GB2277602A - Opthalmic graduated photochromic lens - Google Patents

Abstract

An ophthalmic graduated photochromic lens consisting of two ophthalmic lens blanks; one photochromic (4), the other clear (5), attached in a misaligned configuration and surfaced within the diameter x which coincides with the ophthalmic frame to produce two ophthalmic prisms (2, 3) of equal thickness and power, but with their bases in opposite directions so that they neutralise. This produces a graduated photochromic effect, which is fully obtained within the vertical boundaries of the lens due to its misaligned configuration. The resulting lens may be used for ophthalmic prescription work. <IMAGE>

Description

PHOTOCHROMIC LENSES This invention relates to photochromic lenses.

Photochromic material is a well known optical material which responds to changing light conditions. Typically, photochromic glass darkens when irradiated by ultraviolet radiation and by visible light of wavelengths between 300 and 400 nm, such as that emitted by the sun. The sun is not the only source of ultraviolet radiation; other sources include visual display terminals, high intensity mercury vapour lamps, Xenon arc lamps, fluorescent tubes and other lighting. The accumulative effect of solar radiation and other harmful radiation on the eye over the years has been associated with eye damage, for example, such conditions as brown or sunburn cataracts, and other corneal and retinal disorders.

Photochromic lenses have come into widespread use since the invention of rapidly acting photochromic materials enabling a lens to darken or lighten very quickly so as to correspond to changed lighting conditions. For use as ordinary sunglasses, or for driving in varying lighting conditions without prescription lenses, the photochromic material gives the lens an even colouration throughout which is dark when the light intensity is high and lightens to a minimum colouration which is often almost indiscernible, as the light intensity reduces.

In prescription lenses the thickness of glass in the lens is not uniform, a converging lens being somewhat thicker at its centre than at its periphery while a diverging lens is somewhat thinner at its centre than at its periphery. It follows from this that if a prescription lens were made wholly of photochromic material, as the light intensity increases and the photochromic material darkens, then, in the case of a converging prescription lens, it would tend to become relatively darker at the centre than at the periphery, and vice versa for a diverging lens.

This has been regarded as a drawback to be avoided so that apart from relatively weak lenses where the difference in thickness is relatively small and hence the difference in darkening intensity is not noticeable, photochromic materials should in general be avoided for prescription lenses. Alternatively, a uniform thickness photochromic lens has been cemented to a conventional prescription lens to give a uniform photochromic effect. However, this results in a lens which is relatively thick overall, and hence heavy for the wearer.

Conventional permanently tinted lenses for use as sunglasses or in driving where the tint is graduated are widely available, the lens being relatively dark at the top and virtually clear at the bottom in conformity with the fact that the predominant direction of incident light will be from above the horizontal. Tinted lenses with this form of permanent graduated tint are particularly useful for driving, the darker region at the top of the lens allowing the greater part of the incident light from the sun directly or from a bright sky to be reflected or absorbed while allowing the driver a clear and bright image through the lower part of the lens for viewing his instruments and controls, or for map reading, etc.

The present invention, in one aspect thereof, provides a graduated non-prescription photochromic lens.

In a second and alternative aspect of this invention, there is provided a photochromic prescription lens which darkens on exposure to increased intensity of light in a graduated fashion, the graduation being progressive from a region at or adjacent one edge of the lens to a region at or adjacent an opposite edge of the lens.

Lenses whether according to the first or the second aspect of this invention as identified above are intended to be mounted in a spectacle frame so that when the spectacles are worn the region which darkens most is at the top of the lens and the region which darkens least is at the bottom of the lens.

Graduated photochromic lenses in accordance with either the first or second aspect of this invention suitably comprise a portion made of photochromic material and a portion made of non-photochromic material, the two said materials having at least similar refractive indices, and preferably the same refractive index, and being bonded together to provide the said lens, the thickness of the photochromic portion reducing in a graduated fashion across the face of the lens from a region at or adjacent one edge to a region at or adjacent the opposite edge.

A graduated photochromic lens may be produced by a method in accordance with a further alternative aspect of this invention, the method comprising the steps of taking a photochromic lens blank (which may be of uniform thickness) and a non-photochromic lens blank (which may also be of uniform thickness), bonding the two blanks together, and surfacing or grinding the outer surfaces of the two blanks so as to produce a lens in which the thickness of the photochromic material is thickest at or adjacent one edge of the lens and progressively reduces across the face of the lens towards a region at or adjacent the opposite edge of the lens.

It is the belief of the applicant that graduated photochromic lenses, graduated from a region at or adjacent one edge to a region at or adjacent an opposite edge, have never previously been produced.

Lenses in accordance with the present invention present a number of significant advantages over previously existing photochromic lenses and over previously existing graduated permanently tinted lenses.

For a start, the lens progressively darkens, as do all photochromic lenses, but in the case of lenses in accordance with the present invention, the lens darkens in a graduated fashion across its face. As a result, the lens is able particularly to shield the wearer from incident light predominantly from above the horizon whilst still allowing good transmittance of light from below the horizon, and hence allowing good visibility to the wearer for reading, or viewing vehicle controls, and thus represents a significant advantage over conventional photochromic lenses.

By mounting the lenses in spectacle frames so that the non-photochromic material is on the inner side (ie closest to the eye), the problem caused by perspiration staining the photochromic glass which has previously been noted as a problem of photochromic lenses is wholly avoided.

As compared with conventional permanently tinted graduated lenses, the graduated photochromic lenses of the present invention represent a significant improvement since they operate essentially as successfully as the permanently tinted lenses in the kind of light conditions where the graduated effect is desirable and at the same time allow maximum transmittance in low light conditions, for example within a room at night, where the graduated lenses would not be suitable.

Particularly where a spectacle wearer requires prescription lenses, this means that in order to accommodate all likely lighting conditions, he need only require prescription lenses in accordance with the present invention and so may avoid the need to have two pairs of spectacles, one with clear prescription lenses and the other with permanently tinted prescription lenses. Moreover, for a prescription wearer, the overall lens thickness (and hence weight) need be no greater than for an entirely clear equivalent lens.

In preferred embodiments of lenses in accordance with the present invention, the two portions of the lens, one being photochromic and the other not being, may effectively form ophthalmic prisms, facing each other, the base of each prism facing in the opposite direction to the base of the other prism.

The non-photochromic material employed in lenses in accordance with the present invention may suitably be an entirely clear material, for example standard Crown glass.

Alternatively, the non-photochromic material may be given a slight permanent yellow tint to act as a contrast filter which is particularly useful in hazy weather or for night driving conditions.

Whether the lenses in accordance with the present invention are used in effect as graduated sunglasses by people with normal vision or for prescription work the nonphotochromic material is preferably employed for the backside of the lens for the reason explained above, namely to reduce the potential staining problem with perspiration.

As produced the lens may have an additional portion of uniform photochromic material at one edge and of clear material at the other edge. These additional portions are usually lost as wastage during glazing and allow for a full graduated effect through the whole vertical height of the finished lens when fixed within a spectacle frame.

The invention is hereinafter more particularly described by way of example only with reference to the accompanying drawing, in which: Fig. 1 is a schematic side view of two ophthalmic blanks prior to attachment, photochromic material in this and in the other figures being shown shaded for convenience; Fig. 2 is a side view of the two blanks of Fig. 1 attached together; Fig. 3 is a schematic side view of a lens constructed from the attached blanks of Fig. 2 showing in phantom lines the regions of the attached blanks removed by machining or grinding; Fig. 4 shows schematically the finished lens of the Fig. 3 embodiment prior to mounting within a spectacle frame; Fig. 5 is a view generally similar to Fig. 3 for an alternative embodiment; and Fig. 6 shows how a diverging prescription lens otherwise corresponding to the lens of Fig. 5 may be produced by additional machining of the non-photochromic material.

As shown in Figs. 3 to 6, a typical lens 1 in accordance with this invention has a photochromic material portion 2 and a clear material portion 3.

It will readily be appreciated that for the purpose of illustration all the lens and blank thicknesses are shown exaggerated in the drawing.

The photochromic and clear materials used may be glass.

Photochromic glass may have a refractive index of 1.523nd (for the Helium d' line of 587 nm of wavelength) in its standard form or in its higher index form, an index of 1.6nd. Standard Crown glass has a refractive index of 1.523nd. However, lenses are available with indices of 1.6 1.7, and 1.8 if required. The materials for the photochromic and clear portions should be selected to have similar, and preferably the same, refractive index.

Figures 1-3 show various stages in the production of a lens of Fig. 4. Two ophthalmic lens blanks 4 and 5 are taken, one being photochromic and the other being clear glass. The blanks are attached together (Figure 2) with the photochromic lens 4 on the front and the standard glass blank 5 on the back. The blanks are misaligned with respect to each other as attached for reasons explained below.

The lens blanks are then surfaced and ground. Fig. 3 shows the portions of the lens which are removed by the grinding and surfacing process in phantom. This process can readily be carried out using standard optical surfacing equipment. As shown in Figures 2-4, the lens blanks 4 and 5 are misaligned with respect to each other. The lenses are surfaced along those portions within the diameter x which in this instance coincides with the lens aperture diameter of the ophthalmic frame. Portions 6 and 7 of the lens blanks extending outwards of the lens aperture diameter x are used for glazing wastage. This allows for the full graduated photochromic effect to be obtained within the vertical boundaries of the lens.

In the alternative arrangement of Fig. 5, the lens blanks may be directly aligned and surfacing may be carried out along the whole length of each lens blank. The lens is mounted in a spectacle frame with little or no wastage or else some of the graduated effect will be lost.

Lenses as shown in Figs. 4 or 5 have opposite parallel faces, so that where the refractive indices of the two glasses are identical, the lens is optically neutral. Fig.

6 shows how these lenses can be adapted for prescription use merely by varying the machining of the clear blank. The Fig. 6 lens is a diverging lens with a reduced thickness at its centre 8. For a converging lens the thickness of the clear material at the centre of the lens will be slightly greater than in the Fig. 5 arrangement.

A number of optional features may be included in lenses according to the present invention.

Thus, a special ingredient may be added to the glass mix during production in order to give a slight permanent yellow colouration to the lens and act as a contrast filter.

An inner polarizing or non-polarizing filter lamina may also be introduced between the two lens components in a sandwich fashion to reduce reflected glare, and act as a safety device rather like a car windscreen. On breakage of the lens, the glass splinters would remain stuck to the lamina, hence avoiding eye injury. Chemical toughening may also be used as an alternative safety method.

An anti-reflective coating may be applied to both surfaces of the lens to eliminate reflections and allow at least 80% light transmission for night driving.

A special faster reacting grey photochromic glass may be used, since grey provides good colour recognition, particularly important in driving.

Photochromic materials are now well known and readily available. The present invention may be practised with any such materials, whether glass or plastics, deemed suitable for ophthalmic purposes. Accordingly, it is thought unnecessary to give any detailed descriptions herein of the photochromic materials themselves or of their manufacture.

Claims (4)

1. An ophthalmic graduated photochromic lens, produced from two standard or high index ophthalmic lens blanks; one made of photochromic material on the front or back, and the other made of clear ophthalmic material on the front or back.

2. A graduated photochromic lens as claimed in claim 1,where the attached blanks form a misaligned configuration, and surfaced with optical machinery to produce two ophthalmic prisms, facing each other with their bases in opposite directions so that they neutralise. This produces a graduated photochromic tint with maximum darkening in the upper part of the lens, and maximum clarity at the bottom of the lens.

3. A graduated photochromic lens as claimed in claim 1 or 2 wherein the full graduated photochromic effect is obtained within the vertical boundaries of the finished lens, due to its misaligned formation.

4. A graduated photochromic lens as claimed in claim 2 or 3 which may be used for ophthalmic prescription work.