A white T-shirt under black light, stars on a child's bedroom ceiling, a glow stick, a firefly – they all glow in the dark, but in very different ways. The umbrella term is luminescence: light that is not produced by heat, as in a light bulb or a candle, but because a substance absorbs energy and releases it again as light. Where that energy comes from is what separates the four types. Here we explain them, tell how a glowing stone from Bologna led to today's glow pigments, and show what you can use them for.
Four ways to glow in the dark
Energy: light, usually UV · glows only while illuminated
Fluorescence
Fluorescent substances instantly convert invisible UV light into visible light. Their electrons are excited and fall back within billionths of a second – which is why the glow stops the moment the lamp is switched off. Examples: optical brighteners in washing powder make white clothes shine bluish under black light, highlighters glow vividly, and security features on banknotes become visible.
Energy: light · keeps glowing afterwards, often for hours
Phosphorescence – the afterglow
Phosphorescent substances store the energy of light in so-called electron traps in the crystal and release it bit by bit – long after the light has gone out. This is the afterglow of our pigments. They can be recharged with daylight or lamplight as often as you like, need no electricity and are not radioactive. They glow brightest in the first few minutes; after that the glow slowly fades.
Energy: chemical reaction · glows until the reaction is used up
Chemiluminescence
Here the light comes from a chemical reaction. Bending a glow stick breaks a small glass ampoule, two liquids mix and a dye lights up. Crime scene investigators use luminol, which glows blue when it reacts with traces of blood. Once the reaction is used up, the glow is over – a glow stick cannot be recharged.
Energy: metabolism of a living organism · glows when the organism triggers it
Bioluminescence
Living things such as fireflies, some fungi and marine plankton make their own light: a substance called luciferin reacts with oxygen with the help of the enzyme luciferase, and the energy released becomes visible as light. Bioluminescence is therefore chemiluminescence inside a living body. The anglerfish glows thanks to bacteria living in its "lantern".
A short history of afterglow
For a long time, scholars considered stones that glow in the dark after exposure to light a marvel. Around four centuries of research turned them into a material you can mix into paint, resin or plastic.
- c. 1603
The Bologna stone
Vincenzo Casciarolo, a shoemaker and alchemist from Bologna, roasts stones from Monte Paderno – actually in search of the philosopher's stone. What he gets is a stone that glows in the dark after being exposed to light. Heating the mineral baryte (barium sulfate) with charcoal has produced barium sulfide – the first artificially made phosphor. The "sun stone" becomes a sensation among scholars: around 1612 it is demonstrated in the circle around Galileo Galilei, and in 1634 Ovidio Montalbani devotes a treatise to it.
- 1669
Phosphorus, the "light bearer"
The Hamburg alchemist Hennig Brand extracts a substance from evaporated urine that shimmers greenish in the dark. It soon becomes known as phosphorus, Greek for "light bearer". Ironically, phosphorus does not have an afterglow – it glows because it reacts with oxygen in the air, which is a chemical process. Even so, the name phosphorescence stuck to all afterglow materials.
- 1768
Canton's phosphorus
The English physicist John Canton publishes a simple recipe in the Philosophical Transactions of the Royal Society: roast oyster shells with sulphur. The result is calcium sulfide, which glows brighter than the Bologna stone – for the first time, a phosphor can be reliably reproduced.
- 1852
Fluorescence gets its name
The Irish physicist George Gabriel Stokes shows that some substances convert invisible UV light into visible light of longer wavelength. He names the effect fluorescence, after the mineral fluorite. From now on, instant glow and afterglow can be clearly told apart.
- 1858
Afterglow becomes measurable
The Frenchman Edmond Becquerel builds the phosphoroscope, a device with rotating discs that makes even afterglow lasting thousandths of a second visible. He studies numerous substances and finds that tiny admixtures determine the colour and duration of the glow – the principle of doping on which all glow pigments still rely today.
- 1866
Zinc sulfide
The French chemist Théodore Sidot produces glowing zinc sulfide crystals. Doped with traces of copper, zinc sulfide becomes the most important phosphor for well over a hundred years – in luminous paints, watch dials, switches and toys. Its drawback: the glow becomes weak after a short time.
- 1888
An umbrella term: luminescence
The German physicist Eilhard Wiedemann groups all kinds of "cold" light under the term luminescence – as opposed to the glow of hot bodies. Around the same time, the Frenchman Raphaël Dubois discovers with luciferin and luciferase how glowing animals produce their light.
- from 1917
A dark chapter: radium paint
To make watch dials glow without prior exposure to light, zinc sulfide is mixed with radium. In the USA, young women paint dials with a paint called "Undark" and shape the brush tips with their lips. Many fall seriously ill. In 1927 five of them sue their employer; the 1928 settlement is considered a milestone for occupational safety and liability. Radium was later replaced and is no longer used in luminous paints today.
- 1993
Strontium aluminate, the breakthrough
Researchers at the Japanese company Nemoto & Co. develop strontium aluminate doped with europium and dysprosium. The new phosphor is not radioactive, glows around ten times brighter and many times longer than zinc sulfide. In 1996 they publish their results in the Journal of the Electrochemical Society. Since then, strontium aluminate has been the standard material for glow pigments, escape route signs and watch dials.
- today
Afterglow you can make yourself
Our pigments are based on strontium aluminate too: in grain sizes from fine powder for paints to granules for resin, in colours from green through blue-green to blue and purple. What was a curiosity for scholars 400 years ago is something you can mix into paint, resin or concrete yourself today.
What glow pigments are used for
- Art and painting: pictures that reveal a second motif at night – with glow paints or pigment in clear varnish.
- Resin and casting: jewellery, river tables and decorations made of epoxy resin with glow powder or granules.
- Garden and outdoors: paths, steps, house numbers and stones that give orientation in the evening – with granules and glow stones, embedded in weatherproof clear varnish or resin.
- Children's rooms and decoration: starry skies and planets with glow stickers.
Afterglow is an aid for orientation and effects, not a light source: it becomes visible in real darkness. You will find answers to frequently asked questions in our FAQ, and the mixing calculator works out how much pigment you need.