I'm trying to think of a somewhat plausible way to create artificial "sunlight" in a very large underground city. To summarise some key points:
- The cavern containing this city is hemispheric in shape with a max ceiling height of about 2 kilometres and a diameter of about 5 kilometres. Roughly 60 square kilometres on the ground, give or take.
- There is no possible way for natural sunlight to enter the cavern either by opening a hole in the roof or somehow funnelling it in.
- We're talking about a not-too-distant version of Earth with all of our real-world constraints set 10 - 15 years in the future. Technology has progressed in this time, albeit a little slower than the exponential rate what we're experiencing today. Cars don't fly, we can't teleport, and we still haven't colonised Mars.
- Power requirements and/or money aren't a consideration. We could have multiple nuclear power plants dedicated to powering this one aspect of the city if need be.
- This universe contains no fantasy elements at all, so any magical hand-waving is out of the question.
My first thought was to look at some of the brightest man-made lights which led me to the Sky Beam on top of the Luxor Las Vegas, which is technically an arrangement of multiple Xenon lights. At a reported 42.3 billion candela (10,500 lux as measured from 2 kilometres away, if my calculations are correct) I could put half a dozen of those arranged on the roof with some giant lenses to spread the beams out over most of the city. Assuming the lamps are of the Xenon arc variety, this means they would also produce UV light. Not a hard requirement, but a nice little coincidence given our artificial "sunlight" goal.
10,500 lux is unfortunately only a fraction of the brightness of the sun on average here on Earth (between 32,000 and 100,000 lux), and any spreading of the light through lenses would make it even darker at that distance. While I could install 50 of the Luxor arrays across the entire cavern roof, I feel like there could be a more elegant and/or ingenious way to go about this.
LEDs seem to be the next big leap. Making more assumptions about the Sky Beam using arc lamps at a luminous efficiency of 50-55 lm/W, a theoretical limit for a white LED can reach 260-300 lm/W. That's 5-6 times more efficient (though remember we aren't worried about the power bill). I'm unsure if this translates to "LEDs are brighter" because I'm not sure how this is all measured. Power-in/light-out perhaps, but do they produce the same amount of light? Hopefully someone more intelligent might come along to explain the specifics, because I I'll admit I don't understand how it works.
- Do we have to put the lights on the roof? What about street lights/lamps everywhere?
A viable idea, but it doesn't fit the aesthetic that I'm going for. Many small lights each lighting a highly localised area has a very different kind of feeling to inherent ambient light.
Worth mentioning is that I also came across Artificial Skies in my travels, and while it's a nice idea, we're talking about a size that's orders of magnitude larger than current real world applications (that would also run into the same lighting requirements on our theoretical scale). Also, these people live in a cave and they know/like it, this isn't The Truman Show.
While the ultimate answer to the question may lie somewhere in the above information, I'm trying to think of a solution to the problem that's extra creative or unexpected. Again, given our real-world physics, how would you light the cavern?