Anyone use a concave lens as a beam spreader?

I've experimented with some homemade diffusers such as 'magic tape' (translucent sticky tape) but the common problem is that they absorb more light than they spread out. Has anyone tried a concave lens? If so, did it work, and is there a source for cheap plastic negative lenses?

The following are replies lost when CPF crashed:

Re:Anyoneuseaconcavelensas abeamspreader?
Written by gallonoffuel on 01-21-2011 10:35 AM GMT

You canuse almost any lens
with a curved surface. Even PCX or aspheric lenses in the wrong focal
length (of which there are many) will be out of focus and will spread
the hotspot. You should be able to find some cheap glass lenses at
surplus shed or anchor optics.

Re:Anyoneuseaconcavelensas abeamspreader?
Written by Flying Turtle on 01-21-2011 02:49 PM GMT

I've not had much luck with lenses for diffusers. It's hard to beat the tape for simple and reversible.

Geoff

Re:Anyoneuseaconcavelensas abeamspreader?
Written by LeifUK on 01-21-2011 03:16 PM GMT

Flying Turtle said:

I've not had much luck with lenses for diffusers. It's hard to beat the tape for simple and reversible.

Geoff

Click to expand...

What was not good about lenses? I tried a teleconverter (the only negative What was not good about lenses? I tried a teleconverter (the only negative lens I could find) in front of one torch as a crude test, and it certainly spread the beam. I've seen glass lenses on ebay, but I'd rather an acrylic one for many reasons.

To update this thread, thanks to a CPF member called PhaserBurn (David), I have discovered that a self adhesive film called d-c-fix does a good job of lightly diffusing the beam.

What beam angle are you looking for ? 90, 60, 50 etc ? Most leds natively have a very wide angle like 100 degree or more. I design video lights using leds and perhaps a fresnel made of acrylic would do instead of a lens. What is the maximum diameter you will need, as Edmund Scientific and others have fresnels in many sizes. The beauty of fresnels is that if you vary the distance from the led you will get a really nice variable size circle. If you need a diffuse film, I would get a piece of plexi or lexan which is easy to cut and also cheap, and use the sandpaper trick. The more you sand it down, the more diffuse the light and the more loss (output ) you get. Try a 600 grit under the water tap and check the opacity often untill you get just what you are looking for. Another trick I tried with this method is that I sanded the center portion more than the edges, this way the light fall-off at a flat surface was even without any hot center.Also, plexi/lexan are very durable and keep their shape unlike tape and also act as a led protector.If you really got to have a lens, check out Deal Extreme (DX) as they have many lenses, including aspherics that may be suitable for your application. It ll depends on the beam angle you are looking for.

Yes I agree that a fresnel lens might be good, and there are microlens sheets too. The problem is obtaining some, and then cutting it, and fixing it in place. The lens did a quite good job of broadening the beam, maybe by a factor of two in each linear direction, but it fell off without my realising, so it is gone. The d-c-fix film is a nice thin plastic film which absorbs very little light, whilst creating a nice broadening of the beam, and softening of the hot spot spill transition. Here is a link:

http://www.cpfmarketplace.com/mp/sh...haserburn-s-Diffusion-Film-DC-Fix-CPF-Service

There are example beam shots in that thread.

I dunno, magnifying glasses work wonders....