# Reflector characteristics?



## gcbryan (Jun 11, 2010)

I never seem to get a definitive answer to this question...what characteristics does the width and depth of a reflector vary?

I understand that different emitters have different beam angles and surface brightnesses and that incand is different than led but putting all that aside or rather just consider that I'm asking about XRE-R2 what effect is the reflector having as you vary those two perimeters?

The wider the reflector the more intense and tighter the hotspot ? The deeper the reflector the less spill (tighter spill)?

I understand spill and that a deeper reflector captures more of the beam that would become spill but I also understand that light captured from various places on the reflector aren't all created equal. 

I believe some areas contribute to the very brightest parts of the beam and other contribute bright light but not the brightest.

There is the hotspot and then the corona and then the spill.

Most lights that throw well have wide reflectors and relatively deep ones as well.

I'm not trying to get into the beam angle of the emitters here or the relative surface brightness (even though those factors have much to do with throw).

I'm just trying to get some simple answers only as they relate to reflectors and varying their parameters (specifically diameter and depth).

I think diameter controls hotspot size and intensity and depth controls spill size and intensity (apart from the emitter variables of course).


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## kengps (Jun 11, 2010)

Read this.......

https://www.candlepowerforums.com/threads/264750

You must consider the emmision angle of the LED. A narrow angle LED such as an R2 requires a smaller diameter/depth ratio reflector than say an R5 or SST-50 which emit light at much wider angles. Once you optimize the reflector width/depth for the LED, then the further the edge of the reflector is to the LED the better. It boils down to how far away the edge of the reflector is to the LED. The brightest part of the Hot-spot comes from the outer edges of the reflector. So the further away from the LED until the LED light strikes the reflector, the better it will be colimated.


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## gcbryan (Jun 11, 2010)

Have you read it (I have) and it doesn't answer the question.


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## TorchBoy (Jun 11, 2010)

What kengps is saying is that to design an optimum reflector you need to know what sort of LED it's going to work with. Changing the parameters of a reflector will do different things with different LEDs. But if what he said doesn't answer your question you're possibly only after simple generalisations, so you could have got some answers by posting a question in that thread, or in any one of your multiple threads about aspherics and/or reflectors (instead of starting yet another one), or even on DX. Very basically, the wider the reflector the easier it is for it to collimate the light (especially from large sources) and the deeper the reflector is the more light it will capture from the source. And of course the larger the reflector in both width and depth the better (or more forgiving) the parabola shape will be. Happy now?


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## TorchBoy (Jun 11, 2010)

This post shows very well why the LED type needs to be considered - even a perfect parabola won't focus/collimate an LED perfectly. (There really are the resources available on CPF if you have the time to go browsing.)


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## gcbryan (Jun 11, 2010)

kengps said:


> Read this.......
> 
> https://www.candlepowerforums.com/threads/264750
> 
> You must consider the emmision angle of the LED. A narrow angle LED such as an R2 requires a smaller diameter/depth ratio reflector than say an R5 or SST-50 which emit light at much wider angles. Once you optimize the reflector width/depth for the LED, then the further the edge of the reflector is to the LED the better. It boils down to how far away the edge of the reflector is to the LED. The brightest part of the Hot-spot comes from the outer edges of the reflector. So the further away from the LED until the LED light strikes the reflector, the better it will be colimated.



Yes, the greater the diameter the better it will be collimated. So the greater the diameter the tighter the hotspot (and presumably the brighter it will be)?

Regarding reflector depth little is really mentioned. I have read elsewhere that reflector depth doesn't contribute to throw however it seems to me that it would add to throw to divert some of those beams that would have become spill were the reflector not as deep.

That is not addressed. Very little regarding depth is addressed here or elsewhere.

If it's true that throw isn't improved by increasing reflector depth is it because the increased light being redirected from spill towards the hotspot isn't as collimated and therefore contributes to the corona rather than to the hotspot that is controlling throw?

These things aren't addressed.


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## gcbryan (Jun 11, 2010)

TorchBoy said:


> This post shows very well why the LED type needs to be considered - even a perfect parabola won't focus/collimate an LED perfectly. (There really are the resources available on CPF if you have the time to go browsing.)



I understand that the type needs to be considered but I tried to go to great lengths in my original post to eliminate that from the discussion (by just picking a led for this discussion) because the questions that I have are those relating to any reflector.

In other words the functions of diameter and depth. I'm not trying to pick a specific reflector for a specific flashlight at the moment.


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## kengps (Jun 11, 2010)

Of course increased depth will increase throw, because the edge of the reflector will be further away from the LED. But you cannot increase depth without increasing diameter. You cannot increase diameter without increasing depth. An XRE-R2 typically requires a reflector that is deeper compared to it's diameter. It will therefore have less side-spill. An SST-50 requires a larger diameter reflector compared to it's depth. It will therefore have more spill. I guess you could say that a more open angle reflector (more Dia/Less Depth) Is going to have more spill, at a wider angle. Perhaps the reason "Depth" is not mentioned very often is because Depth is a function of the Parabola. There is no reason we can't talk about reflectors in terms of depth, and never mention diameter at all. But incans were out before LED's and they project their light out the side. So very wide, shallow reflectors were used, and thus Diameter is the most significant dimension to measure them. Thats why with LED's (which need to have deeper reflectors than incans) are still refered to by their diameter dimensions. It's just customery to do so.


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## kengps (Jun 11, 2010)

gcbryan said:


> I think diameter controls hotspot size and intensity and depth controls spill size and intensity


 
No, they are completely inter-related. Of course Diameter controls hot-spot size. So does Depth. You cannot change the diameter or depth of a reflector without changing the dimension of the other as well. You could of course swap reflectors on an R2 for example, from an optimized small diameter/deep reflector, to a reflector that has larger diameter and less depth. That would give you more side-spill, but less throw despite the increased diameter. So in this example Diameter does not contribute to a more intense, and smaller hot-spot. All because the reflector does not fit the emission angle of the R2. Again, you must optimize the reflector for the LED. If you INCREASE diameter/depth, or DECREASE diameter/depth (away from the optimum) you get a less intense hot-spot. NOW....assuming the reflector is proper for the LED. As you increase diameter and depth together in the proper parabola....you will decrease the hot-spot size and therefore increase it's intensity.


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## LukeA (Jun 11, 2010)

kengps said:


> You cannot change the diameter or depth of a reflector without changing the dimension of the other as well. You could of course swap reflectors on an R2 for example, from an optimized small diameter/deep reflector, to a reflector that has larger diameter and less depth.



There. You've just contradicted yourself. You say that the depth and radius of a reflector are directly proportional, and yet in the next sentence you admit the existence of a reflector of larger radius and smaller depth than a reflector of arbitrary dimension, indicating that direct proportionality does not exist.

The depth and radius of a parabolic reflector are given by the relationship d=ar^2. The constant of proportionality, a, can take any value 0<a<∞ for a parabola opening upward in the xy plane. This means that r can have any value 0<r<∞ and d can have any value d<0<∞ simultaneously, indicating linear independence. 



kengps said:


> Again, you must optimize the reflector for the LED. If you INCREASE diameter/depth, or DECREASE diameter/depth (away from the optimum) you get a less intense hot-spot.



This statement is essentially false as well. After the LED die is at the focus of the reflector, the only "optimum" reflector is one that's infinite in radius and infinite in depth, or more practically, one that is as large in radius and depth as is practical for the application.


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## LukeA (Jun 11, 2010)

gcbryan said:


> Yes, the greater the diameter the better it will be collimated. So the greater the diameter the tighter the hotspot (and presumably the brighter it will be)?
> 
> Regarding reflector depth little is really mentioned. I have read elsewhere that reflector depth doesn't contribute to throw however it seems to me that it would add to throw to divert some of those beams that would have become spill were the reflector not as deep.
> 
> ...



Among CPFers, it's not a stretch to say that most agree that throw comes from reflector surface area. The emission angles of LEDs largely determines that a reflector with depth proportionally greater than its width (a "deeper" reflector) is better because the reflector subtends a greater angle with respect to the LED, capturing more light for the beam.


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## kengps (Jun 11, 2010)

EDIT. Never mind. got better things to do today.


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## Tally-ho (Jun 11, 2010)

Flashlight with an enhanced spot beam and a fully illuminated broad beam (link)
*US5459649.pdf* (download)

I know this patent concerns incandescent bulbs but when considering the title of this thread, maybe it will help someone to understand some points.


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## gcbryan (Jun 11, 2010)

LukeA said:


> Among CPFers, it's not a stretch to say that most agree that throw comes from reflector surface area. The emission angles of LEDs largely determines that a reflector with depth proportionally greater than its width (a "deeper" reflector) is better because the reflector subtends a greater angle with respect to the LED, capturing more light for the beam.



Is it your understanding and would you agree with the following...

The most collimated area of a reflector is the outer edge and the further you are from the emitter the better therefore a greater diameter has the most effect (other than emitter) on throw and that greater diameter means a narrower but more intense hotspot?

Would you agree that a deeper reflector (needed with leds because of the smaller beam angles) reduces spill and redirects that light back into the reflector where it essentially ends up at the outer edges of the reflector and therefore contributes to throw? 

Additional question, does all light that hits a reflector ultimately end up at the outer edge? If not then some light is more highly collimated than others and that more highly collimated light would determine throw...the other light would just be "filler". Does the redirected light from the depth of a reflector contribute more to this "filler" light?

Therefore, would it be accurate to say, as I have read elsewhere, that greater diameter determines throw rather than the redirected light from greater depth?


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## LukeA (Jun 11, 2010)

gcbryan said:


> Is it your understanding and would you agree with the following...
> 
> The most collimated area of a reflector is the outer edge and the further you are from the emitter the better therefore a greater diameter has the most effect (other than emitter) on throw and that greater diameter means a narrower but more intense hotspot?



I agree. The smaller the LED die is compared to the reflector the smaller the divergence angle of the beam (read: the better the throw).



gcbryan said:


> Would you agree that a deeper reflector (needed with leds because of the smaller beam angles) reduces spill and redirects that light back into the reflector where it essentially ends up at the outer edges of the reflector and therefore contributes to throw?



A deeper reflector is hit by more light from the LED. Light that reflects off of the parabolic reflector surface becomes the hotspot. So, in short, yes I agree.



gcbryan said:


> Additional question, does all light that hits a reflector ultimately end up at the outer edge? If not then some light is more highly collimated than others and that more highly collimated light would determine throw...the other light would just be "filler". Does the redirected light from the depth of a reflector contribute more to this "filler" light?



I'm having a little trouble understanding your nomenclature here. All the light that hits the reflector (neglecting efficiency losses) is collimated (more perfectly or less perfectly) and all the light that does not hit the reflector leaves the light as spill which is not collimated. The deeper the reflector is with respect to its radius, the more light it captures and the brighter the hotspot will be (for a given radius), and the larger the reflector's radius compared to the LED die, the smaller the beam's divergence angle. 



gcbryan said:


> Therefore, would it be accurate to say, as I have read elsewhere, that greater diameter determines throw rather than the redirected light from greater depth?



Hold reflector radius constant, and the deeper reflector of two otherwise identical ones will throw farther because the deeper reflector will capture more light but the beam divergence angles will be sufficiently identical.

Hold depth constant, and the one with the larger radius will have a smaller beam divergence angle, but the one with the smaller radius will gather more light. So I guess I don't know heuristically which one of those two reflectors will provide higher illuminance of a target at an arbitrary distance based on radius alone.

The entirety of the above only applies to Lambertian distribution and similar LEDs.

I hope some of it helped you understand.


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## Tally-ho (Jun 11, 2010)

gcbryan said:


> Would you agree that a deeper reflector (needed with leds because of the smaller beam angles) reduces spill and redirects that light back into the reflector where it essentially ends up at the outer edges of the reflector and therefore contributes to throw?


I suppose that it is nearly true but...
Eastward YJ-XGR2 and Ultrafire HS-802 (both with XP-E.R2) have reflector with about the same outer diameter but lengh of the reflector of YJ-XGR2 is about 3.5 cm and the one of HS-802 is about 5.5 cm.
The hotspot of HS-802 is much more tighter and brighter but doesn't have more throw proportionally to its gain in tightness and brightness.

I don't know if it has a significant importance but the brightness decreases proportionally to the square of the distance, so the light that strikes the outer part of a deep reflector may loose a part of its brightness, so it probably contributes to brightness but maybe not much than expected.
The light that strikes a reflector that is 2 times more deeper looses 4 times luminosity at the outer part. The light that strikes a reflector that is 3 times more deeper looses 9 times luminosity.

I am far to be a genius especially with optics, so don't pay to much attention to above guesses/conjectures.


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## TorchBoy (Jun 11, 2010)

kengps said:


> But you cannot increase depth without increasing diameter. You cannot increase diameter without increasing depth.


Both not true - you simply change the profile of the reflector's parabola. You'd have to add a condition such as "while maintaining the same focal length" for them to be true, but there's no particularly good reason to do that in practice.



gcbryan said:


> *1.* The most collimated area of a reflector ...
> *2.* ... the further you are from the emitter the better therefore a greater diameter has the most effect ...
> *3a.* Would you agree that a deeper reflector (needed with leds because of the smaller beam angles) reduces spill and redirects that light back into the reflector where it essentially ends up at the outer edges of the reflector and therefore contributes to throw?
> 
> *3b.* Additional question, does all light that hits a reflector ultimately end up at the outer edge?


1. Actually I would use "collimated" to refer to the light after it has been reflected from the collimating surface (the reflector).

2. What are you proposing to attribute that to? It seems to me that you're trying to make generalisations without understanding what's going on, such as how much light is given out at various angles, or how the source more closely approximates a point source the further away it is.

3. Go and find out how a parabola reflects light.



gcbryan said:


> Therefore, would it be accurate to say, as I have read elsewhere, that greater diameter determines throw rather than the redirected light from greater depth?


Once again, throw is determined by the amount of light in a given direction. Throw can be improved by increasing the total amount of light sent in that (approximate) direction AND narrowing the direction in which it is sent.


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## TorchBoy (Jun 11, 2010)

Tally-ho said:


> The hotspot of HS-802 is much more tighter and brighter but doesn't have more throw proportionally to its gain in tightness and brightness.


If it's tighter (= better throw) and brighter (= better throw) but it doesn't throw proportionally better then have you just discovered the inverse square law? Basically, to throw twice as far a light has to be four times as intense. Throw is in inverse proportion to the square of the distance, it's not an inverse linear relationship.


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## gcbryan (Jun 11, 2010)

Thanks guys.

A couple of points. If the HS-802 hotspot is tighter and brighter I think that it must throw further?

Regarding how wide is too wide. I think it's a matter of how narrow is too narrow. I think wider will always throw further but at some point the hotspot will be so small as to be useless (a laser).

Edit: I didn't see Torchboys post before posting this.


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## gcbryan (Jun 11, 2010)

TorchBoy said:


> ...
> 
> 
> 
> ...



I was referring to how the source more closely approximates a point source the further away it is.


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## TorchBoy (Jun 11, 2010)

gcbryan said:


> Regarding how wide is too wide. I think it's a matter of how narrow is too narrow. I think *wider* will always throw further but at some point the hotspot will be so small as to be useless (a laser).


That's just a typo, right? _Edit: Hold on, are you referring to the reflector or the hot spot?_

The diagram on http://en.wikipedia.org/wiki/Parabolic_mirror shows that light doesn't all leave the mirror from the outer edge.


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## Tally-ho (Jun 11, 2010)

TorchBoy said:


> If it's tighter (= better throw) and brighter (= better throw)...


So the ability to throw depends on brightness (square of the distance law), but not only. It also depends on a size law ?

Let's imagine two hotspots from perfect collimated rays of light. The smaller will throw better than a bigger one ?


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## gcbryan (Jun 11, 2010)

TorchBoy said:


> That's just a typo, right? _Edit: Hold on, are you referring to the reflector or the hot spot?_
> 
> The diagram on http://en.wikipedia.org/wiki/Parabolic_mirror shows that light doesn't all leave the mirror from the outer edge.



I was referring to the reflector...you can't get a reflector that is too wide for max throw. You can get one so wide that the hotspot is a laser and not useful.


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## gcbryan (Jun 11, 2010)

Tally-ho said:


> So the ability to throw depends on brightness (square of the distance law), but not only. It also depends on a size law ?
> 
> Let's imagine two hotspot from perfect collimated rays of light. The bigger one will throw better or worst than the smaller one ?



I think he is talking about diminishing returns. Brighter is always better.

Smaller doesn't automatically throw further but nothing is perfectly collimated. Also, if output is the same smaller is brighter.


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## Tally-ho (Jun 11, 2010)

Well, the only thing that I understood is that for better throw, an emitter has to be the smallest and the brightest as possible, but it has to emitte light not in only one direction because in this case there will be less, or no (rays of) light to collimate in a (useable) hotspot.


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## TorchBoy (Jun 11, 2010)

Tally-ho said:


> So the ability to throw depends on brightness (square of the distance law), but not only. It also depends on a size law ?
> 
> Let's imagine two hotspots from perfect collimated rays of light. The smaller will throw better than a bigger one ?


Throw depends on brightness, or intensity. If the intensity of the hotspots is the same they will have the same throw, regardless of how large they are.

You can increase the intensity of the hotspot by focusing the same amount of light into a smaller spot. That can be done using a larger diameter reflector. You an also increase the intensity of the hotspot by increasing the amount of light being focused into the same sized spot. That can be done using a deeper reflector, since it will capture more light from the source.



gcbryan said:


> You can get one so wide that the hotspot is a laser and not useful.


Figuratively speaking, not literally a laser. Lasers are still useful, though, even though they have very narrow beams. Far-throwing torches have their own uses, as do flood lights.


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## gcbryan (Jun 11, 2010)

TorchBoy said:


> You can increase the intensity of the hotspot by focusing the same amount of light into a smaller spot. That can be done using a larger diameter reflector.
> 
> You an also increase the intensity of the hotspot by increasing the amount of light being focused into the same sized spot. That can be done using a deeper reflector, since it will capture more light from the source.



Thanks. That's the best way to think about it.

With aspherics it would just be a matter of increasing the diameter (same amount of light in a smaller spot) and focal length (shorter vs longer) more light vs tigher focus with an optimal balance regarding focal length.

Would that be the correct way to think of aspherics as opposed to reflectors?


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## LukeA (Jun 12, 2010)

gcbryan said:


> ...
> 
> With aspherics it would just be a matter of increasing the diameter (same amount of light in a smaller spot) and focal length (shorter vs longer) more light vs tigher focus with an optimal balance regarding focal length.
> 
> Would that be the correct way to think of aspherics as opposed to reflectors?



You'd want to decrease the focal length and increase the diameter. Minimizing the f-number (focal length divided by diameter) will minimize the beam divergence angle. This is why some CPFers are constantly on the lookout for large lenses with short focal lengths. Those lenses lend themselves to collimation of LEDs.


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