From dinosaurs to lego ghosts, from stickers to yo-yos, a battery of toys now add a cool greenish glow to their attraction.
The key ingredient that causes the glow is a phosphor (from the Greek for “light-bearer”), which has the ability to phosphoresce: it glows when exposed to visible light energy, and continues glowing for a period of minutes or hours after the exposure is stopped.
This phenomenon occurs because the phosphor absorbs energy while exposed to light and later, in the dark, discharges the energy in the form of light. A substance that fluoresces, on the other hand, stops emitting light once the source of energy is removed.
In nature there is a chemical element called phosphorus, discovered about three hundred years ago by a German scientist, which has the remarkable ability to glow. Glow-in-the-dark yo-yos, however, contain a commercially made phosphor, often a compound of zinc or magnesium.
The phosphor is made by heating to above 800 degrees Celsius pure, synthetic ingredients in the form of tiny crystals two to fifteen microns in diameter, along with an “activator,” or trace of heavy metal. After being heated, the atoms of the activator enter the crystal lattice of the phosphor and remain embedded there.
When light strikes the phosphor crystals, the electrons that orbit around the activator’s atoms are thrown into a tizzy. They leap from their normal, relaxed orbits into an excited orbit farther away.
It is a rule of nature that everything wants to be in its lowest energy state, so the atoms continually plummet back toward the nucleus. Because of the lattice structure of the crystal, some electrons get trapped, their descent is delayed, and they continue to release energy, in the form of light, over time. Oddly enough, the smaller the crystal the longer the glow.
“A good glow-in-the-dark bug will shine for as long as thirty minutes after you shut off your lights,” says Robert Majka, project manager at Canrad-Hanovia, Inc., a company in New Jersey that manufactures phosphorescent pigments.
“Eighty-five percent of its luminescence will be expelled by then. But even after that, the bug will continue to glow for as long as eight hours, with the 15 percent of its leftover luminescence. You can only see this low glow if you’ve been in the room with it, so that your eyes have adjusted to the dark and the fading bug.”
Canrad-Hanovia makes a glow-in-the-dark pigment containing an inorganic zinc sulfide phosphor, which is nontoxic and nonradioactive. It comes in the form of a powder, which is sold to compounders who add it to any clear suspension, be it plastic, paint, vinyl, or ink. A carrier material that is not clear would absorb the phosphorescence. From these plastics and inks come glow-in-the-dark Frisbees and mobiles, reptiles and stickers.
The brightest toys with a glow that lasts the longest have a higher concentration of pigments than those with a dimmer glow. Optimal luminosity occurs when the pigment constitutes 30 percent of the product, that is, the ratio of plastic resin to pigment is about 2 to 1. At that point, the carrier reaches a saturation point.
Most kids know how to make their Super Balls and alligators glow. “Simply hold the toy up to any light for about thirty seconds,” says Majka. Kids may be interested to learn that you get the brightest glow if you use fluorescent lights.