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a friend has asked me to help them with a LED billboard type of art piece. It's envisioned to basically be ping pong balls cut in half with LEDs mounted under them. She got some jumbo (10mm) LEDs for their brightness specs, and she's happy with the brightness, but not so happy with the specularity (creates a hot spot). I explained to her, somewhat speculatively, that low specularity (wide viewing angle) and high luminous intensity are competing goals,

So on to the question. What are the parameters / characteristics to focus in on for picking an LED that will brightly and evenly illuminate the inside of a ping pong ball? Are there tried and true ways to take a really bright LED and diffuse its light onto a semi-spherical surface? Am I better off getting a wide viewing angle LED at a lower luminous intensity, or a high luminous intensity and using some kind of external diffusion strategy? Any help / advice would be greatly appreciated!

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  • \$\begingroup\$ You could use mirrors like here? (Sounds like a nice project btw!) \$\endgroup\$ – Keelan Feb 3 '13 at 15:02
  • \$\begingroup\$ @Camil: I don't see how the mirror on the top of that lightbulb is related to the question. It seems to be obscuring exactly what the OP wants to illuminate. \$\endgroup\$ – Olin Lathrop Feb 3 '13 at 16:47
  • \$\begingroup\$ When you put a mirror over the LED, the ground beneath the ping pong ball will be lighted instead of the ping pong ball itself, thereby creating a more diffuse glow. \$\endgroup\$ – Keelan Feb 3 '13 at 17:54
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Diffusers are very inefficient ways to spread light at a wide angle. Proper optics can do this with much less loss.

However, if you really want to use the ping pong ball diffusers, one or more surface mount LEDs without lenses will probably be the best. LEDs in 0603 package tend to be the cheapest, so you can compensate for lower overall emission (and the loss introduced by the diffuser) by using multiple of them. A cluster of three or four 0603 LEDs under the ping pong ball should provide more even light, at a wider angle, and still cost less than the archaic thru hole LED you reference. They will cost even less when you consider they can be machine placed.

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    \$\begingroup\$ +1, but I'm not sure that machine-placement will give cost savings here. The labor rate for artists is even lower than for South-east-asian sweatshop workers. \$\endgroup\$ – The Photon Feb 3 '13 at 16:31
  • \$\begingroup\$ @TheP: Even if the labor is free, soldering surface mount LEDs by hand is still faster and easier than soldering thru hole LEDs by hand, so it's win either way. \$\endgroup\$ – Olin Lathrop Feb 3 '13 at 16:46
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    \$\begingroup\$ I'd agree, for work in a properly equipped lab. But in some cases (working in your garage without a proper microscope and only a RS soldering iron) it might actually be easier to solder one through hole LED than multiple 0603 SMT's. \$\endgroup\$ – The Photon Feb 3 '13 at 17:07
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I agree with Olin that the solution is to place multiple LEDs under each ping pong ball hemisphere. A circuit board could be made to hold the LEDs in position and then also provide a reasonable wire attachment point to connect up the power. Current limit resistors can also be placed on this same circuit board in the case that a fixed voltage drive to the assemblies. Placing the resistors on the board also facilitates building the boards in different versions for red, blue, yellow, green and white LEDs where different sized resistors may be required to accommodate the different forward voltage drops of the various colors.

Noting that ping pong balls are 40mm in diameter you have a lot of flexibility of where to place the LEDs to get an even lighting pattern. Low power LEDs are incredibly low cost and you can fit quite a few under a 40mm diameter hemisphere. Although SMT LEDs are probably a better choice, as Olin has pointed out, I see no reason to also experiment with multiple through hole type components. There is also nothing stopping you from using even larger SMT LEDs as well, such as 0805, which would be easier to hand solder.

Last comment about the circuit board. If the ping pong balls are used in arrays side by side one could make the circuit board into a module that handled a small array of the ping pong balls. Some example layouts could be 2x2, 3x3 or 1x4. Doing this would help to amortize the total cost of circuit boards used for the project. It would also lower the number of wiring harness connections to hook up a whole display.

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