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

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 bending actuators, wherein the or each bending actuator comprises a selectively operable self-bending strip which is attached contiguously to a respective discrete section 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 or each bending actuator is operable to bend itself to impart bending to the respective discrete section of the support ring, thereby to control 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.