If you’ve ever watched a crime show on TV, you might have seen investigators with a UV light, also known as a blacklight. UV radiation causes objects to glow because it ‘excites’ electrons, which give off visible light. Many organic substances glow under UV radiation, including sweat, saliva and urine.

Yellow

We’ve all seen crime shows where investigators search through a dark room with a UV or black light looking for clues. The forensic experts often throw back sheets to reveal a sperm stain that glows in the dark. This is a simple and inexpensive method to detect body fluids but it doesn’t always provide a conclusive answer. Other substances, such as pet urine and thick saliva, can glow under a UV flashlight too.

Normal semen is a cloudy white or gray liquid with a consistency like runny jelly and an alkaline smell similar to bleach. It can turn yellow if it mixes with trapped or leftover urine or a bacteria from the prostate gland called leukocytospermia. This condition can lead to infertility. Yellow semen can also indicate an infection in the bladder or urethra. If you experience this, see your doctor for treatment.

The best way to identify a sperm stain is by using a black light. The best portable black lights are the ones that can fit in your pocket or purse and can be activated with a button. These are the same types of flashers used to detect pet urine and other bodily fluids. If you don’t have a black light, a regular flashlight will work just fine. Just cover the flash with some painters tape and colour it blue to make your phone a useful investigative tool for tracking body fluids.

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Blue

Normally, semen is a thick whitish liquid that looks very different from other bodily fluids. It can also take on a yellow color if a man is experiencing a health problem that changes his sperm production. Using a black light can help detect a suspicious stain of this type. The forensic light emits ultraviolet rays that cause the substance to fluoresce. This allows a person to find the source of the stain and determine whether or not it is sperm.

While a blacklight can identify semen, it is important to remember that other substances can also glow in the dark. Sweat, urine and saliva all absorb UV rays and re-emit them as visible light. Some foods, cleaning products and cosmetics also emit a glow when exposed to UV light.

The simplest way to test for semen is to use a blacklight and place it over the suspected spot. The liquid should glow in the dark if it is sperm. The same technique can be used to identify other body fluids like sweat, saliva and urine.

Black lights are portable and hands-off, making them a popular tool in crime investigations. However, they are not conclusive, as any stains found will still need to be confirmed by other chemical tests. One of the most common is to test for prostatic acid phosphatase, an enzyme that is more likely to be present in semen than other body fluids.

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Green

We’ve all seen crime shows where the detectives wander around a room with a UV, or black light, looking for clues. It’s a common method for detecting evidence of body fluids, as it’s easy to do and hands-off. It’s also a popular method for discovering semen stains in laundry.

While it’s true that sperm glows green under a black light, this isn’t because the sperm itself emits the color. Instead, the stain on the clothes or bedding is glowing because it contains a specific mix of chemicals that react to black light and fluoresce.

A black light is a type of flashlight that emits ultraviolet (UV) rays that are invisible to the human eye. This type of light can illuminate many organic substances, including blood, sweat, urine, saliva, and vaginal secretions. It’s also used to detect sperm stains and other evidence of sexual activity.

A Wood’s lamp (WL) that emits ultraviolet light has been used to evaluate rape-related forensic evidence. The WL is purported to cause semen to fluoresce, making it easier to distinguish from other products. This study was conducted to determine if emergency medicine physicians were able to distinguish a simulated semen sample from other products using the WL. A total of 29 semen samples were evaluated under high-power microscopy and WL. Zero of the 29 samples fluoresced with a standard WL.

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Purple

We’ve all seen crime shows where investigators scour the crime scene with a black light, looking for clues. The detectives inevitably throw back the sheets to reveal a glowing stain that must be semen. Semen does glow under black light, but it’s not the only thing that will. Saliva, urine and many other biological fluids also glow under UV light. If you want to track down that spot where your dog urinated, a simple handheld UV flashlight will do the trick.

While any type of liquid will glow under a black light, sperm stains tend to be brighter than other fluids. This is likely because sperm contains more proteins than other fluids. These proteins react stronger with the black light’s ultraviolet radiation and re-emit it as visible light.

Other types of body fluids, blood and some foods will glow under black light as well. However, if you’re concerned about getting an STD or other sexually transmitted disease from touching dried sperm, it is best to use an at-home testing kit.

Some people even use their smartphone as a blacklight to detect body fluids. There are apps that turn your phone’s flashlight into a UV light, making it easy to find stains and other objects that glow under a blacklight. The apps are used by students, police officers and other people who need to see body fluids in a dark environment.