Focal length of lens needed to focus a beam

I have been searching for a good formula but it seams above my head how to get a filled beam and not an image of the emitter. I am trying to build a light that will have a tight well filled 6-8 in beam 20in from the light. What focal length plano-convex lens do I need to buy?

Info:

Brynite PD30A AAA Q5-WC
11mm diameter front lens
6mm deep reflector
plano-convex lens will be placed 7mm from emitter
currently at 20inches hotspot is 6in and total beam width is 24 in
any other info needed?

Please help me, I am lost and frustrated. Any help or direction would be incredibly appreciated.

Thanks!

I'm not up on optics (for flashlights) really well but let me just see if I know what your want exactly.

You want to get a unifrom (no emitter image or differences in light intesity over the area) square beam 6-8 inches across, at a distance 20 inches form the light? And in theory you don't want any spill outside of the beam. (also, why the simple sphereical lense? and is there a specific application for this light that and emitter image screws up?)

-Well I think I can help you with the spill, try it without a reflector. If I am not mistaken, a reflector behind an optic should casue increased spill.
-On the front of the 6-8 at 20 beam profile, I'm not sure but I think you'll have to place the optic closer to the LED than the focal lenth of the optic. It's hard to calculate what focal lenth lense to use for an unfocused image.
-On getting a uniform square of light, I don't think you can do that. Did you think you saw someone else get that from and LED? If so can you link it?

Thanks for the reply!

I meant to imply a 6 to 8 inch round beam not 6 by 8 inch rectangular beam. I might end up using a very tight TIR lens with some sort of shroud on the outer perimeter in front of the lens to eliminate any spill outside the hotspot.

If anyone knows a place to find a 10mm tight TIR for XR-E I would really appreciate a link!

This might be helpful:
http://www.candlepowerforums.com/vb...ula-for-calculating-throw-using-aspheric-lens

Post 22 is about how you can construct the beamangle from a given setup. But just testing it works best, easiest and quickest.
And better not combine reflector and lens in one build as it gives a lot of straylight messing up the beam.

hazlewob said:

6-8 in beam 20in from the light
11mm diameter front lens

Click to expand...

11 mm is 0.433". With a beam/field angle of zero this would be a 0.4" dia. spot at any distance.
Using the centerline and substracting out the 0.2" gives you ({7/2}-0.2)" in 20", an angle of arcsin(3.3/20) = 9.5 degrees so the beam/field angle is 2x9.5 = 19 degrees, call it 20.
This shouldn't be difficult but I'll have to find my physics book.

DrJones did some quick math for me at dealextreme and came up with a 10mm plano convex with a 20mm fl 12.5mm from the emitter should give me about a 6 in beam at 20 inches. I ordered the lens from surplusshed and will test it out as soon as it arrives.

Still, the idea of a TIR that would fit this light is appealing, if anyone knows where to get a 10mm to 11mm tight beam TIR for a XRE, please let me know.