Why Does Bodily Fluid Like Semen Shine Under UV Light?

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Crime dramas show us shining splatters of blood and semen at certain crime scenes. The scientific reason behind these glow-in-the-dark splatters is due to a chemical phenomenon called luminescence.

Every crime drama fan knows that body fluids are important forms of evidence. However, they aren’t always clearly visible. The blood may have been wiped away or the semen may have dried up, leaving only an indistinct stain behind. To detect these, the forensic experts will shine a special light in a dark room and voila, the bright lights will reveal bodily fluids.

But is that accurate? How and why do bodily stains “light up”?

Hematological analysis with forensic test kit in a murder in a crime lab, positive for human blood, conceptual image
Forensic tests. (Photo Credit : envato)

Presumptive Tests

Despite the name, these tests are real. When forensic experts need to check if certain fluids are present at a crime scene, they will perform certain tests called presumptive tests. The tests help strengthen the assumption that bodily fluid is present, but it is not conclusive. False positives have been found, as certain other substances also react similarly to these tests.

To confirm, forensic experts will then usually perform more rigorous tests.

From among the arsenal of presumptive tests up a forensic expert’s sleeve, one is called an alternate light source. These tests are where an expert will shine a light on a bodily fluid and, if the fluid is present, it lights up. The next question, of course, is how do they light up? Through a phenomenon called luminescence.

Also Read: How Does Science Help Solve Crimes? The Real Life Science Of Crime Scene Investigation And Forensics


Luminescence is a phenomenon where a chemical substance emits light but without being heated. This can happen in two broad ways, chemiluminescence and photoluminescence.

Chemiluminescence, as the name suggests, occurs via a chemical reaction. A subset of chemiluminescence is bioluminescence, in which life, through certain metabolic reactions, creates light. One of the best examples of this is fireflies.

Photoluminescence is when light causes a chemical to in turn release light. There are two types of photoluminescence: fluorescence and phosphorescence. For this article, we’re only interested in fluorescence.

Fluorescence works like this. One would shine light with high energy (light with short wavelengths) on a chemical that can fluoresce, called a fluorophore. That chemical will absorb the energy in the light. The energy will cause the electrons in that chemical to get excited. Excited electrons, though, are unstable, and so the electrons release the extra energy as vibrational energy (by vibrating a lot) and in the form of light energy. The light that is emitted has less energy (and therefore a longer wavelength) than the initial light source, as some energy was given out through vibrating.

Many of our bodily fluids are either chemiluminescent or fluoresce.

Also Read: Why Do Certain Things Glow In The Dark?

Chemiluminescence: Detecting Blood

Blood is readily visible; it is red or brown, depending on how old it is. Why then do we need a special technique to see it?  That’s true, but even if you wipe away blood some residue may remain, in spite of the clean up, even for years.

There are actually several tests that help reveal blood stains, but only one of them works through luminescence.

A chemical called luminol (for the nerds, the IUPAC name is 5-Amino-2,3-dihydrophthalazine-1,4-dione) is frequently used to detect blood stains at a crime scene.

Luminol in action.

When luminol mixed with a little hydrogen peroxide comes in contact with hemoglobin (the molecule in RBCs that transport oxygen and give your blood a red color), or rather, the heme in the hemoglobin, a blue hue is generated.

The best conditions to see this blue hue is in the dark because the light is quite faint. The light is emitted as long as the reaction keeps going, so multiple sprays of luminol may be required.

Luminol is a good detector of blood. According to one estimate, it can help detect 1 micro liter of blood in 1 liter of a solution! For comparison, a single drop of blood is 50 micro liters!

Photoluminescence: Detecting Semen, Saliva And Vaginal Fluid

Bodily fluids like saliva, semen and vaginal fluid do not require a chemical agent to make them emit light. Instead, they fluoresce when exposed to the right (short) wavelength of light.

In 1919, Dr. Wood found that UV-A light, what he called “black light”, could be useful in detecting certain bodily fluids. The technique caught and since then, the light has come to be known as “Wood’s light”.

Semen fluoresces blue between 300-450 nm, in the ultraviolet range. The invisible (to us) UV rays doesn’t interfere with the fluorescence, so forensic experts can see the stains clearly. However, this technique could be misleading, as skin, hair and cloth can also fluoresce under this wavelength.

Semen shining under black light. (Photo Credit : Pedal to the Stock/Shutterstock)

When semen is exposed to wavelengths between 430-470 nm (within the visible spectrum), it generates an orange fluorescence. This can be visualized by using a light filter to filter out all other wavelengths of light besides the fluorescing light. This prevents interference from other sources.

Saliva, urine and vaginal fluid fluoresces for the same reasons as semen—the chemicals, primarily the proteins and lipids (fats), present within them.

Also Read: Why Is Urine Yellow In Color?

A Final Word

These tests are crucial first steps in identifying evidence that could connect a victim to a suspect. However, since they are only presumptive tests, they are liable for inaccuracy. One study found that many misidentified other substances, such as hand cream, soaps and antibiotic creams, as semen.

A similar case exists for blood. Other substances, particularly those with copper and iron, also give out a similar chemiluminescence with luminol.

Other tests, such as DNA testing and checking for specific protein, are much stronger and give far more accurate results.

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References (click to expand)
  1. Khan, P., Idrees, D., Moxley, M. A., Corbett, J. A., Ahmad, F., von Figura, G., … Hassan, M. I. (2014, April 22). Luminol-Based Chemiluminescent Signals: Clinical and Non-clinical Application and Future Uses. Applied Biochemistry and Biotechnology. Springer Science and Business Media LLC.
  2. Webb, J. L., Creamer, J. I., & Quickenden, T. I. (2006). A comparison of the presumptive luminol test for blood with four non-chemiluminescent forensic techniques. Luminescence. Wiley.
  3. Stoilovic, M. (1991, October). Detection of semen and blood stains using polilight as a light source. Forensic Science International. Elsevier BV.
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About the Author

Salama has a degree in life sciences and biochemistry from St. Xavier’s College, Mumbai, which she puts to good use as a science writer and video producer at ScienceABC.com. She’s interested in the history of science; how science has shaped how we understand the world and society.