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Q & A: Focusing LED light

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Most recent answer: 10/22/2007
Q:
Hi, We have science project for physics in which we are trying to focus light from a Radio Shack Jumbo Super Bright Red LED down to the smallest point possible. We read your Q/A on LED Optics but thought the problem we have is different we don’t care about making the light travel further, we just want to make it focus down to a very small spot. We thought about the approach people use when burning a leaf with sunlight and a magnifying glass which focuses light to a small spot, but can’t seem to do the same thing with our setup. The LED is rated at 5000mcd and we put the LED in a spare flash light reflector in order to gather as much light as possible. We tried using a 50mm x 50mm Double Convex Lens to try and focus the light and can not get it to focus down to a tiny spot at all. What kind of optics system would we use to focus this light down to the smallest brightest spot possible. The beam does not need to travel far for the intended spot. The shorter the better since we are creating a mini projection display with the setup. We were trying to avoid using a pinhole because we want the brightest spot possible. It seems easy to be able to do but our optics knowledge seems to be missing something. Do we need different lens types or a combination of lense to do this. Any guidance you can give us would be greatly appreciated. Thanks
- Ken Lancaster (age 16)
Woodstock Georgia, USA
A:
Hi Ken,

Your approach sounds perfectly reasonable -- use a converging (convex) lens to focus light down to a spot. But of course you have stumbled on the primary limitation already of making small spots of light, and that is the size of the source (or lack of control over the optics that comes with the source in this case).

There's a limit to how small a bright spot of light you can make by focusing sunlight with a magnifying glass, and this limit comes from the angular size of the sun in the sky -- light from the sun comes to you from lots of different places and in lots of different directions. An ideal lens can focus parallel light rays down to a point, but if the light rays aren't parallel, then you won't get a point; you'll get an image of the source.

The idea to get as small a spot of light as possible is to make the light impinging on your lens as parallel as possible. You can do this by making the source as small as possible and placing it far away from your lens. (note: jewlery stores use very small, very bright lights place on the cieling in order to make diamond rings sparkle more -- the image of the light source is much more well defined and light reflections come and go if the source and your eye are lined up just right. A diamond ring just doesn't sparkle as much outside on a rainy day).

You could cover up the LED with a piece of paper with a pinhole in it. Then you can focus the remaining light down to a small spot, I'll bet. But this isn't a recipe for a bright spot, just a small one. Bright LED's from Radio Shack and other places come with little plastic lenses molded into their packaging, and some bright ones I have on my bicycle rear-projecting flashing light have a red plastic diffuser all over everything. The whole purpose of these is to spread the light out as far as it will go in all directions. You may want to try removing these obstructing, poorly-controlled, and hard to correct for, optics. The actual light-emitting part of the LED is probably pretty small and bright to begin with.

Tom

Unfortunately, as Tom says each light source has its own intrinsic brightness, which depends both on the intensity of the light at the brightest spot and how big an angle it's direction is spread out over. The best your image can do is maintain that brightness. If you want to get something really bright, use a laser, even a small cheap one. Its light is spreading out over a narrow range of angles, so that it can be focussed down to a tiny spot. You'll want a good lens with a small ratio of focal length to diameter, and then you'll want the beam spread out over most of the diameter of the lens, which you can do with another good lens.

Mike W.

(published on 10/22/2007)

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