Oxygen permeability of a contact lens material. Dk usually depends on the water content of a hydrogel lens material and the silicone content of a silicone hydrogel lens material. Oxygen is more soluble in water than in PMMA, but far more soluble in Silicone rubber than in water. Also known as P (permeability).
P = Dk = diffusion (D) * oxygen solubility (k)
Also see high Dk and low Dk.