In agreement with what Mark just stated, I found that varying intensities of light are pretty handy. Last October, we were doing a long hike (Mono Pass), and we were equipped with headlamps. When the sun went down, the light started to fade, and we turned the lights on at medium intensity, there were still some miles to cover. Once our night vision started to work a little, we turned down to low intensity, although we couldn't really get our full night vision going properly with even that much light. We could not turn the lights off completely, since there wasn't any moon, plus there were trees overhead to block starlight. Once in a while, we got to some junction or obstacle, and we turned up to high intensity for a short period.
One of these days we may be using night vision optics. Where are the Starlight Scopes of forty years ago?