Rugged lens e.g. for spectacles - has main body with diffractive focussing features on side covered with protective e.g. polysiloxane coating of lower refractive index

摘要

A lens for the complete spectrum of visible light, and partic. one for use in spectacles, consists of a basic member of transparent material with diffractive features on at least one face to produce a focal length. This surface concerned has a smooth transparent protective coating on its outside, the refractive index of which is less than that of the main body of the lens. The coating is pref. based on polysiloxane or heateropolysiloxane; the prod. can have a glass or transparent plastic disc. The diffractive body (1) is in the shape of a flat disc of glass. On its inner and focussing side (2) it has the diffractive features in the form of recesses (4) to produce a phase grating. The width of these recesses decreases further from the optical axis (5) which passes through the focal point (3). Parallel rays of light (7) incident on the outer side (6) of the lens are bent inwards towards the focal pt. (3) on the edges (8) of the recesses. The figure portrays the effect with monochromatic light. ADVANTAGE - Suitable for rough usage.