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Q & A: LED Optics

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Most recent answer: 10/22/2007
Q:
I would like to use white LED to replace bulb, but LED is not as bright as light and the Led light con not travel far. Q1. Can I place convex lenses in frant of LED to make the light travel further? Q2. How curve is the convex lenses I have to design ?
- vincent (age 35)
kluang, Johor, Malaysia
A:
Vincent -

The light from an LED will travel just as far as the light from any other source. The difference is that there's just not very much of it. The 'intensity' of the light from an LED is very low - it doesn't emit very many photons per second. Short of using a whole LOT of LED's, there is really nothing you can do to make an LED's light brighter.

The reason that the LED appears to be dimmer from further distances is that the light spreads out in all directions. You could use a convex lens to focus some of the light together so that you could still see the LED from further away, but you will not actually have any more light than you did before. It will just be directed more in a beam in one direction. Basically, you are talking about looking at the LED through a magnifying glass. The curvature of the lens detrmines how far away from the LED it should be placed. The distance from the LED to the lens should be equal to the lens's focal length in order to make the light from the LED come out in a nearly parallel beam. How much light gets gathered into the beam depends of how big the lens diameter is and how far away it id from the LED. Obviously you want a big ratio of lens diameter to focal length to get as much light as possible. That's the same a saying you want what's called a small f-number lens.
Since small f-number lenses aren't cheap, you might consider using a reflector instead. For example, the metallic reflector in a standard flashlight is designed to do just what you want. You could put the LED near where the bulb goes, and adjust the position slightly to get the best beam.

-Tamara (w Mike W.)

(published on 10/22/2007)

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