Seeing as how they use the same die it is reasonable to assume that the same bin for each part would have the same surface brightness at least at lower levels of output.
The Real Reason for Throw - an in depth examination
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easilyled
Flashaholic
Is the die packaged in the same way for the XP-E with the same surface-area as for the XR-E?
Do you know what the beam angle of emission is for the XP-E and whether it is wider than the XR-E?
It would certainly be handy if these 2 different emitters could be used to test this theory.
115 for the XP-E and 90 for the XR-E. Not sure what you mean by "packaged in the same way" as they clearly are very different packages.
easilyled
Flashaholic
That is why I am finding it difficult to know how you can be sure the surface brightness is the same for both of them.
First of all, you have not stated whether the surface area is the same.
Secondly the dome of the XRE would change the apparent die-size anyway.
Seeing as I already said they use the same die I figured it was quite obvious that the surface area would be the same.
As to the difference in apparent die size difference how else do you think the change is going to be made in viewing angle?
easilyled
Flashaholic
That doesn't invalidate the fact that the difference in apparent die size will change the surface brightness too between the XRE and XPE.
So, how will you know whether its the difference in surface brightness or beam emission angle that is altering the throw?
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I see you removed the reference to the SSC there. Good because it doesn't apply here.
Where did I say it invalidated the difference in apparent die size? You were asking about beam angle. Also you never did clarify what you mean by "a well-matched reflector".
I don't pretend to understand exactly how all the parameters play out with every given LED/reflector combination. There is a reason there is ray tracing software.
So if you really want your questions answered you need to test yourself or use the appropriate software.
easilyled
Flashaholic
I see you removed the reference to the SSC there. Good because it doesn't apply here.
Where did I say it invalidated the difference in apparent die size? You were asking about beam angle. Also you never did clarify what you mean by "a well-matched reflector".
I don't pretend to understand exactly how all the parameters play out with every given LED/reflector combination. There is a reason there is ray tracing software.
The point I was making, which I'm sure you know fully well, is that even an XPE cannot be used for comparison with an XRE to determine whether the beam angle makes a difference to the size of reflector needed to achieve a certain throw, because all other conditions are not the same.
You had previously said that an SSC-P4 and XPG couldn't be used because they have different surface brightnesses but implied that an XPE could be used as a comparison.
Midnight Oil
Enlightened
Hmm..
Is it correct to say that increasing the diameter of a reflector, while keeping the depth of the reflector constant, improves collimation but reduces the amount of light distributed to the hotspot; and that increasing the depth of the reflector, while keeping the diameter of the reflector constant, increases the amount of light distributed to the hotspot but decreases the amount of collimation?
If true, then for a given emitter at a given output and a given reflector diameter, there is a unique reflector depth that will maximize the lux of the hotspot; and out of those reflector diameter-depth ordered pairs, there is a unique combination that will yield the absolute maximum hotspot lux?
Is it correct to say that increasing the diameter of a reflector, while keeping the depth of the reflector constant, improves collimation but reduces the amount of light distributed to the hotspot; and that increasing the depth of the reflector, while keeping the diameter of the reflector constant, increases the amount of light distributed to the hotspot but decreases the amount of collimation?
If true, then for a given emitter at a given output and a given reflector diameter, there is a unique reflector depth that will maximize the lux of the hotspot; and out of those reflector diameter-depth ordered pairs, there is a unique combination that will yield the absolute maximum hotspot lux?
MikeAusC
Enlightened
In conventional domestic lighting there is a standard that has been used for many decades. A light is described as either Flood or Spot - the light is spread over a very wide angle (up to 60 degrees) or focussed in a narrow beam (around 10 degrees).
Of course there are many options in between these two extremes and that's why you have many other beamwidths available.
For some reason Torch users don't accept this long-standing standard- it's extremely rare to even see the word "spot" used in torch descriptions for a torch that has a narrow beam.
Throw describes only how bright the beam is at it's centre, but I don't see that being very helpful about describing the usefulness of a beam - a 1 degree beam would be VERY bright, but useless to most people.
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OHHH god lost my 300 word post 
short: Its "effective coolimating surface" that matters.
Its not the diameter which is determinating the throw at leds.
That law is only for omnidirectional lightsources!!! Which all use parabolic reflectors. Parabolic is the perfect shape, the more diameter you add the more of the surface brightness gets seen and used. Adding deepness will only increase the hotspot size, throw and lux remain unchanged.
LEDs have other emitting angles, the surface brightness decreases the further you go away from 0°.
A XR-E is the king of throw under the emitters because of its small viewing angle. For this small angle, the optimum reflectors are not parabolic, they are deep, so the light can hit it in the right angle.
And now the most important point. The deep reflectors, that are the optimum for the XR-E come closer to the 0°, where they can see the most intense brightness, and coolimate it.
Thats why aspheric lenses are so superior. They actually are at 0°, and can see the hightest brightness!!! Adding diameter here means the brightest part of the light source gets "seen" from a bigger "effective coolimating surface", resulting in the best throw.
short: Its "effective coolimating surface" that matters.
Its not the diameter which is determinating the throw at leds.
That law is only for omnidirectional lightsources!!! Which all use parabolic reflectors. Parabolic is the perfect shape, the more diameter you add the more of the surface brightness gets seen and used. Adding deepness will only increase the hotspot size, throw and lux remain unchanged.
LEDs have other emitting angles, the surface brightness decreases the further you go away from 0°.
A XR-E is the king of throw under the emitters because of its small viewing angle. For this small angle, the optimum reflectors are not parabolic, they are deep, so the light can hit it in the right angle.
And now the most important point. The deep reflectors, that are the optimum for the XR-E come closer to the 0°, where they can see the most intense brightness, and coolimate it.
Thats why aspheric lenses are so superior. They actually are at 0°, and can see the hightest brightness!!! Adding diameter here means the brightest part of the light source gets "seen" from a bigger "effective coolimating surface", resulting in the best throw.
I've enjoyed reading and thinking about this thread and I think I do actually have something valuable to contribute, hopefully!
Firstly, I just wanted to say I found it really useful to copy what saablaster did and used a mask on the torch to see how the torch's beam is made up of lots (infinite) different images stacked on top of each other. Half an hour of playing in the dark with this gave me a much better feel for how its working. Using this method I was actually able to figure out why my LD20 R4 has a bit of a donut in the hot spot – it seems to be caused by some parallel lines on the surface of the xp-g, possibly wires or something, which I can clearly see when using a mask like saablaster did. When you rotate the image of the die in the hotspot around, the lines mean a bit less light hits the centre of the hotspot. You can see these lines in photos of the actual xp-g itself, so it was cool to see them projected onto the wall using a mask.
Ok, now to the thing I wanted to contribute. We all know leds (more or less) emit light in a conical shaped arc out from the emitter, with a higher amount of light going out the centre of the cone with lesser intensity as you go towards the edge of the cone. There is a graph of the relatively intensities depending on angle in the xp-g datasheet on the cree website (http://www.cree.com/products/pdf/XLampXP-G.pdf) . Originally I felt this meant that most of the light was actually going straight out the front of the torch without bouncing off the reflector, because we all know more comes out the front.
But, there are two reasons why the amount of light escaping out the front without reflecting isn't anywhere near as much as I'd originally thought.
Firstly, the measurements in Cree's graph are for relative intensity at the different angles, so even at a significant (say 45 degrees) off centre angle the intensity is about 75% of the intensity that is going straight out the front. So less intensity, to be sure, but still a lot of light.
Secondly, the 3D cone of light that the led puts out can be divided into two sections. The first section is the unmodified cone of light emitted (ie assume no reflector). The second section is a smaller cone inside the larger one, which represents light that escapes straight out the front of the lens without bouncing off the reflector. I set out to calculate the relative areas of the curved base of these cones (the "front" of them when looking straight into the lens). The nature of these cones that means that small central cone that escapes has a much smaller base surface area compared to the overall output cone.
I worked with some rough inputs based roughly on my LD20 R4. I roughly estimated the angle of the cone from the emitter to escape without hitting the reflector to be about 40 degrees across (using a protractor and line of sight) (ie 20 degrees either side of centred).
Below is a chart of how much of the total output travels in each angular sector from the emitter. Eg the 10 degree sector is all the light coming out from the central 20 degrees (10 off centre) of the emitter. Looking in the front of the torch the sectors would be a bunch of concentric circles rippling out from the led, however note these circles are really lines on a sphere with the sphere's centre being the led. The green rows are the ones where I estimate the LD20 will let the light straight out the front instead of reflecting. If someone wants to check my maths that would be nice actually, I can send a copy of the spreadsheet The "Cree relative %Intensity in this sector" column is from me looking at the graph on the xp-g specifications and should be accurate to within a couple of %. "Nominal units of output" is a nominal measurement of how much light goes out through that sector, based on relative intensity and the % of total emitter output area. Not 100% on my maths in that column. It doesn't matter what the units are, they are just used for relative calculations between the sectors.
Anyway using those inputs, the results are that the area of the base of the cone of light escaping out the front without reflection is only about 7% of the area of the large cone of light the emitter actually puts out. However about 12% of the total emitted light goes out through this area as the emitter output is higher straight out the front.
So end of the story is - on my LD20, only a smallish proportion of the emitted light goes out the front without being reflected first.
Hope I've made sense to some people! And that the maths is right.
Ps. The formula I used for calculating the 3d surface area of a piece of a sphere marked out by a cone coming from the centre of the sphere with a given angle is:
A = 2 * pi * r * r * (1 – cos(a)) (where r is radius of the sphere and a is the angle of the cone). Just pick any r (as long as you use the same one), then adjust "a" as you need - you're only after a ratio here not an absolute value.
Pps. I haven't taken into account the size of the emitter to simply this a lot. Its not a precise calculation, for that you'd have to do maths way over my head as there's an infinite number of cones centred on the infinite number of spots on the emitter's surface. However its fine for a rough calculation which is all I/we need.
Firstly, I just wanted to say I found it really useful to copy what saablaster did and used a mask on the torch to see how the torch's beam is made up of lots (infinite) different images stacked on top of each other. Half an hour of playing in the dark with this gave me a much better feel for how its working. Using this method I was actually able to figure out why my LD20 R4 has a bit of a donut in the hot spot – it seems to be caused by some parallel lines on the surface of the xp-g, possibly wires or something, which I can clearly see when using a mask like saablaster did. When you rotate the image of the die in the hotspot around, the lines mean a bit less light hits the centre of the hotspot. You can see these lines in photos of the actual xp-g itself, so it was cool to see them projected onto the wall using a mask.
Ok, now to the thing I wanted to contribute. We all know leds (more or less) emit light in a conical shaped arc out from the emitter, with a higher amount of light going out the centre of the cone with lesser intensity as you go towards the edge of the cone. There is a graph of the relatively intensities depending on angle in the xp-g datasheet on the cree website (http://www.cree.com/products/pdf/XLampXP-G.pdf) . Originally I felt this meant that most of the light was actually going straight out the front of the torch without bouncing off the reflector, because we all know more comes out the front.
But, there are two reasons why the amount of light escaping out the front without reflecting isn't anywhere near as much as I'd originally thought.
Firstly, the measurements in Cree's graph are for relative intensity at the different angles, so even at a significant (say 45 degrees) off centre angle the intensity is about 75% of the intensity that is going straight out the front. So less intensity, to be sure, but still a lot of light.
Secondly, the 3D cone of light that the led puts out can be divided into two sections. The first section is the unmodified cone of light emitted (ie assume no reflector). The second section is a smaller cone inside the larger one, which represents light that escapes straight out the front of the lens without bouncing off the reflector. I set out to calculate the relative areas of the curved base of these cones (the "front" of them when looking straight into the lens). The nature of these cones that means that small central cone that escapes has a much smaller base surface area compared to the overall output cone.
I worked with some rough inputs based roughly on my LD20 R4. I roughly estimated the angle of the cone from the emitter to escape without hitting the reflector to be about 40 degrees across (using a protractor and line of sight) (ie 20 degrees either side of centred).
Below is a chart of how much of the total output travels in each angular sector from the emitter. Eg the 10 degree sector is all the light coming out from the central 20 degrees (10 off centre) of the emitter. Looking in the front of the torch the sectors would be a bunch of concentric circles rippling out from the led, however note these circles are really lines on a sphere with the sphere's centre being the led. The green rows are the ones where I estimate the LD20 will let the light straight out the front instead of reflecting. If someone wants to check my maths that would be nice actually, I can send a copy of the spreadsheet
The "Cree relative %Intensity in this sector" column is from me looking at the graph on the xp-g specifications and should be accurate to within a couple of %. "Nominal units of output" is a nominal measurement of how much light goes out through that sector, based on relative intensity and the % of total emitter output area. Not 100% on my maths in that column. It doesn't matter what the units are, they are just used for relative calculations between the sectors.
Anyway using those inputs, the results are that the area of the base of the cone of light escaping out the front without reflection is only about 7% of the area of the large cone of light the emitter actually puts out. However about 12% of the total emitted light goes out through this area as the emitter output is higher straight out the front.
So end of the story is - on my LD20, only a smallish proportion of the emitted light goes out the front without being reflected first.
Hope I've made sense to some people! And that the maths is right.
Ps. The formula I used for calculating the 3d surface area of a piece of a sphere marked out by a cone coming from the centre of the sphere with a given angle is:
A = 2 * pi * r * r * (1 – cos(a)) (where r is radius of the sphere and a is the angle of the cone). Just pick any r (as long as you use the same one), then adjust "a" as you need - you're only after a ratio here not an absolute value.
Pps. I haven't taken into account the size of the emitter to simply this a lot. Its not a precise calculation, for that you'd have to do maths way over my head as there's an infinite number of cones centred on the infinite number of spots on the emitter's surface. However its fine for a rough calculation which is all I/we need.
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wyager
Flashlight Enthusiast
- Joined
- Feb 10, 2010
- Messages
- 1,114
WOW! that's some impressive work!
MikeAusC
Enlightened
Angular Contribution
bbb74 - thanks - VERY educational and interesting work. I'd also assumed that there was greater percentage of light in the central area of the beam, but your column 2 gives hard facts. It's important to think in solid angles - not just angular width !
XPGs and XREs have quite different angular light distribution, so it would be great if you could do the graph also for XREs.
The 3rd colmn would need to labelled "Cree XPG" and "Cree XRE" to make it clear whether they apply to LEDs with 90 deg or 120 deg beamwidth.
Your column 3 needs be
bbb74 - thanks - VERY educational and interesting work. I'd also assumed that there was greater percentage of light in the central area of the beam, but your column 2 gives hard facts. It's important to think in solid angles - not just angular width !
XPGs and XREs have quite different angular light distribution, so it would be great if you could do the graph also for XREs.
The 3rd colmn would need to labelled "Cree XPG" and "Cree XRE" to make it clear whether they apply to LEDs with 90 deg or 120 deg beamwidth.
Your column 3 needs be
MikeAusC
Enlightened
Ideal Reflector for spot beam
Although if you had the ideal reflector designed for a narrow spot beam, every part of the reflector would aim its light at the centre spot.
It's like using an Aspheric instead of a Spherical lens.
Although if you had the ideal reflector designed for a narrow spot beam, every part of the reflector would aim its light at the centre spot.
It's like using an Aspheric instead of a Spherical lens.
MikeAusC
Enlightened
Internal part-Spherical Reflector for brightest spot ?
I tend to think of the Reflector focussing all its light onto the spot and the light that goes straight out the front, forms the nice smooth fill area you get from most Reflector lights.
I wonder if anyone has patented a Twin Reflector design to get maximum light onto the spot and reduce the fill, if you want sharper cutoff.
If you put an Internal part-Spherical Reflector in the middle of the main Parabolic Reflector it would reflect the light, that was going to bypass the main Parabolic Reflector, back onto the LED. The reflected light hitting the LED would be reflected again, maybe this time exiting via the main Parabolic Reflector.
In another thread on CPF, there's a discussion on a Patent to use a part-spherical reflector, in conjunction a lens as the main optic, to reflect light back to the LED. This patent though doesn't seem to make any reference to using a Parabolic Reflector as the main optic.
Another option to focus all light onto the centre spot, is to use a Parabolic Reflector plus a lens in the centre to catch spill light and re-direct it. Since there are already commercial implementations for sale, this method is already patented - or not patentable.
I tend to think of the Reflector focussing all its light onto the spot and the light that goes straight out the front, forms the nice smooth fill area you get from most Reflector lights.
I wonder if anyone has patented a Twin Reflector design to get maximum light onto the spot and reduce the fill, if you want sharper cutoff.
If you put an Internal part-Spherical Reflector in the middle of the main Parabolic Reflector it would reflect the light, that was going to bypass the main Parabolic Reflector, back onto the LED. The reflected light hitting the LED would be reflected again, maybe this time exiting via the main Parabolic Reflector.
In another thread on CPF, there's a discussion on a Patent to use a part-spherical reflector, in conjunction a lens as the main optic, to reflect light back to the LED. This patent though doesn't seem to make any reference to using a Parabolic Reflector as the main optic.
Another option to focus all light onto the centre spot, is to use a Parabolic Reflector plus a lens in the centre to catch spill light and re-direct it. Since there are already commercial implementations for sale, this method is already patented - or not patentable.
MikeAusC
Enlightened
Another useful column to add, particularly for Lens-only applications would be a cumulative column
i.e. 10, 10+20, 10+20+30, etc to show what percentage of the light you capture, as you increase the capture angle.
Re: Angular Contribution
I was thinking of doing something like that actually, but... The XP-E isn't *vastly* different to the XP-G in its light distribution (from memory, after a quick look at the specs sheet a few weeks ago). Really I think most of the xp-e's better throwing power over an xp-g is because of what saablaster was saying - the led die is brighter (because of its much smaller size), less so because of the angular intensity of its output, but I could be wrong. But you are right, the XR-E is quite different to the XP-E/G and it will probably have more of an effect, I'll do it later too. Have a look at the data sheets on the cree website and check the charts.
I disagree, you should try using the mask like saablaster did to understand. Any decent reflector would aim its light at the centre spot. But even the "ideal" reflector you talk of will still have reflector parts closer to the led and the image of the die it projects will be larger (and therefore less light is contributed to the hot spot as the image of the die is less intense due to being spread over a much larger area. Honestly, watch saablaster's video and try it yourself. Poke a small round hole in some plastic or thick paper and find a dark room!
I was thinking of doing something like that actually, but... The XP-E isn't *vastly* different to the XP-G in its light distribution (from memory, after a quick look at the specs sheet a few weeks ago). Really I think most of the xp-e's better throwing power over an xp-g is because of what saablaster was saying - the led die is brighter (because of its much smaller size), less so because of the angular intensity of its output, but I could be wrong. But you are right, the XR-E is quite different to the XP-E/G and it will probably have more of an effect, I'll do it later too. Have a look at the data sheets on the cree website and check the charts.
MikeAusC; said:
I disagree, you should try using the mask like saablaster did to understand. Any decent reflector would aim its light at the centre spot. But even the "ideal" reflector you talk of will still have reflector parts closer to the led and the image of the die it projects will be larger (and therefore less light is contributed to the hot spot as the image of the die is less intense due to being spread over a much larger area. Honestly, watch saablaster's video and try it yourself. Poke a small round hole in some plastic or thick paper and find a dark room!
No, reflected light contributes to spill as well, especially the corona but to overall spill as well. Try it with one of your torches with a mask in dark room. You need to see it to understand. Remember all the images you see on the wall will all be added together to form the beam you normally see.
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