Blood pattern analysis
Even a tiny drop of blood at the scene of a violent crime can give important information to the police. Blood is there either because it has dripped out of a small wound, sprayed out from an artery, oozed out through a large wound, or flown off a weapon. Using blood pattern analysis, police can learn a lot about what happened from the shape of the blood drops.
Sometimes a murderer cleans the crime scene very carefully, and if detectives cannot see any blood they spray a chemical called luminal across the scene. This makes it possible to see the blood in the dark. Luminal can show up very small drops of blood.