Measured LED output vs Cree chart

OhMyGosh · Jul 3, 2007

I measured the output of a new Cree XR-E, P4 bin and compared it to the chart in the Cree spec sheet

Code:

[FONT=Fixedsys]Milliamps Measured Cree Chart   Watts   Volts[/FONT]
[FONT=Fixedsys]      100     175%       219%    0.28   2.850[/FONT]
[FONT=Fixedsys]      200     159%       172%    0.60   2.990[/FONT]
[FONT=Fixedsys]      300     147%       152%    0.92   3.080[/FONT]
[FONT=Fixedsys]      400     138%       137%    1.26   3.155[/FONT]
[FONT=Fixedsys]      500     130%       127%    1.61   3.220[/FONT]
[FONT=Fixedsys]      600     122%       119%    1.97   3.275[/FONT]
[FONT=Fixedsys]      700     116%       113%    2.33   3.325[/FONT]
[FONT=Fixedsys]      800     110%       108%    2.70   3.370[/FONT]
[FONT=Fixedsys]      900     105%       104%    3.07   3.410[/FONT]
[FONT=Fixedsys]     1000     100%       100%    3.45   3.450[/FONT]

They are lumens/watt normalized to the 1 amp level. Note that I used my measured values for both voltages since Cree's chart of Vf is hard to measure.

What is interesting is that they are very close until the lowest current levels. I don't get that big jump in efficacy that Cree's chart shows.

LEDependent · Jul 3, 2007

What equipment are you using to test? It may not be as sensitive at the lower levels as Cree's equipment.

OhMyGosh · Jul 3, 2007

Just a $50 lightmeter. I wouldn't pretend to guess actual lumen values - But it should be close for relative brightness. I had the LED close enough to read 2000 lux at 100 mA, (10x200). It read 13970 @ 1000 mA.

TorchBoy · Jul 3, 2007

That looks very interesting. Which chart in which spec sheet? And why are those voltages lower than the specs I've seen? Are they your own readings?

OhMyGosh · Jul 3, 2007

TorchBoy said:
That looks very interesting. Which chart in which spec sheet? And why are those voltages lower than the specs I've seen? Are they your own readings?

The voltage from one to another varies quite a bit. I just checked 6 @ 600 mA. 5 of them were 3.25 to 3.35 and one was 3.50v. I used a 0.2 ohm 1% resister to measure the current.

The XR-E spec sheet from Cree. There is a chart normalized to 350mA = 100%. I blew it up in Adobe to 50mm = 50% and used a ruler to read the values for each 100mA.

OhMyGosh · Jul 5, 2007

I found a chart on Newbie's awesome, mother of all LED electronics website.
His chart shows similar results to mine; ~149% @ 200mA, ~167% @ 100mA.

I think this will become more important with the new Luxeon Rebels. They are less expensive, and I think you can fit 4 (or more) in a single Cree size reflector. I wonder if the Rebels will have similar low-current efficiency. Driving four emitters @ 250mA vs one @ 1000mA for 40-50% more light output. The rebels lower cost and little extra cost/space for reflectors and heatsinking makes this look very attractive.

TorchBoy · Jul 5, 2007

OhMyGosh said:
I think this will become more important with the new Luxeon Rebels. They are less expensive, and I think you can fit 4 (or more) in a single Cree size reflector. I wonder if the Rebels will have similar low-current efficiency. Driving four emitters @ 250mA vs one @ 1000mA for 40-50% more light output. The rebels lower cost and little extra cost/space for reflectors and heatsinking makes this look very attractive.

And people laughed when I suggested that Luxeon might well have a better long term strategy. (Well not really but it sounds good, especially for those brought up on soap operas.) How long before (dare I mention them) DX starts selling four Rebels on a star? They might need a diffuser on the front of whatever optics were used tho.

Measured LED output vs Cree chart

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