GB2033603A - Improvements in or relating to protective viewing devices - Google Patents

Abstract

A protective viewing device providing observer protection against excessive light intensities comprises first and second polarising optical systems with crossed planes of polarisation and transmissive birefringent means between the first and second systems. The birefringent means comprises a destructible member or other device such as a liquid crystal film which is rendered inoperative in changing the plane of polarisation of transmitted light in the event of excessive light intensity being instant thereon. <IMAGE>

Description

SPECIFICATION Improvements in or relating to protective viewing devices The invention relates to protective viewing devices and more particularly to devices for enabling an observer, or light sensitive device, to view a light source while protecting the light sensitive device or observer's eye or eyes in the event of the light intensity exceeding a desired level.

Such devices are particularly useful for viewing in situations where high intensity flashes may occur such as for example nuclear flash or laser pulses.

In cases where laser illumination is used, viewing devices have been proposed which incorporate a filter for the attenuation of high power laser pulses.

These have used a destructible reflective coating normally formed by depositing a thin metallic film onto a glass or plastics substrate. In the event of the light power density exceeding a required level, the reflecting coating is vaporised so that no damaging light is passed to the observer's eyes. There are however two major disadvantages with this type of filter, namely the cost per unit and the destruction threshold of the film. Since the thickness and quality of the film must be highly reproducible for this type of dynamic filter, the cost of each component is necessarily relatively high. Also the high damage threshold of metallic films necessitates the use of a complex optical arrangement for focussing the incident laser pulse in order to attain the higher power density necessary.Other coatings can be used having a lower damage threshold but these coatings may not necessarily have sufficient reflectivity to pass adequate light to the observer in normal use.

Other protective viewing devices have been proposed including a device using a succession of transmissive polarising devices but this required the use of a carefully selected polarising element arranged to suffer bleaching and thereby controlled change of dichroic ratio when subjected to intense illumination.

It is an object of the present invention to provide a protective viewing device using a transmissive polarising arrangement which avoids the need for materials which exhibit carefully controlled change in dichroic properties but relies upon the simple and effective use of a destructible transmission member.

The present invention provides a protective viewing device for us in viewing at an observation position a light source and arranged to provide protection in the event of the light intensity exceeding a desired level, which device comprises a first polarising optical system arranged to receive light from a light source and including meansforfocus- sing light from the source, a second polarising optical system arranged to receive a light from the first optical system and to direct light towards an observation position, the two optical systems having crossed planes of polarisation, and transmissive birefringent means located on a light path between the first and second optical systems at a position such that light transmitted by the first optical system is focussed and arranged to change the polarisation of transmitted light such that light passes through the first and second optical systems towards the observation position when the birefringent means is operative between the first and second optical systems, the birefringent means being such as to be rendered inoperative in effecting said change in polarisation in any area on which excessive light intensity is incident thereby substantially reducing transmission through said area of the field of view of the device while leaving transmission through the remainder of the field of view unchanged.

Said birefringent means is preferably arranged to respond to ultra short duration light pulses, such as laser pulses. Conveniently the birefringdnt means may comprise a destructible member, such as a plastics film which is physically destroyed thereby forming a hole, in any area receiving excessive light intensity. Alternatively the birefringent means may comprise a liquid crystal film.

Plastics films are relatively inexpensive to manufacture and have a lower damage threshold than metallic films and as stressed pastics films may exhibit birefringent properties, plastics films can be used with the present invention. By choosing a plastics film with a suitable damage threshold the viewing device can be extremely reliable in operation as the only result necessary to achieve immediate and substantial reduction in light transmission through the viewing device is the physical destruction of the part of the film in the optical path. The high transmission of many plastics to light in the visible spectral region can also be used where a transmissive ratherthan reflective technique is used.

The protective viewing device may include three or more polarised optical systems arranged in series between the light source and the observer, each system having a cross plane of polarisation with respect to the adjacent optical system, together with a destructible birefringent member between each pair of optical systems arranged to permit transmission of light through successive optical systems so long as the birefringent material is not destroyed.

Preferably the or each destructible birefringent member comprises a plastics film with a damage threhold selected in accordance with the maximum light power to the transmitted. Such plastics film may comprise polyethylene film or cellophane film.

Preferably each polarised optical system comprises a transmissive polariser together with one or more lenses arranged to produce a converging beam and the birefringent member is located at the focal plane of the optical system which transmits light to the birefringent member.

The invention includes both monocular or binocu larviewing devices. In the case of a binocular device, two light paths are provided for binocular vision each light path being provided with a similar arrangement of polarised optical systems together with one or more destructible birefringent members.

The use of a destructible birefringent member such as a plastics film does not in itself attenuate high power light pulses such as laser pulses, any more than it attenuates the normal intensity of ambient light. The optical arrangement of the protective viewing device is however such that destructin of the birefringent members results in strong atte nuation of the transmitted light beam.

Some arrangements in accordance with the inven tion will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 shows one basic optical arrangement in accordance with the invention, Figure 2 shows a section through a protective viewing device in accordance with the present invention, Figure 3 is a plan view of the optical layout of an alternative embodiment of the invention, and Figure 4 is a side view of the layout shown in Figure 3.

The drawings show examples of optical arrange ments which may be used in a protective viewing device which enables an observer to carry out normal observation through the device but is arranged to provide substantial attenuation of trans mitted light to protect the observer's eye or eyes in the event of the light intensity exceeding a desired level. The light attenuation is effected by the use of two or more crossed polarisers in sequence and a destructible birefringent member is located between the pair of cross polarisers so that when not destroyed by excessive light intensity, the birefrin gent member rotates the plane of polarisation of the transmitted light so as to permit transmission of sufficient light through the device to allow the observer to carry out normal observation.Figures 1, 2, 3 and 4 all show a single optical path which may be used in a monocular device but it is to be understood that in all Figures two similar optical systems may be arranged in parallel light paths in a binocular device.

In the basic arrangement shown in Figure 1,the optical system has an input 11 arranged to receive light from whatever is to be viewed by the observer.

Any light transmitted by the protective viewing device emerges at an output end 12 from which the light passes to an observer's eye. The device incor porates three optical systems 13, 14 and 15 arranged to transmit light in series. The first system 13 adjacent the input end comprises a converging lens 16 and transmissive plane polariser 17. Any light transmitted by the first optical system 13 passes to the second optical system 14 which again consists of a converging lens 18 and a transmissive plane polarising sheet 19.Any light transmitted by the second optical system 14 passes to the third optical system 15 which consists of a converging lens 20 and transmissive plane polarising sheet 21 but in this case the polarising sheet is located on the input side of the lens 20 whereas in the first and second optical systems the polarising sheet is located on the output side of the converging lens.

The polarisers 17 and 21 are arranged to have parallel planes of polarisation whereas the polaris ing sheet 19 has its plane of polarisation perpendicu larto that of the polarisers 17 and 21. In this way, the polariser 19 is crossed relative to the polarisers 17 and 21. Afirst destructible birefringent member in the form of a plain plastics film 22 is located in the focal plane of the lens 16. The film 22 is formed of stressed plastics material such as polyethylene or cellophane and exhibits birefringent properties. The film 22 is arranged to have a low light absorbance so that it transmits light to the second optical system 14 although it rotates the plane of polarisation of the transmitted light between the first and second optical systems.A further birefringent plastics film 23, similar to the film 22, is located between the second optical system 14 and the third optical system 15. The lenses 18 and 20 are arranged such that light passing through the lens 18 is focused onto the film 23 and the position of the film 23 coincides with the focal plane of lens 20. The lenses are arranged so as to give the smallest focused spot size possible on each of the plastics films 22 and 23 and any transmitted light beam emerging through the output end 12 is expanded back to the same size as the input beam.

It will be appreciated that without the birefringent effect of the films 22 and 23, substantially no light will be transmitted through the optical system due to the effect of the cross polarisers. The films 22 and 23 are however arranged so that when they are intact, the plane of polarisation of light passing between the optical systems is rotated such that light may pass from the input end to the output end thereby allowing an observer to see through the viewing device. Should the intensity of transmitted light exceed a desired level, the plastics film 22 will be physically destroyed at the point where light is focused onto it thereby forming a hole in the film.

This will then cease to change the plane of polarisation of the transmitted light so that on reaching the second optical system 14, no substantial light intensity is transmitted through the part of the polariser 19 which is on the same optical path as the hole which has been formed in the film 22.

It will therefore be appreciated that provided the light intensity transmitted through the device does not exceed a predetermined level, an observer, or observing instrument, may view light transmitted through the protective viewing device. However, in the event of the light intensity increasing above a predetermined level due, for example, to a laser pulse, the plastics film 22 will be destroyed in the region on which the excessive light intensity was incident thereby substantially attenuating light transmission through that part of the field of view of the device.

The second film 23 is provided in order to make the viewing device safe even in the event of very high intensity flashes occurring. If a hole in the first film 22 is caused by a very high intensity flash, it is possible that the intensity might be sufficient for the light level transmitted by the polariser 19 to be still excessive for the observer's eye or observing instrument. The second film 23 will then be destroyed in the region on which the light pulse is incident so that a second successive attenuation of the transmitted light occurs before reaching the oservation position.

The use of successive attenuations greatly increases the light intensity levels with which the device can be safely used.

It will also be appreciated that the use of lenses to focus the incident light onto each of the plastics films localises the area of each film which is likely to receive any light pulse of excessive intensity. In this way, any destruction resulting in a hole will only affect transmission through a small part of the field of view and transmission through the remainder of the field of view will be unaffected. In this way, transmission of any high intensity light pulse, such as a laser pulse, is substantially attenuated while the remainder of the field of view is unaffected and may be continuously observed by the observer or observing instrument.

The arrangement shown in Figure 2 illustrates a more compact arrangement for use in a protective viewing device although the basic optical principles and operations are similar to those already described with reference to Figure 1. In this example, the arrangement consists of a tubular housing 30 together with a branch housing 31. The optical components which are similar to those in Figure 1 are marked with the same reference numerals. The lens 16 at the input end of the device is mounted at one end of the housing 30 and the lens 20 is mounted at the opposite output end of the housing 30. The second optical system comprising lens 18 and polariser 19 are mounted at the left-hand end of the branch housing 31. In this example the two polarisers 17 and 21 are formed by two sides of common polarising sheet located adjacent the junction of the branch housing 31 with the main housing 30.Similarly the two birefringent plastics films 22 and 23 are formed by different parts of a common sheet extending across the branch housing 31. In order to achieve a compact arrangement a plurality of mirrors are incorporated into the device. Light passing through the lens 16 is deflected by a mirror 32 onto the polariser 17 and the light is focused at the film 22. Light passing through the film 22 is reflected by a further mirror 33 at one end of the housing 31 before reaching the lens 18. Light transmitted by the polariser 19 is again reflected by a mirror 34 back to the plastics film 23 and light transmitted through the film is reflected by a mirror 35 in the main housing 30 before reaching the lens 20 at the output end of the device. The principle of operation of this arrangement is exactly the same as that already described with reference to Figure 1.

The plastics films 22 and 23 are formed by a common sheet and they may be mounted on a suitable transport system so that in the event of destruction due to a light pulse of excessive power, the transport system may advance the film to a new position so that the destroyed region now forming a hole in the film is moved out of the optical path. In this way the device can be repeatedly used with a new part of the film being located in the optical path after each destructive occurrence. In the arrangement shown in Figure 2 the film providing the birefringent members 22 and 23 may be mounted on a transport mechanism at opposite sides of the branch housing 31 permitting lateral displacement of the film across the housing. Although the transport mechanism is not described in detail, various systems may be used.

In the further embodiment shown in Figures 3 and 4, light from a source being viewed passes along an axis 40 to an observer's eye 41. The drawings show the optical system for one eye only although a second similar system is provided for the other eye.

The axis 40 is inclined to a central axis 42 of the binocular device to facilitate normal binicular vision.

Light enters the device through an objective lens system consisting of two objective lenses 43 and 44.

The lens 44 is immediately followed by a transmissive plane polariser 17. Light is then reflected downwardly through 900 by a plane elliptical mirror 45 located at 450 to the axis 40. The light is then incident on a further plane elliptical mirror 46 again located at 450 to the incident light path so that the light is deflected along the horizontal path 47 through the destructible birefringent plastics film 22 which is closely sandwiched between two converging field lenses 48 and 49. The film 22 is a thin film although it is shown with substantial thickness in the drawings for the sake of clarity. The light is then incident on a further plane elliptical mirror 50 located at 450 to the incident light so that the light is directed upwards to a fourth plane elliptical mirror 51.The mirror 51 is located at 450 to the incident light so that it directs the light horizontally along the path 52 through two eye lenses 53 and 54 to the eye 41. The lens 53 is immediately preceded by the transmissive plane polariser 21. The objective lenses 43 and 44 are adjustable along the axis 40 to permit focussing adjustment. The eye lenses 53 and 54 are also adjustable along the axis 52 to permit focussing. It will be appreciated that in this example, the objective lenses 43, 44, the mirrors 45 and 46 and the field lens 48 form a first optical system which has a focus at the film 22. The eye lenses 53, 54, the mirrors 50, 51 and the field lens 49 form a second optical system having a focus at the film 22.The whole system shown in Figure 3 can be rotated about the axis 52 without impairing the effect achieved. In this example it is preferred to use the mirrors 45,46,50 and 51 to deflect the light paths but it is possible to modify the layout so that prisms are used instead of mirrors to deflect the light.

As the above described examples provide a dynamic filter operating on a transmissive rather than a reflective technique, the polarisers and plastics films used should be chosen to suit the operating characteristics required. When a polarised beam of light passes through a birefringent material, the plane of polarisation is rotated through a certain angle determined by the path length through the medium, amongst other factors. A pair of crossed polarisers will, in general, have an extremely lowtransmittance to an incident light beam. If a sheet of birefringent plastic is placed between the polarisers, they will transmit with minimal attenuation those wavelengths whose polarisation planes have been rotated through 900. Very thin films of birefringent material will only rotate a few specific wavelengths through the required angle but as the film becomes thicker the number of wavelengths rotated through 900 also increased. It is therefore possible to select the thickness of any plastics birefringent film for transmission across most of the visible spectrum.

Should the film be destroyed in any way the cross polarisers will return to their high attenuation state effectively extinguishing the incident light beam.

Ambient light will only be attenuated by approximatey 500 upon passing through the first polariser since the light is not polarsied in any particular way.

With the birefringent sheet between cross polarisers the transmission of ambient light may be of the order of 30% to 40% which is quite sufficient to allow good observation through the device.

In the above examples, the polarisers 17, 19 and 21 may comprise plastics sheets. In an ideal situation, the transmission of non-polarised light by such a polariser should be 50% across the whole visible spectrum, but plastics sheet polarisers tend to have absorption bands in certain regions of the spectrum which can reduce the transmission to approximately 30%. The polariser should therefore be selected to give optimum transmission in the green part of the spectrum where the eye is most sensitive. In designing the filter described in the above examples it is important to consider the extinction ratio of the pair of crossed polarisers which are used.The extinction ratio Re of a pair of crossed polarisers is expressed in terms of the light intensity transmitted when their polarisations are parallel I and orthogonal I Rye = - Dichroic plastics sheet polarisers are available which provide an extinction ratio Re value of approximately 10-5. It is therefore possible by suitable choice of birefringent plastics sheet inbetween the crossed polarisers to produce a device in which destruction of the film results in a 105 times attenuation of the incident beam.

With regard to the birefringent material used for the films 22 and 23, several varieties of transparent plastic film are usable and the birefringent properties arise from the anisotropic orientation of the molucules within the film. Suitable plastics materials include polyethylene and cellophane. The film thickness used in the above examples may be approximately 1 micron as this gives excellent transmission across most of the visible spectrum when situated between crossed polarisers. It is desirable to use films which are as thin as possible in order to provide high speed of destruction in the event of a laser pulse being tranmsmitted. On the other hand it is necessary to choose a film thickness which is sufficient to transmit a major part of the visible spectrum through the crossed polarisers.Owing to the high transparency of such plastics films, it is preferable to introduce a light absorbing species such as a small amount of carbon so that, for example, 10% of the incident light is absorbed by the film. This ensures a high sensitivity to incident power levels. The material which is absorbed into the plastics film may also be a photo-degradable organic molecule selected to increase the speed of destruction at high light intensities. In the above examples, the lenses are used to focus the transmitted light onto the plastics films 22 and 23 so that sufficient energy density results on a small area of the film to achieve satisfactory destruction of the plastics film in the event of a laser pulse being transmitted and to limit the area of destruction to a small part of the overall field of view which remains visible.

Although the above examples describe the use of destructible plastics films 22 and 23, suitable liquid crystal films may be used in place of the plastics films. Liquid crystal films when suitably electrically biased exhibit birefringent properties analogous to stress plastics films. However, these need not suffer permanent damage when exposed to high intensity illumination since the molecular mobility afforded by the liquid solvent permits relaxation of any disorder caused by e.g. high intensity illumination such that the original properties prior to the intense flash are regained. They do however have a similar optical effect in permitting transmission provided excessive light intensities are not viewed.However, in the event of receiving excessive light intensity flashes, substantial attenuation transmission, due to change of plane of polarisation, results in the localised part of the field of view on which the high intensity flash was incident.

The invention is not limited to the details described above. Although carbon black may be used as a useful broad band absorber in the plastics films 22 and 23, both sides of the plastics film may be coated with an electrically conducting material, such as a thin layer of aluminium or other metal or a thin layer of indium oxide or other conducting metal oxide. It is possible to enhance the dielectric breakdown threshold of the plastic by maintaining an electric potential between each conducting layer.

Even a modest static potential difference of 300 volts across one micron thick plastics film is equivalent to an electric field of 3 x 106 volts per cm. The current requirements for this supply voltage may be only a few microamps but the effect on reducing the minimum laser power for dielectric breakdown can be substantial.

The above described examples are advantageous for various reasons. One important factor is the low damage threshold of plastics material which is much lower than that required for reflective metallic coatings referred to above. The overall attenuation of any laser pulse directed at the viewing device is determined by the quality of the polarisers and extinction ratios can readily be achieved permitting attenuations in excess of 2 x 104. Reflective devices are on the other hand always subject to back scatter of radiation.

Claims (14)

1. A protective viewing device for use In viewing at an observation position a light source and arranged to provide protection in the event of the light intensity exceeding a desired level, which device comprises a first polarising optical system arranged to receive light from a light source and including means for focussing light from the source, a second polarising optical system arranged to receive a light from the first optical system and to direct light towards an observation position, the two optical systems having crossed planes of polarisation, and transmissive birefringent means located on a light path between the first and second optical systems at a position such that light transmitted by the first optical system is focussed thereon and arranged to change the polarisation of transmitted light such that light passes through the first and second optical systems towards the observation position when the birefringent means is operative between the first and second optical systems, the birefringent means being such as to be rendered inoperative in effecting said change in polarisation in any area on which excessive light intensity is incident therby substantially reducing transmission through said area of the field of view of the device while leaving transmission through the remainder of the field of view unchanged.

2. A protective viewing device according to claim 1, wherein the birefringent means comprises a transmissive device having a high light transmission in the visible region.

3. A protective viewing device according to claim 1 or claim 2 in which the birefringent means comprises a liquid crystal film.

4. A protective viewing device according to claim 1 or claim 2 in which the birefringent means comprises a destructible birefringent member which may be physically destroyed in a region on which excessive light intensity is incident.

5. A protective viewing device according to claim 4 in which the destructible birefringent member comprises a plastics film with a damage threshold selected in accordance with a required maximum transmission of light power by the device.

6. A protective viewing device according to claim 4 or claim 5 including three or more polarised optical systems arranged in series between the light source and the observer, each system having a crossed plane of polarisation with respect to the adjacent optical system, together with a destructible birefringent member between each pair of optical systems arranged to permittransmission of light through successive optical systems so long as the birefringent material is not destroyed.

7. A protective viewing device according to claim 6 in which the destructible birefringent member of each optical system is provided by a sheet of plastics film common to all the optical systems.

8. A protective viewing device according to any one of claims 4 to 7 wherein the or each destructible birefringent member comprises a polyethylene film.

9. A protective viewing device according to any one of claims 4 to 7 wherein the or each destructible birefringent member comprises a cellophane film.

10. A protective viewing device according to any one of the preceding claims in which each polarised optical system comprises a transmissive polariser together with one or more lenses arranged to produce a converging beam.

11. A protective viewing device according to any one of the preceding claims, said device having a single eye-piece and being arranged to provide a monocular system.

12. A protective viewing device according to any one of claims 1 to 10, said device being arranged as a binocular device and having two light paths provided for binocular vision, each light path being provided with a similar arrangement of polarised optical systems together with one or more birefringent members.

13. A protective viewing device according to any one of claims 4 to 9 in which the destructible birefringent member is mounted on a transport system operable to advance the birefringent memberto a new position such that any hole in the member formed by destruction due to excessive light intensity is moved out of said light path.

14. A protective viewing device substantially as hereinbefore described with reference to any of the accompanying drawings.