# Is it possible to condense a regular light source into a collimated laser beam?



## alltracturbo

I've been doing some reading online and trying to find out if it's possible to condense normal light from something like a flashlight down to a very fine collimated beam around the size of a laser beam.

If so, what would be the easiest way to do this and would it be as powerful as a laser?

I know you can put a parabolic lens behind the bulb and get a spotlight, but I want the beam to be smaller than the bulb. Would it be possible to focus the light from a parobolic lens to a small spot about an inch or 2 in front of the bulb and have it hit a collimating lens there and create a beam?


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## LukeA

You can never focus the spot down to a size smaller than the original light source's size.


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## brucec

LukeA said:


> You can never focus the spot down to a size smaller than the original light source's size.



That is incorrect. There is a simple article in Wiki that may help.

http://en.wikipedia.org/wiki/Lens_equation#Lensmaker.27s_equation

However, it may be more convincing to do a little experiment. Point a camera lens or magnifying glass at the moon and make a image of the moon on a sheet of paper. The size of that image is surely smaller than the original light source (the moon).

Regarding the OP's question, no, the parabolic condenser method that you described wouldn't work. The further you tried to project, the larger the image would be. Basically, lasers work because the light is coherent and collimated to begin with.


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## AnAppleSnail

That's me in a smoky tunnel with an aspheric beamer. The tiny little P4 LED emits onto an inch-big aspheric, and even at perfect focus the best that can be done is to project the LED forward as an expanding cone. You can use lenses to make a hotspot, just like a magnifying glass under the Sun, but it will be very sensitive to focus - nothing like a collimated beam. It's difficult to turn coherent light uncoherent without losses, and hard or impossible to make incoherent light coherent with passive optics.

Now, if your 'lens' is a ruby rod with partial mirrors on the end, and so on, you're in luck. But that's a laser anyway


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## monkeyboy

LukeA said:


> You can never focus the spot down to a size smaller than the original light source's size.



Sure you can (even for the parabolic reflector suggested by the OP), it's just that the reduced size image would lie in between the light source and the reflector.

Another way I can think of would be to use a single lens. The problem with this is that; reducing the image size would require increasing distance between bulb and lens which would reduce the amount of light collected by the lens. You could make the image very small but the distance required would be huge and only a tiny fraction of the light would be collected. (same problem with the reflector actually)

In other words, it's no substitute for a laser.


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## alltracturbo

That pretty much answered my question. I guess the main problem is not being able to collimate incoherent light.

Funny thing, I just searched for "collimate incoherent light" and the second result was a patent explaining exactly what I tried to explain in my original post.

http://www.freshpatents.com/Collima...t20051201ptan20050263784.php?type=description



> A technique for collimating light from a Light Emitting Diode (LED) device involves emitting light from an LED die, collimating the light with a parabolic reflector, and further collimating the light with a lens.


 
I was going to ask why you can't collimate incoherent light, but I read this on the page linked above:



> Another obstacle to collimating light from an LED arises from the fact that LED light, though it may appear to be, is not monochromatic. Photons of different wavelengths have differing indices of refraction. Accordingly, light passing from an LED die through a lens is refracted at different angles according to wavelength. The change of index of refraction according to wavelength is known in the art of optics as chromatic dispersion. Chromatic dispersion is the phenomenon that causes the separation of colors in a prism.


 
That seemed to explain it pretty good. Not sure where the pictures are, maybe you have to be a member to see them.


Also, is sunlight considered collimated? What if you condensed sunlight with a fresnel lens and put a collimating lens at the focal point, would that make a collimated beam of sunlight?


Edit: Nevermind about the sunlight. Even if it was collimated which it probably isn't, it's still incoherent.




monkeyboy said:


> Sure you can (even for the parabolic reflector suggested by the OP), it's just that the reduced size image would lie in between the light source and the reflector.


Thats a good point, I didn't think of that.


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## monkeyboy

> Another obstacle to collimating light from an LED arises from the fact that LED light, though it may appear to be, is not monochromatic. Photons of different wavelengths have differing indices of refraction. Accordingly, light passing from an LED die through a lens is refracted at different angles according to wavelength. The change of index of refraction according to wavelength is known in the art of optics as chromatic dispersion. Chromatic dispersion is the phenomenon that causes the separation of colors in a prism.



An apochromatic doublet lens can reduce this chromatic dispersion to negligible levels but a reflector would have no chromatic dispersion at all. A light source doesn't necessarily have to be coherent to be collimated, for example, you can now get white lasers.

But to get a true laser like collimation, you would require a point source light emitter or something approaching that.


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## 2xTrinity

> I've been doing some reading online and trying to find out if it's possible to condense normal light from something like a flashlight down to a very fine collimated beam around the size of a laser beam.


There will be a tradeoff between the width of your beam, and the divergence. That is, you can have a fat beam that diverges very slowly (eg, the searchlight) or you can focus the light into a tiny beam, and it will diverge very quickly. This is also true for laser light just as it is for incoherent light-- Laser light that is expanded then collimated again using a larger lens will diverge much more slowly than light propagating in a narrow beam.

Another thing. There are also two types of coherence. There is temporal coherence (eg how monochromatic is your light source) and spatial coherence (how much does your light source look like a small point source). For collimation, it is the spatial coherence (small source size) that is most important. One of the articles linked earlier described chromatic aberration as a problem, but this can be compensated for. 

The reason why the big spotlight is able to collimate the light much better than a smaller reflector, is that the LED die is small compared to the size of a reflector, that is it behaves more like a point source (more spatially coherent).



> If so, what would be the easiest way to do this and would it be as powerful as a laser?


There isn't really a way to do what you're describing.

If you're interested in a more rigorous explantion, see this wikipedia article: http://en.wikipedia.org/wiki/Etendue

basically, the Etendue is: (size of the light source) * (angle the source is projected into), and cannot decrease in an optical system. You can focus the light source to a smaller apparent size, but then you will necessarily have a larger divergence angle.





> I know you can put a parabolic lens behind the bulb and get a spotlight, but I want the beam to be smaller than the bulb. Would it be possible to focus the light from a parobolic lens to a small spot about an inch or 2 in front of the bulb and have it hit a collimating lens there and create a beam?


No. If you were to place your eye at that point (with the light OFF of course) you will notice that your LED or filament no longer appears to be a tiny point, but rather a HUGE magnified image of your filament or LED source. If you try to place a second, very narrow collimating lens at this point, that lens will be trying to collimate the large die image it sees, NOT the actual original small point source. You will end up more divergence than you would with the parabolic reflector alone.


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## Juggernaut

The closest your going to get in production is a Maxabeam 1x1 degree beam, my experimental L.Y.L.L. (0.94) degree beam, and the unfinished NightSword should be able to blow all of these away.


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## alltracturbo

Thanks for all the great info, but I give up. I was just hoping it would be something simple and I could make a cheap laser from a flashlight or a real powerful one from sunlight.

I don't have enough knowledge about light to figure out a way to make it work and I don't have the ambition to learn it, but i'm sure someone will firgure it out soon. Once I figured out the correct terms to search, I found that theres lots of people online talking about the same thing and experimenting with different designs.


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## brighter

If it were simple to do that (as you were thinking), than all of the manufacturers would put that kind of optics in their flashlights.
Maybe some day that kind of optics/crystals/whatever will be invented and it will for sure change the flashlight world.


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## saabluster

alltracturbo said:


> Also, is sunlight considered collimated?
> Edit: Nevermind about the sunlight. Even if it was collimated which it probably isn't, it's still incoherent.


Practically speaking, yes, sunlight is collimated. From here on earth anyway. It would not be if you were standing near the sun though.


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## lctorana

How about this:

Mount a small light source (single-die LED, small filament bulb like a 62138, etc) behind nested lenses.

Like a telescope.

I have thought about an aspheric, followed by two bi-convex lenses.

Or two aspherics.

I'll keep thinking about it. I'm *sure *it's possible.


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## fyrstormer

Light emits in all directions from each point on the face of the LED die; it is never a perfect lambertian dispersion pattern like the diagrams show. That makes it impossible to focus perfectly, because light that isn't in-line with the ideal dispersion pattern will be focused improperly by whatever optic you use. There will always be a halo around the edge.

Many people spent their lives trying to get regular light to do what lasers can do. Ultimately the laser was invented because regular light can't do it. The closest you can get is a very tightly-collimated beam of light, passed through a series of lenses and pinhole filters to remove any stray photons that don't line up with the ideal dispersion pattern, and also passed through a series of color and polarity filters to remove photons that aren't in-step or the same color as the rest -- but even that won't be perfect like laser light is. What it _would_ be, however, is an effective lesson in just how "dirty" regular light is and how little of it is actually coherent -- by the time you get done filtering it, you'd have to shine a spotlight through the contraption to get anything as bright as a laser pointer out the other side.

Or you could spend $5 on a real laser pointer that does the same thing and does it properly.


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## mattmagic100

Iv never really heard of a flashlight being focused to a beam but i doubt it since the flashlight would have to keep its originial size and shape. but if you figure out how to make a it a little smaller, let me know


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## lctorana

mattmagic100 said:


> ...since the flashlight would have to keep its originial size and shape...


:huh2:


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## 2xTrinity

fyrstormer said:


> Light emits in all directions from each point on the face of the LED die; it is never a perfect lambertian dispersion pattern like the diagrams show. That makes it impossible to focus perfectly, because light that isn't in-line with the ideal dispersion pattern will be focused improperly by whatever optic you use. There will always be a halo around the edge.
> 
> Many people spent their lives trying to get regular light to do what lasers can do. Ultimately the laser was invented because regular light can't do it. The closest you can get is a very tightly-collimated beam of light, passed through a series of lenses and pinhole filters to removes any stray photons that don't line up with the ideal dispersion pattern, and also passed through a series of color and polarity filters to remove photons that aren't in-step or the same color as the rest -- but even that won't be perfect like laser light is. What it _would_ be, however, is an effective lesson in just how "dirty" regular light is and how little of it is actually coherent -- by the time you get done filtering it, you'd have to shine a spotlight through the contraption to get anything as bright as a laser pointer out the other side.


Well, to do what the OP is requiring, the color and polarization filters would not be necessary or beneficial, but you have a very good point.

All that is necessary for collimation is _spatial _coherence, or light (apparrently) originating from an infinitesimal point in space. This is what the pinhole apertures accomplishes. But of course that also throws out the vast majority of the light in the process. In summary, trying to use a partially coherent light source like an LED, a design can achieve at most two of the following:

narrow beam diameter
reasonable light-gathering efficiency
low beam divergence

Telescope style optics + pinhole apertures as you described:

low divergence, low diameter, horrible light gathering efficiency


big aspheric lens or parabolic reflector:

low divergencee, large beam diameter, high light gathering 


collimator + two biconvex lenses to reduce beam diameter:

very fast beam divergence, high light gathering efficiency, small beam diameter


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