Lens type for adjustable focus

[beanbag]

Hello folks,

I'm looking to build an adjustable focusing system for my Nichia 219 LED. From what I have read so far, there are three main types of lenses to consider for this application: Plano Convex (PCX), Double sided convex (DCX) and Aspheric.

One of the concepts I have read about is that you want a small focal length relative to the diameter so that the lens can capture a wider spread of light from the LED. But I think I will already have to make a small reflector around the LED to redirect light that sprays out at more than, say 40 degrees.

The aspheric lens is supposed to be the best about abberations, but is that really important?

The DCX lens seems best about having a small focal length, but I have read that the light-gathering abilities at the edges are not as good because the light hits the curved surface at a more glancing angle

PCX lens at least has a flat surface that faces the LED, but they are harder to find with small focal lengths.

My guess at how the focusing system would work is that at maximum throw, the LED is at the focal point of the lens, and for more spread, the LED moves closer to the lens.

 

[bshanahan14rulz]

Everything you said makes sense, except for using a reflector to redirect light at the lens. That redirected light will not be added back into the hotspot, but instead create a sort of "spill" in the form of an image of the illuminated reflector, in focus at the plane of the LED. If you used a spherical reflector instead to redirect that wasted light back to the LED, though, that light has a chance of being added back into the hotspot if it is then reflected off the die and up towards the lens.

For a single lens setup, I'd recommend an aspheric.

For 2 or more lenses, that's out of my league, ask a smart person, not a dancing monkey ;-)

 

[beanbag]

By "small reflector", I was thinking of a really small reflector, no more than 1/4" outer diameter, and wedged right against the LED dome.

That's an interesting idea regarding reflecting the light right back onto the die. How "reflective" is the die anyway? And it had better be a diffuse reflecting, or it's just going to hit the opposite side of the reflector.

 

[2xTrinity]

Everything you said makes sense, except for using a reflector to redirect light at the lens. That redirected light will not be added back into the hotspot, but instead create a sort of "spill" in the form of an image of the illuminated reflector, in focus at the plane of the LED. If you used a spherical reflector instead to redirect that wasted light back to the LED, though, that light has a chance of being added back into the hotspot if it is then reflected off the die and up towards the lens.

Simply using a reflector in focus and an LED produces a pretty horrible looking beam which is actaully a bright point in the middle, a big dark donut around it, then a "ring" arouind that (the image of the illuminated reflector).

You can play around with defocusing the reflector so eliminate the gap and have a spot + contiguous corona surrounding it, but the problem is you won't be able to adjustably focus your light on the fly anymore without reintroducing more donut holes again (your LED and reflection of the LED in the mirror will be in focus at different planes so if you defocus the lens it will affect the spot + corona differently)


I have played around with reflecting unwanted spill back onto LEDs (using spherical reflectors and other approaches) and while this will make the LEDs brighter, since light is passing through the phosphor twice and being reemitted at usable angles, the multiple passes through the phosphor often gives the LEDs an unpleasant excessively green tint.

Single element aspheric will work how you describe, although if you try to blur out the LED from being optimally focused once you get to about ~2.5x the original beam diamter you will start to see ugly donut holes and aberrations (color fringes).

Achieving a true narrow to wide angle adjustable beam would require multiple lenses in a zoom configuration. Even if this was non-imaging / aberrations didn't matter something like that is probably way too diffiicult to make as a one-off homebrew light.