I recently replaced the seven 50W GU10 halogen spotlights in my kitchen light fitting with 7W energy-saving bulbs, after managing to google up some which weren't 2cm longer than the originals and hence actually fitted into the space.

The new bulbs take a few minutes to come on fully. I wondered how long, because that way I know how long to go away and wait after flicking the switch. Unfortunately, finding that out by sitting and watching is a bit tricky, because one's attention wanders and in any case one's eyes adjust constantly so it's hard to say whether the lights are still getting gradually brighter or have stabilised.

Solution: the digital camera to which I treated myself at Christmas, in video recording mode. Set it up on a tripod pointing at the kitchen door, with a bit of uniformly lit dining room wall in shot as well to compare against; start it recording; turn kitchen lights on; go away and leave it until it runs out of disk space. Extract the resulting video file, convert into a gigabyte of PNGs, and analyse them in software.

So now I have a nice graph plotting light level against time:

from which I can read off the time to absolute peak brightness as (as closely as I can get by zooming in) 3 minutes and about 39 seconds. Of course, it'll be usably bright long before then, if I'm impatient to eat; but for some things (such as washing up, where I've found bright light really helps for noticing things aren't properly clean) I think I actually would want to make sure I had full brightness.

The glitch at the start of the graph is because I had to actually walk in front of the camera to flick the switch, skewing the average brightness of the picture while I was in shot. Unfortunate but necessary, in the absence of a cloak of invisibility.

An excessive effort, perhaps, but it was fun. Also it vaguely justified the new camera, since the old one's video recording capabilities wouldn't have been nearly up to this job. (That is, even before I broke it beyond repair.)