[rintense]

I saw it on the show, 'ER'. They use black lights as part of a rape kit to detect semen before doing a full gyno exam. Black lights can also be used to detect cat urine on carpet.

[Charles Martel]

Yep, black lights work well to locate beer spills - and other stains. I recently saw a cheap one for sale at one of those "Party Warehouse" stores.

Of course, the janitors would've had to gather that evidence before resigning - you can bet the unfortunate new janitors have had to scrub down every inch of the place.

[LowOiL]

Black lights are handy little object, no doubt, but getting one powerful enough to work in daytime can be a daunting process. You would have to turn the lights off... those little battery powered lights don't do dittly-squat (pardon to our FReeper named similarly).

So basically you would have to get an expensive AC light (or two) and turn off the lights to film it. Doing that without getting noticed perhaps would be hard.