What is lightfastness?
Lightfastness is the degree to which a dye resists fading due to light exposure. Different dyes have different degrees of resistance to fading by light. All dyes have some susceptibility to light damage, simply because their strong colors are indications that they absorb the wavelengths that they don't reflect back. Light is energy, and the energy that is absorbed by pigmented compounds may serve to degrade them or nearby molecules. certain Basic type dyes can cause damage when exposed to light.
Lightfastness is the degree to which a dye resists fading due to light exposure. Different dyes have different degrees of resistance to fading by light. All dyes have some susceptibility to light damage, simply because their strong colors are indications that they absorb the wavelengths that they don't reflect back. Light is energy, and the energy that is absorbed by pigmented compounds may serve to degrade them or nearby molecules. certain Basic type dyes can cause damage when exposed to light.
Unfortunately, ultraviolet is not the only kind of light that causes damage. Visible light is quite sufficient to fade colored materials. In tests using a UV protectant, DyersLIST members Jerry Trapp and Sally Holmes found that the beautiful but poorly lightfast blue-violet, Procion type blue MX-7RX, actually showed an increased amount of damage. This was presumably caused by an interaction between the protectant and the dye; UV protectants in glass windows will surely do no harm, whether or not they are helpful. Most likely, UV protection will help protect some dyes, but not others.
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