I usually watch CSI a lot with my wife so we have this question:

When they start to seek for evidences they always use flashlights, even when they are in a room that has no energy suppression.

I understand that in a place with not so many light sources like a camp a big light could introduce many shadows but in a place like a fraternity room not. They use flashlights even to look at portraits in a table. This doesn't make sense to me.

I know they usually work in the night shift but why don't they just switch on the light?

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    I don't know for sure, but from a pure searching aspect, when you use a flashlight, it can throw shadows from objects. I often use this technique when looking for small things I have dropped on the floor. The shadow is a lot easier to see than the object itself. (I've often wondered this myself, so ...). Commented Mar 23, 2014 at 23:50
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    I worked in movie theaters for a long time, and the best way to see if a floor is really clean is to turn off the lights and shine a flashlight beam horizontal as close to the floor as you can manage.
    – JohnP
    Commented Mar 24, 2014 at 2:27

3 Answers 3


From your question, I'm unclear if you're asking why they use flashlights even when there are lights on, or why they use flashlights instead of using the lights.

The latter is easier to answer. They want to research and document the crime scene exactly as they found it. From Crime-Scene-Investigator:

Documenting crime scene conditions can include immediately recording transient details such as lighting (on/off), drapes (open/closed), weather, or furniture moved by medical teams. Certain evidence such as shoeprints or gunshot residue is fragile and if not collected immediately can easily be destroyed or lost. The scope of the investigation also extends to considerations of arguments which might be generated in this case (suicide/self defense) and documenting conditions which would support or refute these arguments.

They are looking for a single fingerprint, or strand of hair. Even turning a light on or opening a curtain could erase evidence. Therefore, they document the entire scene first by taking pictures, checking for all prints and using only their flashlights rather than other light sources.

Think of the season opener to Series 5, when Greg failed his entrance test to become a CSI for using a toilet at a crime scene - risking corrupting the crime scene and destroying evidence.

If however you were asking why they use flashlights in addition to the lights in the room, I think Paulster2 in his comments above has answered perfectly. The additional light source provides a greater level of illumination and clarity and makes it easier to find things.


The flashlights are a conditioned stimulus for the viewer watching the TV show. Usage of a flashlight by CSI precedes the special FXs of visualizing a crime sequence. CSI has been using visual markers for years to indicate that something is about to happen.

Before the viewer watches an animated sequence of a bullet passing thru a body, there will be a CSI looking at a bullet hole with a flashlight. The flashlight works as a marker to trigger anticipation in the viewer that something is about to revealed.

They do the same thing with the blue UV lights to reveal blood. Before they show a murder sequence they will have the CSI shine blue lights to reveal hidden blood splatter patterns on the wall.

These markers are used in the show to precede spots where CSI science is being used to solve the case. Instead of boring the viewer with scientific information the viewer is rewarded with visual effects or a shocking life action sequence.

It's not the only show doing this. Action and drama shows have been doing it for years.


I think they're doing it on the show Strictly for dramatic effect in a real situation you would need to turn on lights or add lights to reveal the crime scene so that you will not disturb or destroy any evidence by your movements within the crime scene only after this will the more detailed investigation begins.

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