> >They make the point that the radiation pattern and 
> >focusability
>>allows an even greater improvement than this over CFLs on
>>terms of end user light.

> Ah, so this would be a suitable bulb to use in desk lights 
> or those stand
> alone over-your-shoulder type lights. CFLs work horribly 
> in those.


Yes. It has a fairly brought angle (about 140 degrees 
between half power points) and a somewhat fatter than 
Lambertian radiation pattern which is appropriate for this 
sort of use, so would be good for that sort of lamp as is 
and could be lensed down. Increasing angle with lenses is 
something that some attempt but which is largely not 
successful. You can use other optical devices to do this. 
Starting as wide as you want or wider is usually best.

Alas the LED at present costs about $US25 (Mouser /100) or 
if you must buy it from them Euro 25 :-) from somebody else. 
About $US30/1.
No doubt the volume price will be somewhat lower but it's 
still rather dear. At even $US20 that's $2/Watt which is OK.

It would make a nice "torch" :-). 6 x 2500 mAh NimH would 
last for about 45 minutes with a good buck regulator. A 12V 
7AH SLA brick would last for 6+ hours. It would take a 10 to 
20 Watt solar panel to service it well with a 12V 7AH SLA 
but would be a very very good lightbulb replacement.

IF the life is guaranteed it's OK, but this is by no means 
certain.

Luxeon have a "lumen maintenance" specification which is 
essentially a guarantee against degradation. The 10 Watt LED 
has no lifetime data that I can find so far. Dome is 
"silicone resin", which is a good start.

Looking at a data sheet for some Seoul Semiconductor 20 mA 
14 degree 18,000 mCd LEDs revealed a disturbing fact. In the 
test data they run them at 30 mA (the max rating) and test a 
batch of 18 (it seems) for 1000 hours. The spec is for 0 
failures in this time within test limits. Some parameter 
limits are fine BUT the flux limit is 50%. So they are 
saying that all LEDs in the test did not degrade by more 
than 50% in 1000 hours. I would certainly hope not !!! This 
limit is disturbing as they have obviously [tm] set it low 
enough to ensure no failures in 1000 hours. That suggests 
that  a limit of say 60% or 80% or 90% was too tight. With 
lifetimes loosely quoted as 100,000 hours and real lifetimes 
of 10,000 hours plus I'd certainly expect FAR better than 
50% degradation in 1000 hours, even at max permissible 
current. .




        Russell McMahon

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