Short answer
A phosphorescent pigment stores some of the energy of the light it absorbs and releases it slowly as light after the source is removed. Modern long-afterglow pigments are strontium aluminate doped with europium and dysprosium, introduced in the 1990s, which glow green for hours where the older copper-doped zinc sulfide faded within minutes to tens of minutes. Green dominates because the brightest, longest-lasting phosphors emit there and dark-adapted eyes are most sensitive near that wavelength.
Fluorescence re-emits light within nanoseconds. Phosphorescence is slower because the absorbed energy is parked in a state from which return is delayed. In molecules that is often a triplet state; in the inorganic pigments used for glow-in-the-dark products it is a set of traps. In europium- and dysprosium-doped strontium aluminate, ultraviolet or blue light excites electrons from europium ions, some are captured by traps associated with the co-dopant, and they escape gradually with room-temperature thermal energy, returning to europium and emitting light as they do. The glow therefore fades progressively after charging, fast at first and then more slowly, and it depends on how strongly and with what light the pigment was charged: violet-rich daylight charges it well, warm lamplight much less.
For most of the twentieth century the standard phosphorescent pigment was zinc sulfide doped with copper (sometimes with cobalt), which gives a green glow that fades within minutes. Instrument and watch dials that needed to glow all night used radioluminescent paint instead — first radium, then tritium from the 1970s — in which radioactive decay continuously excites a phosphor. In 1996 Matsuzawa and colleagues reported strontium aluminate doped with europium and dysprosium as a new long-afterglow phosphor of high brightness; reviews list its green emission around 480–520 nm lasting 6–12 hours under test conditions. From the mid-1990s it replaced radioactive luminous materials on dials and became the basis of most glow-in-the-dark toys, paints and markings.
Strontium aluminate's europium emission lands in the blue-green to green, and it happens to be the most efficient long-afterglow system. That suits the eye: in the dark the rod-based scotopic system, which peaks near 507 nm in the CIE scotopic luminous efficiency function, is most sensitive to exactly that region, and red is almost invisible to it. Blue-emitting calcium aluminate phosphors exist with afterglows of a few hours. Other colours are usually made by tinting a green or blue phosphor with a colourant or by using less efficient phosphors, so orange, red and purple glow products are dimmer and fade sooner. The same trade-off applies to the daytime colour: pale yellow-green phosphor powders look almost white, and adding a strong daytime colour absorbs some of the charging and emitted light.
Beyond toys and novelty paint, strontium aluminate pigments are used in photoluminescent safety markings — exit and escape-route signs, step edges, low-location guidance — that must remain visible when power fails, and research continues into making them brighter and more durable in coatings. Their performance depends on pigment loading, particle size, layer thickness and a white or reflective backing, and they need charging light to work. This page explains the colour science only: whether a particular marking meets a building or safety standard is a question for the relevant regulations and a competent person, not something a pigment description can answer.
| Phosphor | Emission | Typical afterglow | Notes |
|---|---|---|---|
| SrAl₂O₄:Eu²⁺,Dy³⁺ | Green, 480–520 nm | 6–12 h | High brightness; standard since the mid-1990s |
| CaAl₂O₄:Eu²⁺,Nd³⁺ | Blue, 440–480 nm | 2–6 h | Blue alternative |
| ZnS:Cu (with Co) | Green, 510–530 nm | Minutes | Older standard; shorter afterglow |
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactStrong evidence
Europium- and dysprosium-doped strontium aluminate (SrAl₂O₄:Eu²⁺,Dy³⁺) was reported as a new long phosphorescent phosphor with high brightness in 1996.
FactModerate evidence
A review of long persistent luminescence lists SrAl₂O₄:Eu,Dy with green emission of 480–520 nm and an afterglow of 6–12 hours, compared with minutes for zinc sulfide-based phosphors.
Caveat: Afterglow duration depends on charging, measurement threshold and particle size.
FactModerate evidence
Radium luminous dials were replaced by tritium-based material from the 1970s and by non-radioactive strontium aluminate photoluminescent material from the mid-1990s.
Source: Radium dials
FactModerate evidence
Strontium aluminate phosphors are used to make glow-in-the-dark safety markings; their long-lasting green emission depends on europium being present as Eu²⁺.
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.