If you control the current through the LED, the Voltage Drop on the LED will be varied in a very small percetage. Remember that it is a diode with a very steep curve.

So if you control the voltage drop on the LED, you have to control in steps of mV, in a very small range and NOT in linear fashion.

If you, on the other hand, control the current the voltage drop is left on the LED to be determined. Also current control can be linear.

To measure the current, you just need a small resistor in series with the LED and maybe an opamp to amplify it in the levels your ADC needs.

Then controlling the current (by PWM) of the LED you can get feedback just nicely.

Or use a cheap IC from ZMDI and control it by your PIC.

Soon I will have a power LED from Citizen (10 or 25 watts) controlled this way. I'll try to upload a video also.

Ioannis