Electrically switchable liquid crystal cell, contact lens and method relating thereto

An electrically-switchable liquid crystal cell for changing a focal power of a contact lens, the liquid crystal cell comprising a diffractive optical element, the liquid crystal cell being switchable between an unmatched state and a matched state, wherein:

• in the unmatched state, the effective refractive index of the liquid crystal is different from the refractive index of the diffractive optical element;
• and
• in the matched state, the effective refractive index of the liquid crystal matches the refractive index of the diffractive optical element, such that the diffractive optical element does not contribute a focal power for the contact lens, and
• wherein the diffractive optical element comprises a plurality of diffraction blazes, each one of the plurality of diffraction blazes defining a peak and a trough
• wherein more than one of the plurality of diffraction blazes has a trough with a rounded corner having a radius of curvature of between 5 μm and 50 μm;
• and
• more than one of the plurality of diffraction blazes has a peak with a rounded corner having a radius of curvature of between 0.1 μm and 20 μm.