What does Lightfastness mean on a paint tube?

Lightfastness, LF, and Permanence

Lightfastness, often referred to as Permanence, refers to the ability of the paint pigment to resist gradual fading when exposed to light. Pigments that fade over time are often called fugitive colors.

If you have a favorite fugitive color like Alizarin Crimson, look for the words “new” or “permanent” in the color name, such as Permanent Alizarin Crimson. These are reformulated colors that are meant to be lightfast for that particular color.

There are certain lightfastness standards that you should look for when choosing paint.

ASTM International (American Society for Testing and Materials) classifies colors based on a test that simulates 100 years of gallery exposure.

Color samples are exposed to an accelerated dose of light energy equivalent to approximately 100 years of museum-lit conditions. This exposure is condensed and takes around 15 weeks to complete, depending on the testing methods used.

After the simulation, the color difference before and after is determined using a spectrophotometer and the difference units are calculated mathematically. Less than 4 units of color change gives a color the designation Lightfastness I. In practice, this means that a visual comparison of unexposed and exposed samples, would reveal barely any perceptible color change.