odd thing with an aspheric and luxeon

Long ago I bought an 27mm aspheric from edmundoptics (under "condenser lenses"). It should have a back focal length of 6.9mm, but it is pretty far from the led when it's on its brightest - about 3-4mm from a modified E2E head with stock reflector in focus.

Has this got anything to do with the led's "apparent" source, or what was it called, mentioned to be different in the cree?

hmm. Makes for a longer than planned light so it didn't fit the host.
If I am going to buy another lens I would like to know what to expect.

The difference between a Lux and Cree is the light distribution angle is different. a Cree is ~70 degrees while a Lux is 180. You have to get it further away because you need to get all the light coming from the LE and the reflector in to the lens to refocus. While with the cree there's almost no side emission that hits the reflector so putting it close to the LE you would get most of the light in to the lens.

Take an object that is bright and as far away as you can find. Focus that on a sheet of paper as sharp as you can. Measure the distance from the rear of the lens to the paper. That will be your BFL. Don't get it confused with EFL.

I am guessing it will be 13mm.

• LENS ASPH 27MM X 13 FL NT43-987 $30.40

will do, only why are you guessing 13mm when the bfl is 6.9 according to the edmund list? EFL shold be 13 iirc.

Hello again, I put a 3mm led at 3m distance, and use the lens to project it on a piece of paper. When the dot is smallest, I do think that the distance between the paper and lens could be about 7mm.
So, why then is the optimum place for the lens about 13-14mm from the die? this is where the beam is brightest from the e2e head.

Hi Alisnail, I wanted to try that same Edmund optics lens with a cree but is out of stock until december, I am very interested in your findings about best distance from the die.

Thanks!
AlexGT

What were you trying to fit the lens on? I wanted to try that lens on a Lionheart with a Cree led

BTW is that lens glass?

AlexGT

isnt it quite logical that the distance is different?

Both lights already use an optic element in the beamline: the little glass dome above the die. Different refration indices/curvatures would cause a shift of the appearent emitter position for the distance of the aspheric lense.

I think it's glass - can't hurt it with a file. I was going to put it in a pelican m6 or a 27mm McLux.

IMSabbel, which both lights?

According to newbie, the luxeon die should be at the focal point. That does not seem to be the case with this lens! Maybe it has to do with the reflector in the e2e head?
BTW, McGizmo said that the cree should be placed with the rear of the slug in the focal point, meaning that the lamp would be about 2-3mm shorter with the cree?

BTW also, the tektite fresnell add-on has a FL of about 7mm and a dia of 1.42 inch according to Scott Mele. It would be interesting to compare it to the aspheric!

Even more btw, I think the 27mm lens is brighter than the carclo 50mm 2deg! Does anybody else have any experience or data points with optics like these, especially comparing them, and most especially, where the led would be placed relative to the focal point?

Oh, and a side note, with the 27mm asph and e2e, the brightest does not seem to be when the die is projected most sharply. I am thinking that I need to think more about this, perhaps a different reflector than a parabola would be better with the aspheric?
On the other hand, with such small focal lengths, and a cree, the reflector might not see any light at all!

IMSabbel, I think maybe I didn't describe well enough what I did to determine the bfl as per newbies instruction:
I placed the arc somewhere, then brought the lens and a paper with me, to about 3m away. I then held the lens in front of the paper, and tried to determine the distance, from flat side of the lens to the paper, which gave the smallest projection of the arc on the paper. This turned out somewhere about where it should be according to edmund, about 7mm according to my inaccurate measurement.

The cree defiantly focuses closer to the lens than a luxeon.

Having a reflector light wont affect it much. The reflector shoots forward the light from the sides of the led, and the optic only cares about the light coming from the front of the led itself which normaly creates the sidespill in lights. Using a reflector will only give you an extra ring of light around the "hotspot" created by the optic as that light being channeled into the optic won't be in the direct center.

EDIT: this is with a mag reflectored luxeon. If you have a luxeon in a tiny reflector it can affect the beam more.