Where are the "superbright" blue leds?

[Phaserburn]

Where are the \"superbright\" blue leds?

There are 26K mcd white 5mm on ebay, as you know. But as whites are made from blue leds, why aren't there any blue 26K++ mcd ones available? Where are the brighter other colors?

 

[jtr1962]

Re: Where are the \"superbright\" blue leds?

Here's the problem. My preliminary results after testing those 26K mcd whites show that they put out somewhat more than 3 lumens at 20 mA. 3 lumens of 470 nm blue light is a radiated power of about 48 mW. Accounting for package losses of, say, 20% (just a wild guess), and the die needs to output 60 mW. Now typical Vf for blue and white LEDs at 20 mA is about 3.2V, so the input power is about 64 mW. I think you see where this is going. You would need efficiencies of close to 100% to make a blue LED with brightness and beam angle similar to those 26K mcd whites. Our best die so far (Cree's XT-27 available only as samples) has an efficiency of about 42%. Most blue dies have efficiencies of under 30%. Either you need a blue die with near perfect efficiency, or a package/chip capable of being driven at about 4X the power. Since I assume here that you're refering to making standard 5mm high brightness blues with the usual 20 mA current what you're asking for is close to impossible. In theory LEDs can reach efficiencies of unity. In practice, I think 60% to 80% will be the maximum, and it may take a number of years to get there.

 

[Phaserburn]

Re: Where are the \"superbright\" blue leds?

I understand what you're saying, (I think), but... aren't white leds just blue ones with a coating? If it makes 26Kmcd coated...?

 

[jtr1962]

Re: Where are the \"superbright\" blue leds?

Your eye is more sensitive to certain frequencies (i.e. wavelengths) than to others. Mcd and lumens are weighted based on this sensitivity. You can have 1 watt of light output but 0 mcd and 0 lumens if it's at a wavelength you can't see like IR or UV. 3 lumens of 470 nm blue requires a radiated output of about 48 mW but 3 lumens of the kind of white LEDs put out might only require only 9 mW. And if you want 3 lumens of 555 nm yellow-green, you only need to output 4.4 mW. The phosphor coating in white LEDs converts blue light to wavelengths which your eye is more sensitive to. Phosphors in white LEDs are maybe 50% efficient so figure you need 18 mW of blue light to get out 9 mW of white. Add in the package losses and maybe the blue chip is putting out around 22 mW. The same blue chip in a blue LED might only be rated for maybe 10K or 11K mcd. You didn't perform any magic here, or create something from nothing. You just downconverted blue light to white via the phosphor, and you actually had substantial energy losses doing so. If you could convert all the energy in the 22 mW blue LED to the same kind of white you might have an output somewhat over 6 lumens rather than 3 (or over 15 lumens if you converted it all to 555 nm yellow-green!). Future white LED improvements depend upon making this conversion process more efficient as well as developing more efficient blue chips.

 

[Moat]

Re: Where are the \"superbright\" blue leds?

Thanks for the interesting explanation, jtr1962 - /ubbthreads/images/graemlins/cool.gif !!

 

[Phaserburn]

Re: Where are the \"superbright\" blue leds?

Ahh, thanks. Now I do understand; makes sense.