I think it pertains to reflectors as well. It’s not an issue with the flashlight, it’s an issue with the luminance calculation method.
Electrical thermal resistance must be calculated at certain conditions to be valuable. I don’t think that whatever the manufacturer specifies here is very relevant to our leds over-driven near the limits.
No, I meant the measured value of the XP-G3, which I found, only pertains to the use with aspherics.
I actually state the real thermal resistance, which accounts for the efficiency of the LED. Here is an application note from Osram explaining the differences regarding the real and electical thermal resistance.
The way I understand it…
Let’s say for a while that 1 W of white light is 310 lm.
XP-G3 S5 at binning conditions does about 191 lm/W, 62% efficiency.
Let’s assume also that that their stated thermal resistance is electrical (I don’t know if that’s the case).
For XP-G3 3 °C/W electrical is 7.81 °C/W thermal.
Overdrive it so hard that efficacy goes down to 60 lm/W and electrical thermal resistance rises to 6.3 °C/W.
CFT-90 at binning conditions does 61 lm/W. By overdriving it you won’t make its electrical thermal resistance rise much.
Sorry, I think I edited my post while you wrote your reply. I state the real thermal resistance unless noted otherwise.
The Osram Black Flat HWQP has a stated typical real thermal resistance of 4.3°C/W. They also note a maximum of 5.5°C/W. Do you think this pertains to individual variation of these LEDs or do these values also change when the power goes up?
BTW: I linked all the datasheets in my table.
Since what you state is real thermal resistance - that’s good. Though it would be interesting to know LED efficiency (or efficacy) at peak luminosity to get a more accurate picture of temperature rise.
As to Oslon Black HWQP…
I did a quick search and I see that thermal conductivity of materials changes with temperature. If wikipedia is correct, for metals it improves proportionally to temp rise (in K) while for non-metal solids it stays largely constant. So within LED operating conditions the change can’t be nearly as large. Individual differences? Intuition tells me the difference is too large to be explained this way. Maybe there are different variants of the LED?
Really I have no idea.
I don’t know what you mean with “different variants of this LED”. The datasheet is only for this one type, the HWQP.
So, getting back to the actual topic of this thread, what do you guys think needs to be done in order to improve the maximum luminance? How high can LEDs go considering the properties of the used materials?
Are laser-phosphor modules the only way forward? They offer two distinct advantages:
- A blue laser diode coupled with a focussing lens will put a much higher concentration of blue light onto the phosphor compared to LEDs at a much lower power consumption if one ignores the size of this area
- This design also allows for thermally separating the phosphor from the blue light source and mounting it in a thermally beneficial way (i.e. between two glass plates)
Cree changed production processes of several emitters during their lifetimes. Maybe Osram does the same and what we see is accounting for that?
As to lasers:
There’s always the scare of the laser moving against the enclosure and its beam hitting the reflector, hurting somebody. Personally I’d rather not go this route.
However, there’s a safer way: use Yuji phosphors and UV laser together with UV filter.
Hmm… laser excited phosphor sounds rather expensive and inefficient.
The dielectric layer between chip and LED board could probably be improved further?
Or do they already use nano carbon stuff for that (which is (can be) better than copper and not electrically conductive)?
Cooling on the phosphor side is maybe something to develop.
Diamond top layer is probably good for that, but expensive…
I should have a better look at how an LED is built up…
but as a layered cake:
diamond
phosphor
diamond
blue pump LED
“nano carbon stuff” base
DTP PCB
The diamond layers held in place by thermally conductive stuff with a thermal path to the “nano carbon stuff”, or even integral with the nano carbon stuff base.
Probably too expensive…
Hmm… With a blue pump you use the blue light as a part of the spectrum.
With “Yuji phosphor” all (or at least much more) of the radiation from the UV pump is used to excite the phosphor, and it’s the phosphor that radiates the full spectrum.
I think it’s safe to assume that it will always be less efficient than a blue pump LED.
As for risks of lasers, this depends on how you build it.
I think it’s not hard to mechanically prevent the laser from missing the target.
But the laser could accidentally burn through the phosphor, which sounds like a problem…
You mean someone has measured the beam intensity and divided by the lens area and got 220cd/mm^2?
I measured a sliced XPG3 in a UF1504 and got similar, lower than expected, numbers as in a reflector light.
As far as your question about the future of high luminance LEDs: my understanding is that it’s a matter of internal photon production efficiency and the photon extraction efficiency. Using higher quality InGaN active material and designing better thermal paths can increase the internal efficiency.
Ensuring all photons exit through the die will increase the extraction efficiency and luminance. For example if the LED design is such that a lot of light escapes out the side, like the XPG3, this will lower the luminance.
This happens to some extent in other LEDs. I was working with dedomed SST40s recently and noticed some light escaping out the bottom where the chip meets the package. Not nearly as much light escapes as the XPG3, but I think I have observed the effect on the luminance. Comparing the dedomed SST40 with the dedomed XPL V6 in an eagle eye X6, I measured about 20% more output with the SST40 (because of the higher current), but only 9% higher beam intensity.
Couldn’t that be due to the low Vf of the SST-40, meaning it uses less power with the same current?
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Did you take into account that the SST-40 has a larger die (3.994mm2)? It needs more lumens for the same luminance compared to the XP-L (3.55mm2). You can find both values in my table above (the XP-L HD should have the same die size as the XP-L HI). ;)
I don’t think the electrical power factors in to my reasoning. If the die sizes are the same and all light is emitted from the die area, 20% more output should also mean 20% higher luminance and beam intensity. The fact that they are different could be explained by the fact that some of the light from the SST40 is coming from the side of the die rather then from the die itself. The result of this is a slightly brighter corona.
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That could also explain it. I have not measured the die sizes very precisely.
That would be very interesting, but I don’t think it makes much sense here. These are automotive LEDs. The requirements for such components are very strict and changing specifications would be bad business unless denoted with the product code.
Well yes, but this only pertains to homemade devices.
There are already car headlights and projectors using this technology which of course have all the needed safety features.
Yuji phosphors are nice, but not very efficient at high power density (the more even the spectrum i.e. higher cri, the more inefficient a light source is).
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[quote=The_Driver]
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All die sizes are measured by either sma or koef3. They need to do this rather precisely because otherwise the luminance values would not be of much use. They do this by taking a macro shot of the LEDs at very low power from head on (as straight as possible) and then counting the pixels and using the package size as a reference length.
[quote=The_Driver]
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The dies very well could have areas different by 10, but I wouldn’t be surprised if the uncertainty in the area measurement is greater than 10. For example how do you deal with the brightness gradient near the edge of the die? The point I’m trying to make is that for some LEDs this method of measuring luminance doesn’t really work. In-flashlight measurements do tell us the actual die luminance, but they introduce other sources of uncertainty like reflector quality and focus.
No, sma measured the luminance like he always does, from head on. Using an aspheric lens should allow for the utilization of these high values, because it uses the light from head-on. Of course we don’t know by how much the luminance is reduced at what angle. It would be great if someone were to measure this.
I think the XP-G3, XP-L2 and XHP70.2 are special cases that we need to ignore in this discussion. It’s a step in the wrong direction in this regard (improving lumen density).
So you think that the quality of the InGaN active materials is a major factor? Interesting. Did you ever find any info regarding this?
This is a good point! It concerns all the measurements we discuss here regularily and especially so in this thread. Most people here (except maybe djozz and sma) never talk about what level of error is possible with their measurements and how it effects their analysis of those values.
In-flashlight measurements are of course a great way to check the validitiy of the luminance values. My experience has been that for the older, “classic style” Cree LEDs the luminance measurements are spot on when all the details are considered.
A 10% difference would be easily discernable though on a macro photo, even without counting any pixels.
I think you are misunderstanding. The luminance measured with this method is simply incorrect. With this method some of the measured light is coming from the area to the side of the die and not from the die itself, but the measured intensity is divided by just the die area, thereby inflating the calculated luminance. The actual die luminance is lower. It is not a matter of angular dependence.
Agreed.
Forgive me for not finding references right now. From what I have read the internal quantum efficiency of LEDs can be quite high at low currents. But so-called efficiency droop happens at higher currents, so there is room for improvement here I think. I don’t think the cause of the droop is definitively known, but one possible cause is the inherent electrical polarization present in the common GaN crystal structure (wurtzite). So one direction research is taking is trying to grow the InGaN with a different structure (zincblende) without the inherent polarization.
Anyways, I don’t know the current state of knowledge in this area, but that is one possible way to improve performance at high current densities where we need it most.