have for people who don't wear socks of their own.