GB1589972A

GB1589972A - Spectacles or the like

Info

Publication number: GB1589972A
Application number: GB5389176A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1977-12-08
Filing date: 1977-12-08
Publication date: 1981-05-20

Description

(54) IMPROVEMENTS IN OR RELATING TO SPECTACLES OR THE LIKE (71) We, STANDARD TELEPHONES AND CABLES LIMITED, a British Company of 190 Strand, London W.C.2, England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to filters, for use for instance in sun-glasses, safety goggles, or for the protection of sensitive optical equipment from high-intensity incident light.

One of the difficulties with conventional filters is that they reduce the light intensity which reaches the eye or the optical equipment from relatively dim objects in the field of view. The present invention seeks to minimise this disadvantage.

According to the present invention there is provided a light filter arrangement which includes light sensitive means directly responsive to ambient light to be filtered, a liquid crystal cell and connections from the light sensitive means to the liquid crystal cell whereby the optical transmissivity of the cell is varied electrically in accordance with variations in the intensity of the light falling on the light sensitive means, in which the cell has a plurality of electrodes on at least one of its faces so that the light transmissivity of different areas of the cell can be separately varied, and in which the light sensitive means includes a plurality of light sensitive elements each corresponding to one of the different areas of the cell, the light sensitive elements being connected respectively to different ones of the electrodes of the cell for effecting said separate control.

According to the present invention there is also provided a pair of spectacles or safety glasses in which light-sensitive means directly responsive to ambient light to be filtered is provided, in which each of the lenses of the pair of spectacles or safety glasses includes a liquid crystal cell with connections to the cell from the light sensitive means whereby the optical transmissivity of the cell is varied electrically in accordance with variations in the intensity of the light falling on the lightsensitive means, in which each said cell has a plurality of electrodes on at least one of its faces so that the light transmissivity of different areas of the lens formed by that cell can be separately controlled, in which the light-sensitive means includes a plurality of light-sensitive elements each corresponding to one of the different areas of the cell, the light-sensitive elements being connected respectively to different ones of the electrodes of the cell so as to effect said separate control, and in which the light sensitive elements are mounted in the frame of the spectacles or glasses, which frame also carries one or more batteries plus electrical circuit elements associated with the control exercised by the light sensitive elements on the filter.

The invention will be described in its application to sun-glasses, although as already indicated it has other applications. Embodiments of the invention will now be described with reference to the drawings accompany- ing the Provisional Specification, in which Figs. 1 and 2 are diagrams explanatory of the operation of different types of liquid crystals, Figs. 3 and 4 illustrate the use of liquid crystals in sun-glasses, and Fig. 5 is an electrode lay-out for a liquid crystal cell embodying the present invention for use as a lens for a pair of sun-glasses (or safety goggles).

Conventional sun-glasses and safety goggles are of three types or a combination thereof. The first is a simple absorption or reflection type, in which incoming light is attenuated by absorption or reflection at a transparent medium, which may have neutral or selective properties in the spectrum of the incoming radiation. The second employs polarizing filters which transmit light vibrating in one plane only, thereby rejecting part of the light which has been reflected and partially polarized from a surface. Thirdly there are filters whose absorption varies with the intensity of the incident light (e.g. photochromatic glasses). These all have the disadvantage of reducing the light intensity from dim objects as well as bright ones, and in the case of the variable filters the response time is slow when the incident light intensity varies.

The arrangement described herein use the electro-optical characteristics of the liquid crystals, which can be contained in thin glass cells with internal transparent contacts, so made that when placed between crossed polarizing filters they transmit light until a small alternating or direct voltage is applied, when the incident light is wholly or partially absorbed. One type of liquid crystal cell known as "twisted nematic" is widely used for numeric displays, and another type using a "twist and bend" orientation would be suitable for the present purpose, because the light transmission varies smoothly with the applied voltage.

The arrangements described below consist of a spectacle, goggle or face-mask frame in which the optical components or "lenses" consist of transmissive liquid crystal cells with built-in polarizing filters.

In its simplest form, such a cell has transparent electrodes all over both internal surfaces. The spectacle frame carries photovoltaic cells (such as silicon or gallium arsenide photodetectors or solar cells) mounted to be exposed to the ambient illumination, their terminals being connected directly to the liquid crystal electrodes. In the dark, there is little or no output from the detector and the spectacles remain at their maximum transmissivity. As the ambient illumination increases (say to bright sunlight) the voltage from the photodetector also increases, and the liquid crystal cell becomes more opaque. The response time is about 0.1 second, which is virtually instantaneous to the eye. Note that the reduced light transmitted by the cell is polarised, thus preserving the "antiglare" properties of normal polarizing glasses.

The power requirements of liquid crystal cells is so low (a few microwatts per square centimetre) that the photovoltaic cells used are each less than 1 mm square for the average sizes of spectacle lens. This small power requirement also allows the detector's signal to be attenuated, amplified, modified or chopped using miniature circuit components, and a battery can be built in to the frame. Similarly the light detector may be a photoconductor, which may be a discrete component, or an extra layer produced on the liquid crystal cell. A photoconductor needs a separate source of EMF, e.g. a small battery.

Sun glasses made as described are no more bulky or heavy than normal sun glasses, and have the following advantages: (1) Optical transmissivity varies almost instantly to adjust to ambient illumina tion.

(2) Much of the "glare" from the in cident light is removed by the polarizers.

(3) The optical properties of the sun glasses, e.g. variation of transmissivity from light to dark conditions, can be easily adjusted in the design of the circuitry and an adjustable component can be introduced to enable the wearer to modify them to suit his own comfort and requirements.

(4) It is easy to incorporate special properties such as non-linear variations of transmission or a cut-off point such that ambient illumination below a certain level leaves the transmission at maxi mum. Such properties can also be made adjustable by the wearer.

The foregoing description relates to spectacles in which the transparent electrodes cover the entire "lens" area, so that variations of transmissivity with ambient illumination take place over the entire lens area and are non-selective. A selective action whereby only the brightest parts of the scene are attenuated is achieved by dividing one or both of the transparent electrodes into separate conducting areas with individual contacts. The photodetectors are divided into a similar number of elements each controlling a liquid crystal area and each confined to illumination from the direction appropriate to its associated liquid crystal area.

This is achieved by using several detectors in the frame, each in a housing to receive light only from a given direction. Another and better method is to make the detector as a mosaic on a single area of semiconductor onto which an image of the scene is focussed by a small short focal length lens mounted on the frame.

When a person wearing such spectacles looks at a scene containing one or more very bright sources (say the sun or car headlights) the photodetectors receiving that area of the field of view respond to the light and apply a voltage (either directly or from a battery) to the corresponding area of the liquid crystal "lens", increasing its light absorption. Thus the wearer's view of the bright source is reduced in intensity without obscuring the remainder of the field of view.

Again, the magnitude of the effect may be pre-set or adjustable by the wearer.

The discrimination with which a source is dimmed depends on the number and shape of the photodetector and liquid crystal elements. Because the darkened liquid crystal element is too close to the eye to focus, it appears to have blurred or soft outlines and is not likely to be mistaken for a real object in the field of view. In the case of spectacles or goggles, of course each lens has its own photodetector and liquid crystal mosaic, so both eyes are similarly protected.

The relationship between the direction of the incoming light and the specific area the liquid cell is determined with reference to the spectacle frame, since the photodetector is attached to the frame.

Normally the darkened part of the liquid crystal cell attenuates a bright source only if the wearer is looking straight ahead through the centre of the spectacles. Movement of the head automatically results in a new element of the spectacles being activated so that the bright source still appears dimmed. Movement of the eye ball however is not compensated, so that if the source is large or the liquid crystal element small some light may reach the pupil of the eye without being attenuated. In some cases this is an advantage, as it allows the user to "recapture" an unattenuated view of the source if he wishes. In other cases the effect could be irritating and is overcome by making the liquid crystal elements large enough to continue to obscure the source over the whole or part of the range of movement of the pupil of the eye.

The liquid crystals used herein are free flowing liquids in which the molecules retain a degree of spatial ordering similar to that of anisotropic solid crystals and which thus have optical properties depending on the orientation of incident light. There are many liquid crystals known and with some of them an applied electric field tends to orientate the molecules either along or perpendicular to the field. This phenomenon is used in many electro-optic display devices.

One example is the twisted nematic device.

Fig. 1, which consists of two closely spaced transparent electrodes 1, 2 with a nematic liquid crystal 3 between them. The molecules of the liquid crystal are arranged to be roughly parallel to the two surfaces but with a 900 twist so that they have a helical formation as in Figure 1(a). In this form they rotate a beam of plane polarized light through 900. Polarizing sheets 4, 5 are arranged on the two sides of the liquid crystal cell, crossed with respect to each other. Light can now pass through the cell. When a small alternating or direct voltage is applied to the electrodes, the liquid crystal molecudes align themselves parallel to the electric field, thereby destroying their helical structure as shown in Figure l(b). In this condition there is no rotation of polarized light and since the input and output polarizers are crossed, no incident light is now transmitted. Close control of the applied voltage gives rise to intermediate states in which part of the incident light is transmitted.

Another arrangement which allows more gradual control of the transmitted light is similar to that described above, but the liquid crystal molecules are orientated in a "twist and bend" mode as shown in Figure 2. The molecules are parallel to the surface of one electrode and normal to the other, and the addition of some cholesteric liquid crystal or other chiral material (e.g. Canada Balsam) provides the desired amount of molecular rotation through the thickness of the layer. Application of a small voltage to the electrodes produces a progressive "untwisting" of the arrangement so that polarized light is rotated by an amount depending on the applied voltage.

In both systems the effect only occurs between electrodes so one or both electrodes can be shaped or divided so that the optical effect can be confined to a selected part of the area.

Figure 3 shows two situations when the user is looking straight ahead, only the right hand eye and right hand side of the spectacles being shown. Figure 3(a) shows the situation when there is a small bright light source in the direction A. Light from this source falls or is focussed on one or more elements of the photodetector, which then apply a voltage across the appropriate elements E of the liquid crystal cell LC. For clarity the polarizers on each side of LC have been omitted. The element E then becomes more opaque according to the light intensity, so that the direction A in the field of view is shaded, with the penumbra represented by AB A1B1 while the outer field BC and B1C1 will remain clear. Figure 3(b) shows that a similar situation applies if the relative direction of A moves to the right.

Figure 4(a) and (b) shows two similar situations with the eye looking to the right.

In this case the bright source A is unobscured or only partially obscured, so that A re-appears in the periphery of the field of view.

Thus obscured light sources can be recaptured by eye movement and the magnitude of this effect is determined by the size of the opaque elements.

The liquid crystal elements may be of any shape or size, according to the patterns of transparent conductors on the inner surfaces of the liquid crystal cell. It has been shown experimentally that dark areas less than one centimetre in diameter will satisfactorily cut out most unpleasantly bright objects, such as the sun or car headlights, without excessive interference in the rest of the field of view. In the case of safety goggles such as those used by welders, the element size may be much smaller and still cut out the bright point of light. In other applications such as general outdoor use where the source of glare may be a brightly diffusing sky rather than direct sun, the elements could be much larger. For some purposes it may be preferred to use an annular configuration of electrodes as in Figure 5. Tn this example, one of the transparent electrodes is divided into four electrically isolated quadrants as indicated by the broken lines and the other into three annular or circular sections. The photodetectors are in a similar configuration but very small, and are mounted into the spectacle frame with a small lens focussing the image of the scene onto it. Each photodetector element thus controls the optical density of the corresponding liquid crystal element.

All of the characteristics of the above system such as the degree of optical darkening in a given illumination is open to design control and can even have personal control by the user.

In some cases a single set of light-sensitive devices may be used to control both of the lenses of a pair of spectacles, in which case the light sensitive devices are located on the spectacle frame between the lenses.

WHAT WE CLAIM IS : - 1. A light filter arrangement which includes light sensitive means directly responsive to ambient light to be filtered, a liquid crystal cell and connections from the light sensitive means to the liquid crystal cell whereby the optical transmissivity of the cell is varied electrically in accordance with variations in the intensity of the light falling on the light sensitive means, in which the cell has a plurality of electrodes on at least one of its faces so that the light transmissivity of different areas of the cell can be separately varied, and in which the light sensitive means includes a plurality of light sensitive elements each corresponding to one of the different areas of the cell, the light sensitive elements being connected respectively to different ones of the electrodes of the cell for effecting said separate control.

2. A filter as claimed in claim 1 and in which each of said light sensitive elements is incorporated into the filter as a layer thereon.

3. A filter as claimed in claim 1, and in which the liquid crystal cell is a twisted nematic device.

4. A filter as claimed in claim 1, or 2 and in which the liquid crystal molecules in the cell are orientated in a twist and bend mode as herein defined.

5. A pair of spectacles or safety glasses, in which the lenses each consists of a filter as claimed in any one of claims 1 to 4.

6. A pair of spectacles or safety glasses as claimed in claim 5, and in which the light-sensitive elements are mounted in the frame of the spectacles or glasses, which frame also carries batteries plus electrical circuit elements associated with the control exercised by the light sensitive elements on the filter.

7. A pair of spectacles or safety glasses in which light-sensitive means directly responsive to ambient light to be filtered is provided, in which each of the lenses of the pair of spectacles or safety glasses includes a liquid crystal cell with connections to the cell from the light sensitive means whereby the optical transmissivity of the cell is varied electrically in accordance with variations in the intensity of the light falling on the lightsensitive means, in which each said cell has a plurality of electrodes on at least one of its faces so that the light transmissivity of different areas of the lens formed by that cell can be separately controlled. in which the light-sensitive means includes a plurality of light-sensitive elements each corresponding to one of the different areas of the cell, the light-sensitive elements being connected respectively to different ones of the electrodes of the cell so as to effect said separate control, and in which the light sensitive elements are mounted in the frame of the spectacles or glasses, which frame also carries one or more batteries plus electrical circuit elements associated with the control exercised by the light sensitive elements on the filter.

8. A pair of spectacles or safety glasses as claimed in claim 6 or 7, and in which a single set of light sensitive sub-elements is used to control both lenses of the spectacles or glasses, said single set being located on the frame and between said lenses.

9. A pair of spectacles or safety glasses as claimed in claim 5, 6, 7 or 8, and in which a measure of manual control is provided for the liquid crystal cells in addition to the control exercised thereon by said light sensitive means.

10. A light filter substantially as described with reference to Fig. 5 of the drawings accompanying the provisional specification.

11. A pair of spectacles or safety glasses substantially as described with reference to Fi. s of the drawings accompanying the provisional specification.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. indicated by the broken lines and the other into three annular or circular sections. The photodetectors are in a similar configuration but very small, and are mounted into the spectacle frame with a small lens focussing the image of the scene onto it. Each photodetector element thus controls the optical density of the corresponding liquid crystal element. All of the characteristics of the above system such as the degree of optical darkening in a given illumination is open to design control and can even have personal control by the user. In some cases a single set of light-sensitive devices may be used to control both of the lenses of a pair of spectacles, in which case the light sensitive devices are located on the spectacle frame between the lenses. WHAT WE CLAIM IS : -

1. A light filter arrangement which includes light sensitive means directly responsive to ambient light to be filtered, a liquid crystal cell and connections from the light sensitive means to the liquid crystal cell whereby the optical transmissivity of the cell is varied electrically in accordance with variations in the intensity of the light falling on the light sensitive means, in which the cell has a plurality of electrodes on at least one of its faces so that the light transmissivity of different areas of the cell can be separately varied, and in which the light sensitive means includes a plurality of light sensitive elements each corresponding to one of the different areas of the cell, the light sensitive elements being connected respectively to different ones of the electrodes of the cell for effecting said separate control.