Nachleuchten

Nachleuchten, or afterglow, refers to the continued emission of light by a material after the excitation source has been removed. In scientific terms, it is a form of persistent luminescence produced mainly by phosphorescence. Unlike fluorescence, which stops emitting almost immediately after excitation ends, phosphorescent materials trap some of the excited energy in defect states and release it slowly over time, causing light to persist for seconds to hours.

Mechanism and materials

The effect depends on trap states in a material’s crystal lattice and the energy landscape of dopants. When light or another energy source excites the material, electrons can be captured in metastable states. Thermal energy over time allows gradual release, producing a decaying afterglow. Common phosphors include zinc sulfide doped with copper (ZnS:Cu) and more modern long-lasting phosphors such as strontium aluminate doped with europium and dysprosium (SrAl2O4:Eu2+,Dy3+), which can emit green or blue-green light for extended periods.

Applications and limitations

Nachleuchten is used in safety signage, emergency lighting, and glow-in-the-dark decorative items and watch dials. The brightness and duration depend on the material, temperature, and prior exposure. Afterglow intensity decays over time and eventually falls below perceptible levels. Modern persistent phosphors aim for higher luminance and longer duration, while older materials typically produce shorter-lasting glows.

History

Awareness of afterglow dates back to early luminous paints and pigments, but significant advances occurred in the late 20th century with the development of long-lasting phosphors that offer practical lifespan and brighter emission.

See also: phosphorescence, luminescence, persistent luminescence.