GB2220500A - Optical devices - Google Patents

Abstract

An optical device including a pattern of material which, depending on the applied potential difference, can vary from substantially light-transmitting to substantially light-absorbing. Thus, the device may be a zone plate on which suitable material has been printed. The device may be used, in conjunction with an auto-ranging device, to compensate for less accommodation, on ageing.

Description

OPTICAL DEVICES This invention relates to optical devices including a light-diffracting pattern, e.g. zone plates having one or more annular zone plate patterns which may be produced by mechanical action on the plate.

Such zone plates are known. See, for example, Figs.

2 and 3 of GB-A-2129157 and Fig. 4 of GB-A-2183246. If the annular pattern is opaque, the zone plate is a cancellation or obstruction zone plate. If the annular pattern is of material that causes light transmitted through it to be phase-retarded by n compared with the light passing directly through the sheet, the zone plate is a phase-reversal zone plate. Both types of zone plates form a primary image, at the first focus, and also further images, at multiples of the primary focal length.

It is now known that patterns other than a simple circularly-symmetric pattern may provide focusing.

Printed rather than machined annular patterns, or holograms, can be constructed, which reduce the tendency cf a zone plate to provide multiple foci. A zone plate may be considered as the simplest hologram for focusing power operating by diffraction.

According to the present invention, an optical device (a light-transmitting and focusing system of variable focal length) includes a pattern of material which, depending on the applied potential difference, can vary from substantially light-transmitting to substantially light-absorbing. In an electro-optical device of the invention, there is also means for applying an electric potential across the material.

The invention is based on the principle that by varying the potential across the pattern, the pattern can be made opaque, so that the system serves as a diffractive, e.g. zone, plate, and brings the light to a focus, or can be made substantially transparent, so that the material has no significant effect upon the transmission of the light. Thus, by altering the potential, the focal length of the system can be changed between two values. The sheet may additionally carry further diffractive, e.g. zone, plate patterns, each having means for applying electric potential across it, so that by applying electric potential across the appropriate patter or patterns it is possible to vary the focal length between a wide range of values.

Conventional liquid crystal constructions can be used for forming the diffractive pattern. For instance, the devide may comprise a light-transmitting sheet, e.g.

as part of a light-transmitting cell, for instance of glass or polarised glass, filled with liquid crystal material and having, for instance, tin oxide printed on the inner surface of one of the sheets of the cell.

Combined with the use of a simple auto-ranging device, an opthalmic lens system can be produced which can substantially compensate for the loss of visible accommodation experienced by all people in later life.

In this case, the electro-optical system would select the most appropriate diffractive pattern, e.g. zone plate or hologram system, for the patient's focal requirements, and anticipate this system electrically or electronically, using suitable control means. This is inherently most likely to be appropriate for spectacles, owing to the need for a ranging device which requires a fixed base line as in a-rigid structure.

The light-transmitting sheet may itself be part of some other light-transmitting and focusing system. For instance, it may be part of a lens that may be itself of fixed or variable focus.

According to a second aspect of the invention, a zone plate has one or more annular zone plate patterns which both diffract light and also refract light to the primary focus. Such zone plates can be produced by careful machining of the plate or sheet, and have the advantages that an increased proportion of incident light is focused (rather than scattered) and that, consequently, the primary focus is enhanced with respect to the secondary and other foci.

According to a third aspect of the invention, an optical device such as a zone plate, preferably a phase-reversal zone plate, including a light-diffracting pattern, is of variable curvature. In one embodiment of the invention, the focus of such a zone plate is varied by causing the zone plate to adopt a concave or convex curvature. In another embodiment of the invention, such a zone plate is part of a variable focus lens system, as described in more detail below, in which event it may be supported by a part of the lens system that has variable curvature or by a part of fixed curvature or that is planar.

The zone plate pattern may be formed electronically by the use of an elastomeric screen loaded with magnetic or electrostatically-chargeable particles. As is known, such a material may be deformed into the desired pattern by the action of an electro-magnetic field.

Alternatively, the pattern may be formed from a sheet of photoresist photo-sensitive material by projecting the desired pattern on to the photosensitive material and removing unfixed material by washing in a suitable solvent. Either of these methods may be used for forming the zone plate itself or, more usually, for forming a mould against which the zone plate can be formed by casting.

According to another embodiment of the invention the phase reversal zone plate is produced by casting a silicone elastomer from an appropriate resin catalyst system against a mould having the desired zone plate pattern, the mould being produced in conventional manner or by one of the methods described above.

When the zone plate of the invention is to be part of a variable focus lens system the lens system may involve a light transmitting cell defined on its opposing faces by sheets at least one of which is deformable through different radii of curvature, the cell being filled with a fluid material. Devices in which there are means for forcing fluid material into or out of the cell, and thereby deforming the deformable sheet and varying the focus of the lens, are described in, for instance, British Patent Specifications Nos. 1,209,234, 1,226,003 and 1,289,704. The means for forcing fluid material into or out of the cell may comprise a piston.

The lens may alternatively comprise a light transmitting closed cell defined on its opposing faces by sheets of which at least one is deformable through different radii of curvature and filled by a first deformable material, means for deforming the deformable sheet or sheets through different radii of curvature and a second deformable light transmitting material having a refractive index that is different from that of the first material.

The means for deforming the deformable sheet or sheets may comprise means for adjusting the separation of the opposing faces of the cell around the periphery of the cell. For instance the periphery may be defined by telescopically engaging walls and, since the cell is a closed cell, changing the depth of these walls will inevitably result in changing the radius of curvature of deformable faces of the cell. In another construction the cell is formed of deformable sheet material and the means for deforming the sheets may comprise means for varying the plan area of the cell for instance by pulling the cell outwardly so as to increase the plan area, while reduction in the area may be achieved by relaxation of these means in combination with elastic properties of the deformable material-or materials of the sheet or sheets.

One of the deformable materials in the lens is generally a liquid or elastic salid while the other is generally air or some other gas. The lens may consist of the cell filled with air and surrounded by elastic fluid or solid enclosed within an outer cell, but more usually the lens consists solely of a cell at least one face of which is deformable and that is filled with deformable material having a refractive index different from that of air, the lens being surrounded by the atmosphere.

The means for deforming the cell may be such as to result in uniform deformation so as to change solely the focal length of the lens or may deform the cell differently in different parts of the plan area, so as to change the focal axis of the cell or generate an aspheric lens.

When a zone plate system according to the invention is combined with any of these systems it may be combined in such a way as to alter the focal length or the focal axis. For instance a zone plate may be printed on part only of an ordinary lens, so as to form a bifocal lens for opthalmic use.

The transmitting and focusing systems of the invention are useful for, for instance, opkhalmic purposes, and eye protection devices such as industrial and other goggles. However since they tend to generate weak secondary and other images they may find particular value in cameras, office copying machines computer read-out devices and other situations where it is possible to observe the primary focus only and to ignore, and not record, any secondary or other focuses.

Claims (6)

1. An optical device including a pattern of material which, depending on the applied potential difference, can vary from substantially light-transmitting to substantially light-absorbing.

2. An optical device according to claim 1, in the form of a zone plate, i.e. carrying one or more annular zone plate patterns of the material.

3. An electro-optical device comprising an optical device according to claim 1 or claim 2 and means for varying the potential difference.

4. An electro-optical device according to claim 3, in the form of a pair of spectacles, and including an auto-ranging device.

5. A zone plate having one or more annular zone plate patterns which both diffract light and also refract light to the primary focus.

6. An optical device including a light-diffracting pattern, of variable curvature.