Thanks again David, I was not aware about the way we perceive brightness. I will certainly implement another law when I come to mapping values in the project 
I would also like to use 3 of the channels to control some RGB strips. I have a TIP122 wired up as per the attached schematic (not using a regulator though just a simple led load), linked from the arduino playground (base connected to TLC output via 1k and base pulled down to gnd through a 12k). But it does not work how I had hoped.
I have a LED hooked up through a resistor to 12v and the cathode to the emitter, but it is on full time with the TLC set anywhere between 0-4095.
Any ideas what could be happening here?
TIP122 datasheet: http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000911.pdf