Luminol and hydrogen peroxide are combined. This solution is then sprayed in an even distribution over an area where it’s suspected that blood is present. If any spots glow blue in the dark, this is an indicator that blood has come in contact with it.‍

This glow is the result of a chemical reaction called chemiluminescence. This is the same type of chemical reaction that happens when we see fireflies blinking.‍

There’s just a short window of time that the affected area will glow, with luminol generally only giving off light for a brief 30 seconds. Crime scene investigators may document this evidence by using a long shutter speed when taking a photo of the affected area.‍

Though its glow may be brief, luminol can illuminate traces of blood even after a long time has elapsed, giving investigators the proof they need to show that a crime has been committed. And it won’t interfere with the DNA analysis of this blood evidence.