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The Science Behind Black Light Effects… in Plain English

Black light effects can add a powerful dimension to any dramatic presentation. Whether it’s an invisible image that appears out of nowhere, or a subtle day/night transition, black light effects often leave audiences picking their jaws up off the floor. Unfortunately, there’s a lot of misunderstanding about how black light effects work. Without that understanding, it can be difficult to create a really powerful effect. Obviously, there’s an artistic element to it, but before you can unleash your creative genius, you need to have a little grounding in the science behind how it works.

Black light is the common name for long-wave ultraviolet light. Okay, so what’s long-wave ultraviolet light? Simply stated, it’s a form of light that exists just beyond what the human eye can see. Humans see a range of visible colors from red to violet. Remember high school science class where you had to learn the colors of the spectrum, ROYGBIV? (Red-Orange-Yellow-Green-Blue-Indigo-Violet.)

Red light has the longest wavelength, and lowest energy level, of all the colors in the spectrum. Violet light, on the other hand, has a shorter wavelength and higher energy level than other visible light. In fact, violet light is sometimes called high-energy visible light.

There’s a lot more to light than what we actually see. There is a whole range of light extending from low-energy radio waves, which can have wavelengths many miles long to high-energy x-rays and gamma rays which can have wavelengths shorter than the diameter of an atom (which is really, really small).

Light waves just a bit too long for us to detect with our eyes are referred to as ultraviolet light. (“Ultra” is Latin for beyond.) Ultraviolet (UV) light is arbitrarily divided into three bands: short-wave, medium-wave, and long-wave. (Sometimes called UV-C, UV-B, and UV-A respectively.)

Short-wave UV light (200-280nm—see chart) is dangerous to the skin and eyes. For bacteria and other germs, it kills on contact, which is exactly why it’s used for sterilizing water, food, and many kinds of surfaces. Fortunately for us, short-wave UV rays from the sun are filtered out in the upper atmosphere. Some minerals react spectacularly to short-wave UV light.

Medium-wave UV (280-315nm) is what’s responsible for sun-tanning, and what doctors have traditionally attributed to skin cancer (although recently, in light of new studies, there’s been some debate about whether that’s entirely true). However, there is no debate that medium wave-UV is dangerous to look at. So don’t stare into the sun!

Long-wave UV (315-400nm) is what we’re concerned with here. This is the so-called black light—“black” because we can’t see it. Interestingly, some animals such as certain birds, reptiles, and insects do see in the ultraviolet range. Imagine being able to see another color!

I’m sure this begs the question, “If we can’t see black light, why are we able to see in a room lit only by a black light?” Two reasons:

No light source emits only UV light. There is always a certain amount of visible light present.

Black light sensitive materials emit light we can see. This is the “glow” effect, which we’ll discuss in the next section.

If, somehow, you had a light source that emitted only black light, and you were in a room with no UV sensitive material whatsoever, you’d be in complete darkness.

Fluorescence is the technical term for the black light “glow” effect. It will take a couple of minutes to explain how this works, so stick with me.
In the last section, we referred to light waves. One of the most fascinating properties of light is that it can behave both as a wave and as a particle. A particle of light—which you can think of as a little packet of energy—is called a photon.

When a photon collides with an electron in an atom, the electron absorbs the photon as extra energy. It doesn’t hang on to it for long. It turns around and releases most—but not all—of that energy as another photon. The lower-energy photon has a longer wavelength. In short, fluorescence happens when electrons absorb high-energy photons and release low-energy photons.

Fluorescence can happen anywhere in the light spectrum. Any kind of light can be absorbed by an electron, then re-emitted as a lower frequency light wave. The black light effect happens when an electron absorbs a long-wave ultraviolet photon and releases a photon of visible light.

The important thing to keep in mind here is that the UV sensitive material is actually giving off light. It becomes its own light source. This is why it can be difficult specifying black light fixtures. It’s more than a matter of how much wattage the light produces. Equally important is the degree of UV sensitivity of the materials being used.

What Factors Contribute to the Brightness of a Black Light Effect?
There are four basic elements you need to take into account to create a black light effect bright enough for your application:

When choosing a black light fixture, it’s important to choose one that actually produces black light. Some so-called black light fixtures don’t really produce black light at all—at least not very much of it. Incandescent lamps, for example, produce very little black light in comparison to visible light. So any fluorescence is washed out.

As we mentioned earlier, long-wave UV light is in a band from 315nm to 400nm. You can actually see 400nm, but it’s on the very edge of visible light. Some UV LED fixtures give off 400nm, and you will see white shirts and some other materials glow. But you won’t get an effect that really “pops.” LEDs in the 385-390nm range are also common, and slightly better than a 400nm fixture.
The “sweet spot” for most UV sensitive materials is about 365nm—right in the middle of the long-wave UV range. Most good black light fluorescent lamps peak at around 365nm. A good metal-halide lamp will, too.

In general, the higher the wattage, the higher the output. The thing to remember, however, is there’s a lot more to a fixture than wattage. Wattage refers to power consumption, which is directly related to output. If that output isn’t in the right UV range, you won’t get the brightest possible effect.

This one’s obvious. The closer the light source to the material, the brighter it glows. For stage applications, you’ll need a metal-halide long-throw fixture, such as those made by Altman or Wildfire. For closer, more intimate applications, a good fluorescent will serve nicely. You just don’t have as much flexibility in shaping the light with a fluorescent as you do with a metal-halide long-throw.

Another obvious point, but one you need to take into account. If you have a lot of light bleeding into the sanctuary from windows, open doors, exit signs, stand lights, etc., this will wash out the effect. Try to get rid of as much ambient light as possible. If it’s unavoidable, you’ll need more high-output fixtures to counteract the ambient light.

It doesn’t matter how much black light you’re throwing on a material if it’s not UV sensitive. Different materials have differing degrees of sensitivity. Some react better at different wavelengths (which is why 365nm makes a good compromise in a black light fixture). So choose your materials carefully. Experiment a little. The vignette on page 54 shows some common black light sensitive materials. You’ll also want to be sure you’re purchasing UV sensitive materials from a quality manufacturer.

By keeping these points in mind, and using a bit of experimentation, you should be able to set up a powerful effect. I hope this article has helped clear up some confusion, and more importantly, that it’s piqued your interest in further exploration. If that’s the case, you may be interested in a special report Wildfire has created entitled The Ultimate How-To Guide To Creating Spectacular, Ultra-Bright UV Effects, which explains many of these points, and more, in much more detail. You can download the PDF by filling out a brief survey at There really is a world of possibilities, and a little scientific understanding will go a long way toward helping you pull off a stunning black light effect.

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