XM-L Color Separation

[edroedro]

Has anyone noticed the large amount of color separation on the XM-L?

The low angle light on my XM-Ls is very warm and the high level light is very blue.



I'm sure it mixes together to give the the nominal color temperature in a sphere.



I have noticed this on most other LEDs, but it seems much worse on the XM-L.

Has anyone noticed this phenomenon?

 

[SlowPoke808]

I've witnessed this first-hand and there are numerous posts in which people are reporting greenish or yellowish colors around the hotspot and purple in the spill area of the beam. We're probably just seeing the chromatic dispersion of the light as it exits the XM-L dome. The large surface area of the die relative to the dome guarantees that a lot of light will pass through the surface of dome at an angle and disperse (only light emitted from the very center of the die will pass through perfectly perpendicular to the dome's surface and thus not disperse).

I'm guessing that all XM-L emitters are doing this but some reflectors happen to focus in such a way that the separate colors are concentrated, making the colors very noticeable. Other reflectors probably focus differently and mix the light to make it appear white again.

 

[qwertyydude]

Throwy SMO reflectors really amplify this with a hotspot that's noticeably yellower than the spill.

 

[richpalm]

For me, orange peel reflectors or frosted optics reduce or eliminate the variation.

Rich

 

[HooNz]

I just checked mine , its blue-ier on low and white-er on high , a T6

 

[Mike S]

All the T6 bin XML's I've tested show the same green/yellow light at its edges.

SST-50/90 orange peel reflectors (12° and 28°) seem to reflect that light into the hotspot.

 

[SlowPoke808]

Now I'm irritated with this. I just finished my first Mag 3C mod with a 5000K XM-L T6 from Cutter. Check out the colors in this photo with the head removed:

The hotspot is VERY yellow/green!

These photos were taken with an old, low-end camera but they look fairly close to what I'm seeing in real life. The first photo was taken with the Mag almost on it's side with a piece of white paper underneath. In the second photo I'm just aiming the assembled light at the ceiling.

It looks to me like this reflector (stock plastic Mag incan) is concentrating the yellow/green light coming out the sides of the emitter right in the hotspot and the blue/purple light is mostly going out the front without being reflected. Since the blue/purple is not concentrated enough, especially where the yellow/green is, it can't recombine in equal proportions to make white light again.

I may have to add some texture to the reflector or something because I can't stand that color! It's too bad though, because I didn't want to give up any throw.

 

[Harold_B]

I recall this coming up before the crash and there's an abreviated thread still out there. I have a model of an XM-L for lighting simulations although I haven't fine tuned the phosphor parameters to exactly match the manufacturer specs with regard to the spectrums. I ran some simulations anyway as part of the discussion as to why there is color seperation in this product. The reason the XM-L has color seperation isn't the dome and some kind of rainbow effect, it is the path length of the source light through the phosphor layer. There is a good reference to what I mean in this paper by Osram: http://www.osti.gov/bridge/servlets/purl/963890-7R3mLc/963890.pdf

The off-axis color seperation can be simulated in software that has volumetric fluorescing / down conversion capablity. In this image you can see the ray are rather squiggly at the emitter surface as they make their way through the phosphor particles but they do not deviate much if at all when exiting the dome: http://i1083.photobucket.com/albums/j390/Harold_LF/XM-L-RAY.jpg

These are some screen shots of the detector with the CCT measured at various angles off-axis:
http://i1083.photobucket.com/albums/j390/Harold_LF/XM-L-CENTER.jpg
http://i1083.photobucket.com/albums/j390/Harold_LF/XM-L-MID.jpg
http://i1083.photobucket.com/albums/j390/Harold_LF/XM-L-LOW.jpg

For kicks I stuck a Carlco optic in the path and ran a simulation and replicated the same basic patterns as shots above:
http://i1083.photobucket.com/albums/j390/Harold_LF/XM-L-WITHOPTIC.jpg

Doesn't fix the problem but now you know why it's there.

 

[SlowPoke808]

Now that is really interesting! Thanks for sharing that paper. I'm still reading through it. I'm hoping to see an emitter configured like the one on page 13 in production in the near future!

Are you able to run a simulation on the XM-L without the dome? How different does it look?

I don't see what makes the XM-L unique as far as color separation goes. Shouldn't we be seeing the same thing with the XP-G for instance? Maybe it's happening on a smaller scale so it's less noticeable?

I'm also curious about this picture: http://i1083.photobucket.com/albums/...F/XM-L-RAY.jpg
You are right when you say that the path does not deviate much when it exits the dome. What refractive index did you plug in for the dome material? Shouldn't we be seeing much more refraction (unless this is an underwater simulation )? It's obvious that there's a lot of refraction going on when you look at an XM-L in real life because the die looks all distorted when viewed at an angle, mostly on the far side of the die as expected...

 

[Harold_B]

The light entering the dome, reflecting from the die, exiting the dome and entering your eye will take a completely different path than the light being emitted from the die. The purpose of the dome and the reason it is effective and the shape of choice for most LEDs is that the interface surface between the air and the silicone encapsulant is near normal to the light emitted from the die. One thing to keep in mind about the screen shots is that they are CAD models and lighting simulations, not renderings. That is to say that they are dimensionally correct to the micron when possible, every surface and material is defined by optical parameters, but the distortion you would actually see is not represented correctly.

This is the RI I used for the dome. It is a silicone encapsulant specific for high temp / high power LEDs: http://i1083.photobucket.com/albums/j390/Harold_LF/SILICONERI.jpg
can't post the name due to NDA.

I haven't used the XP-G to any great extent so I can't say that I have or have not had the effect in our samples. The off-axis color seperation can be more pronounced depending on the size of the die, the thickness of the phosphor layer, and the layering method. I know there are ways to mitigate the problem (Rebels don't have an issue that I am aware of) but any source stuck in a parabolic reflector will suffer from some systematic chromic distortion.

I've read a lot of patents and papers about filters on or in LEDs like the one you mention on page 13 and they seem like they would be really expensive to produce, particularly on a dome.

 

[SlowPoke808]

The light entering the dome, reflecting from the die, exiting the dome and entering your eye will take a completely different path than the light being emitted from the die.

This is true only up to the point where the light leaves the die. If the paths of the light entering your eye were different after that point then the die would appear to change shape when you turn the LED on.

One thing to keep in mind about the screen shots is that they are CAD models and lighting simulations, not renderings. That is to say that they are dimensionally correct to the micron when possible, every surface and material is defined by optical parameters, but the distortion you would actually see is not represented correctly.

I think your representation should still be very accurate. I took another look at the picture and noticed that some paths do indeed refract significantly. These are the ones that are emitted from one edge or corner of the die and exit the dome on the extreme opposite side. I didn't didn't notice this at first, probably because the number of paths drawn are relatively sparse.

Do you still have easy access to the simulation software? It would be really interesting to run a simulation with light being emitted from only one point on the very corner of the die. I'm almost certain that we'd see a very distinct pattern appear. I would still like to see the simulation run without the dome as well.

In any case, I'm only brainstorming here to try to come up with a new way to mitigate the color separation when using XM-L emitters.:thinking: I'm just a consumer so I have to work with what I can afford to buy and the XM-L seems to be producing the most bang for the buck at the moment.

Thanks for sharing your research. Keep it coming!

 

[SlowPoke808]

Has anyone seen or used this multifaceted reflector? This might do the trick! Too bad they don't have Mag-sized ones...

 

[Canuke]

If this phenomenon were a function of refractive index, wouldn't it be much less prominent with the XR-E package? I see just as much of this effect with those emitters.

On the other hand, my USV0I Seoul emitters do not seem to show this issue much at all, despite having the same sort of dome.

I've always thought that this effect was due to phosphor having a thicker accumulation around the edge of the die than on its top face.

I seem to remember some folks around here removing domes from emitters; do bare dice show this pattern?

 

[SlowPoke808]

Harold's point about the particular path the light takes through the phosphor layer totally makes sense and it explains why you would get different colors before it even exits the dome. Assuming that your die is flat and the phosphor layer is uniform in thickness, you would expect a bluer color from the light that's passing through the phosphor layer perpendicular to the phosphor plane. Light exiting at high angles (more parallel) to the plane will have to pass through much more of the yellow phosphor material and result in a yellower tint (if I'm understanding things correctly).

Refraction from the dome would further enhance the color separation, but you would still expect this to be happening with the XP-E if the die/phosphor layer construction is similar to the XP-G and XM-L. The only way to really tell would be to run some tests without the domes. Has anyone out there been able to de-dome any of these LEDs without damaging the phosphor layers? If so, we need some side profile shots without reflectors!

I don't know how the dies are manufactured but if it were possible to create a die with a textured surface (dimpled perhaps) that might help to randomize the amount of phosphor the emitted light passes through at any angle to create a uniform color.

 

[bshanahan14rulz]

I don't know how the dies are manufactured but if it were possible to create a die with a textured surface (dimpled perhaps) that might help to randomize the amount of phosphor the emitted light passes through at any angle to create a uniform color.

Good thinking! They do that. This is an old sample teardown, but still neat pics
http://www.yole-dev.com/pagesAn/products/Report_sample/mu4.pdf

 

[saabluster]

Harold's point about the particular path the light takes through the phosphor layer totally makes sense and it explains why you would get different colors before it even exits the dome. Assuming that your die is flat and the phosphor layer is uniform in thickness, you would expect a bluer color from the light that's passing through the phosphor layer perpendicular to the phosphor plane. Light exiting at high angles (more parallel) to the plane will have to pass through much more of the yellow phosphor material and result in a yellower tint (if I'm understanding things correctly).

Refraction from the dome would further enhance the color separation, but you would still expect this to be happening with the XP-E if the die/phosphor layer construction is similar to the XP-G and XM-L. The only way to really tell would be to run some tests without the domes. Has anyone out there been able to de-dome any of these LEDs without damaging the phosphor layers? If so, we need some side profile shots without reflectors!

I don't know how the dies are manufactured but if it were possible to create a die with a textured surface (dimpled perhaps) that might help to randomize the amount of phosphor the emitted light passes through at any angle to create a uniform color.

Cree's HEW XP-E does essentially do this. Not via dimples but a raised structure. This effect of the changing color based on angle has been known and understood here on CPF for many many years. Sorry I didn't chime in any earlier but yes Harold got it right.

 

[SlowPoke808]

Good thinking! They do that. This is an old sample teardown, but still neat pics
http://www.yole-dev.com/pagesAn/products/Report_sample/mu4.pdf

This paper has some very good pictures. I didn't see any reference to a dimpled surface though. Figures 4.2 and 4.18 reveal some microscopic texture on the surface of the phosphor layer but it is flat in shape overall so you will still have yellower light being emitted at high angles. I was thinking larger dimples. Imagine that same picture of the die having a surface like a golf ball with deep dimples. This would cause the yellower light to be emitted at a wide range of angles relative to the overall die plane, not just very high angles. The same goes for the bluer light. It would be emitted at all sorts of angles, not just the low angles. The resulting mixing should produce a much more uniform, whiter light overall.

 

[SlowPoke808]

Cree's HEW XP-E does essentially do this. Not via dimples but a raised structure. This effect of the changing color based on angle has been known and understood here on CPF for many many years. Sorry I didn't chime in any earlier but yes Harold got it right.

I looked at the datasheet for the High Efficiency XP-E but I didn't notice anything different from the regular XP-E other than the diffused dome (that's definitely another way to reduce apparent color separation though!) Do you have any more info on this raised structure?

 

[saabluster]

This paper has some very good pictures. I didn't see any reference to a dimpled surface though. Figures 4.2 and 4.18 reveal some microscopic texture on the surface of the phosphor layer but it is flat in shape overall so you will still have yellower light being emitted at high angles. I was thinking larger dimples. Imagine that same picture of the die having a surface like a golf ball with deep dimples. This would cause the yellower light to be emitted at a wide range of angles relative to the overall die plane, not just very high angles. The same goes for the bluer light. It would be emitted at all sorts of angles, not just the low angles. The resulting mixing should produce a much more uniform, whiter light overall.

You are correct. The rough texture on that LED is not to reduce the variance in color but to aid in light extraction.

I looked at the datasheet for the High Efficiency XP-E but I didn't notice anything different from the regular XP-E other than the diffused dome (that's definitely another way to reduce apparent color separation though!) Do you have any more info on this raised structure?

I tore down a HEW the other day and was amazed at what I found. The structured is raised and has cut angles and is just beautiful to look at. It looks like a cut gem stone. I was going to post some pictures but then Cree released their royal blue version the other day and along with it was a nice picture that shows it well.

 

[SlowPoke808]

I tore down a HEW the other day and was amazed at what I found. The structured is raised and has cut angles and is just beautiful to look at. It looks like a cut gem stone. I was going to post some pictures but then Cree released their royal blue version the other day and along with it was a nice picture that shows it well.

Whoa, that looks cool!!! I hope they start making the XM-L emitters like that! That should fix the color problems!