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Old 11-02-2015, 11:41 PM #1
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Default The difference focal length makes with our highly divergent diodes.

Just a quick test with 2 NDB7875's
One with a 3 element lens and one with one of my scavenged 1 inch lenses.

As we know the beam diverges rapidly from our favorite blue diodes and just like our G7 with its longer focal length produce a tighter infinity focal point than our G9 with it's shorter focal length, an even wider, longer focal length lens produces a wider exit beam diameter that can be focused tighter at an infinity focal point.

Being multi mode there are always the wings, but the hot spot in the center can be focused to a much higher energy density with a wider longer fl lens.

This is not a perfect solution, cylindrical pairs to correct beam shape would produce a more workable beam, this is just a basic crude demonstration that most of you have probably already done yourselves.

Please share any results you may have, I know some of you have some massive lenses

This is just an expedient slide focus arrangement.





You can see the exit beam is wider.



The focused beam width is reversed of the exit width.



The distance is about 65 feet.
You can see the 1 inch lens, even with its edge clipping and substantial overspray burns the cardboard, the 3 element has no chance.



Again this is just a basic test and not a perfect solution, please share your own results and/or ideas.

I'm going to try a pair of cylindrical lenses and see how well that works.



Last edited by RedCowboy; 11-03-2015 at 07:04 AM. Reason: typo
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Old 11-03-2015, 11:46 PM #2
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Default Re: The difference focal length makes with our highly divergent diodes.

I was about ready to make the same experiment, but it seems you already proved my theory. Just curious, but how far is the surface of the lens from diode? My suspicion is the larger lens area is grabbing a much bigger chunk of the emitter area, and the results are obvious downstream with more energy hitting the cardboard. Exit pupil is going to be bigger, but it makes up for it at the focal point. If I recall my optical design stuff (been a lot of years) it's possible to design a lens with a wide field objective *and* small exit window, but requires a fairly complex intermediate grouping in the middle. Telescope eye lenses can do this, but aren't cheap. Nice tests.....shows the limits of our typical optics.
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Old 11-04-2015, 05:15 AM #3
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Default Re: The difference focal length makes with our highly divergent diodes.

Quote:
Originally Posted by blasterman View Post
I was about ready to make the same experiment, but it seems you already proved my theory. Just curious, but how far is the surface of the lens from diode? My suspicion is the larger lens area is grabbing a much bigger chunk of the emitter area, and the results are obvious downstream with more energy hitting the cardboard. Exit pupil is going to be bigger, but it makes up for it at the focal point. If I recall my optical design stuff (been a lot of years) it's possible to design a lens with a wide field objective *and* small exit window, but requires a fairly complex intermediate grouping in the middle. Telescope eye lenses can do this, but aren't cheap. Nice tests.....shows the limits of our typical optics.
Yes I am grabbing the diodes diverging output at a wider spot at a longer focal length to get a tighter focus at a longer distance.

But not any lens will work, multi mode diodes are a mess, one axis diverges faster than the other and there are multiple bars of output.

An aspheric lens that is flat on the back won't work, it has to be aspheric and slightly concave on the back, or a combination of lenses.

It's not a matter of getting all the diodes output into the lens, the G2 gets all the output into the lens at 2.39mm rear focal length, and it will reproduce a picture of the pn junction at a specific distance based on the math, somewhere in the 2 inch range...my estimation, I have not done the math...
BUT, if it achieves unity at 2 inches then you can not get that tight of a spot at 4 inches, your 4 inch spot, focused as well as you can get it will be twice as wide and twice as long for 1/4 the energy density.

So any given single lens based on it's shape/power and it's focal length it will only reproduce a multiple of the original emitter area based on the math.

That's why starting wider lets you finish tighter and further away.

My 1 inch lens focus at 65 feet at about 1.75 inches from the diodes emitter, so if you look at the raw output of the 7875 I am losing the edges, yet it still burns a lot further away because of focal length.

So the picture it is reproducing of the pn junction is much tighter, i.e. higher energy density because it's focal length is much longer than the smaller lens.
At 65 feet the smaller lens with a shorter focal length is reproducing a larger picture of the pn junction, i.e. lower energy density.


I hope this makes sense, I am still learning all the time myself.

Last edited by RedCowboy; 11-04-2015 at 05:40 AM.
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