Why do TV CSIs use only flashlights - even for the initial/cursory search? Why not just turn the lights on?

I can see why you need darkness and a flashlight for delicate and hard to see evidence, blood, etc... But they always enter dark or darkened rooms and buildings using only those tiny flashlights. Right from the start, when they are doing cursory/general overview searching. I especially hate when they do that at... show more I can see why you need darkness and a flashlight for delicate and hard to see evidence, blood, etc... But they always enter dark or darkened rooms and buildings using only those tiny flashlights. Right from the start, when they are doing cursory/general overview searching. I especially hate when they do that at night!! I want to scream at the TV - TURN THE LIGHTS ON!! (I can't believe it's because there might be a finger print on the switch.) I would expect the initial walk through to be a general sweep with lights blaring and then they'd come back with the luminol and teeny flashlights and do the detail search. Is this just for dramatic TV effect, or do real life CSIs do it this way as well? If so, why? Does light destroy evidence?! I would think the more light the better the initial search is going to be.
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