When a material absorbs light, usually ultraviolet or blue, and gives it back out at a longer, visible wavelength.
The emitted light adds to ordinary reflection, so a fluorescent surface can return more visible light at some wavelengths than falls on it, which is why fluorescent colours look brighter than any ordinary paint.
Fluorescence stops the moment the exciting light is removed; phosphorescence, which continues to glow, is a different effect.
Optical brighteners in paper and detergents fluoresce blue under daylight's ultraviolet, making white look whiter.
Fluorescent materials measure and look very different depending on how much UV the light contains, which complicates matching and printing.
A fluorescent sample cannot be described by a reflectance curve alone, because what it sends back depends on the spectrum of the light exciting it. The CIE treats the total as two parts, the reflected and the luminescent, and the sum can exceed 100% of the incident light at the emission wavelengths. Instruments therefore have to control the ultraviolet content of their lamp: the ISO 13655 conditions distinguish D50-like illumination (M1), tungsten-like (M0) and ultraviolet-cut (M2). Comparing a reading with and without ultraviolet shows how much of a paper's whiteness is brightener.
A print proof on brightened paper is approved under office lighting and compared with the job under a D50 booth.
The amount of ultraviolet differs between the two lights, the brightener glows more in one than the other, and the paper white and every pale tint shift between blue-white and cream.
Fluorescent signage, safety clothing or a highlighted document is expected to stay bright for years.
Daylight-fluorescent colourants have comparatively poor lightfastness and brighteners in paper break down with exposure, leaving a duller, yellower result. Replacement intervals belong in the plan.