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Ultraviolet Light vs Black Light: What’s the Difference?

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Ultraviolet light vs black light

Ultraviolet LED strips, black light torches, and UV sanitizer lamps are just a few products sold online and in stores for a variety of purposes. From detecting pet urine stains to bringing out fingerprints and even revealing subcutaneous (deep skin) bruises, they have many uses. But what’s the difference between ultraviolet and black light?

Black light—also known as UVA light—is a type of lamp that emits long-wave ultraviolet radiation. But not all ultraviolet lights are UVA, or black light.

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Overview of Ultraviolet Light

Plants Ultraviolet Light
Image Credit: kukurund, Shutterstock

Ultraviolet light is the part of the electromagnetic spectrum that spans from just below violet (visible light) all the way to short-wave UVC. In other words, it is light that has longer wavelengths than X-rays, but shorter wavelengths than visible light.

How is Ultraviolet Produced

Around 10% of the total energy output from the Sun is ultraviolet radiation, but that’s not the only source. Artificial sources include mercury-vapor lamps, tanning beds, black lights, and electric arcs produced through the process of welding, plasma cutting, and electric discharge machining.

Artificial UV lights are used in a wide range of commercial and industrial applications, including sanitization, water treatment, phototherapy, and more. 

Ultraviolet waves measure between 10 nm and 400 nm. Depending on the wavelength, ultraviolet is split into three subtypes: UVA, UVB, and UVC.

Ultraviolet A (UVA)

Measuring between 315–400 nm, ultraviolet A (UVA) light has the longest wavelengths in the UV range. UVA rays from the sun do not get blocked by the Earth’s atmosphere.

All kinds of UV rays cause collagen breakdown in the skin, and, therefore, will accelerate aging. But, UVA is not responsible for sunburn. Although it can’t directly cause skin cancer or eye damage over prolonged periods, exposure to UVA can cause negative reactions in deeper skin layers, which can eventually lead to DNA damage and skin cancer. UVA from the sun can negatively impact the eyes.

Ultraviolet A lamps are otherwise known as black light lamps, and have many applications in different fields, such as forensics, medicine, and biology. 

Sun Light
Image Credit: jplenio, Pixabay

Ultraviolet B (UVB)

The wavelengths in ultraviolet B (UVB) rays measure between 280–315 nm. Also known as medium-wave UV, UVB is responsible for stimulating essential vitamin D production in humans and most animals.

However, prolonged exposure to UVB can also cause sunburn, DNA damage leading to some types of cancer, and eye irritation and damage. Most of the Sun’s UVB rays are blocked out by the Earth’s atmosphere, but not all, which is why it’s important to wear sunscreen and sunglasses, and avoid prolonged exposure to direct sunlight.

In medicine, ultraviolet B lamps are used for the treatment of dermatological diseases such as psoriasis, eczema, and vitiligo, to name a few.

Ultraviolet C (UVC)

At between 100–280 nm, ultraviolet C (UVC) rays have the shortest wavelengths and the highest energy in the ultraviolet range. This is the most dangerous type of ultraviolet light, but it is entirely absorbed by the oxygen in the Earth’s atmosphere, a process that generates ozone.

Artificial UVC radiation is commonly used for disinfection of air, water, nonporous surfaces, and tools.shutter camera divider 2

Overview of Black Light

Black Light uv light lamp
Image Credit: Andrey Sukhachev, Shutterstocks

Black light refers to lamps that create UVA light. Most of these lamps contain a filter that stops visible light from being emitted, though the small amount that does escape usually produces a dim violet or blue glow. They’re essential in processes where UV light is needed to observe fluorescence, but they’re used widely in other applications too.

Types of Black Light

Fluorescent

Also known as BLB tubes, these tubes contain a purple filter coating that stops most visible light from being emitted—though they usually appear to glow with dim violet light. Varying phosphor compositions are used to produce different wavelengths of UVA—from a peak of 450 nm used to treat jaundice, to 310 nm used in medical applications and polymerization.

Bug Zapper Tubes

Commonly referred to as Blacklight or BL, these lights also use a phosphor composition to emit UVA. However, unlike the BLB tubes, they don’t contain a filter to block out visible light. These lights usually emit a light blue glow.

Because of the amount of visible light that these lamps produce, they can’t be effectively used for applications where low visible light is required—for example, in detecting fluorescence.

Bug Zapper Black Light
Image Credit: S and S Imaging, Shutterstock

Mercury Vapor

Mercury vapor black lights are bulbs that use optical filter coatings to block out visible light and short wavelength ultraviolet light. They’re usually used in theaters and concerts to create special effects and displays, and are generally more efficient than fluorescent tubes.

Where It’s Used

  • Medical Applications – Black light is used for a variety of medical applications, from diagnosis to treatment. For example, some dermatological diseases can be detected through fluorescence that appears under black light. Neonatal jaundice is usually treated with phototherapy, using blue light within a range of 420–470 nm.
  • Authentication – Black light is used often for authenticating pieces of art and currency. Stolen items may be marked with invisible ink that only shows up under black light. Similarly, many countries use banknotes that are marked with an invisible stamp that glows under black light. Meanwhile, an unusual amount of fluorescence can be a mark of counterfeit notes—as authentic paper that is used to print banknotes doesn’t glow under black light the way commercial paper might, because the chemicals used to process the paper differ.
  • Engineering – In engineering, a black light can be used to quickly identify leaks in air conditioning systems and refrigerators. In structural analysis, fluorescing liquids are painted over structures, then, using a black light, cracks and weaknesses in the materials are quickly and easily located.
  • Geology – Most rocks and minerals glow with an array of bright beautiful colors under black light. Geologists take advantage of this by sometimes prospecting for fluorescent minerals, such as scheelite at night, using a black light. Additionally, most gemstones are fluorescent, making them easier to spot under black light.
  • Forensics – Crime scene investigators use a black light to hunt for evidence that would be otherwise invisible. Blood, saliva, semen, and urine all glow under black light. Fibers, detergents, and other toxic oils and poison can also be detected in the same way.

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Can I use my phone as a black light?

If you can block all visible light from your phone’s torch—for example, through a filter—you may be able to create a black light. With some transparent tape, a blue marker, and a purple marker, you can follow these simple steps to create your own black light:

  • Cover the light-emitting LED on the back of your phone with a small piece of clear tape. 
  • Use the blue marker to color the tape over the flash.
  • Carefully add another layer of tape without smudging the blue color, and color this one purple.
  • Repeat steps 2 and 3 so that you have a couple of more layers.
  • Switch off the lights and try your new black light!

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What color glows best under black light?

White and fluorescent colors glow best under black light. For example, fluorescent green, pink, yellow, and orange are all very bright under black light. Some materials glow better than others, too.

Why does urine glow under black light?

A black light can be a great way for pet owners to detect any nasty surprises left behind by their cats or dogs, but you may have found yourself wondering why urine glows so well. Urine contains phosphorus, a chemical that glows yellow when it reacts with oxygen. With the help of a black light, phosphorous becomes naturally fluorescent.shutter camera divider 2

Summing Up

The main difference between ultraviolet light and black light is that, whereas ultraviolet light encompasses the entire ultraviolet range, from 10 nm to 400 nm, black light only refers to long-wave ultraviolet, or ultraviolet A.

Although black lights, UVB, and UVC lamps all emit ultraviolet rays, it’s important to remember that each one has its own uses and benefits. Black light, or UVA lights, have the longest wavelengths in the ultraviolet range, and are therefore least harmful, while ultraviolet C rays are short-wave, and are therefore the strongest and most harmful types of ultraviolet radiation. That said, even UVC light can be used beneficially for disinfection and sanitization.


Featured Image Credit: Left: Plume Photography, Shutterstock, Right: S and S Imaging, Shutterstock

About the Author Cheryl Regan

Cheryl is a freelance content and copywriter from the United Kingdom. Her interests include hiking and amateur astronomy but focuses her writing on gardening and photography. If she isn't writing she can be found curled up with a coffee and her pet cat.