During the last few years the maximum luminance which can be reached with an LED in a flashlight, without additional aiding measures, increased by about 25%. It's specified in Candela per square millimeter of die area, the actual glowing part part the LED (cd/mm2). Together with the front surface area of the reflector (or optic) it determines the throw of a light (just multiply the values). The current leader of the white LEDs manages at least 300cd/mm^2. Phosphor-converted green LEDs can reach an even higher luminance, but with a large reduction in light quality and color contrast. In comparison to years past these are of course very high values, but they're still much lower compared to even low powered short-arc HID bulbs. The 75W Xenon short-arc bulb in the Maxabeam does around 784cd/mm2 and even more at 85W (~1300cd/mm2). The 120-350W UHP bulbs in projectors easily manage 2000cd/mm2.
In this thread I want to discuss why some LEDs manage a higher luminance than others and what can be done to increase these values even further.
Generally luminance values can be found in this thread by sma (which lists values for a lot of other light sources as well giving a great overview!) and here in the tests of Köf3 (he has recently switched over to BLF). Most datasheets unfortunately don't offer this data. Those which do are usually missing values for the maximum possible current.
So how does one get a high luminance?
One needs a cool-white LED with low CRI (high proportion of yellow/green light in the spectrum) and without a dome on it, which tolerates a high power density (Watts per square millimeter of die area).
Limiting factors:
- Temperature of the LED and everything that influences it
- Temperature of the phosphor
- Not fully understood/researched current limit of some LEDs (see the Luxeon V for example)
By far the most important criteria is heat. Most LEDs are rated for a maximum temperature of 150°C.
Here I have created a table where I compare the currently best LEDs in this regard:
|
LED |
max. luminance [cd/mm2] |
Die-area [mm2] |
max. Power [W] | Power density in regard to the die-area[W/mm2] | Current density in regard to the die-area [A/mm2] | Area of the heat conducting solder pad [mm2] | Power density in regard to the heat conducting solder pad[W/mm^2] | real thermal resistance, calculated values assume 20% efficiency and 1.293 °C/Wmm² thermal resistance of solder[°C/W] | min. LED-temperature at max. luminance and 25°C heatsink temp[°C] | max. rated LED temperature Tj [°C] |
|---|---|---|---|---|---|---|---|---|---|---|
| [>=455] | 1,0609 | 24 | 22,6 | 6,13 | 2,64 | 9,09 | 4,59 | 135,2 | 150 | |
| Osram KW CULNM1.TG | ~336 | 1,0609 | ||||||||
| Osram "White Flat" KW CSLNM1.TG | [>=298] | 1,0609 | 20,01 | 18,9 | 5,42 | 2,64 | 7,58 | 4,59 | 98,5 | 150 |
| Osram Oslon Boost HX KW CULPM1.TG | [>=278] | 1,9875 | 6,84 | 1,99 | 150 | |||||
| Osram KW CSLPM1.TG | [>=255] | 1,9875 | 2,64 | 3,09 | 150 | |||||
| Osram Black Flat HWQP | 246 | 1,122 | 22,1 | 19,7 | 4,99 | 2,67 | 8,28 | 4,78 | 108,1 | 150 |
| Osram Ostar LE UW Q8WP | 245 | 1,91 | 33,9 | 17,8 | 5,03 | 5,26 | 6,45 | 2,85 | 121,6 | 150 |
| Luminus CFT-90 | 230 | 9 | 178 | 19,8 | 5 | 45,9 or 747,6 | 3,88 or 0,24 | 0,56 | 105** | 150 |
| Cree XP-G2 R5/S4 de-domed | 200 | 2,16 | ~24 | 11,1 | 2,78 | 4,245 | 5,65 | 4,31 | 107,8 | 150 |
| Osram SYNIOS P2720 KW DMLQ31.SG | 195 | 0,503 | 9 | 17,9 | 4,97 | 2,58 | 3,49 | 10,5 | 100,6 | 150 |
| Luminus SST40 dedomed | 180 | 3,994 | 31 | 7,8 | 2,1 | 13,44 | 2,31 | 2,6 | 89,5 | 150 |
| Lumileds Luxeon CZ | 1 | 1,08 | 135 | |||||||
| Lumileds Luxeon Z ES | 173 | 1,774 | 16,3 | 9,2 | 2,59 | 1,81 | 9,01 | 4,46 | 83,2 | 135 |
| Cree XHP35 HI E2 | 171 | 5,32 | 49 | 9,2 | 2,41 | 4,11 | 11,92 | 2,12 | 108,1 | 150 |
| Osram SYNIOS P2720 KW DMLN31.SG | n.a. | 0,225 | 3,17 | 14,1 | 4,22 | 2,58 | 1,23 | 20,5 | 77 | 150 |
| Cree XP-L HI V3 | 168 | 3,55 | ~31,5 | 8,87 | 2,11 | 4,11 | 7,66 | 2,52 | 88,5 | 150 |
| Cree XP-G3 S5 shaved | 140-160? | 2,06 | 34 | 16,5 | 4,13 | 4,245 | 8,01 | 3,31 | 115 | 150 |
| Cree XP-E2 Torch U5 dedomed | >141 | <0,85 | ~9,75 | 11,5 | >3,06 | 4,11 | 2,37 | 9,32 | 97,7 | 150 |
| Osram Square Flat KW CSLPM2.PC | ? | ? | ? | ? | ? | 2,629 | ? | 3,49 | ? | 135°C |
| Luminus SBT-70 WCS PB | 7 | 54 | 8,14 | 2,14 | 45,9 | 1,18 | 0,9 | 63,9 | 150 | |
| Lumileds Luxeon V shaved | 6,43 | 41,9 | 6,52 | 1,65 | 7,63 | 5,49 | 1,17 | 56,8 | 135 |
I got these values from the datasheets, from tests in the forums and some I have calculated. Please tell me if you find any mistakes.
What I noticed:
- Most of the LEDs reach their maximum luminance at around 90-108°C
- Only the LEDs from Lumileds reach their maximum at temp less than that
- Only a few reach their maximum above that
- The Luminus CFT-90 seems to be a step ahead of the other LEDs from a technological standpoint. Its luminance is almost unbelievably high for such a large die. It does have the largest thermal pad though (I calculated using the thermal pad size of the SST-90 and also using the size of the integrated copper pcb)
- The Osram "White Flat" KW CSLNM1.TG has the highest power density (of the whiter LEDs) regarding the die size, but not regarding the thermal pad size
What I want is a LED with a small die (2mm2), no dome, directly mounted on a copper pcb like the CFT-90. This pcb would optimally have the 16mm standard footprint we often use here.
What do you guys think?
Notes:
- Calculation of real thermal rsistance which includes solder: real_thermal_resistance = typ_datasheet_therm_resis + (1,293°C/Wmm2 / area_of_heat_conducting_solder_pad)
- Calculation of min temp in table: min_temp = (max_power * 0,8 * real_therm_resis) + 25°C
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Nice summary!
But i keep following this thread, and am curious about new additions!
I’m not in the position to find or correct mistakes, that is a speciality of you german members
link to djozz tests
Nice compilation, but I suspect your luminance value quoted for the XPG3 (and possibly others) is too high. Often the luminance is calculated by measuring the intensity above the LED (in candela) then dividing by the die area. But if light is being emitted by area other than the actual die than the calculated luminance will be inflated. I showed this with the XPG3. In these cases measuring the resulting beam center intensity from a flashlight and dividing by the reflector area is a better way of determining the luminance since, if properly focused, this method measures the light coming from the die itself. This phenomenon happens with other LEDs, too. If you look at the lit LED and see light coming from parts other than the die than the above luminance calculation will be inflated. This stray light is contributing to the total output from the LED, but contributes to the beam corona rather than the beam center intensity.
Yes, I realize this problem. I will add that it only pertains to asphercis.
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A question…is thermal resistance specified using watts consumed by the LED or watts wasted as heat?
Some data sheets state both. I think I tried to State the electrical thermal resistance to be consistent. I will double check.
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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.
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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.
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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:
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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.
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).
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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.
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).
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.
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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?
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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.
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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.
How is light coming from the side of the die going to hit the sensor of the lux meter during a correctly done luminance measurement?
Correctly done means that only direct light coming straight from the front of the die will actually hit the sensor. Thus dividing the measured Candela by the the visible die size will result in the actual luminance that the lux meter is “seeing”.
Here in the first post you can see a picture of sma’s test setup for measuring the luminance. He has implemented measures to prevent any stray light from hitting the sensor and thus inflating the measured values.
Using an aspheric with a high f-number should make it possible to make use of this high “front luminance”.
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The light is coming from the area to the side of the die, so it is exposed to the luxmeter. I’m not talking about light going to the side.
Imagine looking down on the sliced XPG3 from above. The phosphor covering the package to the side of the die is lit and gives off light which the luxmeter detects. I showed with my measurements that the light from the side phosphor is significant in inflating the luminance calculated using this method.
Ok, I get you now. Partly the reason or this is the excess phosphor which is around the die, but not covering it. For a better measurement this should either be removed or one could use a pin-hole type cover with the exact size of the die (or 1mm^2 circle).
EDIT: I just noticed that you do state the current which you measured at, 2.8A. From this and other test results I will calculate the real maximum luminance.
EDIT2: ~157cd/mm^2 at 8.5A and 85°C.
EDIT3: maybe instead of back paint you could try something reflective on the sides of the shaved XP-G3, basically turning it into a “laser”.
BTW: I just calculated the real thermal resistance of the Luminus and Lumileds LEDs where the datasheets don’t have these values. I just assumed an efficiency of 20%. Based on this I also fixed the values in the column “min. temp at max. luminance and 25°C”. The LEDs are actually reaching their maximum at about 90-107°C.
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Perhaps it turns out that a luminance value that can be defined as the total light flux divided by the light emitting surface area does not 1 to 1 predict the throw because it does not take the spatial distribution into account, and even then spectral shifts with changing angle can have an influence. In theory, an ideal spatial distribution for throw may be different for reflector lights and for lights with aspheric lenses.
If the aim is getting a value for any particular led that directly gives potential throw, maybe an empirical approach for measuring it must be devised. I have an abandoned project for testing reflectors (as always I found designing the method way more fun than doing series of measurements using the method) that when used the other way around, with a fixed reflector and a fixed aspheric lens (say, a C8 reflector and Brinyte B158 lens) and varying led, can be made into a standard method for measuring throw. Spot intensities can be measured with all leds at a standard current, and the current/output graph from the output test can then be used to calculate the maximum throw.
http://budgetlightforum.com/node/42458
link to djozz tests
will heating ever go away?
I^2 * R losses?
it would be nice to design without having to consider heating and all its problems…
wle
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