Ultraviolet light is the light at shorter wavelengths than visible light, past the violet end of the spectrum. The thickness of the lining material is also a factor in UV resistance. In exposed lining materials, a thickness over 40 mil (1.0 mm) is usually recommended for long term UV resistance. In materials where the strength is provided by a fabric, an opaque coating of typically 0.4 mm on each side is required to prevent UV degradation of the fabric's strength.