GlowintheDark

Glow in the dark refers to a property of certain materials that can absorb energy from light and then re-emit light after the light source is removed. This persistence of luminescence is called phosphorescence. In glow-in-the-dark products, the brightness and duration depend on the material and the amount of light exposure. The emitted light results from electrons being excited to higher energy states and then becoming trapped in metastable states within the material’s lattice, slowly returning to their ground state and releasing photons over time. The effect contrasts with fluorescence, where light emission ceases almost immediately when excitation ends.

The best-known modern long-afterglow pigments are strontium aluminate doped with europium and dysprosium, which produce bright greenish light and can glow for hours after a short recharge. Earlier phosphors used zinc sulfide doped with copper (and sometimes silver), which emitted a shorter, dimmer afterglow. Other aluminates and related hosts exist, but strontium aluminate remains the industry standard for persistent glow.

Applications include safety and emergency signage, watch and instrument dials, toys and novelty items, clothing and footwear, and decorative or outdoor markings. The quality of glow depends on pigment concentration, particle size, coating, and the spectrum of the charging light.

Safety and environmental considerations are generally straightforward for commercial phosphorescent pigments, which are non-toxic as solids. Historical luminous paints that used radioactive materials are no longer in use in consumer products.

Related technologies include self-luminous systems such as tritium gas tubes and electroluminescent materials, which offer continuous light without external charging but differ in mechanism.