# Making a light with a collimated beam



## CKOD (Sep 23, 2010)

I'm looking to make a light with a collimated beam, I know its not possible to get perfect collimation without a true point source, but trying to do the best I can with a LED. Also, I'm going for collimation here more then throw. 

My thought was a convex lens pair of different focal length to provide a reduction in the projected die image at the focal point of the 2nd lens. Place a ground glass diffuser at this focal point and this would become a virtual "die" for the rest of the system. Then simply put a lens in front of the diffuser, with the diffuser surface at its focal point, to better emulate a point source. Obviously 3 lenses and a diffuser will cause a decent amount of lumen loss, but I think brute force and just shoving more down the pipe to begin with will be the solution for that. 

On something like this, how big would the output beam be? Would it just be as big as the active optical area of the final lens? 

Thanks for any input!


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## AnAppleSnail (Sep 23, 2010)

CKOD said:


> My thought was a convex lens pair of different focal length to provide a reduction in the projected die image at the focal point of the 2nd lens. Place a ground glass diffuser at this focal point and this would become a virtual "die" for the rest of the system. Then simply put a lens in front of the diffuser, with the diffuser surface at its focal point, to better emulate a point source. Obviously 3 lenses and a diffuser will cause a decent amount of lumen loss, but I think brute force and just shoving more down the pipe to begin with will be the solution for that.
> 
> On something like this, how big would the output beam be? Would it just be as big as the active optical area of the final lens?
> 
> Thanks for any input!



It'd be neat for prisms, maybe. Definitely would show LED spectral output.
LED to lens focused on diffuser, and a lens focused on the diffuser? I think a big problem with using a diffuser like this is that you can't have one smaller than the LED die.

I can't draw pictures here but follow the light from the LED. It spreads out, hits the first lens, and converges. But at the place where that light focuses, you have an image of the LED, not a point of light. I'm pretty sure that some optics thing keeps that image larger than the LED, but it won't be smaller.

The closest to your goal I've seen was a bare LED half a meter from a large fresnel lens. It had dispersion comparable to most laser pointers (Not the nice ones). You want a very small apparent size of source from the lens to get collimation. That is, a smaller light source, further away, collimates better. There is a huge tradeoff in lumens to get the lux.


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## Walterk (Sep 23, 2010)

I went through the steps you are taking now I think.
I think you should read the thread * Formula for calculating throw using aspheric lens. *You won't get it much easier, although you can make it more difficult and less efficient by adding lenses.

Edit: And note there is lumen and candlepower, but assume you have figured out the differences between them.


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## MikeAusC (Sep 24, 2010)

No matter what you do with optics, you can't get a projected image smaller than the original LED.

All the extra optics would do, is lose light - you may as well just use good lenses to focus the LED image.

There are quite a few examples here of people doing that.


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## JP Labs (Oct 2, 2010)

MikeAusC said:


> *No matter what you do with optics, you can't get a projected image smaller than the original LED.
> *
> All the extra optics would do, is lose light - you may as well just use good lenses to focus the LED image.
> 
> There are quite a few examples here of people doing that.




Really? That surprises me. Using a lens, you can get a projected image of the sun which is obviously much smaller than the real thing. Why wouldn't the same apply to an LED die? I believe the limitation is that you can't get a higher brightness than the source, but a smaller image is easy, in my understanding. 

While I'm no certainly not very well versed in optics, common experiences tell me the OP's idea could function as he intends, relative to creating a secondary, smaller light source with the lens & diffuser.


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## ma_sha1 (Oct 2, 2010)

You can reduce the beam size/reduce the die image size to as small as you want pretty much.

I tried this by putting a 7x monocular in front of an Aspherical flashlight.
The image at 5ft was about 1 ft in size & after going through binoculars,
it become a tiny spot about 1 inch in size roughly. 

If you use high magnification binocular or monocular, you can probably reduce the beam to the size of a sesame.

But the smaller image was't any brighter, as the two more lens caused lots of light loss.


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## Dr.Jones (Oct 2, 2010)

Yes, the smaller image can't be any brighter, even if we neglect all losses. That's a fundamental principle in optics.

Let's assume all lenses have the same diameter and we compare a single (ideal) lens to your proposal with two (ideal) lenses.

If the first lens projects a smaller image, the rays from that image must have a wider angle (just draw the beams to see that). To catch all these rays with the second lens of a given lens diameter, the focal length of that lens must be short, and a short focal length results in a bigger magnification and thus a wider beam after the lens. The smaller 'virtual' die must be magnified more to catch all light, and these two effects cancel each other out, nothing is gained in comparison to using just one lens of the same diameter.
(A keyword for further searching&reading might be 'etendue'.)


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## ma_sha1 (Oct 2, 2010)

Dr. Jons, is it possible to add a PCX lens with super long 10 meter EFL over the arpheric light and make a brighter spot at 10 meter?


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## Dr.Jones (Oct 3, 2010)

If that lens isn't any bigger in diameter than the 'original' main lens, then no. 
If that lens was directly in front of the main lens it would have the same effect as a slightly stronger main lens (the optical powers add).

Edit: Oops, misread the question. The answer is yes, see below.


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## ma_sha1 (Oct 3, 2010)

I Just tested this with a 10mm efl lens & it worked


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## Dr.Jones (Oct 4, 2010)

Oops... 
I just reread your post, I did misread it first, I'm sorry:
*"... make a brighter spot at 10 meter?"
Yes. A well collimated flashlight projects an image of the die into infinity. An additional lens with f=10m will project it into 10m distance.
Slightly moving the main lens away from the LED (<1mm) would have the same effect.*


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## ma_sha1 (Oct 4, 2010)

At the focal point, the tiny spot looks 10x brighter than aspheric alone. 

Imagine if I put a PCX lens with 1 mile EFL in front of the Main Aspheric lens. I can get a 10x brighter white laser like spot at 1 mile . 

It'll diffuse before & after that focal point, but with the 1 mile long EFL, I'd imagine it still have very narrow beam spead vs. Aspheric alone? at least a few hundred before & after the 1 mile point, be significantly brighter than ASpherica lens along. 

The longest I've seen are the huge telescopes with objective lens up to 20,000 mm EFL, that's only 65 feet. 
Do you know if 1 mile EFL lens exist?


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## ma_sha1 (Oct 4, 2010)

Dr.Jones said:


> Oops...
> *Slightly moving the main lens away from the LED (<1mm) would have the same effect.*




This is something I could not achieve, after adjusting led to Aspheric lens distance, one can get a image but still rather large. 

Then put a 10cm EFL lens over it, the image become a tiny spot at 10cm, 10x smaller & 10x brighter. 

Because the aspherical beam is almost parallel but not quite, still has 2-4 degrees depend on lens, 
I wonder if a PCX lens exist that can correct the aspheric beam spread down to almost zero degree. 

I think a one mile EFL lens, if it exist, would turn the aspherica beam into nearly parallel in order to focus at 1 mile?


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## Dr.Jones (Oct 4, 2010)

That tiny spot will only be that bright as long as it's that tiny; and that can only be achieved by lenses with relatively short focal lengths. Using a 10m efl lens for the wall in 10m distance will have a much(!) weaker effect, barely noticeable. That's because 10m is so much longer than the lens diameter.
At 1 mile I doubt there is any measurable difference.

You can get that small bright spot in 10m target distance if you held that 10cm efl lens 10cm in front of the target; that will concentrate all light that hits the lens into a small and much brighter spot... But that's not what you want 

>> _Slightly moving the main lens away from the LED (<1mm) would have the same effect._
> _This is something I could not achieve_

Focus at a wall far away, say 20m, to get a nice projected image of the die there. Then target a much nearer wall, say 2m, and you get a blurred spot. You can refocus by either adding a lens with 2.2m focal length - or just shift the main lens a bit, as you usually do to refocus.

> _Because the aspherical beam is almost parallel but not quite, still has 2-4 degrees depend on lens,_
That's not because of lens errors etc, but very fundamental, it depends only on die size and focal length. If the die takes an angle of 2° as seen from the lens center, then the resulting beam will have 2° beam divergence - even with a 'perfect' lens. For the FAT NEEDLE I used a 50cm efl lens and got a very thin beam, only 3.4 mrad, that's 0.2° - but that has disadvantages...


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## ma_sha1 (Oct 4, 2010)

You burst my bubbles, I :fail:.

Thanks for the explanation :thumbsup:


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