From: Stef Mientki > Dave Tweed wrote: > > The 18" Eyeball spins at 30 revs/sec and can put about 500 pixels around > > the circumference, giving a pixel rate of 15000/sec. or 67 us/pixel. A > > turn-off time on the order of 10 us would not be too slow. But it uses RGB > > LEDs (no phosphors), so it wasn't even an issue for us. > > > > I don't recall how this thread started, but if the application is for a > > planetarium (projected on the inside of a dome), > > That's exactly the idea, > although we're not sure yet to use projection, because we'll loose a lot > of brightness. > > > you'll probably want a > > *lot* more pixel density than that -- at least 10x linear density, if not > > 100x, which begins to approach the spatial resolution of the human eye > > (about 200 pixels/degree, or 6e-9 sr/pixel). This means somewhere between > > 10e6 and 1e9 total pixels in a half-sphere. > > If you look at a star-map on a computerscreen, the stars vary (depending > on the brightness of the star) from 0.02 .. 0.5 degrees (I mean > star-degress, but don't know if that's a good english word). > Looking at a starmap on paper, the size varies from 1 .. 2 degrees. I was thinking about this a bit more after I sent my previous message. My laptop has a pixel density of about 75/inch, and I sit about 18" away, giving an optical density of about 0.042 degree/pixel, or 24 pixels/degree. > So I think we get a quiet good image if we set our goal at 1 degree for > the brightest stars. In that case we need about 100 white LEDs over a 90 > degrees arc. To also display moon and planets, we need another 100 > yellow or orange LEDs. But it isn't just a question of how big the stars are, but also how accurately you can position them in the "sky". And a planetarium show isn't just a big star map; the goal is to reconstruct the actual appearance of the night sky. Excessive pixellation will destroy that illusion very quickly. > If we're aiming at a sphere with a diameter of 40 cm and use 3 mm LEDs, > we can't get 100 LEDs on a quarter, so we need in totally 4 rows of each > 50 LEDs. Huh? You should be able to place 419 x 3mm LEDs around the circumference of a 40 cm circle. > Now fortunatly stars twinkle, so we can choose a lower rotation speed, > than in normal propellor clocks, maybe even as low as 5 rev/sec. But stars twinkle randomly, not periodically. I've seen the Eyeball running at 15 to 20 revs/sec., and it gets really hard to maintain the illusion of any sort of continuous image at that flicker rate. -- Dave Tweed -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist