Improvements in or relating to variable focusing power optical devices and an augmented reality headset or helmet incorporating such a device

A membrane assembly for a variable focusing power optical device comprising a distensible membrane that is held under tension around its periphery by at least one bendable membrane support ring and one or more selectively operable curvature controllers;

• wherein the support ring defines an (x,y)-plane in an undeformed state, and the or each curvature controller is mounted on a respective discrete section of the support ring for actively controlling the curvature of the respective section of the support ring on a z-axis that is normal to the (x,y)-plane of the support ring and tangential to the periphery of the membrane, wherein the or each curvature controller comprises a linear actuator that is attached to the support ring at two locations that are spaced apart along the ring such that operation of the linear actuator causes the support ring to bend between the two locations.

A variable focusing power optical assembly comprising a compressible fluid-filled envelope having a first wall formed by a distensible membrane that is held under tension around its periphery by at least one bendable support ring, an inflexible second wall opposite the first wall, the support ring being held at a fixed distance from the second wall at three or more hinge points which are spaced apart around the support ring, and being bendable towards or away from the second wall intermediate the hinge points, and a side wall between the first and second walls that is collapsible to accommodate such bending, and one or more curvature controllers mounted to respective discrete sections of the support ring between the hinge points;

• wherein the membrane is arranged to distend in a direction towards or away from the second wall upon bending of the support ring between the hinge points, a surface of the membrane forming an optical surface having variable focusing power owing to such distension, and the one or more curvature controllers being operable for actively controlling the curvature of each respective discrete section of the support ring in a direction parallel to an optical axis of the optical assembly and tangential to the periphery of the membrane.