As pointed out by jking, just varying the voltage does not really work: you need to adjust the current flowing through the LEDs; this needs a transistor as shown in Hew's circuit. You then either use the transistor to vary the current through the LED or use a PWM (Pulse Width Modulation) signal to switch the LED on/off at high speed using an adjustable duty cycle.

I'm sure a google search will throw up useful circuits, or check the data sheet for the ubiquitous 555 timer chip - IIRC it can be configured as a PWM generator - there is a dual version (556?) allowing both channels to be driven from one chip.

I think Hew went down the route he did to allow integration with his RH Designs timer/analyser. Plus, using the PIC microcontroller means he can add as many bells and whistles as he likes simply by changing the firmware. The PIC only costs a few quid/dollars so if you already have the development tools and are familiar with its programming language the additional cost is not great. If you need to buy & learn it all however then it does become far too much hassle....

Nick: Hew made 4x5" heads, so scale up from there for larger sizes, taking the area to be illuminated in to account (e.g. an 8x10 head would require 4 times as many LEDs for the same level of illumination). I think you are probably right about the colour printing - I don't do colour and I've never looked deeply in to it so I'm not at all sure if the specific LED colour wavelengths would be suitable for colour work. Someone needs to try it...

Jolly interesting, but I don't think I'll go down the route of building my own head myself. I could do with better and brighter safelighting in my darkroom however so I might have a look at the Red and Amber Lumileds...

Cheers, Bob.