I was wondering how consistent the vf is? My plans is to connect a number of leds in series, lets say 15 rebels in a string. This is fed by a 54vdc regulator at 650ma allowing 3.6v for each led. Do you think some leds will have a bigger or smaller vf than others in this string? I havent been experimenting with leds that much yet and have not much practical experience.
What problems could I encounter?
If they are in series then slight differences in Vf don't matter. They all get the same current which is the important thing. You should have some resistance in series to limit the current for the worst case (i.e. highest) voltage and lowest total Vf (i.e. all LEDs hot).
When LEDs are in parallel the slight Vf differences can lead to current hogging which can overdrive the low Vf LEDs. But in series there is no equivalent problem.