[teel]

While this seems to be a neat thing, I am just wondering how common nowadays are door locks that actually have a hole through them? Where I come from, those locks could be seen in doors of old cabinets and other furniture. Or perhaps at the summer cottage, but even there, it is usually just an old shed or something; any doors that are supposed to keep anything valuable safe, have solid lock housing, i.e. no through keyholes.

[_Microft]

"Keyhole Imaging" is only a catchy name for the technique. It works with any other opening, like a door crack for example, as long as you can shine a laser through it. I would expect that it is even possible to use an opening that is covered by a light diffusing material like paper or tape on the inside. Instead of measuring reflections from the far wall, light diffused by this material would be used instead.

[TeMPOraL]

Most of the locks I've seen in my area still have a path for light to bleed through, but it's very, very tiny. Most likely not enough for this application. But peepholes are still a thing, not everyone has a cover on the apartment side, and the effects of the fisheye lens in the middle could be compensated for with a bit of extra math...

[djmips]

You probably don't need even need math, another lens might do.

[silisili]

Same thought. The only house I had with an actual keyhole was built in 1948. Every other house I've lived in has had no such thing.