LED
You can see the plates, LED, wiring, holes, stnadoff and cover here. It's all very simple, but effective. And very efficient. These LEDs are 109lm/W nominal. I'm running then at 700mA rather than 350 so that's more like 100lm/W, and you lose some in the diffuser too. The worst bit of the system is the Driver unit which is just ">60%". It's this one: http://www.leds.de/en/Accessories/Power-supplies/Constant-current-supply-700mA-IP20.html which has a nice form factor and is only 13€. Better supplies exist.