URL explaining WHY large heads/reflectors have better throw?

[mzil]

I'm looking for a technical web page, that I can relay to a friend, explaining the physics behind why a large head (reflector, really) on a flashlight provides more "throw" compared to a smaller one. Anyone know of such an URL I can pass along?

[Unfortunately, links to discussions on this forum, or other similar forums, blogs, etc. wont be of any use in this case. He insists it be from more of an academic source like a text book or a scholarly journal, I guess.]

I would imagine this is explained in text books on physics, and that's exactly what I'm looking for, but I don't know where to find such references and I was hoping some of our readers here might know.

Thanks.

[yellow]

I dont have any links, but maybe You could do a drawing, that should be enough

* Usually bigger reflectors are also deeper - the deeper the reflector, the more light emitted goes into the focused part of the light
* the bigger reflector makes it easier to point the individual spot You are checking into a certain direction. With a smaller object, slight irregularities are extremely "bigger" in ratio

[Torchaddict]

You don't really need a physics paper to explain it to him. For the sake of this conversation, this applies to flashlights with just reflectors right (no special lenses).

The physics is really simple:
1. Light is reflected (duh).
2. In order to get perfect throw, you must have the light rays leave the flashlight parallel to each other (perfect collimation).
3. Light always obeys the law of reflection (the angle of incident light always equals the angle of the reflected light against a normal surface)
4. So in order to get collimation, you must always have a specific shape (angles must be met). Only a parabolic reflector can do this.
5. A larger reflector therefore gathers more light and therefore produces more throw.

You can imagine that a 1cm reflector will miss a whole lot of light vs one that is say 10cm (remember both have to be the same shape to produce collimation). This means that shorter reflectors can be thought of as basically larger reflectors that have been "cut down" from the front.



Both reflectors MUST meet this light at a specific angle, and since the reflector is one piece, you cannot design a smaller reflector that possesses the same characteristics (say, if you try to make it smaller, you'll sacrifice collimation near the LED). You cannot make a smaller reflector that has all of its surface parallel to the "perfect" reflector shape and collect the same/more amount of light.

The above depicts a perfect light source (point) and a perfect reflector. Of course LEDs are not point sources. This allows flashlight makers to divert slightly from the "perfect" shape since an LED is essentially multiple point sources. So you can sacrifice angles (and therefore collimation) near the LED without a huge loss (but there is loss!), but nonetheless, the same principle still generally applies (you still would collect more light and more collimation from larger reflectors).

Conversely, this can be used to explain why larger LED dies exhibit floody characteristics. A Cree XM-L, although brighter, can generally be outhrown by it's XT-E/XP-G brethren. This is because the larger die size of the XM-L makes it harder (actually impossible) to achieve the same degree of collimation as smaller die sizes. A reflector with an XM-L will not be able to meet all of the light rays at the correct angle, therefore, it tends to be floody. This is even more exaggerated in smaller reflectors.

[tatasal]

Very nice, Torchy

[mzil]

* Usually bigger reflectors are also deeper - the deeper the reflector, the more light emitted goes into the focused part of the light

Yes, thanks you two guys. I can certainly see how all things being equal a deeper reflector has a more focused spot, ie more throw, and also that the further away the reflector surface is the more it treats the light source (LED) as if it were correctly a true point source and not a circle/square area of light, but I still need an URL to pass along to him, not your forum posts or my relayed explanations of them.
---

What is the scientific study of reflected light/optics called anyways? Photometry? [No, I dont think so, but I'm at a loss for what it is.]

Maybe I can find a text book on this science on line. I just need to know what it is called!

[Th232]

I'd say looking for a textbook on optics should do it. "Optics for Dummies" might be a good start, although I've never read it myself.

Out of curiosity, why is this friend so adamant that it has to be a scientific paper or something? All of this is like me demanding Newton's original paper on F = ma before I believe that it's true...

[mzil]

The study of light seems to have no better word for it than "optics", yet we all know an "optician" isn't a person that studies what I'm talking about! "Optical engineer" is closer but still not quite right.

The study of lighthouses is called : "pharology" [edit to add: and "signal lights, their construction and illumination" so not too far off, really]

The study of light refraction is called : "dioptrics".

Google has let me down. :sad face:

[Torchaddict]

Well, a deeper reflector is only ONE aspect. It has to have a certain shape. I can design a deep reflector that won't throw very effectively. My point was that it takes much more than just any parabolic shape to make a light "throw."

[Norm]

Try here there's a paragraph on reflectors that might help.

Norm

[Th232]

The study of light seems to have no better word for it than "optics", yet we all know an "optician" isn't a person that studies what I'm talking about! "Optical engineer" is closer but still not quite right.

"Optics" is the study of light and how it interacts with materials. This includes reflection among other things. The principle that you're interested in is dead simple (reflection is just angle of incidence = angle of reflection), and with a reflector that's all you need. From there it's only a matter of geometry and ray tracing, so maybe you could try those keywords?

I suspect you won't find a more specific term than that because they're such a simple and basic principles that there isn't a person or field that's dedicated to it. Using my F = ma example, you won't find a dedicated "F = ma-ist", instead you'll find everyone from physicists to mechanical engineers who use it, likewise with reflection and refraction you'll find everyone from physicists to jewellers who uses it.

[angelofwar]

How about this: http://www.school-for-champions.com/...refraction.htm

Or this: http://www.learnnext.com/class10/sci...eflection2.htm

[ikeyballz]

If he's saying that because he's an engineer/engineering/physics major, its because in an ideal world if all the light is generated from a single point, the parabolic shape can be infinitely small (the head) and light will all trot out in a perfectly linear path. In reality, because light is not being emitted from a singular spot the size of the parabola has to be much greater than the space the light is being emitted from, so the light emitting area is as close to a point as possible.

Main idea: light emitted from a singular point can use a much smaller focusing dish. Since this isnt true in incan OR leds, the smaller (in ratio) you can make the emitting point to the dish, the more "ideal" it will be.

torchaddict, your drawing is close but the smaller dish (in your drawing, since its a point emission) needs to just be more concaved and it'll work exactly like the larger dish

[MichaelW]

The larger [deeper] reflector collects and redirects more light for useful work-throw.
For the same size light source.

[mzil]

If he's saying that because he's an engineer/engineering/physics major, its because in an ideal world if all the light is generated from a single point, the parabolic shape can be infinitely small (the head) and light will all trot out in a perfectly linear path. In reality, because light is not being emitted from a singular spot the size of the parabola has to be much greater than the space the light is being emitted from, so the light emitting area is as close to a point as possible.

Main idea: light emitted from a singular point can use a much smaller focusing dish. Since this isnt true in incan OR leds, the smaller (in ratio) you can make the emitting point to the dish, the more "ideal" it will be.

This! [You pretty much nailed my situation. He's had college physics and I haven't.]

[saabluster]

Here you go.

[AnAppleSnail]

And if your friend objects to the above post, show him what Saabluster does.

[djozz]

quite a thread already, and the precise answer isn't there yet, just hints, so here is why bigger reflectors make for better throwers:

The parabolic reflector of a flashlight in fact projects an enlarged image of the led in infinity, it behaves no different than a lens (in another type of flashlight an actual (aspheric) lens is used to do that, usually referred to as an 'aspheric' flashlight). So it abides the simple general lens formula ( 1/objectdistance + 1/imagedistance = 1/focal lenghth of the lens). This means in flashlight country that the weaker the lens (in this case the weaker the curvature of the parabolic reflector) the smaller the enlarged image of the led in infinity, which is the hotspot, and presto: the more throw! And in order for your weak-curved reflector to pick up a significant percentage of the light to be projected into the hotspot it needs to be wrapped around the led sufficiently and with its weak curvature it consequently will be big.

To correct one of the above explanations: you can just as easy catch the same amount of light with a small reflector with a stronger curvature, but the reflector's focal length is then shorter and the projected hotspot will be bigger -> less throw.

Please imagine some nice graphics to go with this story, and a bit better english (sorry), and there you have it all explained without following college classes :-)

djozz

[HotWire]

There is some general information about reflector shape in a flashlight here: http://www.maplesoft.com/application...5523&view=html

I didn't see any reference to size on that URL.

[Esko]

If he's saying that because he's an engineer/engineering/physics major, its because in an ideal world if all the light is generated from a single point, the parabolic shape can be infinitely small (the head) and light will all trot out in a perfectly linear path. In reality, because light is not being emitted from a singular spot the size of the parabola has to be much greater than the space the light is being emitted from, so the light emitting area is as close to a point as possible.

Main idea: light emitted from a singular point can use a much smaller focusing dish. Since this isnt true in incan OR leds, the smaller (in ratio) you can make the emitting point to the dish, the more "ideal" it will be.

This! [You pretty much nailed my situation. He's had college physics and I haven't.]

I, on the other hand, was pretty sure, that at least he can't have an engineering background. Because, this really is not a question of scientific research. Some basic (and very old) knowledge of optics and reflections should be enough, along with some logical thinking and the knowledge, that leds are not point sources of light. For a point source (singularity), the reflector could be infinitely small. For a surface, it can't.

Leds have a relatively big surface. The light is emitted to every direction from every single point of that surface. If we choose a single point on the reflector we can see, that the light emitted from different parts of the led arrive to that single point in different angles. And are therefore also reflected in different angles. Big light source surface, big differences in angles. Reflector surface close to the light source surface, big differences in angles. Small light source surface and(/or) long distance to the reflector surface (ie. big reflector), small differences in angles and hence the possibility to have a very narrow beam.

Really, if your friend can rely on his abilities to think logically and evaluate the data he is given, this should be enough. If the subject is new to him, a physics textbook about optics/reflections could be the scientific source he is looking for. Any university grade book should do the work.

[ikeyballz]

True, but in mathematics/physics you cover the idealistic cases before the "actuals" and in an ideal world, any sized perfect parabola will have a perfect focal point, and if light is emitted from that point, a parallel beam...etc etc. the theoretical stuff sticks much more than the actual in most cases. From my experience, engineers fall into two categories - those who have an amazing theoretical/equations side and those who are much more practical. I suppose the theoretical ones should've gone into physics/math though.

I, on the other hand, was pretty sure, that at least he can't have an engineering background. Because, this really is not a question of scientific research. Some basic (and very old) knowledge of optics and reflections should be enough, along with some logical thinking and the knowledge, that leds are not point sources of light. For a point source (singularity), the reflector could be infinitely small. For a surface, it can't.

Leds have a relatively big surface. The light is emitted to every direction from every single point of that surface. If we choose a single point on the reflector we can see, that the light emitted from different parts of the led arrive to that single point in different angles. And are therefore also reflected in different angles. Big light source surface, big differences in angles. Reflector surface close to the light source surface, big differences in angles. Small light source surface and(/or) long distance to the reflector surface (ie. big reflector), small differences in angles and hence the possibility to have a very narrow beam.

Really, if your friend can rely on his abilities to think logically and evaluate the data he is given, this should be enough. If the subject is new to him, a physics textbook about optics/reflections could be the scientific source he is looking for. Any university grade book should do the work.

[Esko]

True, but in mathematics/physics you cover the idealistic cases before the "actuals" and in an ideal world, any sized perfect parabola will have a perfect focal point, and if light is emitted from that point, a parallel beam...etc etc. the theoretical stuff sticks much more than the actual in most cases. From my experience, engineers fall into two categories - those who have an amazing theoretical/equations side and those who are much more practical. I suppose the theoretical ones should've gone into physics/math though.

True, too. This reminded me of Richard Feynman and his book "Surely you are joking, Mr Feynman!". In the book, he was briefly a visiting physics lecturer in Brazil. His students knew the theory very well - in theory. They had just memorized everything, without understanding the subjects and with no skills to put the knowledge into practice or recognize the phenomena in real life situations. At the end it was clear that even the author of their physics book - a highly regarded scientist in his country - had never done the experiments described in the book.

It was a good book (the Feynman's one that is).

Another thing that came to my mind was this comic.

Also tried to do a few quick searches in Sciencedirect and Google Scholar but didn't find any specific article for OP.

[fishndad]

rofl, the funniest thread ive read on cpf

[djozz]

Actually, very basic theory of optics applies very well in the practice of flashlight-making. Even the most cheapy toy-camera needs better optics than a flashlight. Optical complications such as edge-unsharpness and chromatic/spherical abberations of lenses/reflectors are not much of a problem, A blurry image of the led is even desired (soft hotspot), so leds are placed just out of focus, and the optical quality of the reflector is often even intentially ruined to obtain this (OP-reflectors). As long as reflectors are more or less parabolic with the right curvature, and the led is more or less neatly in the focal point of it, your beam pattern will be alright.
So IMO there is a lot of phyics theory that is not easily apllicable in practice, but for flashlight design, the basic general lens equation is more than adequate.
djozz

True, but in mathematics/physics you cover the idealistic cases before the "actuals" and in an ideal world, any sized perfect parabola will have a perfect focal point, and if light is emitted from that point, a parallel beam...etc etc. the theoretical stuff sticks much more than the actual in most cases. From my experience, engineers fall into two categories - those who have an amazing theoretical/equations side and those who are much more practical. I suppose the theoretical ones should've gone into physics/math though.