Joules thief circuit

Last weekend I started some experiments with the "Joules thief" circuit. The circuit is very simple to make and it should work first time, if not exchange the connections of one of the windings. It works down to 1.2V but it probably works down to 1V. It should be possible to design this circuit with higher efficiency once some of the circuits components are better defined.
Here's a picture of my Joules Thief circuit with a white LED...Some parameters can be determined experimentally, like saturation of the inductor (max ampere turn), gain of the transistor, the frequency of oscillation can be controlled with a capacitor on the base circuit. I've also experimented with a solar cell powering the circuit, but the it needs to be under a strong light.
One of my project ideas was to build an "daylight extender" with a two or three white LEDs, a Ni-Cd battery and a light sensor, maybe with a microcontroller.