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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?

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  • $\begingroup$ "Pumping" in sunlight through fiber optics is not an option? dosdesigngroup.com/fiber-optic-skylights $\endgroup$
    – Gillgamesh
    Oct 9, 2023 at 14:44
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    $\begingroup$ Why would you want one small but insanely powerful lamp instead of a multitude of lamps covering a large part of the ceiling? (And anyway, the average illuminance on any given patch of land on Earth is nowhere near 100,000 lux. 100,000 lux is absolute tops, to be encountered only in summer, on a very clear day, at noon. In summer, at reasonable latitudes and on clear days, the daytime average illuminance is more like 40,000 lux. On a cloudy day summer day, with light clouds, 20,000 lux. And daytime is only half the time, with nighttime illumination being not more like 1 lux. $\endgroup$
    – AlexP
    Oct 9, 2023 at 16:08
  • $\begingroup$ I am asking why not multiple lamps covering most of the ceiling because on Earth in daytime some 20% to 30% of the light comes from the blue sky, not directly from the sun. $\endgroup$
    – AlexP
    Oct 9, 2023 at 16:10
  • $\begingroup$ I also raised that issue here. Some proposals and mainly some cool answers can be viewed in this link worldbuilding.stackexchange.com/questions/103535/… $\endgroup$ Oct 9, 2023 at 17:43
  • $\begingroup$ Out of curiosity how is heat produced by lighting and city's activity brought out ? And how do people and goods get in or leave ? $\endgroup$
    – user35577
    Oct 9, 2023 at 22:31

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I would suggest a Sulfur lamp. This is a golf-ball with a small amount of sulphur in it, heated to a plasma by microwave radiation. It gives off an incandescent white with a very smooth spectrum. Each lamp is about 6 KW. You cannot make them smaller without losing efficiency. It is too bright to look at, and light pipes are often used to spread out the light over a larger area.

The sulfur lamp manages about 100 lumens per watt.

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As one commenter pointed out, Light-Tubes might be an option, and are essentially the closest thing to natural lighting below ground that we could achieve.

With the right techniques, i think it might be possible to light a city with natural sun-light by constructing tubes that are long and large enough. Coating most surfaces which are touched by the light in a layer of reflective paint could help to distribute it more evenly.

There's another type of lamp which could potentially illuminate large, open areas at once, the moonlight-tower. They were more commonly used in the late 19th century, fitted with (especially harsh and bright) electronic arc-lamps, making cities at night be almost as bright as they would be during the day. Though i'm sure with more modern technology they might be a bit easier on the eyes, yet equally bright.

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    $\begingroup$ Moonlight towers were ordinary lamps, just placed as high as possible. They were definitely not "built to amplify natural light given-off by the moon"; it is not possible to do that without consumption of energy. $\endgroup$
    – AlexP
    Oct 9, 2023 at 16:00
  • $\begingroup$ @AlexP I must've misremembered that then. Though I distinctly remember hearing this, i might be confusing/combining it with another type of lighting. $\endgroup$
    – NimRad
    Oct 9, 2023 at 16:24
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In Kim Stanley Robinson's novel "2312", habitats were made from hollowed-out asteroids through the center of which were run "sunlines" which were essentially long filaments with a blocking backing them so that as the asteroid spun around it, portions would be under light while others were in shade. If your cavern is not rotating, you could still run a line from end to end and have it turn off in sections that run the length, so that you could simulate dawn and dusk in areas not directly under the currently lit portion.

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Billions of LEDs

Two 10 lumen LED lamps together make 20 lumen. It is a gross oversimplification, but it is the tool I would use to light a cavern.

They are difficult to pack in tightly enough to make it feel like daylight with the same shadows, but that is difficult to do anyway. Packing enough high powered lamps to light up an area like the sun is also wildly more expensive and more difficult to maintain than a huge amount of LEDs spread over a large area. Many LED strips also allows more options in the distribution of power towards the LEDs

Add strip after strip of LEDs to the ceiling. Together they will light up the 'sky'. With enough together you'll get sunlight strength in no time, and they have other advantages. You can use coloured LEDs as well as their dimmable qualities to easily create different atmospheres. From dawn to noon and sunset, to stars at night or a party colour or even an important announcement if you make them addressable.

Cooling

Is it that easy? Of course not. Heating will be a huge problem with any implementation of light. Though LEDs are again the best option in light vs heat, they are still susceptible to heat damage and inefficiencies. To truly set it up correctly, you want cooling.

Cooling can be done in multiple ways. Though the rick surface might be a good enough cooling for a single strip, it will not suffice for strip after strip on the rocks, even with a little distance between them. You'll want to place them on some metal, with an active cooling solution behind it. You'll have to create a way yourself where you want to dump the heat. Homes for heating, an underground river, the open air at the surface or simply enough rock somewhere where the heat will be radiated away.

Summary

With billions of LEDs you'll have a huge distributed lamp. This offers many advantages in maintainability, heat distribution, longevity, power distribution, communication and day simulation. Some can break without it being noticed on the ground floor, where the noon light still floods the subterranean streets.

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  • $\begingroup$ As a baseline calculation: To have 40,000 lux on the surface you need 40,000 lux on the roof. So 40,000 lumens/m^2. Assuming an LED efficiency of 200 lumens/watt, you'd be generating 200W/m^2 of heat X 60km^2 = 12GW of heat / power required. $\endgroup$
    – IronEagle
    Oct 21, 2023 at 0:53
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I'd suggest to build a tokamak, but instead of gaining energy it could spread a light as an artificial sun. Didn't solve the lux variables, but I guess, this could be many of them. You only need 400 tonnes of copper (mainly) to build ~1MW unit. The fusion is of course kept by magnetic field and the temperature is about 200M Kelvins. I have no idea if it's safe (the light) - maybe on appriopriate distance it is.

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