The Dk value measures oxygen permeability, the Dk/t value measures oxygen transmissibility of a contact lens. Dk/t is calculated using the oxygen permeability (Dk) of the material and the thickness (t) of the contact lens. Usually, the Dk/t value refers to a lens with a certain power, therefore making it easier to compare lenses.
In general, the higher these values are, the better is the oxygen permeability/transmissibility of the material. A high level of oxygen flow to the cornea is important as it improves wearing comfort and longer-term tolerance.
Dk and Dk/t further explained
The Dk value is a measure of the oxygen permeability of contact lenses indicating how much oxygen is passing through the material under certain given conditions. The primary factors for a low or high Dk value are the type of lens material and the temperature of the contact lens. The Dk value is measured on a piece of material the size of one cm³.
Why is there a Dk and a Dk/t value?
The Dk value is measured based on the Irving Fatt Method and indicates the absolute amount of oxygen diffusing through the material. For better distinction, this oxygen permeability measure is sometimes called Dk (FATT). Dr. Irving Fatt (1920 - 1996) was a pioneer in the research of corneal physiology working with Dr. Richard Hill at Berkeley Optometry to quantify oxygen uptake in the living human cornea. Fatt made significant contributions to the understanding of how oxygen flows through blood and other materials, helping to investigate oxygen supply to the cornea and designing the first corneal deswelling studies in humans. The Irving Fatt Method resulted in various ISO standards which are still present in the current ISO 18369-4 standard.
On the other hand, the Dk/t value stands for the oxygen permeability (Dk) in relation to the thickness of the material measured at the center of the contact lens (t = thickness). The contact lens specification sheets on Optix-now usually show the Dk/t value for a lens at -3.00D. Most manufacturers provide a Dk/t value for the -3.00D version of a lens. That way it is easier to compare lenses from different manufacturers.
How important is the Dk/t value?
Getting enough oxygen through the lens material to the cornea is a crucial factor in the design of healthy lenses with long-lasting wearing comfort. However, other factors must not be ignored when looking at the specifications of a lens. E.g. The amount of water embedded into the lens and maintained during the day impacts how your eyes will tolerate the lens throughout the day. Also, some eyes can deal better with a lower level of oxygen while others are fine with a lower amount of water content. Every eye is different, so let's not be 'blinded' by the highest Dk/t value. Modern silicone hydrogel contact lenses have pushed the limits and most of them have a Dk/t value of 100 or more.
Irving Fatt, PhD, FAAO - Biography
- Charter Member, Berkeley Optometry Hall of Fame
- Ophthalmic optics, Contact Lenses - Part 4: Physicochemical properties of contact lens materials