valguskindlus

Valguskindlus, or lightfastness, is the ability of a material to resist color or property changes when exposed to light, especially ultraviolet and visible light. It is a key property for dyes, pigments, paints, plastics, paper, textiles, and coatings, affecting appearance and longevity.

Fading results from photochemical reactions triggered by light energy. Dye molecules can degrade or detach from substrates; pigments can desaturate; polymers may oxidize or undergo chain scission. Color shifts, loss of brightness, and changes in gloss or mechanical properties can occur. The degree of fading depends on the light spectrum, intensity, exposure duration, temperature, humidity, oxygen, and the presence of stabilizers, contaminants, or additives.

Measurement and standards vary by material type but share common features. Lightfastness is typically tested in controlled lab aging using light sources such as xenon arc or UV lamps under specified conditions, with color changes assessed by colorimetry (delta E) or fade ratings. For textiles, blue wool standards are commonly used. International standards from ISO and ASTM provide guidelines for textiles, paints, coatings, and plastics.

Applications and strategies: For art, archival, and outdoor use, materials with high lightfastness are preferred. Formulations may include UV absorbers, HALS (heterocyclic amines), antioxidants, and protective coatings to improve photostability. Trade-offs can include cost, hue shifts, and compatibility with other properties.

Limitations: Laboratory tests offer comparative indications but may not perfectly reproduce long-term outdoor exposure; extrapolation to real-world conditions should consider environmental variability and sample geometry.