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Old Dec 1st 2014, 03:12 PM   #1
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Question How to focus multiple light sources as one?

Hello everyone,

I have a simple diy projector project. I got crashed at optical side of the project. First of all, (unfortunately) i decided to use a power LED matrix as light source, because of efficiency, life time etc. Photo of the matrix is given below.




Each LED is 1W power LED and produces ~100 lumens of light. A lens covered each LED, and thus, LED lights are propagating with 120 degrees angle.

I need to parallelise, or at least focus that beams as one single light source. The outgoing light will be go across an lcd panel, so power of light must be the same at each point of lcd panel.

I bought acrylic fresnel lenses for this, but not get a good result. The fresnels' data is below.

size 163*107mm focal length 120mm, 2pc
size 150*90mm focal length 185mm, 2pc

Thickness: 2mm
Material: PMMA (acrylic)
Groove pitch: 0.5mm




So, is there any way to do this with theese lenses?

Thanks in advance...
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Old Dec 2nd 2014, 05:26 AM   #2
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I would have thought that you could probably treat each LED as being (almost) a point source (at the centre of the LED lens).
If you place this point source at the focal point of the Fresnel lens then the result should be reasonably parallel.
Note that you will require a separate Fresnel lens for each LED.
Also note that the Fresnel lenses do not need to be round, so you should be able to tessellate them in a reasonably sensible fashion
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Old Dec 2nd 2014, 07:14 AM   #3
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MBW has the right idea, but it appears that would be impossible to use lenses on the order of 150 mm in diameter and placed 120 - 185 mm from the LEDs and still cover a significant portion of the 120 degree projection of the LEDs. I think you need to find lenses that have a diameter less than or equal to the pitch of the LEDs (about 8mm) and a focal length of not more than about 4 mm. I don't know whether such lenses exist.
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Old Dec 2nd 2014, 10:11 AM   #4
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It may be possible to alter the LEDs.
The plastic dome covering the actual diode is partly for protection of the delicate electronics, and partly to spread the light.
With care it is possible to re-shape the profile of this plastic cover.

However, you could easily end up with a pile of ruined LEDs...
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