A Blood Detector

The Great Crime Scene Showdown....

A Blood Detector - Luminol

An investigator taking photos of a crime scene

When police arrive at a crime scene, they can find a LOT of blood evidence. But sometimes, the sneaky criminal has tried to wash off the blood to cover their tracks. Did you know that police can still find this invisible blood? It is called "latent" blood and police have special sprays that will glow when they come in contact with the iron in blood. The glow is caused by a chemical reaction between the iron and the chemicals in the special spray. This is called "chemoluminescence," a really long word which really just means "a glow from a chemical reaction."

Bloostain on a t-shirt

Glow of Bloodstain when sprayed with Luminol

The Crime Lab that I toured uses a latent blood detection spray called "Luminol." Luminol was discovered in 1902, but police didn't start using it at crime scenes until the 1940s. Luminol is tricky to use because the room has to be completely dark to see its glow. That means police have to cover up windows, doors and any other light sources before they spray with Luminol. The glow only lasts 30 seconds, so police have to move fast to take samples. A batch of Luminol lasts 30 minutes, so police will have to make more if they are spraying a large crime scene.
These photos were taken during my tour of the lab. The photo on the left is of a bloodstained t-shirt, and the one on the right the glow when it is sprayed with Luminol.

What does the FBI do???

They e-mailed and told me!!

The FBI helped me with my research

I e-mailed the FBI to ask them what kind of latent blood sprays they use. A Supervisory Special Agent from Quantico wrote me back, which was very exciting!! He said that the FBI lets their agents choose what blood detection spray to use, Luminol, Bluestar or Fluorescein. I could not test the last kind of spray called Fluorescein because only professional labs can order it. Fluourescen is not used that often because it takes a special light and 2 different sprays. You probably see it a lot on TV because that extra light looks cool. Bluestar and Luminol are the two most popular sprays, so I tested them.