How Glow in the Dark Stuff Works

Have you ever wondered how glow in the dark stuff works?

I'm talking about materials that truly glow after you turn out the lights, not the ones that glow under black light or ultraviolet light, which are really just converting invisible high energy light into a lower energy form visible to your eyes. There are also items that glow because of ongoing chemical reactions that produce light, like the chemiluminescence of glow sticks. There are also bioluminescent materials, where the glow is caused by biochemical reactions in living cells, and glowing radioactive materials, which may emit photons or glow because of heat. These things glow, but how about glowing paints or the stars you can stick on the ceiling?

Things Glow Because of Phosphorescence

Stars and paint and glowing plastic beads glow from phosphorescence. This is a photoluminescent process in which a material absorbs energy and then slowly releases it in the form of visible light. Fluorescent materials glow via a similar process, but fluorescent materials release light within fractions of a second or seconds, which is not long enough to glow for most practical purposes.

In the past, most glow in the dark products were made using zinc sulfide. The compound absorbed energy and then slowly released it over time. The energy wasn't really something you could see, so additional chemicals called phosphors were added to enhance the glow and add color. Phosphors take the energy and convert it into visible light.

Modern glow in the dark stuff uses strontium aluminate instead of zinc sulfide. It stores and releases about 10 times more light than the zinc sulfide and its glow last longer. The rare earth europium is often added to enhance the glow. The modern paints are durable and water-resistant, so they can be used for outdoor decorations and fishing lures and not just jewelry and plastic stars.

Why Glow in the Dark Things Are Green

There are two main reasons why glow in the dark stuff mostly glows in green. The first reason is because the human eye is particularly sensitive to green light, so green appears brightest to us. Manufacturers choose phosphors that emit green to get the brightest apparent glow.

The other reason green is a common color is because the most common affordable and non-toxic phosphor glows green. The green phosphor also glows the longest. It's simple safety and economics!

To some extent there is a third reason green is the most common color. The green phosphor can absorb a wide range of wavelengths of light to produce a glow, so the material can be charged under sunlight or strong indoor light. Many other colors of phosphors require specific wavelengths of light to work. Usually, this is ultraviolet light.To get these colors to work (e.g., purple), you need to expose the glowing material to UV light. In fact, some colors lose their charge when exposed to sunlight or daylight, so they aren't as easy or fun for people to use. Green is easy to charge, long-lasting, and bright.

However, the modern aqua blue color rivals green in all of these aspects. Colors that either require a specific wavelength to charge, don't glow brightly, or need frequent recharging include red, purple, and orange. New phosphors are always being developed, so you can expect constant improvements in products.

Thermoluminescence

Thermoluminescence is the release of light from heating. Basically, enough infrared radiation is absorbed to release light in the visible range. One interesting thermoluminescent material is chlorophone, a type of fluorite. Some chlorophane can glow in the dark simply from exposure to body heat!

Triboluminescence

Some photoluminescent materials glow from triboluminescence. Here, exerting pressure on a material imparts the energy needed to release photons. The process is believed to be caused by the separation and joining of static electrical charges. Examples of natural triboluminescent materials include sugar, quartz, fluorite, agate, and diamond.

Other Process That Produce a Glow

While most glow-in-the-dark materials rely on phosphorescence because the glow lasts a long time (hours or even days), other luminescent processes occur. In addition to fluorescence, thermoluminescence, and triboluminescence, there are also radioluminescence (radiation besides light is absorbed and released as photons), crystalloluminescence (light is released during crystallization), and sonoluminescence (absorption of sound waves lead to light release).

Sources

• Franz, Karl A.; Kehr, Wolfgang G.; Siggel, Alfred; Wieczoreck, Jürgen; Adam, Waldemar (2002). "Luminescent Materials" in Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH. Weinheim. doi:10.1002/14356007.a15_519
• Roda, Aldo (2010). Chemiluminescence and Bioluminescence: Past, Present and Future. Royal Society of Chemistry.
• Zitoun, D.; Bernaud, L.; Manteghetti, A. (2009). Microwave Synthesis of a Long-Lasting Phosphor. J. Chem. Educ. 86. 72-75. doi:10.1021/ed086p72