Glowing, Glowing, Glowing, . . . . Gone?
In the original glow in the dark materials, zinc-based compunds were used for the phosphors. The ZnS crystals in these compounds readily absorbed ultraviolet light and produced some greenish-yellow light in turn. The transition from the ZnS metastable state back to the ground state is reasonably direct, giving it a fairly short half life. That is why the visible glow produced by zinc-based phosphors is relatively short lived. Also, there are other, competing processes besides phosphorescence that can lead back to the ground state for the atom. The existence of these other, radiationless pathways reduces the brightness of the compound's glow.
Newer phosphors, based on alkaline earth aluminates, have higher efficiencies than the zinc compounds - leading to brighter glow and longer glow duration. Because of the enhanced brightness, the glow color can also be modified by adding other fluorescing dyes into the earth aluminate mixture that fluoresce in response to the phosphorescence of the mixture. (Altering the glow color through other fluorescent dyes was impractical with the zinc compounds because their phosphorescence was too dim to excite much fluorescence in the dyes.) To some extent, we can also "tune" the glow color by altering which kind of impurity atom gets mixed into the earth aluminate crystals. The results of this are, to say the least, dramatic!
Time-lapse photographs of phosphorescent samples, initially excited by black light (UV).
The phosphorescent samples shown in the time-lapse sequence above are (from left to right) green earth aluminate paint, generic zinc glow paint, purple earth aluminate paint, generic zinc glow glue, and another generic zinc glow glue. As you can see directly, the earth aluminate samples glow visibly for much longer than the zinc samples! The actual glow intensity readings (measured by 4Physics) and the evidence of your eyes when the samples are right in front of you is much more dramatic! (Photographing in the range of purple through green with a conventional digital camera, as was done here, is not completely accurate as the camera's sensitivity falls off quickly as you move toward shorter wavelengths: green → blue → purple.)
So, more accurately, here is the change in glow intensity with time:
Light intensity measurements comparing earth aluminate phosphors with zinc phosphors.
Generic GID = zinc phosphor; Glow, Inc. = earth aluminate phosphor
While the zinc phosphor compounds glowed visibly for about half an hour, the regular (purple glow) earth aluminate phosphor glowed visibly for about 12 hours and the ultra fine earth aluminate phosphor was still glowing more than two days later!
So, if you found glow in the dark products disappointing before, take another look. Just make sure you are looking at the alkaline earth aluminates! They keep glowing and glowing and glowing. . .!
© 2004-2008 - 4Physics