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Thread: LED long-throw design suggestion

  1. #1
    curiousCrandall
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    Default LED long-throw design suggestion

    Subject: long-throw LED flashlight design

    This is an open letter to Peter Gransee, Paul Kim, and all of the terrific experimenters and innovators out there who are making the world of hand-held illumination such a strange, exciting, and fast-moving one these days. I want to propose and advocate a general strategy for improving the quality of the beams coming from the new-generation LED lights, like Arc's pathbreaking LS, and SF's KL4.

    My proposal comes in 3 parts: 1), why incandescent flashlights are better at providing a hot-spot of light, which in turn gives the light a longer 'throw'; 2) why that design won't work for LED's; 3) what will work instead.

    1) How come incandescents have such a beautiful hot-spot of light? How come even a cheapy $20. Auto-Mart spotlight can throw a beam of light farther than a $150. LED miracle? Simple: incandescents use parabolic reflectors with the filament at the focus of the parabola.
    The magic is definitely not in the filament. An incandescent filament is basically a point-source of light, radiating 360 degrees in a sphere. The filament in the bare GE 60-watt bulb hanging from a work light provides no hot spot at all, and the filament in my spot-light is no different from it as far as that goes.
    All of the trick comes from the reflector. A parabolic surface (paraboloid, if you prefer) has the cool property of being able to collect light from a point-source located at one of its focuses, and direct all of it outward in a bundle of parallel rays. If you want the most extreme, unmodified spot, that's the design you use; prior to the invention of lasers (which produce parallel bundles of light in a whole different way), that was the best way to get a tight, intense beam of light from one place to a far distant place. Of course, for flashlight purposes, we usually want some combination of hot-spot and corona, an intense focus dead ahead, with a decent amount of ambient light for our peripheral vision. And designers get this combination by stippling the parabolic reflector, and by shifting the filament a little shy of the focal point (like your old "spot-to-flood" aluminum behemoth where the whole bulb assembly pistoned back and forth). Still, what gives the basic character to incandescent flashlights--and what still gives a dime-store AA incandescent the long-reach advantage over an L4--is the fact that the filament is roughly at the focus of a reflector that is roughly parabolic.

    2) So why don't we just put a Luxeon LED at the focus of a parabolic mirror and get all the benefits outlined above? Because of one minor problem, and one major problem. Minor first. An LED circuit like the Luxeon 5-watt is not a point-source. It is mounted on a chip, and only radiates in the hemisphere above the chip. So if we mounted the chip facing outwards at the focus of a paraboloid reflector, very little of the light would even interact with the reflector on its way out the door. It would just stream out the aperture of the paraboloid in the relatively diffuse way we're all familiar with. (Indeed, I think that is roughly what is happening with the KL4 head, assuming its reflector is roughly parabolic; I doubt if the reflector is doing that much work at all, and it certainly is not producing a hot-spot. And moving the chip forward, from the vertex of the parabola to the focus, would only make the problem worse). Still, that's only a minor problem: to circumvent it, we could simply mount the chip facing **backwards**, i.e. into the reflector, and then the reflector could capture it and focus it into a tight beam. With a little careful focusing and stippling, we could even remove the shadow of the chip from the center of the beam (you may have noticed that a lot of incandescent lamps have a frosted or even opaque apex to them, but this does not leave a hole in the beam if you make allowances for it).
    But that leaves the major problem unsolved. As Peter Gransee has told us repeatedly, these new high-powered LEDs produce considerable heat, and are extremely sensitive to heat build-up. They need careful heat-sinking to keep them running. And while it might be possible to mount an LED at the focus of the parabola without the bulk of the chip obscuring the beam, we would certainly not be able to mount the chip **and** a simple and efficient heat-sink at the focus. Here's an area where incandescent filaments do have an advantage; they don't need a heavy heat-sink, so they can stand up into the focus on their own spindly leads without a lot of bulky undercarriage. Once we have provided a 5-watt LED with proper heat-sinking (and wiring), the package is too bulky to put at the focus of a parabolic reflector--at least one you could contemplate putting in your pocket.

    3) What can we do instead? My suggestion is that we borrow a trick from telescope design. Keep the LED at the vertex of the parabola, as it is in the KL4 head, and keep it facing forward. Now, mount a small hemispherical mirror at the focus of the parabolic reflector, facing backwards towards the LED. (This is roughly like a Schmidt-Cassegraine telescope design). The small hemispherical secondary reflector collects light from the LED, but it makes the primary, parabolic reflector think that it has a tiny point-source of light located at its focus. This puts us in the running with the basic incandescent design; it allows the parabolic reflector to create a hot-spot of light, and then the normal modifications and stippling can both provide for an adequate corona, and provide for 'erasing' the shadow of the secondary mirror. The LED can remain where it is, in the body of the flash-light, with simple wiring and adequate provisions for heat-sinking.
    The secondary mirror could be mounted in the parabolic primary with a set of spider-legs. Or, it could mount on a post projecting backwards from the front lens, like a mushroom on a stalk. Depending on the proportions and focal lengths, there might not be a stalk at all; for a turbo-head version with a 2.5 inch lens, it might simply be a matter of gluing a dime-sized mirror directly to the back of the lens.

    Well, that's the proposal, in outline. Practically all engineering is in the details, and I haven't given you any. I don't have the time or resources to try this idea out and see how quickly it hits a dead end. But not all engineering is in the details; there is also having a vision to work towards and a leading idea to follow. I think this basic idea is sound--and I have some confidence in that claim, because it is not my idea to begin with. It's just a standard telescope design run in reverse, with the LED located at the eye-piece. Maybe the idea can be scaled down and put in your pocket, maybe it can't; I'm hoping that one of you people with a good machine-shop and an R&D budget--or just time on your hands--can give it a try and tell us all how it works out. (And I wouldn't say 'no' to a beta-version….)
    In closing, let me take this opportunity to thank all of you out there for your willingness to share ideas, take risks, spend money, and sometimes lose money. Although I've never met him, I especially want to thank Peter Gransee, who is not only a fine engineer and entrepreneur, but also a really admirable communicator and educator. No one can read his posts without learning things and wanting to teach others. And no one can watch how he runs his company without feeling a renewed faith in basic principles of fairness, decency, boldness, innovation, and profit motive that are sometimes summed up under the heading of 'the American way'. That's another source of light in these dark times. Best wishes, curiousCrandall

  2. #2
    Flashaholic*
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    Default Re: LED long-throw design suggestion

    Or we can just copy what Streamlight is doing NOW with their TL-2 LED, TL-3 LED, and 3AA Luxeon Task Light.

  3. #3
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    Default Re: LED long-throw design suggestion

    There is the side emitter (SE) LS LED available in 1W. This has a lens built on the LED that replicates what a secondary reflector would do. It was designed to work with standard flashlight reflectors.

    I don't think it is available in 5W just yet.

    Is that what Streamlight is using?

    If you're still looking to use a secondary reflector, I don't think a concave or parabolic type would work. IIRC, the bare LS LED has a 120deg beam. That might be way too wide to effectively collect. Or you could collect it, but the secondary reflector would probably have to be quite large, which would make the primary even bigger and hence the whole package would be rather unwieldy.

    However, a convex reflector could be used close to the bare LS to "push" the beam outwards, towards the primary reflector. This, in effect, is probably similar to the workings of the SE LS LED. It garners a mention here only because this is probably what one has to do to use a 5W with a reflector to get long throw (since those slackers at LumiLeds still haven't designed a 5W SE).

    The difficulty would be finding the correct convex angle to use, the distance from the LED at which to place it, and how to ensure that it is centered (and remains centered) over the LED.

    Anyway, that's my 2cents.

  4. #4
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    Default Re: LED long-throw design suggestion

    I think 5 watt SE's are available. There's an Arcmania modded mag with an X3 binned SE in japan somehwere...

  5. #5
    *Flashaholic* McGizmo's Avatar
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    Default Re: LED long-throw design suggestion

    Welcome curiousCrandall,

    I believe it is safe to say that your proposal and assesment of the differences between incandescent and LED light sources have been investigated to quite an extent by many who are capable of understanding the nature of the beast as well as the physics of taming it. As a garage hacker, I have experimented along some of the lines you have discussed with various levels of sucess and failure.

    The SideEmitters are available in 5W but due to the die size and configuration of the integrated lens, they require an even larger optic or reflector to treat the light source as a relative "single point source" of light.

    As I now understand it, the SE's were designed to be embedded in acrylic or glass and to utilized in light pipe types of illumination. It was never intended to be used as a focal point light source.

    I guess I want to say that I don't think there is a simple solution or magic bullet here. I believe there are experts already involved in the physics of the optical output from these LED's and I suspect that they are aware of various methods available of photon redirection.

    - Don

  6. #6

    Default Re: LED long-throw design suggestion


    Good proposal, you've done alot of work.

    The problem I see is the flood beam of an LED light is NOT a problem, it's an advantage.

    Why is it that when a flashaholic or newbie gets a new flashlight they take it outside and try to illuminate a distant object? If it lights up that utility pole they say, hot damm this must be a good flashlight! Then they walk inside and use the flashlight inside over half of the time, where they would be better suited with an LED's flood beam!

    Even those people who walk at night, need a flashlight to illuminate their immediate path, say 20 feet ahead. On the occasion they hear something in the distance and feel better when they can illuminate something 100 ft in the distance. But they are better suited with a flood beam so they don't trip over something.

    I guess this is why manufacturers like Maglight tout their adjustable beam feature so highly. But we all know that doesn't produce a quality beam we all want. I adjust the beam for the smoothest beam and leave it there, unfortuniately that's usually a tight narrow beam.

    Personally I think that LED's flood beams are one of the best things that have happened to flashaholics, they will get over this quest to throw a pencil thin beam 500 yards, while tripping over the garden hose at their feet they can't see.

    Just my opinion,
    GregR

  7. #7

    Default Re: LED long-throw design suggestion

    The most practical approach may be a compound lens rather than fancy reflector arrangements. However, single reflectors work pretty well too, if they're big enough.

  8. #8

    Default Re: LED long-throw design suggestion

    [ QUOTE ]
    Double_A said:

    ....the flood beam of an LED light is NOT a problem, it's an advantage...Personally I think that LED's flood beams are one of the best things that have happened to flashaholics...

    [/ QUOTE ]

    Greg,

    My recent tests of a variety of lights in a rural setting demonstrates the truth of your statements. The quest for long throw LED lights (already successful with SN II) is important, but the ability of LEDs to illuminate large areas remains one of their most appealing characteristics.
    This is why the L4 was chosen by my little test group as their favorite walking light.

    Brightnorm

  9. #9
    *Flashaholic* McGizmo's Avatar
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    Default Re: LED long-throw design suggestion

    Double A and BrightNorm,

    I couldn't agree more with your assesments. Obviously different beams of light are needed depending on the task at hand. If there is something making noise in a tree 150 feet away or you need to do a search grid on the other side of a canyon, a tightly focused and intense beam of light is required. As often as not though, illumination is required at shorter distances and in wider fields of view.

    The spoof thread about the PhotonKing (a real person [img]/ubbthreads/images/graemlins/grin.gif[/img] ) is based mostly on the somewhat silly approach to the design of a light that will win the title of LuxKing.

    Binoculars and telescopes serve a purpose in bringing distant objects closer for view. It's not a good idea to comport oneself over unknown terrain while looking through either. If you are moving and working within your environment, I propose that an even flood of light is desired. If you are stationary and seeking information of distant objects, then a tighter focused and more intense beam is required. If you want to play watch the bouncing ball on the wall of an already well lit room, you will need a tightly focused and intense beam. [img]/ubbthreads/images/graemlins/grin.gif[/img]

    - Don

  10. #10

    Default Re: LED long-throw design suggestion

    Rather than having a focusable flashlight with a terrible flood beam (M*glite), I prefer to carry multiple lights with me (Arc LSH-P, Arc AAA, E2e, etc.). The Minigaglite may throw farther than the LSH-P with fresh batteries, but it is useless closeup, and the LSH-P works great for distances of 5-50 ft. (less than 5 feet away it is too bright).

  11. #11
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    Default Re: LED long-throw design suggestion

    I have to agree with the usefulness of the flood-like beams od LEDs for standard short to medium range! nothing beats a light like the L4 there!

    and if you want a LED with a throw, consider Don's DB750/R2H in the PM6 and your jaw will drop to the floor. although I do not own a SNII, this thing should fall in the same category.

    and as time goes by, the solution will eventually be found to make the LEDs the king of throw. be patient, watch and enjoy.

    bernhard

  12. #12
    Flashaholic
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    Default Re: LED long-throw design suggestion

    Fraen optics and EL's 30mm optics come pretty close. I believe many of these can out-throw the lower-end reflectored incandescents.

    That said, I use a McFlood myself. [img]/ubbthreads/images/graemlins/smile.gif[/img]

  13. #13
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    Default Re: LED long-throw design suggestion

    LEDs are in many ways, superior to incandescants. They just have different applications.

  14. #14

    Default Re: LED long-throw design suggestion

    I'm seeing the inverted mirror cup over point source in many automotive headlight applications. This what you mean?

    As for doing both (flood and longthrow spot), there is already a single flashlight that can do this (Pelican M6), although with several different screw-on (Mc)Modules, of which a couple "longthrowers" are still presently being developed.

  15. #15
    *Retired* The_LED_Museum's Avatar
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    Default Re: LED long-throw design suggestion

    Most automotive headlights have a round metal piece over the filament so that oncoming drivers and pedestrains are not blinded. The headlamp's reflector still functions as it should, except that there is less peripheral or spill light as the filament itself is obscured from the front of the bulb.

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