Well, one possibility is using something known as Fabry-Perot filter. It allows an extremely narrow frequency to pass, due to multiple reflections and interferences inside the material. Put the light source material within this filter, and you get a laser. That’s essentially the main difference between a led and a semiconductor laser. The filter makes only a narrow band of the emission be “stuck” there, creating a feedback effect that eventually tends to infinity, and a good chunk of that power passes through the filter reflectors, which are intentionally not perfect.

Other than that, I don’t think there is a filter that could be as narrow as the line emission from vapor lamps. Maybe using metamaterials, but a laser would be so much cheaper and easier. A vapor laser would certainly get the job done, but they are large and hard to maintain.

A laser, maybe, but definitely not LEDs. Vapor/gas lamps produce the narrowest frequency bands possible, because it comes from very well defined atomic transitions (Hz range). LEDs produce frequency bands with widths in the GHz/THz range, while semiconductor lasers can maybe reach KHz if they are really good. So, unfortunately, for this type of applications, vapor lamps would probably still be needed.

Source: I work with lasers and spectroscopy.

Edit: very good idea about using non-visible light!

My story is kinda cool. TLDR: she stole my cat.

We lived in the same street, but other than the occasional nod when passing by each other, never really talked. I always thought she was cute, though. Oh, and she was married to a typical abusive guy.

One day my cat escaped, and after a week of searching, I almost gave up. Then one day she knocked on my door and asked if that cat was mine, because she thought she was a stray, and took her in. She gave her back, apologized, and that was it. But that cat would always find a way to go back to her place, and we kind of agreed to let her have 2 homes. Still didn’t talk too much or anything.

Years passed, and she finally had enough of her husband, got a divorce, and moved out. Came to ask if she could take the cat with her, and I said that’s ok, she barely came to my place anymore anyway.

More time passed, we kept a little in touch, but still didn’t really talk to each other. And then I decided to have a barbecue at home for my birthday… I invited mostly people from work, but then thought: “it would be cool to invite her”. And so I did. We talked, and talked, and when we realized, it was already 10 pm (barbecue was supposed to be from noon to sunset). She went home, the next day I invited her out, and long story short, we’ve been together for almost 5 years, living together for 3 and a half years, have 6 cats, and our first son is about to be born.