The Science Behind Glow In The Dark Products (Click to Shop Paint / Click to Shop Powder)
Have you ever wondered why some objects glow in the dark? The simple explanation is that what we refer to as glow-in-the-dark is the process of a material “charging” by absorbing energy through ultraviolet rays and then later, when the source of light is removed, releasing the stored energy slowly in the form of visible light at a longer wavelength than the original light. This process is called phosphorescence.
In scientific terms, radiative emission derives from the decay of electrons at quantum levels of lower energy. The process is a process of energy storage and release, usually composed of visible light or ultraviolet radiation, exciting atoms, pushing some electrons to external orbitals. When these return to the inner orbital they emit light.
In addition to other phenomena of non-radiative decay, the return to the stable fundamental state with radiant emission can occur essentially in two ways: a singlet-singlet conversion or, alternatively, a transition to a triplet configuration through an internal conversion and the subsequent decay at the fundamental state of singlet. In the first case there is fluorescent emission while in the second one the phenomenon of phosphorescent emission occurs. The decay that produces phosphorescence is temporally longer than fluorescence. Phosphorescent emission occurs longer than fluorescence because in the latter the effect is immediate and stops as soon as the energy source is interrupted, while in the phosphorescence the effect continues even after the decay transition.
How long does the glow last?
The light emission is proportional to the concentration of excited electrons. Exponential phosphorescence decay is expected when the concentration of valence band holes is much larger than the concentration of excited electrons in the conduction band. Another words, fluorescent light is released simultaneously with the “charging” process. Whereas phosphorescence can last from just a few minutes up to more than ten hours depending on the semiconductor, compounds, type and length of exposure to the light source
What can be “charged”?
A phosphor is a type of material that can absorb energy, and then, later release the energy. A phosphor is composed of a base compound (a pure insulating material or a wide energy gap semiconductor) that is activated with the addition of a small percentage of impurities called doping. These impurities deform the crystal struction, producing various types of metastable energy levels between the conduction band and the valence band of pure crystal. A previously excited electron in a trap level reaches the conduction band by thermal excitation and then subsequently decays to a recombination center.
What are examples of phosphors?
So glad you asked. Zinc Sulfide is an inorganic compound found in nature and one of the most common phosphorescent materials. It is used as phosphor in several applications, including X-ray screens, cathode ray tubes, paints, children’s toys and safety markings. Copper-doped zinc sulfide is used also in electroluminescent panels. Zinc Sulfide, ZnS, is the semiconductor associated with the eerie green-light, universally, recognized color associated with glow-in-the-dark items.
Recently discovered and made more popular is strontium aluminate (SRA, SrAl, SrAl2O4). This monoclinic crystalline powder is solid, odorless and nonflammable. It is heavier than water and pale yellow in color naturally. When activated with a suitable dopant (Europium or Dysprosium), it acts as a photoluminescent phosphor with long persistence of phosphorescence. For many phosphorescent-based purposes, strontium aluminate is a vastly superior phosphor to its predecessor, copper-activated zinc sulfide; it is about 10 times brighter and 10 times longer glowing, however it is about 10 times more expensive than Cu:ZnS. Strontium aluminate phosphors produce green and aqua hues, where green gives the highest brightness and aqua the longest glow time. Strontium aluminate can be formulated to phosphoresce at longer (yellow to red) wavelengths as well, though such emission is often dimmer than that of more common phosphorescence at shorter wavelengths.
What are some common compounds used in doping?
Different impurities create various colors. Doping with copper, silver or manganese introduces intermediate electronic energy levels within the forbidden band. Using silver generates a bright blue; doping with manganese yields an orange-red color, and adding copper delivers the familiar, intense greenish glow in the dark color.
Europium(III) oxide (Eu2O3), is a chemical compound of europium and oxygen. It is widely used as a red or blue phosphor in television sets and fluorescent lamps, and as an activator for yttrium-based phosphors. It is also an agent for the manufacture of fluorescent glass. Europium fluorescence is used in the anti-counterfeiting phosphors in Euro banknotes.
Dysprosium is a rare-earth mineral found in the earth’s crust. As a phosphor is emits a white light, so therefore, it has a potential application in white LEDs.
In closing we have learned that glow-in-the-dark phenomenon is a result of energy released from compounds that have absorbed energy from UV or ambient light. These compounds can be found in commercial products such as X-ray or television tubes, children’s toys, and safety markings. In the past, these basic components were not available for private use. Now, ViziGlow has sourced these compounds making them available to our customers in the form of powders to use in paints, screen printing and many other projects.
Acknowledgements:
Thank you to the authors of the below referenced articles for sharing their knowledge and experiments about phosphorescence and authorizing the use of this information as stated on the bottom of their web page.
Zinc Sulphide Phosphorescence, February 6, 2019 –
/https://physicsopenlab.org/2019/02/06/zinc-sulphide-phosphorescence/
Glowing in the Dark, February 5, 2019 –
https://physicsopenlab.org/2019/02/05/phosphorescence/