Dk: Oxygen Permeability
Contact lens is the barrier which reduces the amount of oxygen reaching the cornea. The value of oxygen permeability is the indicator for measuring this barrier: higher value indicates that more oxygen can reach the cornea. "Oxygen Permeability" is the value for the diffusivity (D) of the oxygen goes through the lens times the solubility (k). Therefore, the oxygen permeability is determined by the "material" of the lens, and has nothing to do with its shape or curve design.
Hydrogel lens transports oxygen mainly by the water in the lens, therefore, high water content means high oxygen permeability. Even when the water content of the lens is as high as 99%, the oxygen permeability would not go over the theoretical value, which is 40. As for the silicone hydrogel lens, although it also contains water, oxygen is not transported by water; instead, it is delivered by the polymer which consists of silicon molecule chains. The theoretical value of silicon molecule's oxygen permeability can be more than 100; therefore, silicone hydrogel lens' oxygen permeability is much higher than hydrogel lens', which depends on water to transport oxygen.
Dk/t: Oxygen Transmissibility
Compared to oxygen permeability, oxygen transmissibility takes the value "t" (the lens' thickness) into consideration; that is, oxygen transmissibility is equally related to the thickness, material, and geometric design of the lens. The value of its thickness is the harmonic mean of the measurements of its thinnest central optic zone. But since the lens with different power has different thickness, we use -3.00 D as the standard reference. (The central optic zone's thickness of soft contact lenses falls between 0.07~0.10mm)
Ideally, if the lens is designed to be very thin, its oxygen transmissibility can be greatly elevated even the material has low oxygen permeability. However, if considering the factors such as optical property, physical property, yield rate and usability, more problems would occur if the lens is too thin.