Light enters the front of the tube in the form of photons. The number of photons depends on the ambient light levels. The photons strike an input screen in the tube and are converted into electrical energy.
This screen is phot-sensitive and multiplies the number of electrons. These electrons are projected onto a phosphor coated screen which converts the electrons back into light in the form of an image.
As previously stated they do not work in complete darkness. On an overcast, no moon night in the desert, you're lucky if you can see the ground from 50'. However in populated countries such as the UK, the cloud reflects the light from built up areas so visibility is usually very good.
Once you've flown with them, you never want to fly in the dark again.