Aspheric lens wanted

Does anyone know of an aspheric lens that is concave on one side, and convex on the other? So long as the convex side was more convex than the concave side, it'll have a focusing effect.

The reason I want this is so that I can use the lens as close to the emitter as possible, thus collecting the maximum light possible with the smallest diameter lens. Even an ashperic that was totally flat on one side would be better than the lenses I am currently using, which are convex on both sides, although not equally. These are about 20m in diameter, but have a 10mm focal length, meaning a significant amount of light is not falling onto the lens and is thus not being used. Hence my search for a focussing aspheric that can be placed much closer to the LED and capture more light without needing to be much larger in diameter.

Anyone know of such a beast? Failing that, anyone know someone can can injection mould acrylic to order like this? Defining the lens mathematics should be easy, as the geometry of the lens would not be complex, so anyone who can mould a lens to order should be able to do this.

What you're talking about is a positive meniscus lens and it won't work for you. Positive meniscus lenses have extremely long focal lengths (hundreds of mm).

Your best bet for efficiency and throw is a TIR optic. A TIR optic will have the same throw as an aspheric of the same diameter, and will do it with much higher efficiency.

An extra problem you may have noticed with your double convex 20mm lens is that a lot of light that hits the edge of the lens will be reflected because the angle is so steep.

Have a look at at the lenses I'm selling (see my signature below). I don't think you'll find better value anywhere else since I bought them in bulk and am now offering them pretty close to cost price.

LukeA said:

Your best bet for efficiency and throw is a TIR optic. A TIR optic will have the same throw as an aspheric of the same diameter, and will do it with much higher efficiency.

Click to expand...

Is there a thread that discusses this? I've heard it a few times but couldn't replicate the results myself. I have also used several different 30mm lenses with different focal lengths and different beam width (and hence throw). I really want it to be true so that I can get a nicer beam than the aspherics offer.

As for efficiency, I don't think that my lenses are losing much since they are made of BK7 and have a very high diameter/focal length ratio. At least they don't lose much with an XR-E.

FWIW, I'd also like to know if there're some TIRs that I'm missing. In my (admittedly limited) experience, for similar diameter aspherics and TIRs the TIRs appear to put more total light out the front (duh!), but the aspherics have them beat in terms of throw/on axis intensity. That said, I'm working on the smaller end of things (15-19 mm dia stuff), so the diameter:focal length ratio is probably dragging down the efficiency of the aspheric, but by how much I'm not sure. Could probably integrate the output curve on the datasheet and use some geometry, but it's nearing 12:30 in the morning here.

Th232, you may be interested in a few plots I did a while ago of beams given by different optics. They take into account the light collected by the aspherics I had on hand at the time.

https://www.candlepowerforums.com/threads/210383

Interesting plots, thanks for the link!

Going to have to go off and rethink some of my choices now.