The XP-G3 and the mystery of the disappearing luminance

The XPG3 created a lot of excitement when it was first tested because of its high output and low Vf. With a die area very close to the XPG2 but with more output, the XPG3 was anticipated to beat the XPG2 for thrower applications. But the performance of the XPG3 in flashlights was disappointing in some ways: the measured throw when dedomed was significantly less than the dedomed XPG2 and there was a pronounced angular/spatial “tint shift” in the beam which was distracting and hindered object identification. In this post I will explain these observations as resulting from the new “flip-chip” architecture that the XPG3 uses.

Throw measurements:
In djozz’s test thread there are a couple reports of throw measurements of the dedomed or sliced XPG3 in flashlights and the candela was about 60-64% of the candela of the dedomed XPG2 (both in direct drive situations) even while pulling considerably more current than the XPG2 and presumably outputting more lumens. So the question was: how are the throw numbers for the XPG3 so bad when it is outputting more lumens from the same size die?

The mystery was deepened when measurements of the actual die luminance were performed. The luminance (like surface brightness) of the die is what determines the throw. See here for explanation of these concepts. The luminance (with units cd/mm^2) tells you the candela that you’ll get with a given area of illuminating surface. So there are multiple ways one can measure the luminance of the die: one could directly measure the candela above the bare die and divide by the die area or one could measure the candela of a flashlight beam and divide by the area of the reflector. These measurements should give you essentially the same quantity, after taking into account some losses from the reflector and glass in the flashlight. The XPG3 die luminance was measured here at 186 cd/mm^2 at 5.05A which is around what the dedomed XPG2 has been measured at with typical direct drive currents (4-4.5A). I also measured the bare sliced XPG3 die luminance at around 230 cd/mm^2 (with 2mm^2 die area) at 7A, which is a typical direct drive current. So these measurements suggest the dedomed XPG3 is outputting more lumens and should actually beat the dedomed XPG2, but luminance measurements in actual flashlights are much lower. I measured around 115 cd/mm^2 for the sliced XPG3 in a C8 smooth reflector at direct drive current (~120Kcd, ~1050mm^2 reflector area), significantly below the bare die luminance measurement. So, again, how is this discrepancy possible?

LED architecture:
Traditionally the InGaN light emitting junction(s) is grown on top of a sapphire or SiC substrate and phosphor and bond wires are put on top of the junction. See this article. To aid in light extraction efficiency there can even be a reflecting silver layer between the InGaN and sapphire to aid in “photon recycling”. The XPG3 uses a new architecture called “flip-chip”. In this architecture the InGaN junction is grown on clear sapphire or SiC substrate then that chip is flipped over so the the substrate is now the material on top. In this configuration there doesn’t have to be bond wires on top. Phosphor is then applied to the sapphire/SiC substrate on top. There are pictures in the above article.

What I have found is that this “flip-chip” architecture of the XPG3 allows light to escape out the sides of the clear sapphire/SiC layer. This layer is relatively thick (~200 microns in the XPG3) compared to the thickness of the InGaN layers which is on the order of a micron or less. This escaped light out the sides is the cause of all of the peculiar observations regarding the XPG3: the tint shift, the low throw performance, and the discrepancy between the different luminance measurements. In the XPG3 the phosphor covers the entire package in order to capture and convert the blue photons that escape out the side of the die. This arrangement might actually increase the overall light extraction efficiency of the package relative to the traditional architecture, but it is bad for flashlight use. The excited phosphor at the side is a different color than the actual die, and this is what causes the tint shift observed in flashlight beams. In my testing I removed the side phosphor and the tint shift was directly affected. The corona turns from warmer to blue as the phosphor is removed. When the phosphor is removed the blue light escapes unconverted causing the blue corona.

The escaped light to the sides affects the actual die luminance and also causes errors in the bare die luminance measurement. Light escaping out the sides reduces the die luminance because it’s not contributing to the light coming directly from the die. When measuring the bare die luminance one measures the candela coming from the package and divides by the measured die area. With the XPG3 light is also emanating from the area to the sides of the die, and this fact artificially inflates the luminance measurement if one uses just the die area for the calculation.

My measurements:
I performed bare die luminance measurements and luminance measurements with an Eagle Eye X6, all with a constant current 8x7135 driver. In the table below are my measurements. First I measure the apparent bare die luminance by measuring the candela above the bare die. To do this I took off the bezel and reflector of the X6 and placed a lux meter 40cm above the LED. This way all measurements use the same electrical setup and should have the same current determined by the 8x7135 driver. I charged the 30Q cell between measurements to ensure the current was the same for all measurements. Light reflection from the X6 head interior should be minimal since it is black and the interior walls are vertical so should have minimal area apparent to the lux meter. Then I measured the candela of the assembled X6 with reflector and stock AR glass. The X6 area includes the (approximately) measured reflector area plus the area of the die itself. I ensured the focus was optimal with the sliced XPG3 by varying the spacer height and measuring the candela. I varied the height by +/- 0.005” and found the candela decreased in either direction, showing the focus was near optimal with the stock X6 centering ring height.

For reference I tested a XPL V6 1A emitter. The luminance as measured in the X6 is 88% of the measured die luminance which is consistent with some losses from the reflector and glass. Then I performed measurements on the XPG3 S5 3A with different modifications. I sliced the dome off using a razor blade. I estimate there was about 0.15mm of silicone remaining on the die. Then I scraped off most of the phosphor on the side, labeled in the chart “scrape 1”. Then I removed all of the phosphor on the side so the side of the clear sapphire/SiC was exposed. Then I painted the exposed sides with a spray paint to prevent light from escaping out of the sides.

Because of the error associated with measuring the bare die luminance, this value is artificially inflated and the luminance measured in the X6 is actually a better measure of the actual die luminance. With the as-sliced XPG3 there is a large discrepancy between the measured bare die luminance and that measured in the X6. Scraping most of the side-phosphor off decreased the total light coming from the package and also increased the luminance as measured in the X6 by about 10%. Scraping all of the side-phosphor off further decreased the measured candela above the package but left the actual luminance unchanged. Significant amounts of blue light escape from the sides without any side-phosphor. The picture below-left shows the blue light escaping the sides. Next I used “mirror finish” paint to coat the sides of the exposed sapphire/SiC substrate. On the right is a picture (same exposure settings) with the sides painted. I hoped this would aid in photon recycling and actually increase the die luminance, but it seemed to just absorb most all of the blue light because the X6 luminance was practically unchanged. The bare die luminance measurement, however, decreased significantly and agrees much better now with the X6 luminance measurement. Blocking the escaping light to the side mostly removed the error with this method of measuring the die luminance. Ceiling bounce tests (proportional to the total lumen output) were consistent with the changes in bare die candela.

The actual XPG3 die luminance is relatively low (compared to dedomed XPL V6 and XPG2), as several in-flashlight measurements suggested. The flip-chip architecture used by the XPG3 causes light to escape to the sides which caused errors in bare die luminance measurements. The total lumen output is high, but significant amounts of that light emanate from the area to the sides of the die. This fact causes the undesirable tint-shift in flashlight beams.

So, I hope this answers some questions/confusion for those that had them. Personally I was really bothered by the performance discrepancies associated with this LED and am happy to have figured them out.

Thanks for the testing mate. I have used heaps of XPG3s in different lights domed they throw good. I have sliced a few but didn’t see a point to it throw is down a tad compared to other de-domed LEDs but they output a lot of light and are cheap.

“The total lumen output is high, but significant amounts of that light emanate from the area to the sides of the die.”
As most other new leds, XP-G3 was not made to work with reflectors (that cant focus this side light as tight as you want). New leds need different optical system. I think those who like throwers need to check how do fresnel lenses work with such “bad” leds.

The fact is the peak luminance is lower with the new LEDs and so will be the throw.

Sure they still work and do output a lot of light. They are just not as suited to most lights because of the tint shift and lower throw.

Awesome post, I understand it, thanks!

Thanks for the effort EasyB. Nice detective work. :+1:

Rather than having a flat layer of phosphor, the Xp-G3 has a sort of “dome” underneath the main dome which is filled with phosphor, this is how it achieves higher luminous flux.

And that’s also why it loses both lumens and intensity when shaved, you lose phosphor.
If you try to dedome it you would probably be left with the small dome on, which would give worse intensity than a flat phosphor LED.

Source is saabluster’s XP-G3 research on CPF.

Enderman, I don’t think that is true of XPG3s in general. Maybe he used some sort of solvent which redistributed some phosphor. With mine the phosphor is quite flat and I didn’t remove any of it when I shaved. Also, in the picture in this post I think it’s apparent it is just a layer.

Based on ceiling bounce tests I’m losing about 10% of the output upon slicing the XPG3. I lost a similar amount with the XPL V6 1A I dedomed. Now, with most of these XPLs I’ve been dedoming with gas, the dome does not come off cleanly and I’ve had to scrape the silicone off the die, so there is typically a small amount of silicone left on the phosphor, which I imagine lowers the output some. But I think even with a perfect dedome there is output loss. I know you have posted djozz’s test of an XPL V6 2C which shows practically no output loss upon dedoming, but I think that is the exception rather than the rule. I could be wrong though.

Thanks for the investigation EasyB, that was an interesting read, I never dived very deep in how leds are build up and you gave a clear summary, and why it is so relevant for flashlights. The experiments and measurements are fantastic, you must have a pretty steady hand for this led surgery! :slight_smile:

That’s a top down image, you can see there is some sort of glossy silicone in a circle shape within the yellow phosphor circle:

I marked with red pixels where I see the border of this “inner dome”
I am pretty confident if he took a side image of it we would see that it is not perfectly flat, it bulges. (unless the chemical dedoming process ruined the silicone bulge)

If the phosphor is mixed with silicone, (which is what the domes are made out of i think?) then dedoming might remove some of the inner dome which is mixed with phosphor, reducing the output.
Even if it didn’t touch the inner dome, simply having the bulging shape instead of flat would give worse throw.

From the XPG3s I have looked at and modified the phosphor layer is flat enough to be called flat, and I’m pretty sure I have not removed any phosphor from slicing the dome. And I don’t think the finite thickness of the phosphor is the cause of the poor throw performance, given what I have explained in the OP.

Have you dedomed any?
Chemical or heat?

I’ve only sliced.

I’ll emphasize one result that explains why the XPG3 is a poor thrower. The data in the last three rows in my table shows that 20% or more of the total light output is not coming from the die itself but from the light that has escaped the sides. The actual die luminance was unaffected evidenced by the X6 luminance being unchanged, but the total light output dropped significantly when the side-escaping light was blocked. You can’t expect to have high die luminance when 20+% of the output escapes to the sides.

I think we’re pretty much talking about the same thing…
Since it emits light to the sides that’s why they used a blob of phosphor rather than a flat surface, so that it goes over the edges and then produces the white light.
The first two images I posted have the die IN that dome, it is not simply the phosphor from on top of the die.
Here is a better image to explain:

You’re right though, dome shaving probably wouldn’t touch this phosphor dome.

I have dedomed a few XPG3s the phosphor layer is liquid like and not hard like the old style domes. Much easier to move around it doesn’t flake of?

When i done mine they where flat the bulge is from the LED die.

Thanks, I’m glad for this community that I can share my thoughts and work with.

I have gotten pretty good with a loupe and razor doing LED surgery. :slight_smile:

Kd/lm is fixed parameter for optical system&led combination.
If you have enough lumens but not happy with candelas - just change optical system.
Fresneles are good to focus big cobs like cree cxa tight, why not try them with side light distribution leds?

Fresnels are only good for very large diameters where an aspheric lens becomes too heavy or expensive.
Other than that, they are much worse than a single aspheric.
And no, the XP-G3 still sucks for any high intensity application regardless of the type of optic you use.

Surefire dont think so