Are Rebels more efficient than K2 TFFC?

Looking at the data at Future, I saw this data for the best available Neutral emitters:

Rebel 100 bin @ 700mA = 180L
K2 180 TFFC @ 1000mA = 180L

And Rebels seem to be running lower Vf than K2, so this tells me that the Rebel above is far more efficient than the K2. Am I looking at this wrong?

By my estimation the best low Vf Rebels should be over 91L/Watt, while the best K2 220 will only be just over 61L/W. Is my math OTL?

The new Rebel ES is alledgedly 100Lm/watt in neutral white and can be driven at 1 amp (I cannot immediately see this part on future's website though)

HL

Remember that the rebels are Vf binned @ 350mA and the K2 TFFC @ 1000mA.

You also need to compare lm/w at the same current for a fair comparison which is difficult since the specs are given at different currents.

There are different bins too, the top K2 available right now iirc is 220 lumes @ 1A. Overall both packages should be using the same technology so they should be relatively equal to each other.

I quoted neutral bin specs since that is what I'm interested in. The Rebel 100 neutral is the spec I quoted (they don't say SE anywhere I could see on Future).
Even looking at K2 220 cool bin with an optimistic Vf of 3.15 (lowest I've seen for K2, but only in K2 200's) then K2-220 gets 70L/W while Rebel 100 gets 91L/W. Vf should be higher at higher amps for a given LED, but if an LED is designed to operate at 1Amp, then that is where it's efficiency matters. And Neutral K2 (K2-180 are available) are actually around 57L/W with a low Vf of 3.15 @1A if you can get them. Some run up to 4V, you can't spec the Vf bin on Future, and I don't want to risk a Vf that high.

Higher Vf and more efficiency at lower amps can't account for 30% difference in efficiency.
As an exercise to show this, I took an established Lumens/Amp curve and estimated 144L at 700mA for the neutral K2 180. Assuming a lower Vf of about 3.0 (the Amp/Vf curve is very steep at & just below Vf for diodes, so the Vf will not change a lot for a 30% drop in current) then I get 69L/W. For the best cool bin available (220) even with a Vf not seen in the K2-220, then I get 84L/W. Getting closer, but I'm using vaporware LED bins to do so. But if I wanted to drive something at or below 700mA, I'd still use the Rebel 100 and be ahead.

180 lumens is 180 lumens, and if it takes only 700mA in a Rebel, but takes 1000mA in the K2, then that tells me something. The Vf would have to be 30% lower for the K2 to make up that difference, and I don't see that anywhere. Rebels are the ones with consistantly lower Vf, even though some low Vf K2 exist - TV0D's (200L, 3.15Vf) are out there.

I assumed since they use the same technology they would be the same efficiency too, and I am still mystified as to why they apparantly are not.

Can you provide a link to the low Vf rebel 100?

By my calculations, 91lm/w @700mA corresponds to Vf of ~2.8V @700mA. That corresponds to ~ 2.5V @350mA which is lies within the "A" voltage bin which I haven't seen in a white rebel.

Also the 180 bin of the K2 is minimum spec whereas the 180lm rating of the rebel is typical.

It's 91L/W at 350mA using 3.15Vf which I have seen. I calculated 79L/W at 700mA assuming Vf goes up to 3.25V. I saw test data from another member showing Rebel, Cree SSC, etc, Vf's and hear the Vf run low - usually D-bin (3.03-3.27V). I have seen K2 TFFC up to 3.99V (G-bin), but they do come in as low as D-bin as well so I give them the benefit of .

monkeyboy said:

Can you provide a link to the low Vf rebel 100?

By my calculations, 91lm/w @700mA corresponds to Vf of ~2.8V @700mA. That corresponds to ~ 2.5V @350mA which is lies within the "A" voltage bin which I haven't seen in a white rebel.

Also the 180 bin of the K2 is minimum spec whereas the 180lm rating of the rebel is typical.

Click to expand...

You are right! the Rebel 180L at 700mA is typical, not min. I missed that. So the K2 is min 180 @ 1A with typical Vf of 3.65V...
... crunch## ... crunch### ... (crunching numbers)

Still by my estimation including some realistic estimated MIN Lumens from output charts:

Emitter . . . Lumens . @mA . Vf . Watts . Lum/W
Rebel 100 . . . 100 . . 350 . 3.15 . 1.1 . . . 91 (min Lum)
Rebel 100 . . . 180 . . 700 . 3.25 . 2.3 . . . 79 (typical Lum)
Rebel 100 . . . 166 . . 700 . 3.25 . 2.3 . . . 73 (min Lum - estimated)
K2 180 . . . . . 144 . . 700 . 3.00 . 2.1 . . . 69 (min Lum - estimated)
K2 180 . . . . . 180 . .1000 . 3.15 .3.15 . . 57 (min Lum)
K2 180 . . . . . 250 . .1500 . 3.30 . 5.1 . . .49 (typ Lum)

Rebel still beats K2 Min numbers. Compare the estimated Rebel Min at 700mA (166L) and the K2 estimated Min at 700mA (144L even with a possibly optimistic lower Vf)

I like the K2 output overall and wish it was just as efficient since it is easier to mod with. If it was as efficient it would put out 285Lumens (Min!) at 1A. Hope to see some of those soon.

BTW, I am having a K2 TV0D (200L @ 1A, 3.15Vf) dropped into my avatar light as we speak...
I hope it survives at 3.8V from an RCR (for short bursts only)!

Great stuff here guys. I came to much the same conclusion myself a few weeks ago before I sent some of the neutral Rebels to Milky. Now there seems to be another reason why the Rebels may be better than the K2s. Check out the reports from Tebore regarding tint shift with drive current for the K2. It remains to be seen if the Rebel is any better.

The tint shift is really terrible when current drops for the neutral White series of K2T. I imagine it'd be worse without PWM (the EDC uses PWM).

Driving at the LED at ~1.7A at ~3.8-3.9V. (I didn't care about killing it I just wanted to see how much the color could shift). The LED would still much greener than a 5A MC-E but it'd finally be acceptable for a neutral white.
At <1A the LED just gets horribly green. :green:

I'd also like to see if the Rebel suffers from this kind of tint shift.

I got my fingers crossed and tossing Pennies in to a wishing well for the day a V-bin high CRI SSC P4 is common place. :devil:

tebore said:

The tint shift is really terrible when current drops for the neutral White series of K2T.

At <1A the LED just gets horribly green. :green:

I'd also like to see if the Rebel suffers from this kind of tint shift.

Click to expand...

Interesting - I ordered a pair of the 220L cool white and a pair of the 180L neutral TFFC from Future to experiment with. One of the 220L went into my ROV Extreme 2xAA adjustable focus light to replace the non-TFFC K2. The other 220L went into a Dorcy PR drop-in to replace the Lux I that I use in a Mag 2D. The ROV benefitted nicely from the upgrade, but I believe I got a real high Vf bin on the 220L - the highest bin on the spec sheet, if I recall, K, with a V0 color bin (not 100% sure on this info).

The 180L went into another Dorcy PR drop-in that I use in a Gel-Brite 2AA around the house. I have not measured the current on the Dorcy PR lamps, but it can't be more than 350mA, maybe 500mA tops. It's definitely a warmer tint, but I certainly wouldn't call it green - maybe a wee bit yellowish-green. Can't say I like it any better than a good non-bluish cool white, but it's nice, nonetheless. The neutral whites were a much lower Vf bin, but I don't recall the exact Vf and color bins.

Chris