How well a colourant resists fading or changing when exposed to light.
Commonly rated against the ISO blue wool scale, eight reference textiles each about two to three times as sensitive to light as the next one up. A rating of 1 fades quickly; 8 is very resistant.
Lightfastness is a property of the colourant in a particular material, not of a colour name. Two reds can differ enormously.
Many historic dyes rate 1–3 on the blue wool scale; most mineral pigments rate 7–8.
It decides how long a colour survives in a window, a gallery, a car interior or outdoors.
The sample is exposed to light alongside a set of eight standard blue-dyed wool strips, and is given the number of the strip that has faded to the same extent. Each strip is about two to three times as resistant as the one below it, so the scale from 1 to 8 covers a very wide range of durability. Artists' materials also carry ASTM ratings from I to V, where I corresponds to blue wool 7 to 8. Because the test compares fading with fading, it can be run under intense lamps to save time. Museums use a just noticeable fade, a colour difference of about ΔE 1.8, as the unit of damage.
A watercolour, a print or a textile is framed and hung in a bright room.
Light damage is cumulative and irreversible, and depends on the total dose: twice the light for half the time fades just as much. The most sensitive colourants reach a just noticeable fade in about a year and a half of display even at a museum's 50 lux, and fugitive pigments go first, shifting the balance of the whole picture.
A product is specified by pigment name on the assumption that the name fixes its durability.
Products sold under the same Colour Index name can differ in fastness. The rating has to come from the manufacturer for that product in that medium.