Brian, it's purpose built. At our old house I used to 'operate' in our bathroom but that had a skylight (which I could pretty well block with cardboard and material) but the exhaust fan was a problem due to a tiled roof that let a lot of light in during the daytime. I only went to the trouble of covering it a couple of times and used to restrict my darkroom time to night time, which is a problem in summer (sunset about 9pm) not to mention getting all the stuff out of the cupboard and setting it up, then cleaning up ready for the morning showers. My new room has cost a few $$$ more than it could have been done in that I had the sink made rather than doing it myself, and I got the cabinients made by the kitchen suppliers where something home brew or 2nd hand would have really surficed. Now I only have to find time to get in there!
Marsh, everyone says the Thomas safelights are the best.. I wouldn't know myself but I've got a 10x8 Premier (I think that's the brand) mounted on the wall pointed up at the ceiling. It's got the original 25w globe in it and it quite bright. I did a safelight test and it seemed ok. I need to revisit this and confirm how long I can keep paper out (for those multiple dodge and burn jobs!
One thing I do need to look at modifing my exhaust fan. I have the small Dorian (once again, I think that's the name) exhaust fan but it's a noisy little mongral. I'm thinking of moving it by putting it at the end of some ducting which should quieten it down.