Future development of the maximum luminance of LEDs

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.

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.

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.

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:

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

Nice, please keep us posted! :wink:

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.

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:

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.

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.

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.

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.

I meant using it on the thermal pad of a led while joining electric pads otherwise. If the other joining method is soldering - LM seems off the table. If it’s glueing heat is not a problem.

There was quite a number of people reporting how Conductonaut leaked and caused shorts inside their laptops. That’s why people recommend CLU there - it’s thicker.

I don’t know where he got the number, different places state different things. A range makes more sense to me than a single value. The highest I have read is 2500.

I do find it interesting though that you stated that it doesn’t work that well because this one company can’t or won’t do it that good.

In the end polishing the surfaces will reduce the effect of the paste anyhow. Vinz preferred this stuff especially for potting his lights.

I think it could be used for replacing a DTP pcb. That would be interesting.