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Can purple light be used for black light?

Can purple light be used for black light?

Black lights, also known as UV lights, are lights that emit ultraviolet radiation. The most common use for black lights is to cause fluorescent objects and materials to glow or fluoresce. Black lights have a number of uses including detecting counterfeit money, finding hidden stains, seeing fluorescent minerals, enjoying fluorescent artwork, and creating unique lighting effects for events or entertainment venues.

What is Black Light?

Black lights emit mostly UV-A light which has wavelengths between 315-400 nanometers (nm). This range is just beyond what is visible to the human eye which can see light approximately between 390-700 nm. Because black lights emit at the edge of human visibility, they appear to be black when turned on in a normally lit room.

There are a few key characteristics that distinguish black lights from normal white lights:

  • Black lights emit primarily long wave UV-A light with very little visible light.
  • This causes many materials to glow brightly when illuminated by a black light.
  • Black lights are weaker than sun exposure so they are relatively safe to use.
  • Compared to sunlight, black lights emit very specific wavelengths optimized for fluorescence.

These features make black lights uniquely useful for creative, scientific, security, and inspection applications.

What Causes Fluorescence Under a Black Light?

Many substances contain molecules that absorb UV photons from black lights and re-emit that energy at visible wavelengths to our eyes. This causes them to glow brightly under black light illumination. Some common fluorescent materials include:

  • Optical brighteners in clothes, detergent, paper, cosmetics
  • Organic compounds in minerals, gemstones, fossils, rocks
  • Dyes, pigments, inks in artwork, posters, signs
  • Vitamins, bacteria, bodily fluids
  • Fluorescent markers used in security, forensic analysis

The specific molecules that absorb and emit UV light are often referred to as phosphors. They fluoresce brightly under black lights while non-phosphorescent materials appear dark. This contrast allows black lights to be used to locate and analyze fluorescent materials.

Can Purple Light Work as a Black Light?

Normal purple lights or grow lights that appear purple do not work well as black lights. While these lights produce some light in the UV range, they primarily emit visible purple light between 400-450 nm. This overwhelms the UV emissions needed to cause fluorescence.

However, specially engineered LED black lights are available that use a purple filter to remove most visible light. This filtering allows the UV light to pass through and provide black light effects. The visible light emitted by the LEDs themselves is removed by the filter, leaving primarily long wave UV-A emissions.

Here is a comparison of the emissions from different light sources:

Light Source Approx. Wavelength Range Appearance
Incandescent Bulb 360-830 nm White
Halogen Bulb 350-850 nm Slightly whiter than incandescent
CFL Bulb 350-750 nm White
LED Grow Light (Purple) 400-500 nm Purple
Black Light 315-400 nm Little visible light emission

As the table shows, while purple grow lights emit light partially in the UV range needed for black light effects, they also emit a lot of visible purple light that washes out the fluorescence. Only black lights designed to filter out most visible light while letting UV through will work well.

What Wavelength is Best for Fluorescence?

The optimal wavelength for fluorescent effects depends on the specific materials being illuminated. However, there are some general guidelines:

  • Shorter UV wavelengths around 350 nm work best for inorganic minerals and some artificial highlighters.
  • Longer UV wavelengths around 370 nm work best for organic materials like vitamins, bacteria, and bodily fluids.
  • A broad spectrum UV-A black light works well for most general fluorescence needs.
  • Avoid shortwave UV-B/UV-C as this can be damaging. UV-A is safest.

For versatile black light effects, a broad UV-A spectrum between 350-400 nm is recommended. This covers the peak absorption ranges for most common phosphors.

Can Any Purple Colored LED Work as Black Light?

Not reliably. While some purple LEDs emit partially in the UV-A range needed, they also emit a lot of visible purple light. This visible emission overpowers the UV component and prevents clear fluorescent effects.

However, specially engineered black light LEDs use purple phosphors along with filters to block most visible light. This allows the UV emissions to pass through and provide black light effects. But normal unfiltered purple LEDs do not work well.

How to Make a DIY Black Light

It is possible to make a simple black light from an existing visible light source using proper filtration. Here are some options:

  • LED Flashlight: Cover the flashlight with 2-3 layers of purple cellophane or craft film. This will filter out most visible light.
  • Compact Fluorescent Bulb: Use thick purple acrylic plastic or glass between the bulb and items to be fluoresced.
  • Mercury Vapor Bulb: These emit some UV already – add purple cellophane to increase UV ratio.

However, commercially made black lights with specialized LEDs and filters will provide much better UV intensity and fluorescence.

Purple LED Strip Lights

Purple LED strip lights and tape are popular for decorative effects and growing plants. However, most flexible LED strips are not suitable as black lights.

This is because the LED chips themselves emit in the visible spectrum, along with some light in the near UV range. The purple color comes from a phosphor coating on the LEDs. But phosphors continue emitting some visible light, reducing UV intensity.

There are specialty UV LED flexible strips available. But standard purple LED strips are not optimized for UV emission and fluorescence.

Black Light Flashlights

Handheld black light flashlights can be very useful portable UV sources. They use LEDs paired with filters to emit primarily long wave UV-A light.

Key features of quality black light flashlights include:

  • UV-A wavelength between 365-370nm.
  • Deep purple filter to block visible light emission.
  • High UV intensity rating: at least 8000 μW/cm2 at 15cm.
  • Adjustable beam and brightness settings.
  • Runs on AA or lithium batteries for portability.
  • Durable aluminum construction.

Black light flashlights are affordable options for exciting fluorescent minerals, finding pet stains, detecting counterfeits, and exploring fluorescence.

Growing Plants with Purple Lights

Purple grow lights do provide some UV radiation, but not enough for robust black light effects. However, they can still offer benefits for growing plants:

  • Promotes compact, bushy growth and thicker stems.
  • Increases production of beneficial plant compounds and antioxidants.
  • Enhances color and flavor in fruits and veggies.
  • Earlier flowering and budding in many species.

The exact purple spectrum varies among grow light products. But wavelengths between 400-500nm complement the red and blue chlorophyll absorption peaks.

While purple grow lights provide UV, they are not specifically optimized for fluorescence applications like black lights are.

Safety Tips for Black Lights

Black lights are generally safe when used properly, but excessive exposure can pose some risks:

  • Avoid looking directly into the light, especially at short distances.
  • Limit direct skin exposure, as UV can cause burns.
  • Wash hands after touching glowing objects illuminated by black light.
  • Supervise children using black lights.
  • Work in well ventilated area to avoid inhaling vapors from fluorescent liquids.
  • Turn off black lights before checking for damage or leaks.

Safety glasses offer additional eye protection from UV exposure. Responsible use minimizes any potential risks of black light radiation.


In summary, specialized black light LEDs that filter out most visible light are required to produce fluorescence effects. Normal purple grow lights and LED strips emit too much visible light, overwhelming the UV emissions needed. Purpose-built black light sources with UV specialized phosphors and filters block visible wavelengths while transmitting UV-A rays to create unique fluorescent effects.

With prudent safety precautions, black lights are versatile tools for glowing artwork, detecting authentication markers, exploring minerals, finding organics, and more. The future is bright for innovative black light applications across many fields.