Future development of the maximum luminance of LEDs

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The_Driver
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djozz wrote:
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.

Well it’s not based on lumens. It’s based on Candela. You only need a lux meter to measure the luminace, not a sphere. But still, you make a valid point regarding the spectral shifts although experience has shown that de-domed emitters have less spectral shift at high angles.

For lambertian emitters (perfectly flat) there is a somewhat accurate method though using lumens. Simply divide the lumens by the surface area times pi. It gives you a ballpark figure.

djozz wrote:
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

The used approach works for all LEDs except these new, “weird” ones from Cree (although the Luxeon V might also fit into this category). It’s based on physics. The luminance is a constant at all angles for a lambertian emitter. Otherwise it would never work with reflectors…

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EasyB wrote:
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.

I ask me if it is about the influence of the yellow lateral light areas, which I have noted in the sketch and their relevance in practice in a Thrower.
Does this light amplify the intensity of the hotspot in a Thrower?

What about measuring the illuminance to calculate the luminance?
Is the light from the side surfaces still being detected by the luxmeter?

The_Driver
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A note to everyone:

You can now click on the luminance values. It will lead you to the sources of where I got them (sometimes requiring additional calculation for the maximum value). Some of the linked tests are in German.

Also, there is a new king of luminance, just recently tested by Köf3, the Osram SYNIOS P2720 KW DMLQ31.SG with a 0.5mm2 die. It does up to 316cd/mm2.

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Taking some data from here:
http://www.cree.com/led-components/media/documents/XLamp_PCB_Thermal.pdf
we can calculate thermal resistance of the solder layer under the thermal pad.
This assumes quite good thermally SNC solder, for many builds results will be worse.

75 μm / 58 (W/m°C) / x mm² ~= 1.293 °C / Wmm².
So thermal resistance of DMLN31.SG from MCPCB to junction is 20.5 °C/W and XHP 35 HI – 2.14 °C/W. At peak power this leads to the increase of temperature of 1.6 °C and 15.4 °C respectively. I guess it’s significant enough to include in the emitter temp calculations.

Below the solder level, what is the effect of thermal pad size? There must be some temperature gradient….I don’t know really.

And I can’t help but think about djozz tests that showed MOSX MCPCB as the performance-limiter of Blackie. Doing calculations like I do here, it looks that at peak power MOSX gives 31 °C temperature for this LED. It may be worse, led4power just specified thermal resistance to being equivalent of 250μm of solder without telling which one.

I guess that with some LEDs it may be worthwhile to think again about picking a better solder paste or go with thermal glues…Compared with SNC, SnAg would give a 4°C improvement for XHP35 HI. SnPb is another 2.4 °C worse. Though the difference is less at sweet spot.

ADDED:
Note that the following assume no voids in solder. This assumption is totally unrealistic. I am unable to tell how much difference does it make though.

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BTW Conductonaut on the thermal pad and solder on electrical ones may be interesting too.

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Thanks a lot for you help!
I will revise the temperature column.
EDIT: done!

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In the meantime: pure indum is the best TIM that I found so far. 86 W/m°C. Compared with SnAg it drops the temperature by another 1 °C. So 7.4 °C better than SnPb at peak output.
But as far as I can tell it is not available as a paste, only wire (it seems to be the case with SnAg too). How about this:
1. Put MCPCB on the hotplate
2. Put paste on electrical pads
3. Cut a tiny bit of indum wire and put in on the thermal pad
4. Place the LED on the hotplate next to the MCPCB
5. Heat up until the solder melts
6. Place the LED on the MCPCB. It should self-center
7. Tap it
8. Let it cool

This way should ensure:

  • no cold joints
  • not overheating the LED
  • not heating / cooling too fast

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German Modder Vinz uses self-made diamond based thermal glue. It has 85% synthetic diamond content. The diamond has a thermal conductivity of 3300 W/(m·K). It theoretically has a better thermal conductivity than copper. He doesn’t use it for the central thermal pad though. He uses normal DTP copper pcbs and a solder with a high silver content, Mundorf MSolder Silver Gold. This solder is very expensive unfortunately (50€ for a 100g roll). MSolder Silver Gold Supreme has an even higher Silver content (9.6g instead of 3.8g), but it’s twice as expensive.

I would prefer we talk about more fundamental changes. Using better solder will not increase the luminance by visible amounts. How do we get from 300cd/mm2 to 500 or even 1000?

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MSolder Silver Gold Supreme also has much higher melting temp.
ADDED: pure indium solder wire is even more expensive.
Indium scraps are more expensive too, but available in small packages:
https://www.aliexpress.com/item/10-grams-High-Purity-99-995-indium-in-Me...
100g Indium blocks are cheaper than than MSolder wire:
https://www.aliexpress.com/item/100-grams-High-Purity-99-995-indium-in-M...
but who needs 100g?

I would expect Indium-Silver to have better conductivity than pure indium, but according to Indium Corp that’s not the case:
https://www.indium.com/thermal-management/thermal-k-list/

Too bad MSolder doesn’t list conductivity of their solder….though Indium Corp shows a similar 96.5Sn3.5Ag to have 33 W/mK. And this one at 78 W/nK Facepalm

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It says 60W/mK for Msolder SilverGold in your link…
Which is not a bad value, but SnPb 63/37 is almost as good (50W/mK ) and doesn’t cost much.

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This “almost” is 7.4 °C for XHP35 at full power. It’s hard to look at the Cree charts, but to my eye it’s very roughly half bin upgrade. For Blackie the difference would be 4 °C. 2% flux difference? Not a big deal, but if I were chasing records and could verify purity, I would spent the $10 without a blink.

ADDED:
Well, though without knowing solder joint thickness that we get with our methods, the numbers above may be significantly off in any direction.

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Agro wrote:
This “almost” is 7.4 °C for XHP35 at full power. It’s hard to look at the Cree charts, but to my eye it’s very roughly half bin upgrade. For Blackie the difference would be 4 °C. 2% flux difference? Not a big deal, but if I were chasing records and could verify purity, I would spent the $10 without a blink.

Yes, in that case it’s worth it to do every little thing. That’s why Vinz does it. His Mjölnir light used to have an XM-L2 which ran at 7.5A for 30s (after that the drivers reduces to 7A). Nobody “pushed” more current through that LED!

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I have added the Luminus SBT-70 because it's unique in that it has many of the right features (large solder pad, low thermal resistance, no dome, cool-white low cri), but is hindered by it's low efficiency (43.7 - 46.7 lm/W at max brightness). I still like it though because of it's unique beam when used with aspheric lenses and the high light quality of the WDH (cool white, high-cri, high R9) version.

Getting those values was a lot of work. 

Here sma measured the luminance of an SBT-70 from 2012 in an Olight SR-95S UT host. It did 71cd/mm^2 at 10.6A. Back then the LED had just come to the market and Olight couldn't have gotten the best bins which were available in the following years. Unfortunately they don't state the used bin in the description of the light. This promotional picture says "up to 1700 lumens". I checked an older datasheet and it has a Bin which goes up to 1710 lumens at the maximum current of 10.5A, the NA-Bin. In the latest datasheet version the best Bin is the PB-Bin. LEDs in this bin are 22.8% brighter compared to those in NA-Bin. So now we are at 87.2cd/mm^2. Now we need to find out at what current the LED reaches it's maximum brightness and how much brighter it becomes. Unfortunately there are no real tests of this LED. DB Custom has reported that it does 18.75A in direct-drive with a 26650. I took a closer look at the brighness curve in the datasheet and also compared it to the SST-90 (slightly larger 9mm^2 die) which is rated up to 18A. From 3.5A to 10.5A the SST-90 becomes 125% brighter and the SBT-70 only 122%. This makes sense because it has a smaller die. I concluded that the maximum current must also be lower. By extrapolating the curve I could see that it reaches it's maximum at around 15A and 3.91V. Other people and also myself have measured much lower Vfs compared to the datasheet values (which are measured at 25°C though...). So I assume a Vf of 3.6V at 15A and a brightness increase of 20% from 10.5A to 15A. That gives us 105cd/mm^2, 15A, 54W, the values I put into the table. 

I really wonder why the brightness curve is so flat. At 54W and 25°C heatsink temp it only has junction temperature of ~64°C. The main reason why the newer CFT-90 is so much better is because it can handle almost twice the temperature. 

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My guess is that a new generation of 445nm laser pumped white light diodes will probably come out at some point that increase the luminance by a few 100% there should be not that much of a problem to produce this in a similar package than the LEDs are now

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Lexel wrote:
My guess is that a new generation of 445nm laser pumped white light diodes will probably come out at some point that increase the luminance by a few 100% there should be not that much of a problem to produce this in a similar package than the LEDs are now

Yes, I agree that lasers can be very small as well, although I don’t think this it too important. They are small enough already. The problem with lasers is that they are less robust and require better cooling compared to LEDs.

I just had the idea that one could seperate the phosphor of an LED from the die by using a piece of short glass fiber. This could result in higher output.

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you need to heatsink the heat from the phosfor though, while you loose the pathway via the led die.

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djozz wrote:
you need to heatsink the heat from the phosfor though, while you loose the pathway via the led die.

That depends. How hot does the Phosphor get when it is part of a LED? Is it hotter or cooler than the die? Does it heat up more because of the heat emitted by the blue die or by the inefficiency of the conversion process?

We need some rest results regarding this.

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Quote:
What I want is a LED with a small die (2mm^2), 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.

At first I dismissed this comment. After all, we BLFers won’t have our own custom LED. But then I started thinking…LED construction may be too hard for us amateurs, but there are quite a few companies which have all the skills and tools. Especially that we don’t need to build everything from scratch. Companies like Cree sell individual dies or phosphors.

Now the question is: what are R&D costs? What are unit costs? What volume would be needed to make this possible?
I have no idea what are the answers to any of these questions. I’d like to have it.

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Agro wrote:
Quote:
What I want is a LED with a small die (2mm^2), 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.

At first I dismissed this comment. After all, we BLFers won’t have our own custom LED. But then I started thinking…LED construction may be too hard for us amateurs, but there are quite a few companies which have all the skills and tools. Especially that we don’t need to build everything from scratch. Companies like Cree sell individual dies or phosphors.

Now the question is: what are R&D costs? What are unit costs? What volume would be needed to make this possible?
I have no idea what are the answers to any of these questions. I’d like to have it.

I don’t know. I would imagine that it is unbelieveably expensive.
The LED I mention above would probably need to be made by Luminus or maybe Osram. As far as I know only they offer direct mounted LEDs. Those LEDs are different compared to Cree, Nichia etc.

Some links:

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Thanks, I mailed Seenov, may go to prophotonix later.

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Agro wrote:
Thanks, I mailed Seenov, may go to prophotonix later.

Nice, please keep us posted! Wink

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djozz wrote:
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.8

Theoretically, if the LED has no dome or flat dome they will all have the same spatial distribution following cos(theta)
However I have calculated the lm/mm^2 of many LEDs and it does not match up at all with the measured cd/mm^2 so there is definitely some other variable(s) affecting the intensity.
Real world testing is the most accurate method atm.
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Seenov doesn’t make custom LEDs but only custom PCBs. I asked prophotonix, though looking at their site more closely I suspect they do the same.

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Agro wrote:
Seenov doesn’t make custom LEDs but only custom PCBs. I asked prophotonix, though looking at their site more closely I suspect they do the same.

Thats too bad, but I guess it was to be expected. Thanks for keeping us up to date. Wink
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I added the new Osram Square Flat. Together with the Luxeon Z ES it’s the only XP-G2 sized LED which comes without a dome. It has a low thermal resistance, only the XP-G2 is even better.

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If you list pad size to 3 decimals, your number for Osram Square Flat KW CSLPM2.PC is wrong.
You missed that the thermal pad corners are rounded, so the pad is 2.629 mm².

ADDED:
Also, there’s some controversy regarding Osram Ostar LE UW Q8WP NB, so the listed luminosity may be too high.

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Agro wrote:
If you list pad size to 3 decimals, your number for Osram Square Flat KW CSLPM2.PC is wrong.
You missed that the thermal pad corners are rounded, so the pad is 2.629 mm².

ADDED:
Also, there’s some controversy regarding Osram Ostar LE UW Q8WP NB, so the listed luminosity may be too high.

You are correct, thanks. I guess I was too lazy for that:)
I agree regarding the fact that there is controversy, but I would prefer actual throw measurements with a reflector to back those values up.

I will also add Köf3’s mneasured values of the Luxeon V.

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The_Driver wrote:
German Modder Vinz uses self-made diamond based thermal glue. It has 85% synthetic diamond content. The diamond has a thermal conductivity of 3300 W/(m·K). It theoretically has a better thermal conductivity than copper. He doesn’t use it for the central thermal pad though. He uses normal DTP copper pcbs and a solder with a high silver content, Mundorf MSolder Silver Gold. This solder is very expensive unfortunately (50€ for a 100g roll). MSolder Silver Gold Supreme has an even higher Silver content (9.6g instead of 3.8g), but it’s twice as expensive.

I would prefer we talk about more fundamental changes. Using better solder will not increase the luminance by visible amounts. How do we get from 300cd/mm2 to 500 or even 1000?


I know this post is a bit old but I wanted to mention that diamond is 2200W/mk not 3300, and that thermal compounds made of diamond do not actually work very well.
ICdiamond only has ~10w/mk.
The way to get the best thermal connection is to use liquid metal such as coolaboratory liquid ultra/pro or thermal grizzly conductonaut.
The pro and conductonaut have ~76W/mk.
Using this is a better choice than solder because being a liquid it can contact all the small cracks and crevices.
If you tried to solder a copper MCPCB to a copper heatsink it would be almost impossible to get a perfect joint due to the large area and how fast the heat moves away.
High silver content in solder also gives a higher thermal conductivity, about 70-80 IIRC, but it makes pretty much no noticeable difference.
I use 3% silver/tin solder for all of the stuff I do.

Applying and removing the liquid metal is actually really easy to do.
Having an easily removable MCPCB is great, I don’t recommend people solder their MCPCBs to a heatsink.

Agro
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There are 2 problems with Conductonaut though:

  • it is rated to 140 °C, so you can’t use it together with soldering (although I don’t know what happens once you exceed that temp).
  • it is fluid which means there’s a risk of run-out. Or pump-out.

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Agro wrote:
There are 2 problems with Conductonaut though:
  • it is rated to 140 °C, so you can’t use it together with soldering (although I don’t know what happens once you exceed that temp).
  • it is fluid which means there’s a risk of run-out. Or pump-out.

The LED should never exceed 140C.
If you use liquid metal there is no need to solder anything, that’s the entire point of using it instead of soldering.
If you mean soldering the LED to the MCPCB, you do that before you put the MCPCB on the heatsink and apply thermal compound.

Also if you apply it correctly there is nothing that comes out.
If you overapply and some comes out around the MCPCB you can just wipe it off.
Nothing else from below the MCPCB will come out so it is completely safe.
This is assuming the MCPCB is securely screwed down of course.

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