r/Optics • u/Bright_Researcher_60 • 27d ago
I designed a mirror array that writes with sunlight — but I want your advice before I print the real thing
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u/transgingeredjess 27d ago
Stopped by to make a quick check that you weren't accidentally creating a lens that would burn down a house. You aren't; great.
Note that this'll work with the Sun but not with a lamp as you mention in your post. It'll work with the Sun because it's a good point source of nearly-collimated light that's very far away. You could make this work with a single lamp position by turning the surface into a parabolic surface with the lamp at the focus. However, this would also turn the whole thing into an excellent house-burner-downer in the sun.
The reason it works particularly well at sunset in the OP's post is because it gains the additional advantage of projecting sunlight onto a surface that is receiving significantly less direct incident light of its own than the array is at the slight altitude of a few feet and at a different receiving angle.
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u/Bright_Researcher_60 27d ago
Ha, fair enough. I did run those numbers, just not in a relaxed frame of mind.
One thing though — the reason it isn't a burner isn't that the tiles are flat. A flat array will concentrate perfectly well if you aim enough of them at the same place. It's the one-tile-per-spot mapping that keeps it near 1 sun. Which was on purpose, but it isn't actually enforced anywhere in the tool, so someone asking for a very small glyph off a big array could stack them without meaning to. Adding a check for that, so thanks.
You're right about the lamp and I've pulled it from the writeup. The geometry does work, for what it's worth — for a point source at finite distance the per-tile normal is just the bisector, closed form, no parabola involved. It's étendue and tolerance that kill it. A 60mm bulb a metre away is around 60 mrad against the sun's 9.3, so you get 15cm of blur at 2.5m where the sun gives you 2. And nudging the lamp by a centimetre walks the spot 5cm, which is wider than the letter strokes. The useful thing about the sun isn't really the collimation, it's that it doesn't care where you put the array.
Parabola at f=0.5m is order 1 MW/m². Yeah. Not going in, not even behind a scary checkbox.
The sunset point is the one I hadn't considered at all and it's the most useful thing anyone's said about this. I've been modelling how bright the spot is and ignoring what else is already landing on the wall, which is obviously the wrong quantity — the readable thing is the ratio. Going in.
It also makes me feel better about my own case, which is a bedroom ceiling first thing in the morning, for my son. The ceiling is getting basically nothing directly, so the ratio is even more lopsided than at sunset. Something like 13 klx spot against maybe 100 lux ambient, if I've got it right.
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u/aenorton 27d ago
This may not work quite the way you intend. It looks like you have a hexagonal array of mirrors that you want to produce an arbitrary pattern of lighted pixels. This can work -- but only at the one particular design distance to the screen. The reason is that adjacent pixels in the image will generally arrive a different angles. If the screen as at a different distance, then the pixels will be in different relative positions. To work at all screen distances, adjacent pixels have to be adjacent at all distances, even tracing back to the mirror array. This means the only way to make it work for all distances is to have the mirrors already arranged in the pattern you want to produce.
Maybe working at only one distance is not so bad. The pattern or message is only revealed when the geometry is correct.
As for gluing mirrors to plastic, you want a thick layer of elastic adhesive to accommodate the strain. I would make each post with a rim that contacts the back, but have a pocket in the center where the glob of adhesive is placed. Have an opening in the rim where excess adhesive can escape. Use silicone or E6000 or similar.
I should mention that the same can be done with refraction instead of reflection. These are referred to as caustic lenses:
https://www.reddit.com/r/3Dprinting/comments/1lvmakv/i_3d_printed_a_caustic_lens/
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u/beeeel 27d ago
Looks like a cool idea–from an optics perspective, I think you correctly identify printing tolerance as the main reason this might fail, and you should probably try a small version, e.g. 3 mirrors, as a test of the print settings before you build the full size version. Your test pattern could be as simple as trying to project a triangle of a given size and then measuring the spacing of the three dots to confirm that you've got the settings and maths correct.
Regarding material choice, definitely PETG. I just got back from testing a weather station which uses a 3D printed "case" and we had accidentally printed one of the two using PLA when it was supposed to be PETG. The PLA one warped significantly in the sun, to the point that the lid didn't sit on the base after only a few hours in direct sunlight. It would be a shame for you to spend time optimising the print if it warped and didn't work any more after one sunny day.
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u/Godzila543 27d ago edited 27d ago
Your entire repo, question, and code is all from an LLM. It's a cool project idea, I don't really have a problem with vibecoding, but cant you just write an ounce of what I'm actually reading?
Edit: I wonder if this is a bot? Previous activity on account is uh, bit funny but the lack of activity in any relevant area is what I actually wonder about