# For the optical experts: How would you construct such a beam?



## gcbryan (Jun 30, 2011)

I've been experimenting a bit lately trying to learn just what the perfect beam would be for me and how to get there.

I've noticed that the best beam when throw isn't an issue is when you take the reflector out of a LED flashlight (totally even flood) but it still is like looking through a tunnel due to the hard edge of the head of the flashlight. I'm talking about the point where you go from light to no light. It's the large circle you see in your peripheral vision. 

To fix this you put a piece of diffusion film where the front lens would be so now you have a bare emitter, no optic or reflector and then the diffusion film.

Throw comes from using a reflector and those rays going forward and not hitting the reflector make up the spill. This spill is not diffused as in the perfect beam described above.

How would one construct a beam as in the perfect diffused no edge wide beam above and yet have a brighter hotspot in the middle for throw?

This would be a seamless beam in that the diffused part of it would be so wide that as you moved around it would not seem to move. There would just be a lower level of even illumination everywhere you looked. It would be diffused and wider than your peripheral vision so it would feel more like the illumination you would get on a full moon. As you focused your attention directly in front of you the light would be more intense and therefore you would be able to see much further and in greater detail but it wouldn't seem unnatural as it does with a bright focused hotspot and then a sudden drop off to spill and then a sudden drop off from spill to darkness. It would all be wide angle and diffused except for the center hotspot.

How would one accomplish this? If you were able to diffuse the top most portion of the emitter dome would that have this effect? That's the portion that results in spill.

I can visual the effect I'm looking for but I'm not sure (whether practical or not) how one could (at least theoretically) accomplish this.

To be even more specific as to the exact effect it would be this. Take two BD Storm headlamps (I'm being very specific since this is what I have and what I've experimented with) and put a piece of Scotch "Magic" Tape over the main "spot" optic. This is the same effect as putting a piece of tape over a light that has a reflector. Now you have a diffused but still directed beam as opposed to pure flood. It now has an angle of 40 degrees lets say. It throws 75 feet. You don't really see a spot anymore. It's diffused but it's still diffused light that is coming out of an optic (or reflector) so it still throws.

Take the other headlamp and turn it on in flood mode which in the Storm is two bare emitters with some diffusion material over them. They are located on each side of the headlamp so the end result is a very wide diffused beam. When in this mode and you are wearing the headlamp and turn your head you never see the end of the beam since it is wide and diffused and outside of your peripheral vision.

If you are now wearing both headlamps, one with the totally diffused wide non-reflectored beam on at lets say 25 lumens and if you also have the diffused optic directed beam on at 75 lumens you have the effect I'm talking about.

It lights up the area around you in a natural way and it does the same but with more throw in the area just directly ahead of you out to 75 feet. There are no circles, tunnels, sharp light drop-offs. It all looks more like the moon is just illuminated where ever you are and where ever you walk.

How would one construct a light to do that? Again, would it be as simple as diffusing just the top of the dome of the spot emitter? If not why and how would you do it?

RA, where are you when I need you


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## MikeAusC (Jun 30, 2011)

Most LED-reflector torches I've seen have this beam - I bright centre spot from the light which has been focussed by the reflector and a large even flood from the direct light off the LED.


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## gcbryan (Jun 30, 2011)

MikeAusC said:


> Most LED-reflector torches I've seen have this beam - I bright centre spot from the light which has been focussed by the reflector and a large even flood from the direct light off the LED.


 
They also have a spill beam angle of 40-60 degrees instead of 180 degrees and they have a distinct edge when going from spill to darkness.


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## bshanahan14rulz (Jun 30, 2011)

That distinct edge is formed by the outer lip of the reflector, or the edge of the bezel. You could put fake eyelashes on the outside of your flashlight bezel. Instead of an assault crown, it would be a style crown


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## MikeAusC (Jun 30, 2011)

For really wide spill put a small Aspheric in front where the focal length is at 4 times the diameter.

The spot won't be as bright because it's only capturing a small part of the LED's output - though the brightest part.

The spill will be obstructed by the aspheric so to avoid a shadow around the spot, a diffuser around the aspheric will be needed.


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## gcbryan (Jun 30, 2011)

If I just wanted a wide beam I certainly wouldn't pick an aspheric. Just removing the reflector would do that and a diffusion filter would do the rest (regarding eliminating the hard edge...no false eyelashes required).

The question had to do with diffusing only those (direct) rays that contribute to the spill as in (if it were possible) puting a small dot of diffusing paint (for instance) in the center of the dome over the emitter thus targeting (diffusing) only those direct rays that aren't going to be hitting the reflector (spill).


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## MikeAusC (Jun 30, 2011)

gcbryan said:


> If I just wanted a wide beam I certainly wouldn't pick an aspheric. Just removing the reflector would do that and a diffusion filter would do the rest . . . . .


 
The reason I didn't mention a reflector in the second post, was because I was looking at using the aspheric instead of a reflector to get the spot.


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## gcbryan (Jun 30, 2011)

MikeAusC said:


> The reason I didn't mention a reflector in the second post, was because I was looking at using the aspheric instead of a reflector to get the spot.


 
Yes, I know how the LED Lenser and other flood-to-throw lights work but that's not what I was describing at all. The spot on those lights isn't a "spot" and the flood (as well as the spot) is like looking down a tunnel. They have the hard edge I'm trying to eliminate in both the spot and flood modes. Aspherics eliminate (waste) all spill.


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## MikeAusC (Jun 30, 2011)

gcbryan said:


> . . . . . Aspherics eliminate (waste) all spill.


 
Normal aspherics where you want the brightest spot, yes. If you want spot and flood I'm suggesting a small aspheric say 30mm dia 100mm from the LED. This allows a lot of the light to provide smooth flood.




gcbryan said:


> Yes, I know how the LED Lenser and other flood-to-throw lights work but that's not what I was describing at all. . . . . .



LED Lensers use TIR optics that stop any direct light.


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## ASheep (Jun 30, 2011)

To get rid of the hard edge, you would literally need to get rid of the hard edge at the head of the light. Some arrangement like Zebralight used with the H501, where the LED is almost on the surface of the light, set into a slight conical bore. As MikeAusC said, if you put a small aspheric in front of the LED, mounted on say a piece of perspex, you could have a diffused flood emitting from all around the aspheric, and a focussed spot coming out the front. This setup would leave the lens and LED ridiculously exposed though. 

The only other way I can think of to get both a pure flood and diffused spot beam is to make/mod a light which has some small bare LEDs with a diffuser, and a second reflectored LED with diffusion film.

Cheers,
Alex


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## MikeAusC (Jun 30, 2011)

I agree - I've never built a multiLED light before, but to produce this beam I'd use two LEDs - one bare to produce the flood and with reflector or aspheric to produce the spot. it would let you individually control the brightness of each component and adjust the size of the spot.


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## gcbryan (Jul 1, 2011)

Regarding the multiLED that's what I would actually prefer. I have a headlamp that does this and I like both beams but as it is currently set up I can't use both beams at once. That would be ideal however.


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## beerwax (Jul 1, 2011)

......


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## hellokitty[hk] (Jul 1, 2011)

Mmmm...A diffuser tip glued in the middle of the lens.
You'd have throw the reflector out the edges of the lens, and the top would diffuse what would normally go into the spill.


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## gcbryan (Jul 18, 2011)

By the way MikeAusC I wasn't quite following your initial comments which I now get so sorry if I sounded dismissive.

I have another related question. I'll test it out later tonight after its dark but in case it spurs more questions I'll throw it out here.

If the end goal was to get diffused light and eliminate that hard cut-off (spill to darkness) you would just use a diffuser but then you would have no throw.

What if throw was all that important but you wanted a bit more than you would have with a totally diffused beam (for hiking for instance...you might not need much but a bit of throw can be nice). This is a bit of throw while still not having that sharp cut-off so diffusion is still needed.

What if you cut a large hole in the middle of the diffusion material? This would still keep diffusion material around the perimeter and would (I guess) eliminate that sharp cut-off. This would also let some of the collimated rays out and should result in throw. Would this work?

I realize (I think) that the most collimated rays come from the outer edge of the reflector which the light appears more point like and therefore diffusion material there would disrupt those rays. However the rest of the reflector would still be collimating rays, just not as effectively which should be fine.

Should this approach work? If not why does this not work?
Thanks.


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## gcbryan (Jul 20, 2011)

It turns out that punching a hole in the center of the diffusing material does result in the diffused edges I was looking for and greatly increased throw (increased over not punching the hole in the first place).

If the hole were very small then the only light coming out would be the light that contributes to spill anyway so no advantage. However by making the hole larger but not so large that there is no diffusion material around the perimeter then you get the collimated beams from the area of the reflector under the hole and you have a diffused edge so no hard cut-off.

You aren't getting the most highly collimated beams from the edge of the reflector but collimated beams are coming off all parts of the reflector so you are getting some of those.

It was easy to test this effect since I could slide the head of the light from part of the diffusion material with no hole to see the effect and then slide it to the part with the hole punched...throw increased while still having a diffused edge.

I was able to make the hole just large enough to further test this effect so that the diffused edge effect was starting to go away (hole almost too large). So you need to leave a certain percentage of diffusion material around the perimeter for the edge effect to disappear while still keeping the hole as large as possible for enhanced throw. To test this effect I actually kept the hole the same size and just switched to a smaller diameter light so proportionately the size of the hole (and the effect) increased. Since I was using a standard size hole punch this was the easiest approach for this test.


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