Why don´t they make the glowing dice a bit bigger ?

I was looking at the dice where the light emmits from a led and was wondering why don´t they make this bigger.

Judging from what I see from the leds when they are on (Looking thru shaded glass ) I think that the dice could be made bigger and the reflector could be made more of high quality like the SF ones but in miniature, that would help make them be brighter

what do you guys think ?

Jeez I´m just a "liuttte drunk right now and maybe I´m not thinking straigth.

AlexGT

They have done! In those neato new LumiLEDs products like in that other thread that talks about them.

As for the little reflector inside, well you have to realise that it is maybe over 50 times smaller than that of a surefire`s lens (probably- don`t quote me on that though cos I don`t have any surefires), and at that sort of scale it is very hard to make things super accurately. Plus not always does the light emitted by the die come from the sides. More often the bulk of it comes from the top of the teeny cube shape inside, so the reflector does little to maximise the light output, instead it is the shape of the resin body that affects the beam shape- that curved top acting as a lens. Basically it aint easy making an led with a tight even beam!
With the new generation of LEDs (see that "newer brighter leds" thread) they have an external collimating/focussing lens as well as that larger die (more light to begin with) inside making for a much brighter led indeed and a smoooooth beam too.

Hiya,

Large junction devices have actually been around for some time, and the product range manufactured LumiLeds / Philips Lighting / Agilent employs this technology.

You can make an LED glow brighter by driving it at overcurrent but that makes the die get hot, and its efficacy and life quickly diminish. What is unique about LumiLeds products is that they pioneered the so-called large junction technology. Standard 5mm LED's employ a cubic die of side 0.25mm, whereas LumiLeds are up to 1mm. That immediately gives you up to 16 times greater junction area, and a good deal more luminous flux. Because current per unit area is the same, life is not reduced.

The large die does however pose several potential manufacturing difficulties. Not only does its differential expansion and contraction with respect to the encapsulant cause problems, there is also a lot of heat to be removed. So their "Luxeon" product range is built on a superb heatsink material (sometimes even man-made diamond) attached to a metal-cored PCB, and the die itself is encased in an optically clear compliant silicone gel, and then encapsulated in UV-stabilised clear epoxy. Because the chip is too big to be fitted inside a reflector cup which can be formed out of the wires traditionally used for 5mm LED's, a special separate large reflector cup is used.

The attached photos show some stages in LumiLeds LED development together with the associated flux increase. Some of you may have heard these products referred to by their internal names - the large junction device is a Barracuda, and the Truncated Inverted Pyramidal die with bevelled 55-degree sides is known as Prometheus. They are packaged in the Luxeon envelope, with or without a collimator to deliver a homogeneous and narrow beam. All devices are supplied on a 25mm square metal-cored PCB which is essential for adequate heat dissipation, and must be secured to a suitable heat-sink. Chris Millinship posted photos of these Luxeon packages a couple of days ago.



Hope this is of interest!

James

FYI here is another diagram which shows the constructional differences between traditional LED's and the high-flux devices made by LumiLeds. BTW the slug is the heat sink-reflector in which the LED die is mounted, and the clear silicone is located above this, encapsulated within the outer plastic lens.

James.

The slug is the heatsink?

The slug is the heatsink?

Yep. Well part of it anyway. Not only does it form the reflector but also conducts the large amount of heat produced by that big fat die away and down into the metal core PCB it is to be fixed to. They aren`t going to be supplied without that special PCB because of this- they have to be bonded to it to ensure good thermal conduction. Then the heat spreads throughout the metal and is dissipated to the surroundings, perhaps through whatever it gets bolted to (for example a flashlight head).

Thanks for the info!

-Peter Gransee

Wow!

That´s a lot of info there, thanks !

So if I got it correctly, the dice can´t be made bigger because there is heat and heat related expansion of the material in the dice involved.

Well why do they make plastic or resin filling instead of something more heat resistant like pyrex or some other heat resistant stuff.

Another thing if I coat the leds with AR coating like the SF´s will it transmit more light ? I can take the leds to some lenscrafter to be coated

thanx

AlexGT

I can see where the heatsinking is superior with the high flux. I wonder if it would be possible to have a space around the slug/reflector to allow for expansion/contraction. Maybe a vacuum in the space or a gas, so the heat would not transfer to the plastic. It might be possible to make the die/slug much larger. Either way I can see this is a much better design.

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by KenB:
I can see where the heatsinking is superior with the high flux. I wonder if it would be possible to have a space around the slug/reflector to allow for expansion/contraction. Maybe a vacuum in the space or a gas, so the heat would not transfer to the plastic. It might be possible to make the die/slug much larger. Either way I can see this is a much better design.<HR></BLOCKQUOTE>

They actually use a liquid silicone compound in the space between the lens and the die.
The primary purpose of this is to help prevent drastic thermal changes (mainly expansion/contraction) from ripping apart the ohmic contact & bond wires off the LED chip & leadframe as the LED is used.

http://ledmuseum.home.att.net

LED chips can be made bigger than this, and indeed have been in the laboratory. And yes they do generate more heat when they're bigger. But the problem is not so much finding an encapsulant which will withstand this (like Pyrex you mentioned), instead it's important to makes sure the LED chip never gets that hot.

As you might have noticed when overloading LED's, they actually get dimmer if you load them too much that they overheat. Light output decreases as the chip becomes hotter so the aim at present is to make better and better heat sinks to conduct the heat away more efficiently.

Craig is correct about the silicone filling around the die, at present that's the easiest way to combat the thermal expansion with the bigger chips - inert gas fillings are not good at conducting heat and can be difficult to manufacture and maintain in service. You can see the silicone-filled region quite easily in white Luxeon LED's. In these ones, the phosphor is dispersed within the gel and it can be seen as a yellowish region into which the contact wires disappear.