I’m planing very simple rear light for my bike and wondering which RED diode would match the driver’s characteristics best? I’m not that deep into Vf and efficiency things so I’ll gladly accept any help.
The aim is long run so efficiency comes first. Doesn’t matter if 3535/5050 or 620/660nm.
But it has to be available at Ali or KD at this moment to make the purchase possible.
I’m not an expert on red light emitters 
but there are a few aspects that set it apart from the rest.
First of all, they are not very efficient, so don’t expect a gigantic red beam from an ordinary flashlight.
Second, they have a relative LOW Vf compared to blue and “white” led’s (2.0V vs 3.5V) so I think you need a special low voltage driver.
You can find 2 types of red led’s and a special red light driver in the Convoy shop on Ali.
Thank you Hank.
I have this Convoy driver but blinkies are a disaster there. Fastest way to get kicked by a furius driver.
Anyway, 105C is a must. But if I remember good, there was 5050 once which was more towards white leds but I can’t find the thread.
Yes, that’s the one. Thank you.
Great help, I owe you a 
But it doesn’t seem to vary that much…
620nm will be more visible than 660nm, all else being equal.
(Even if they have the same efficiency, which is the ratio of power converted to light, the 620nm will be more efficacious which means you will have more visible light for the same power because our eyes are more sensitive to 620nm than they are to 660nm.
When talking about how bright a light appears at a certain power efficacy is more relevant then efficiency.)
Red LEDs are low voltage so not a good match to the 105C (if it uses 7135s as my brain tells me) as the difference in voltage between the battery and LED Vf (forward voltage) will be dropped across the 7135s which, with a fullish battery and representative figure for easy maths, would be (4.1v - 2.1v = ) 2v.
Mutiply this 2V by the 350mA a 7135 passes through and you get 0.7W of heat per 7135 which is a lot to dissipate.
I don’t know where the point is for more power than the heatpath can handle in your light but this is where you would have problems.
Also, because any excess voltage is dropped across the 7135s the LED Vf makes no difference to overall efficiency as any energy saved by sending less voltage to the LED gets wasted in the 7135.
There’s a chance that this C50 Red LED is phosphor converted so would match the 105c better due to having a higher Vf.
I haven’t seen one in action so the fact that it’s phosphor converted is a guess, but i have the C50 Amber and Lake Blue versions which look phosphor converted to me as they contain most of the spectrum of colours, and the listed Vf of the C50 Red is in the region for a white LED rather than a red LED.
I also have no idea how it performs, or how it fits in the with any legalities relating to what light you can have on the back of a bike.
Red LEDs have a low forward voltage, so the 7135s would be burning up waaaaay too much power as heat.
Try to find a phosphor-converted red to play nicely with the '105C.
Tkank you guys, very helpful input.
Is there anything you’d recommend to mitigate the loss of power? Like taking out AMCs or using more than one led?
I assume it would not be very easy to adjust the driver to lower Vf, right?
And not sure how big the loss would be? Can there be any estimation made on which % of my battery capacity would turn into heat instead of light?
From my figure above you would have a 50% loss in the 7135s with a full battery, dropping to 44%ish when the battery reaches 3.6V, which is as low as i like to run mine. Basically, a lot.
Changing this would mean a driver redesign and new components, basically a new driver.
But the multiple LED idea is interesting. ILooking at the current vs voltage chart on page 16 of the Cree XP-E2 datasheet you might be able to run 2 x photo reds in series.
It looks like they’d start at a max of 250-300mA (each) with a full battery and drop in relation to the drop in battery voltage, down to around 50mA at 3.7V, alkaline stylee 
You’d have to find out whether this would be bright enough for you, but you’d only be losing minimal amounts of power through the 7135s as they basically wouldn’t be doing anything.
KD also do the XP-G3 photo-red which has a slightly lower Vf (page 18) so you’ll get some more light out of these compared to the XP-E2, but it will probably be less intense as more light would go to the sides and less directly out the front.
All depends on how it works in practice, and only you can know that 
Thank you Marc for the input.
If I understand correctly I need to use 2 LEDs in series. Hmmm… I’m sure I have some mcpcb for 2 but need to check if in series or parallel. And probably mule in this option since I’m not hoping for matching and ref or TIR optics.
Unless I find the way to power 2 separate flashlights from one driver 
Still worth the effort to try I think.