Blood does not inherently glow under ultraviolet (UV) light, commonly known as black light. The popular culture depiction of glowing blood is a misconception. Any visible effect associated with blood and black light involves a chemical reaction, not a natural property of blood.
What Happens When Blood Meets Black Light?
When exposed to black light, fresh bloodstains generally appear dark or black. Hemoglobin in blood absorbs UV radiation instead of re-emitting it as visible light. Surrounding surfaces, which reflect UV light, can appear brighter, making the dark bloodstain more noticeable. Blood does not naturally contain strong phosphors, which cause a visible glow.
However, a dramatic blue-green glow occurs when blood contacts a chemical reagent like Luminol. This visible reaction is a chemical process, not natural fluorescence. The light produced is temporary, typically lasting about 30 seconds per application.
The Chemistry of Chemiluminescence
The glow from Luminol and blood is an example of chemiluminescence: light emission from a chemical reaction without significant heat. This differs from fluorescence, where a substance absorbs light at one wavelength and immediately re-emits it at a longer wavelength. In chemiluminescence, the energy for light emission comes directly from chemical bonds breaking and forming.
Luminol, a chemical compound, reacts with iron in hemoglobin, an oxygen-carrying protein in red blood cells. To initiate the light-producing reaction, Luminol must be mixed with an oxidizing agent, typically hydrogen peroxide, in an alkaline solution. Hemoglobin’s iron acts as a catalyst, accelerating hydrogen peroxide’s decomposition into reactive oxygen species.
During this process, Luminol is oxidized to form an energized compound called 3-aminophthalate. The electrons in this 3-aminophthalate are in an excited, higher-energy state. As these electrons return to a lower, more stable energy level, they release their excess energy as photons, perceived as a blue glow.
How Forensic Science Utilizes This Principle
Forensic investigators widely employ Luminol to detect trace blood at crime scenes, even if diluted or cleaned. The solution, containing Luminol and an oxidant, is sprayed as a fine mist over suspected areas. The reaction is highly sensitive, detecting bloodstains invisible to the naked eye.
For effective visualization, Luminol application typically occurs in darkened conditions, such as a dark room or at night. The resulting blue glow indicates potential blood, allowing investigators to identify hidden patterns, like blood spatter. The transient nature of the glow necessitates quick documentation, often through long-exposure photography.
Luminol is generally non-destructive for subsequent DNA analysis. While some studies have explored its potential effects, Luminol typically does not interfere with obtaining DNA profiles from treated bloodstains. This allows forensic scientists to locate unseen blood evidence before genetic testing for identification.
Substances That Mimic Blood Reactions
Luminol is a powerful but presumptive test for blood; a positive reaction suggests its presence but doesn’t confirm it definitively. Several other substances can cause Luminol to react and produce a similar glow, leading to false positives. These interferences can complicate crime scene analysis.
Common examples include household bleach and other cleaning products containing oxidizing agents. Certain metals, such as copper or iron, can also catalyze the Luminol reaction, producing a glow. Some plant materials, notably the pulp of turnips, parsnips, and horseradish, contain enzymes that can trigger a Luminol reaction.
Due to these potential false positives, forensic scientists always follow a positive Luminol test with confirmatory tests. These tests verify the detected substance is blood and, if possible, determine its origin. They also help differentiate true blood reactions from other glowing substances, ensuring accuracy.