Ultraviolet (UV) light remains unseen by the human eye. Popular culture often depicts blood glowing brightly under UV light, leading to a common belief about its detection. However, this widely circulated image is largely a misconception, as blood does not visibly glow when exposed to UV radiation alone.
The Truth About UV Light and Blood
Blood does not naturally glow or become fluorescent under UV light. The central reason lies with hemoglobin, the protein in red blood cells responsible for oxygen transport. Hemoglobin primarily absorbs ultraviolet radiation rather than re-emitting it as visible light. This absorption means fresh bloodstains often appear dark or black when illuminated by a UV light, standing in stark contrast to surrounding materials that might reflect UV light. Blood lacks the specific chemical compounds that would allow for such a reaction.
The Role of Forensic Luminol
The “glowing blood” portrayed in crime dramas is the result of a chemical reaction. Forensic investigators use substances like luminol to reveal hidden blood traces. Luminol reacts with the iron found within hemoglobin, a component of blood, in the presence of an oxidizing agent such as hydrogen peroxide. This chemical interaction produces a temporary blue light, known as chemiluminescence.
Chemiluminescence differs from fluorescence because it generates light through a chemical reaction, whereas fluorescence involves the absorption of light at one wavelength and its re-emission at a different, longer wavelength. The luminol reaction is highly sensitive, allowing forensic teams to detect even minute or diluted bloodstains. This blue glow is best observed in a darkened environment, helping discover cleaned-up or faint blood evidence at a crime scene.
What Can UV Light Actually Reveal?
While blood does not inherently fluoresce, UV light is a valuable tool for revealing other substances. Many bodily fluids, such as semen, saliva, and urine, naturally fluoresce when exposed to UV radiation. This occurs because these fluids contain specific compounds, like flavins or tryptophan, that absorb UV light and then emit it as visible light.
UV light can also make various common household items visible due to their fluorescent properties. Examples include certain laundry detergents, some types of paint, and security features in currency. Petroleum jelly, some minerals, and certain vitamins can also exhibit fluorescence under UV light. These substances glow directly under UV light, distinguishing them from blood, which requires a chemical reagent like luminol.