GB2146135A - Improvements in or relating to light transmitting filters - Google Patents

Abstract

A light transmitting filter unit arranged to provide an increased optical density in response to light of an intensity exceeding a desired level comprises a first polarising system (41) and a second polarising system (43), the two systems having crossed planes of polarisation. A transmissive birefrigent member (45) comprising a thin layer of linear dye on a transparent substrate is located between them. The dye has a unidirectional ordered molecular structure which exhibits birefringent properties but is destroyed by excessive light intensity. <IMAGE>

Description

SPECIFICATION Improvements in or relating to light transmitting filters The invention relates to light trasmitting filters and particularly to light transmitting filters arranged to provide increased optical density in response to light intensity above a desired level. Such filters are particularly applicable to protective viewing devices of the type described and claimed in UK Patent 2033603.

UK Patent 2033603 describes and claims 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 focusing light from the source, a second polarising optical system arranged to receive 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 focused 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 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.

As is explained in Patent 2033603, such devices are particularly useful for viewing in situations where high intensity flashes may occur such as for example nuclear flash or laser pulses. It is also useful for viewing intense flash lamps used in photochemistry or conventional high power white light sources such as the sun or a welding arc, and in avoiding accidental exposure to beams of intense light such as laser beams.

Viewing devices have previously been proposed which incorporate filters employing crossed polarising systems. Some have incorporated reflective metallic coatings and others have used bleachable dichroic members.

Reflective metallic films suffer from the disadvantage of cost per unit as well as the extra optical elements required to compensate for the inversion of the image upon reflection. In the case of dichroic members which are bleached by excessive light intensity, these have the disadvantage that the dichroic members normally exhibit birefringence and a compensating birefringent element must be incorporated in the system so that the overall effect on visibility is due to dichroism alone. The provision of reliable compensation for the birefringence effect can present problems particularly as the birefringence exhibited by the dichroic material can vary in response to thermal distortion caused by incidence of excessive light intensity.

To avoid the above problems, UK Patent 2033603 proposed the use of a birefringent member in which the birefringent properties were destroyed in response to incidence of light of excessive intensity. Such birefringent members included plastics films or liquid crystals. This does however rely on absorption of sufficient heat due to excessive light intensity in order to destroy the birefringent structure.

In some cases this may require doping of the birefringent film in order to achieve sufficient absorption of visible light.

It is an object of the present invention to provide a light transmitting filter which is arranged to provide increased optical density in response to light of an intensity exceeding a desired level, in which an improved birefringent member is incorporated and which does not require additional doping with absorbent materials in order to destroy the birefringent properties in response to excessive light intensity.

The present invention provides a light transmitting filter unit for use between a light source and an observation position and arranged to provide increased optical density in response to light of an intensity exceeding a desired level, which filter unit comprises a first polarising optical system arranged to receive light from a light source, a second polarising optical system arranged to receive 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 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, characterised in that the birefringent means comprises a thin layer of linear dye on a transparent substrate, the dye having a unidirectional ordered molecular structure which exhibits birefringent properties while the ordered structure remains, the ordered structure being destroyed by absorbed light energy when the light intensity incident on a region of the birefringent means exceeds a desired level, whereby the birefringent means is rendered inoperative in effecting said change in polarisation in any area on which excessive light intensity is incident thereby substantially increasing the optical density of the filter in said area of the field of view.

Preferably the birefringent means comprises an amorphous non-crystalline layer of a linear dye on a substrate formed of plastics material or on a glass substrate.

Preferably the layer of linear dye is such that the light absorption due to the dye itself is sufficient to destroy the ordered molecular structure and thereby the birefringent properties when the incident light exceeds a desired level, while permitting use of a layer sufficiently thin which permits substantial light transmission while the unidirectional ordered molecular structure remains.

By use of linear dyes having a unidirectional ordered molecular structure it is possible to obtain satisfactory operation with a layer of dye having a thickness less than one micron.

The invention is particularly applicable to viewing devices of the type described in Patent 2033603 and may be used in place of the destructible birefringent members described in Patent 2033603.

Some embodiments of the invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 shows schematically a light transmitting filter unit according to the present invention, and Figure 2 shows a section through a protective viewing device in accordance with the present invention.

In the filter unit shown in Figure 1, light from a source S passes through a first optical system which comprises a polarising plate 41 arranged to polarise the light in the direction indicated by the arrow 42. Before reaching an observation position 0 the light passes through a second optical system comprising a second polarising plate 43 arranged to transmit light polarised in the direction of the arrow 44. It will therefore be seen that the polarising plates 41 and 43 provide crossed polarisers arranged to provide substantial optical density with little light transmission unless means is incorporated between the two polarising plates to rotate the plane of polarisation between the plates 41 and 43.For this purpose, a birefringent member 45 is located between the plates 41 and 43 and is arranged in normal use to cause a rotation of the plane of polarisation of light transmitted by the plate 41 so that the light is subsequently transmitted by the plate 43 to the observation position 0. The birefringent member 45 is arranged to be rendered inoperative by destruction of the birefringent properties in response to light intensity in excess of a desired level. In this example, the birefringent member 45 comprises a transparent plate formed of glass or plastics material carrying a layer of amorphous non-crystalline linear dye.

In forming the birefringent member, the dye molecules are orientated such that they have a unidirectional ordered molecular structure.

This may be effected by rubbing the substrate material before the dye layer is deposited from a dye solution, or by rubbing the preformed dye layer with a suitable material. In either case, a film is produced in which the dye molecules have a unidirectionally ordered structure exhibiting highly birefringent properties even where the film has a thickness of less than one micron. The amorphous layer of dye provides absorption of visible light without the need for any additional doping for absorption purposes, that excessive light intensity causes heat generation sufficient to destroy the ordered molecular structure and thereby destroy the birefringent properties and provide rapid increase in optical density of the filter unit.By use of the aforesaid dye layer, the layer may be thin and for example may be less than one micron thereby allowing transmission of up to 20%, usually 5 to 10%, of the incident light through the filter while the unidirectional ordered molecular structure remains, while providing a very substantial increase in optical density, typically to an optical density of up to 4, corresponding to a transmission of 0. 01%, and thereby a reduction in transmitted light upon destruction of the ordered structure in response to a preselected light intensity level which may be relatively low if desired.It will be appreciated that where the destruction of birefringent properties merely requires sufficient heat generation to destroy an ordered molecular structure, the filter can be arranged to respond to lower light intensities to cause the increase in optical density without the need for doping with additional light absorbing material which might be necessary where physical destruction of the entire substrate is necessary. By avoiding the use of such additional light absorbers to generate higher heat levels, a higher normal light transmission level can be achieved.

Specific examples of suitable linear dyes are Neutral Red, Brilliant Yellow, Pseudo isocyanine, Methylene Blue, Congo Red, Azombine.

The thickness of the dye is typically from 0.1 to 1,um, preferably about 0.2,us. Examples of suitable substrate materials are glass, polystyrene, polymethylmethacrylate, polyethylene glycols; and non-crystalline solids such as solid hydrocarbon and waxes which have a melting point below 1 00'C, hereinafter referred to as non-crystalline low melting point solids.

The above described filter unit is particularly applicable in the optical viewing device shown in Figure 2 which corresponds to Figure 2 of Patent 2033603 and in which similar reference numerals have been used. Figure 2 illustrates a compact arrangement having a tubular housing 30 arranged to receive from a light input 11 and to direct light through an output end 12 to an observer's eye or other light detector. A branch housing 31 is connected to the housing 30. Light passing through a lens 16 is deflected by a mirror 32 onto a polariser 1 7 corresponding to the polariser 41 in Figure 1. The light is focused onto the birefringent plate 22 corresponding to the plate 45 in Figure 1. Light passing through the member 22 is reflected by a further mirror 33 at one end of the housing 31 before reaching a lens 18.The light then passes through a further polariser 1 9 (similar to the polariser 41 in Figure 1) and is again reflected by a mirror 34 back to a birefringent member 23 similar to the member 22. Light transmitted through this polariser 23 is reflected by a mirror 35 in the main housing 30 before reaching a lens 20 at the output end of the device. The principle of operation of this arrangement is as described in Patent 2033603. However in that patent, the birefringent members 22 and 23 are described as destructable films such as polyethylene or cellophane (Registered Trade Mark) or a liquid crystal device.In the present examples, the birefringent members 22 and 23 comprise substrates of plastics material or glass carrying a thin layer of a linear dye having an amorphous unidirectional ordered molecular structure as described for the member 45 shown in Figure 1.

It will be appreciated that the polarising members 41 and 43 shown in Figure 1 and the corresponding polarising members shown in Figure 2 should not be rendered inoperative by light intensity up to that which causes destruction of the birefringent properties of the member 45. The polarisers 41 and 43 are therefore formed of material which is resistant to high light intensity and are preferably formed in accordance with the invention described in our copending Application No (Our Ref: 54748) filed on the same date as this application.

It will be appreciated that the light absorption heating and consequent disordering and destruction of the birefringent properties of the dye layer are localised at the region of the film where excessive light intensity is received. The remainder of the film therefore retains the ordered structure and normal open state transmission of up to 20% through the filter remains unaffected in those other areas.

Although reference has been made to the monocular system shown in Figure 2 of Patent 2033603 it will be appreciated that the invention of the present application may be applied to the other optical arrangement, including binocular arrangements, shown in Patent 2033603.

The above examples provide very efficient light filtering devices and systems may be provided in which the absorption, with extinction coefficients of the order of 10,000 cm;-1 in the visible spectrum and consequent short path length results in sufficient sensitivity to light that for example, 20 mJ cm-2 of laser light at 530 nm in a pulse 35 ns FWHM causes complete disordering of the aligment and destruction of birefringence. Similarly 40 mJ cm;-2 of a 590 nm laser pulse of less than 2 ys FWHM is sufficient to disorder the dye layer, particularly where the dye is deposited on a thermally insulating substrate, such as polymethylmethacrylate, or polystyrene.

Greater intensities of light will vaporise the dye layer frcm the substrate destroying any birefringence and resulting in high attenuation of light between the crossed polarisers. Such vapourisation or disordering of the dye layer takes place during the passage of the damaging laser pulse through the dye provided the energy density of the laser is sufficient. The energy densities indicated in the above examples are sufficient to ensure disordering of the birefringent layer during the passage of the laser pulse.

Claims (11)

1. A light transmitting filter unit for use between a light source and an observation position and arranged to provide increased optical density in response to light of an intensity exceeding a desired level, which filter unit comprises a first polarising optical system arranged to receive light from a light source, a second polarising optical system arranged to receive 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 optical systems 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, characterised in that the birefringent means comprises a thin layer of linear dye on a transparent substrate, the dye having a unidirectional ordered molecular structure which exhibits birefringent properties while the ordered structure remains, the ordered structure being destroyed by absorbed light energy when the light intensity incident on a region of the birefringent means exceeds a desired level, whereby the birefringent means is rendered inoperative in effecting said change in polarisation in any area on which excessive light intensity is incident thereby substantially increasing the optical density of the filter in said area of the field of view.

2. A filter unit according to claim 1 in which said birefringent means comprises an amorphous non-crystalline layer of a linear dye on a substrate formed of plastics material.

3. A filter unit according to claim 1 in which said birefringent means comprises an amorphous non-crystalline layer of a linear dye on a glass substrate.

4. A filter unit according to claim 1 in which said birefringent means comprises an amorphous non-crystalline layer of a linear dye on a substrate formed of solid hydrocarbon, a wax or other like non-crystalline low melting point solid.

5. A filter unit according to any one of the preceding claims in which the layer of linear dye is such that the light absorption due to the dye itself is sufficient to destroy the ordered molecular structure and thereby the birefringent properties when the incident light exceeds a desired level, while permitting use of a layer sufficiently thin which permits substantial light transmission while the unidirectional ordered molecular structure remains.

6. A filter unit according to claim 5 in which said layer of linear dye has a thickness less than one micron.

7. A protective viewing device for use in viewing or sensing 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 filter unit according to any one of the preceding claims.

8. A protective viewing device according to claim 7 including three or more polarised optical systems arranged in series between the light source and the observer or light detector, each system having a cross plane of polarisation with respect to the adjacent optical system, together with a destructible birefringent member formed by use of a layer of linear dye on a transparent substrate between each pair of optical systems arranged to permit transmission of light through successive optical systems so long as the molecules of dye have a unidirectional ordered structure.

9. A protective viewing device according to claim 7 and claim 8 in which the optical system arranged to direct light onto the birefringent means incorporates means for focusing light onto the birefringent means.

10. A protective viewing device according to any one of claims 6 to 9 in which a single eyepiece is provided together with a monocular system incorporating the said polarising optical systems.

11. A protective viewing device according to any one of claims 6 to 9, 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.

1 2. A light transmitting filter unit for use between a light source and an observation position substantially as hereinbefore described with reference to the accompanying drawings.

1 3. A protective viewing device substantially as hereinbefore described with reference to the accompanying drawings.