When the investigators walk into a potential crime scene to investigate, why don’t they turn on the damn lights? They walk around with flashlights all the time. I don’t get it.
What is the source of your information? How many potential crime scenes have you been present at? Or are you talking about how these are depicted on TV? (If the latter, this is more appropriate for CS.)
Well, that’s mostly TV. But not uncommonly, the power is off. And, you have to make sure that there aren’t prints on the lightswitch before you go fooling with it.
That being said, they take that sort of care on very few crimes a year. Not even every murder.
Too often, on a home burgalry, you could even point out a perfect set of prints, and the officers would just take your report and move on.
I have always thought those scenes were dumb. The have the actors running around in bright sunshine with a flash light. But it looks cool but dumb.
Sometimes the investigators (in real life, as well as on CSI) are using flashlights that are designed to make certain materials fluoresce (semen, blood, fingerprints etc). While they have goggles or screens to assist, these systems are easier to use in low-light conditions. Using Luminol (which glows in the presence of blood) requires almost total darkness for the best results.
So there is a mix of high light visual examinations and low-light fluorescence - and one of those looks more dramatic on a TV screen. 
Si
It’s a convenient tool on TV and movies to allow the director to show the audience only the part(s) of the room/scene that actually matter, rather than assuming the audience will be able to tell that, for example, the detective is looking at the teddy bear by the wall under the window rather than at the flower pot on the windowsill.
My complaint about the CSI shows in general is the unreality of the labs. Look at CSI Las Vegas. Turn some fucking lights on ferchristsakes! I love Abby’s lab in NCIS … she has lights … and work tables, and mass spectrometers and stuff. Granted she can instarun almost any test, but at least her lab looks more functional than blundering around in the dark. And she doesnt go out investigating and arresting and questioning suspects [well I think she has a few times, but those were exceptionally rare]
I have found when I have dropped something on the floor, that it is easier to look for it with a flashlight, even with all the lights on. So I can understand them using a flashlight all of the time. Why they have all of the lights turned off in their lab, that is just silly.
Moving to CS from GQ.
Colibri
General Questions Moderator
The CSI folks (at least on the original; I stopped watching when all the spinoffs started, especially the egregiously awful Miami one) tended to hold the flashlight up by their ear. Was that done to keep the camera focused on their faces? Aren’t flashlights normally held out on front of you?
I seem to recall in an early episode of CSI Las Vegas Grissom said something to the effect of, if you use the flashlight instead of turning on the room lights it makes you focus on where you’re looking and makes it more likely you’ll actually notice what’s in front of you.
Of course, it was just writer BS to justify, “We think it looks cool.”