Why fluorescent colours look brighter than white

Borrowed light

Most colour is subtraction. Light falls on a surface, the pigment absorbs some wavelengths and the rest bounce back to your eye. A red apple soaks up most of the spectrum and returns the red. A white wall returns nearly everything and a black one returns very little. On these terms no painted surface can ever send back more light at a given wavelength than a perfect white would, because reflection can only give back what arrives.

Fluorescent paint gets round that rule. Its dyes absorb short wavelengths, ultraviolet and also visible blue or green light, and almost immediately re-emit the energy as light of a longer wavelength. The surface is still reflecting in the ordinary way, but on top of that it is emitting. So in the band where the dye emits, the paint can return more light than a perfect white surface.¹ Conservators describe these colours as appearing to glow because they give off more than 100% of the visible light falling on them in that range.² No energy is created. It is moved from the parts of the spectrum where your eyes are weak to the parts where they are strong.

The numbers can be striking. In a 1992 article for Optics & Photonics News, the optical scientist Susan Houde-Walter described a yellow-green coumarin dye called Alberta Yellow. Without fluorescence it would reflect about 80% of the light between 500 and 550 nanometres. With fluorescence it sends back about 77% more light in that band than a perfect white, a peak radiance factor of 177%. Some combinations of dyes, in which one dye absorbs blue and passes the energy to another that emits red, go beyond 300%.¹ That is why a fluorescent orange can look brighter than white paint in daylight, and why it has an odd, slightly lit-from-within quality that ordinary pigments do not.

Stokes and the stone

The effect was named by the Irish physicist George Gabriel Stokes in a long 1852 paper in the Philosophical Transactions of the Royal Society, "On the Change of Refrangibility of Light".³ Stokes showed that substances including fluorspar (the mineral fluorite) and uranium glass could turn invisible light from beyond the violet end of the spectrum into visible light. In the main text he called this "dispersive reflexion". In a footnote he admitted he did not like the term and was "almost inclined to coin a word", suggesting "fluorescence" after fluor-spar, in the same way that opalescence is named after opal.³⁴ The glow of fluorite, it later turned out, does not come from the calcium fluoride itself but from traces of other elements in the crystal, such as europium, which gives the blue fluorescence.⁴

The same physics explains why white shirts light up under a club's black light. Many laundry detergents contain optical brighteners, compounds that absorb ultraviolet and emit blue light, which makes fabric look whiter than it really is.⁵⁴

The Switzer brothers

Fluorescent paint as a product began with an accident. In the summer of 1933 Bob Switzer, a chemistry student at the University of California, Berkeley, was working at a H. J. Heinz lab when he fell while unloading tomatoes from a freight car. He fractured his skull and damaged his optic nerve, and doctors told him to stay in a dark room until his sight came back.⁶ His brother Joe, an amateur magician, wanted glowing materials for his act. In a darkened room in the basement of their father's pharmacy the brothers went through chemicals under an ultraviolet lamp, found that Murine eyewash mixed with alcohol glowed, and thickened fluorescent compounds with shellac into the first black-light paints.⁷⁶

In 1934 they set up Fluor-S-Art to sell glowing advertising displays, including posters for cinema lobbies.⁷ Those paints only worked in the dark, under ultraviolet lamps. The bigger step came in the 1940s, when the Switzers developed "daylight fluorescents" that glowed in ordinary sunlight. The business moved to Cleveland, Ohio, and became the Day-Glo Color Corp. in 1946.⁶ During the Second World War the colours were used on aircraft and equipment where being seen mattered, and Bob Switzer made an early high-visibility garment by dyeing his wife's wedding dress.⁶⁷

From posters to painting

Artists took to it, though not always with the critics' blessing. Andy Warhol used the colours in his advertising work before he was famous, and Frank Stella tried fluorescents early on. In the early 1960s painters such as Richard Bowman and Herbert Aach used Day-Glo to push painting towards a more physical, sensory experience, and their work was often dismissed as being in bad taste.⁷ In 1982 Keith Haring and LA II (Angel Ortiz) filled the basement of Tony Shafrazi Gallery in New York with ultraviolet light and art made with Day-Glo paint.⁸

Hi-vis and the eye

Under daylight the human eye is most sensitive to light at about 555 nanometres, a yellowish green.⁹ A fluorescent yellow-green that pours extra light into that region is about as conspicuous as a surface can get, which is part of why it is so familiar on roadsides. It is not the only option, though. The American high-visibility clothing standard, ANSI/ISEA 107, allows three fluorescent background colours, yellow-green, orange-red and red, and advice on choosing between them is about contrast with the surroundings, such as woodland, desert or a work zone already full of orange cones.¹⁰

The same quality that makes these colours useful makes them short-lived. Fluorescent dyes are not lightfast enough for most fine art use. Over time the fluorescence weakens, the colour fades, or both.² A painter using them is working with a surface that is brighter than anything around it and will not stay that way.

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1. Susan Houde-Walter, Fluorescence and the Color of Day Glo Paints, Optics & Photonics News, 1992.
2. Museum of Fine Arts, Boston, Fluorescent paint, CAMEO: Conservation and Art Materials Encyclopedia Online.
3. G. G. Stokes, On the Change of Refrangibility of Light, Philosophical Transactions of the Royal Society of London, vol. 142, 1852.
4. Wikipedia, Fluorescence.
5. Tim Mouw, Textiles and optical brightening agents, X-Rite, 2016.
6. Wikipedia, Bob Switzer.
7. Liz Stinson, How DayGlo Went From Utility Pigment to Design World Darling, AIGA Eye on Design, 2018.
8. Art Gallery of Ontario, Keith Haring exhibition large-print text.
9. Georgia State University, Perceived Brightness, HyperPhysics.
10. 3M, ANSI/ISEA 107-2020 Made Easier, 3M.