How to drive luxeon Rebel LED
You should measure the voltage across the 3 ohm resistor rather than across the LED. That will represent the current, and current is pretty much proportional to brightness. 

Make sure that your 3\$\Omega\$ resistor is non-inductive (not wirewound) type. 

Voltage is only loosely related to brightness in an LED. If you insist on measuring the voltage across the LED, put a 200\$\Omega\$ 1W non-inductive resistor across it. 







simulate this circuit &ndash; Schematic created using CircuitLab

Plot without R4



With R4