Diffractive Lens

The central region of the replica lens

Diffractive lenses are used to focus monochromatic light (often laser light). A diffractive lens consists of a series of radial rings or "zones" of decreasing width. The structure focuses light because rays hitting the outer zones of the lens are deviated more than rays hitting in the centre. Light of different wavelengths will be deviated by different amounts so different colours are focused at different distances behind the lens. This is why these lenses have been used as bifocal contact lenses and wavelength sensors. The focal length of the lens also depends on the refractive index difference between the material of the lens and its surroundings. By immersing the lens in a liquid and changing the refractive index of that liquid by applying an electric field across it, the focal length of the lens can be varied in a controlled way.

The key to making lenses for this application is to match the refractive index of the lens material to that of the liquid in its natural state, with no field applied. Epigem was able to do this by careful formulation of the UV resin mixture. In addition the replica lens had to be embossed onto a thin sheet of ITO-coated glass so as to provide one of the electrodes in the cell structure.