Led convex lens optics

CfabStudios · Jul 15, 2013

Yesterday I received my first Cree xml I have mounted it on a heatsink and used a 1" copper coupling and a 28mm glass convex lens to explore its optic capabilities. I have observed the the xml is very difficult to focus to create a small narrow sharp projection of the dye. Basically I manuall moved the lens back and forth and discovered that at 20.5mm to 24mm its optimally focused. Now I used a smaller lens thats 23mm of a Cree Q5 adjustable Led flashlight and it seems to focus it a lot sharper within a shorter focal length. Why is a smaller convex lens focusing the beam better than a larger convex lens on the Xml?
Thanks

bshanahan14rulz · Jul 16, 2013

The diameter of the lens is only part of the equation. The focal length is the other part. You found that ~24mm the lens is optimally focused. You can use this as an estimation of that lens' focal length. If you were to draw a cone using the diameter of your lens and a point 24mm away from that circle, that would be the cone of light accepted by the lens, basically.

You will notice that if you use a smaller circle with the same focal length, the cone angle gets smaller. Likewise, if you use the same sized circle, but with a closer focal length, the cone angle gets bigger. The bigger that cone angle is, the more light the lens can collect from your LED. This part is geometry.

With this, you will now probably notice that the smaller lens' angle of acceptance is a bit larger than the larger lens, even though the larger lens has a larger diameter.

One last thing to note: there are plano-convex lenses which are usually a sphere section, and then there are plano-aspherical, which are made using a curve that isn't a sphere section, but is optimized to project images with less spherical aberrations.

CfabStudios · Jul 16, 2013

wow thanks that makes perfect sense, The lens was described as a 28mm convex but its good to know those other versions exist. I have also looked into the aspherical lens by anhorton and most people have gotten nice results. I will post pics later this week. Thanks for the help man.

bshanahan14rulz said:
The diameter of the lens is only part of the equation. The focal length is the other part. You found that ~24mm the lens is optimally focused. You can use this as an estimation of that lens' focal length. If you were to draw a cone using the diameter of your lens and a point 24mm away from that circle, that would be the cone of light accepted by the lens, basically.

You will notice that if you use a smaller circle with the same focal length, the cone angle gets smaller. Likewise, if you use the same sized circle, but with a closer focal length, the cone angle gets bigger. The bigger that cone angle is, the more light the lens can collect from your LED. This part is geometry.

With this, you will now probably notice that the smaller lens' angle of acceptance is a bit larger than the larger lens, even though the larger lens has a larger diameter.

One last thing to note: there are plano-convex lenses which are usually a sphere section, and then there are plano-aspherical, which are made using a curve that isn't a sphere section, but is optimized to project images with less spherical aberrations.

cekic · Jun 11, 2014

Hi, this is because the focussing is directly proportional to the radius of the convex lens and therefore the smaller is the radius of the lens more is focussing due to more divergence of light passing through the lens.

smt pcb

Led convex lens optics

CfabStudios

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bshanahan14rulz

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cekic

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