WO2014122424A1

WO2014122424A1 - High transmittance polariser

Info

Publication number: WO2014122424A1
Application number: PCT/GB2014/050184
Authority: WO
Inventors: Simon Robert WELLS
Original assignee: Wells Simon Robert
Priority date: 2013-02-05
Filing date: 2014-01-24
Publication date: 2014-08-14

Abstract

According to the invention there is provided a circular or elliptical polariser including at least one partially polarising element having a structure configured so that the polariser i) transmits circularly or elliptically polarised light of a defined handedness with a transmittance which is close to 100%, ii) transmits circularly or elliptically polarised light of the opposite handedness with a transmittance which is substantially greater than 0%, and iii) transmits unpolarised light with a transmittance that exceeds 42% and preferably exceeds 50%.

Description

High Transmittance Polariser

The present invention relates to a high transmittance polariser, with particular, but by no means exclusive, reference to linear, circular and elliptical polarisers and their use in vehicles to reduce dazzle from vehicle headlights.

A polariser is an optical filter which only permits the passage of light of a specific polarisation. Light waves not of the specific polarisation are blocked. A polariser can be used to convert a beam of light of mixed polarisation into a beam of light with well-defined polarisation.

Known types of polarisers are linear polarisers and rotating (e.g. circular or elliptical) polarisers. Linear polarisers can be absorptive or beam-splitting. A known type of absorptive polariser is J-Sheet, which is herapathite, an iodoquinine sulphate, buried in a nitrocellulose polymer film. A linear polarising material known as H-sheet is made from polyvinyl alcohol (PVA) plastic with an iodine doping. During manufacture, the sheets are stretched causing the polymer chains to align to form an array of linear molecules. The iodine dopant is attached to the PVA molecules and makes them conducting along the length of the chains. Valence electrons from the iodine are able to move along the chains, but not traverse them. Any light which is polarised parallel to the chains is absorbed by the sheet and light polarised perpendicular to the chains is transmitted.

Rotating polarisers can be used to create clockwise or anti-clockwise circularly polarised light and to absorb or pass clockwise or anti-clockwise circularly polarised light. The most common way to create circularly polarised light is to place a quarter-wave plate after a linear polariser with light first passing through the linear polariser. The polarised light leaving the linear polariser is transformed into circularly polarised light by the quarter-wave plate.

The most common form of a circular polariser is PVA film, or a glass coating, used with a quarter wave retarder.

As any road user knows, the dazzle of lights from an oncoming vehicle can briefly blind them. It is obvious that this poses danger to all road users. It is possible to wear sunglasses or apply a heavy tint to a vehicle windscreen to avoid the dazzle but this prevents ambient light being received by the road user.

Polarisers have previously been suggested in documents such as GB365507 and GB2287005 to deal with the problem of glare from vehicle lights by applying them to headlamps and windscreens to reduce this glare.

Known circular filters or polarisers can be used to remove glare from oncoming vehicles. However they are currently at 42% efficient to unpolarised light and 99.9% efficient to orthogonal (or crossed) polarised light, the orthogonal transmittance being only 0.1 %. This poses two problems, particularly in the field of transport. Firstly, the transmittance of ambient (i.e. unpolarised) light is reduced to 42% which causes visibility problems when looking through the windscreen in low light conditions. Secondly the 99.9% efficiency of the orthogonal (or crossed) polarisation creates too great a reduction in the perceived direct light output of the oncoming headlight.

Therefore, there is a need to provide a polarising filter which allows more ambient light and more polarised light (from the oncoming headlight) to pass through thereby reducing the dazzling effect whilst still allowing a road user to view the surroundings in ambient light. The present invention provides polarisers according to the appended claims. The present invention also provides a vehicle screen, a visor, spectacles, an IR imaging device, a mirror, a headlight and a vehicle according to the appended claims.

In some embodiments the partially polarising element has one or more apertures formed therein which allow the passage of unpolarised light therethrough. The apertures may be holes or slots. In embodiments in which the polariser includes a retarder and a linear polariser as the partially polarising element, apertures are formed in the linear polariser although it is possible to additionally provide apertures on the retarder. Typically, apertures on the retarder are in register with corresponding apertures on the linear polariser.

A dopant may be applied to the partially polarising element in a desired pattern so that one can determine where ambient (unpolarised) light can pass through. In other embodiments, the partially polarising element is stretched during manufacture to interrupt the polarising properties of the partially polarising element. The stretching may be controlled to control the cross sectional thickness of the partially polarising element. For example, it is possible to control the stretching of doped polymeric films to determine the polarising properties. In some embodiments, the polarising efficiency and transmittance of dye-containing polyolefins such as polyethylene are controlled by controlling the draw ratio (Y. Dirix, Ph. D. Thesis, Technical University of Eindhoven, 'Polarisers Based on Anisotropic Absorbance or Scattering of Light', 1997, ISBN 90-386- 6868-3, the entire contents of which are herein incorporated by reference). Further, one can apply a magnetic and/or electric field to the partially polarising element to affect the polarising properties of the partially polarising element. It is also possible to use a combination of these methods to produce a polariser according to the present invention.

The structure of the partially polarising element can be selected according to the desired purpose. For example, the structure of the partially polarising element can be selected to provide a circular polariser which is well suited for use in vehicles to reduce dazzle from headlights whilst still allowing sufficient ambient light to pass through the circular polariser for the driver to be able to properly observe his or her surroundings.

In an embodiment, the regions of the partially polarising element which are substantially transparent to unpolarised light are distributed randomly across the surface area of the partially polarising element.

In a further embodiment, the regions of the partially polarising element which are substantially transparent to unpolarised light are distributed in a desired pattern.

In a further embodiment, the regions of the partially polarising element which are substantially transparent to unpolarised light are varied across the surface area of the partially polarising element. For example, the circular polariser may be relatively transparent to unpolarised light (i.e., have a transmittance that exceeds 42% and preferably exceeds 50%) in certain regions, and not others. This will be of particular use when applied to a vehicle's windscreen so that in the periphery of the driver's view is through a portion of the circular polariser that transmits more ambient, unpolarised light. This enhances the driver's ability to see the sides of the road whilst not being dazzled by the lights of an oncoming vehicle.

Generally, the partially polarising element is a linear polariser. The linear polariser may be a polymeric film, optionally a doped polymeric film. A preferred but non-limiting embodiment is a doped PVA film. Doped polyolefin sheets, such as doped polyethylenes, may be used. However, the use of other kinds of linear polarising materials, such as H sheet, J sheet, K sheet, XP sheet, and coated glasses, is within the scope of the invention. Examples of dopants include iodine and dyes, such as di-azo dyes.

The polarisers of the invention may transmit polarised light in the visible and/or the infra-red (IR) regions of the electromagnetic spectrum. Generally, the visible region corresponds to the 380nm - 740nm wavelength range, and the IR region corresponds to the 740nm - 1000 micron wavelength range. The IR region includes the near-IR (0.74 - 1 .4 micron), short-wavelength IR (1.4 - 3 micron), mid-wavelength IR (3 - 8 micron), long-wavelength IR (8 - 15 micron), and far IR (15 - 1000 micron) regions. In principle, a polariser of the invention may transmit in any one of these regions of the IR, or across a number of these IR regions. In some embodiments, the polariser transmits light in the near-IR region. The polariser may transmit light in both the visible and near-IR regions of the electromagnetic spectrum.

The invention will find particular use on or in screens of vehicles as the problem of dazzle is associated with road vehicles in particular. The screen may be the windscreen of the vehicle, although the invention can be used in conjunction with rear screens and side screens. The invention is particularly suited to automobiles, although the invention may be used in connection with other kinds of vehicles such as airplanes, ships and trains.

The circular or elliptical polariser may be retro-fitted to existing vehicle windscreens. However, most windscreens are already tinted and applying a filter to a tinted windscreen could render the windscreen too dark. In this situation, it is possible to apply a circular or elliptical polariser with a partially polarising element having a structure which transmits unpolarised light with a suitable efficiency to compensate for the tint. However, it is preferred to use the invention in conjunction with windscreens which are untinted. This is because such windscreens tend to allow more ambient light to pass through. Additionally, a separate tint treatment is generally unnecessary because the polariser of the invention acts as a tint.

In another embodiment, an IR imaging device is provided with a circular or elliptical polariser of the invention which transmits polarised IR light. The IR imaging device may be a camera having an array of IR-sensitive sensors.

In yet another embodiment, the circular or elliptical polariser is retro-fitted to a visor of a helmet, such as a motorcycle safety helmet. Alternatively, the circular or elliptical polariser is made integral with the visor.

In a further still embodiment, the circular or elliptical polariser is incorporated into spectacles. The driver of a vehicle can wear these spectacles instead of having the filter in or on a windscreen or visor.

The invention also relates to a vehicle including a windscreen having a polariser as described above and, optionally, a headlight having a polariser which outputs light of the opposite handedness. The polariser associated with the headlamp may be a conventional circular or elliptical polariser or a circular or elliptical polariser of the invention.

The circular or elliptical polariser associated with the windscreen can be placed on the inside or outside of the windscreen, or made as an integral part of the windscreen. Generally, the circular or elliptical polariser includes a linear polariser and a retarder, with the retarder being closest to the exterior of the windscreen.

In order for the circular or elliptical polariser to achieve its purpose it must receive polarised light from an oncoming vehicle's headlamp(s). To achieve this, a circular or elliptical polariser is placed near to a headlamp. Generally, the circular or elliptical polariser includes a linear polariser and a retarder placed near a headlamp with the linear polariser being closest to the headlamp. The circular or elliptical polariser associated with the headlamp needs to produce light of the opposite handedness to the handedness of the light which is preferentially passed by the circular or elliptical polariser of the invention used on or in a windscreen, visor, glasses etc. It is apparent that manufacturers will have to agree on the handedness associated with the circular or elliptical polarisers used in or on windscreens etc and on headlamps so that all vehicles are consistent, thereby permitting the problem of dazzle to be addressed in accordance with the invention.

The circular or elliptical polariser can be placed on the cover of the headlamp or made as an integral part of the headlamp cover.

The vehicle may include fog lights provided with a linear or elliptical polariser oriented such that the reflected light from the water droplets in fog is blocked by the circular or elliptical polariser of the windscreen. This results in light from the fog lights, which is reflected by fog, being blocked by the windscreen.

The bandwidth of the fog light source, the polariser associated with the fog light source and the polariser of the windscreen may be a narrow bandwidth to increase the transmittance of ambient (unpolarised) light. Typically, the fog lights are provided with a linear polariser orientated at approximately 45 degrees to the linear polariser in the circular or elliptical polariser associated with the windscreen. Tests have shown that it is only reflected light from the water droplets that behave in this manner. The same fog light viewed through a windscreen of an oncoming vehicle is unaffected by the water droplet, and the fog light is not blocked.

The invention also relates to a vehicle including:

an IR imaging device having a polariser according to the invention attached to or incorporated therein which transmits IR light of a defined handedness incident thereon with a first efficiency, and transmits light of the opposite handedness incident thereon with a second efficiency in which the second efficiency is substantially lower than the first efficiency; and, optionally, at least one headlight and/or IR projection device having a polariser attached to or incorporated therein so that the headlight and/or the IR projection device outputs IR light of the opposite handedness.

In this way, problems related to IR 'dazzle' associated with IR night vision imaging systems can be reduced or alleviated. The IR 'dazzle' may be caused by IR light from oncoming headlights and/or IR light from IR projection devices on oncoming vehicles.

It is preferred that the polariser used in a screen, visor or mirror of the invention includes a linear polariser which blocks horizontally polarised light. In these arrangements, sunlight reflected from surfaces such as wet road surfaces and reflective vehicle bodywork is blocked. Additionally, ambient light passing through the screen, visor or reflecting from the mirror of the invention would not be blocked by polarised sunglasses worn by, e.g., the driver of a vehicle.

The present inventor has also realised that useful IR 'anti-dazzle' systems can be provided which do not necessarily utilise circular or elliptical IR polarisers of the type described elsewhere in the present document, i.e., circular or elliptical polarisers which can transmit circularly or elliptically polarised light of a defined handedness with a high transmittance and also transmit circularly or elliptically polarised light of the opposite handedness with a transmittance which is substantially greater than 0%. Thus, according to a further aspect of the invention there is provided an IR imaging system including;

an IR imaging device having a circular or elliptical IR polariser attached to or incorporated therein which transmits IR light of a defined handedness incidents thereon with a first efficiency, and transmits light of the opposite handedness incident thereon with a second efficiency, in which the second efficiency is substantially lower than the first efficiency; and

a vehicle having at least one headlight and/or IR projection device having a circular or elliptical IR polariser attached to or incorporated therein so that the headlight and/or the IR projection device outputs IR light of the opposite handedness.

The second efficiency may be 0% or close to 0%.

Typically, the IR imaging device comprises part of the vehicle itself. However, in principle, the IR imaging device may be differently provided, for example as a visor, spectacles, or other device worn by an occupant of the vehicle.

Generally, the circular or elliptical IR polarisers each include an IR retarder and an IR linear polariser.

Whilst the invention has been described above, it extends to any inventive combination of the features set out above, or in the following description, drawings or claims.

The invention will now be described with reference to the accompanying figures wherein:

Figure 1 shows three possible perforation patterns applied to a linear polariser;

Figure 2 shows a linear polariser material having four possible dopant patterns;

Figure 3 shows a linear material, which has been stretched intermittently to produce an interruption pattern;

Figure 4 shows a) a linear material which has been fully stretched and b) a linear material which has been stretched incompletely;

Figure 5 shows a pattern with decreasing density, when viewed from top to bottom, which is applied to a linear polariser; Figure 6 shows performance envelopes for, on the left-hand side, a prior art linear polariser and, on the right-hand side, a linear polariser of the invention;

Figure 7 shows performance envelopes for, on the left-hand side, a prior art circular polariser and, on the right-hand side a circular polariser of the invention;

Figure 8 shows the locations and orientation of circular polarisers in the headlight cover and the windscreen of vehicles; and

Figure 9 shows spectral transmission properties for a band-pass linear polariser.

The invention includes a number of methods for preparing linear, circular and elliptical polarisers which, in addition to providing a desired polarising effect, also permit the passage of desired proportions of unpolarised light and/or light of the opposite polarisation state to the polarisation state primarily passed by the polariser. Typically, these effects are produced by suitable modification or production of a linear polariser. Circular and elliptical polarisers can be produced by combining a linear polariser of this type with a suitable retarder. However, the invention is not limited in this regard, and it is possible to apply the techniques described herein to retarders and other polarising elements. The invention finds particular utility in the reduction of glare through vehicle screens caused by headlights. However, the invention is not limited in this regard either.

Figure 1 shows a linear polariser having three possible perforation patterns. The perforations can be made by any suitable method such as etching, drilling, or by laser. The perforations will allow the passage of all light. Figure 2 shows a linear polariser having four possible dopant patterns. The areas of the film having dopant applied will act as a linear polariser. The areas where no dopant is applied will allow passage of all light. The approach is suitable with polarisers such as H sheet which employ a dopant to provide a polarising effect.

Polarising materials such as H-Sheet are stretched during manufacture to provide a molecular alignment which gives rise to the polarising properties. Figure 3 shows a linear polariser material which has been stretched intermittently to produce an interruption pattern. The stretched regions are indicated by straight lines. Light falling on these lines will be polarised. Unstretched regions are shown in Figure 3 by dotted lines, and will allow the passage of all light. Intermittent stretching can be achieved using conventional manufacturing and processing equipment for sheet material, such as rollers, wheels and pinch rollers.

A further way of patterning a linear material to have selected regions which have a polarising effect is to use electric and/or magnetic fields which are applied intermittently during the manufacturing process. This can produce the same response pattern as shown in Figure 3. Light falling on regions which the applied electric/magnetic fields have caused to align will be polarised. Non- aligned regions allow the passage of all light.

The stretching of materials such as H-Sheet can be controlled in further ways in accordance with the invention. Figure 4 shows a linear material in which the stretch has been halted before optimum polarisation has been reached. A greater percentage of all light will therefore be passed. For comparison, Figure 4a shows a standard polariser where the stretch is complete and the molecules are fully aligned. Reference is also made to Y. Dirix, Ph.D. Thesis, ibid for a further discussion of the control of stretching during manufacture.

Linear polarisers can be produced by placing a polarising coating over an essentially transparent substrate such as a glass substrate. By applying the polarising coating to selected regions of the substrate it is possible to provide a polariser of the invention. For example, it is possible to achieve the same patterning as shown in Figure 2 by selective coating of a glass substrate. The areas of the glass having coating applied will act as a linear polariser. The areas where no coating is applied will allow passage of all light.

In a further embodiment, a polariser is produced by adding a dopant during manufacture. However, the concentration of dopant used is deliberately sub-optimal in terms of producing a polarising effect. This might be achieved by using relatively dilute solutions containing the dopant. As a result, a relatively high proportion of unpolarised light is transmitted by the polariser. The skilled reader will appreciate that the proportion of unpolarised light transmitted can be varied by varying the concentration of the dopant.

Figure 5 shows a pattern, which varies in density across the polariser material. In particular, the pattern has decreasing density moving from the top to the bottom of the polariser material. The pattern may be made by perforations or by a dopant or by any other suitable method. The light coloured portions can be holes, areas of dopant or the dark area can be where dopant has been applied and the light areas have been masked to prevent dopant being applied. Figure 9 shows the spectral transmission properties of a band-pass type linear polariser. In the embodiment shown in Figure 9, the band-pass linear polariser is configured to pass light of wavelength in the range 570-590nm with high efficiency. The lower curve shown in Figure 9 represents the degree of polarisation of light passing through the polariser as a function of wavelength. In the band-pass region 570-590nm, the transmission of light approaches 100%, and a very large proportion of the transmitted light is not in a defined linear polarisation state. Accordingly, the degree of polarisation approaches zero. Any polarisers of this type can be incorporated into circular or elliptical polarisers which are configured to transmit light of a defined handedness, but block light of the opposite handedness. The skilled reader will appreciate that circular or elliptical polarisers incorporating linear polarisers having spectral properties shown in Figure 9 will perform this role outside of the band-pass region. However, within the band-pass region, light of the opposite handedness (together with unpolarised light) will be transmitted with high efficiency. In one embodiment the effective frequency bandwidth of the light source, its associated polariser and the analysing circular or elliptical polariser may be the defined narrow bandwidth within the visible spectrum, thus allowing a greater transmittance of ambient (unpolarised light).

In one embodiment several narrow band pass frequencies are used simultaneously, typically Red 620-750nm, Green 495-570nm and Blue 450- 495nm. Further these bands may be subdivided to allow a limited frequency of Red, Green and Blue to be transmitted with high efficiency. Circular or elliptical polarisers utilising a linear element of this type find particular (but non-limiting) utility in systems in vehicles for enhancing a driver's perception of ambient light whilst reducing glare from headlights. In a further embodiment the headlight is arranged to produce light with a defined intensity in the same or excluded band pass frequencies. Such systems are described below.

Figure 6 shows performance envelope comparisons for a standard prior art polariser (left-hand side of Figure 6) compared to a high transmittance polariser of the invention (right-hand side of Figure 6). The performance data shown on the left-hand side of Figure 6 are for a prior art polariser XP42, with an average single transmission of 42 +/- 2%, and a cross transmission (DP) average of less than 0.004%, and a polarising efficiency of greater than 99.8%. The performance data shown on the right-hand side of Figure 6 are in relation to a high transmittance polariser of the invention having a surface area comprising 50% perforations (or any other structure according to the invention which provides equivalent transmission characteristics with respect to unpolarised light). It can be seen that the average light transmittance for ambience light is raised from 42% in the prior art to 71 %, and the DP transmittance is raised from less than 1 % to 50% with the polariser of the invention. It will be appreciated by the skilled reader that the characteristics of the polariser of the invention, such as the proportion of the surface area of the polariser which is perforated, can be easily varied in order to fine tune the transmission characteristics of the polariser to any given application. Figure 7 shows the performance envelopes for a prior art circular polariser (left-hand side), and a high transmittance circular polariser of the invention (right-hand side). The prior art circular polariser is of the CP42 type comprising a linear polariser of the kind described in relation to Figure 6 in combination with a quarter wave plate. The circular polariser of the invention comprises a linear polariser of the invention of the type described in relation to Figure 6 in combination with a quarter wave plate. The prior art circular polariser has a single transmittance average of 42 +/- 2%, a cross transmittance (DP) average of less than 0.04% and a polarising efficiency of greater than 99.98%. The circular polariser of the invention has a higher average transmittance for ambient light of 71 %, and the average cross transmittance is raised from less than 1 % with the prior art circular polariser to 50%. This value is independent of orientation. For the avoidance of doubt, the cross transmittance is a measure of the transmittance of the circular polariser with respect to circularly polarised light having the opposite handedness to light of the handedness which the circular polariser is primarily is intended to transmit.

Figure 8 shows arrangements for reducing the dazzling effect of vehicle headlights by incorporating circular polarisers in the headlights and the windscreens of vehicles. The topmost part of Figure 8 shows an arrangement in which prior art circular polarisers are used. In particular, a prior art circular polariser is used as a headlight cover to ensure that light emanating from the headlight is of a defined polarisation (right-hand (RH) polarisation as shown in Figure 8). The windscreen of an oncoming vehicle incorporates a prior art circular polariser which in this example is configured to transmit left-hand (LH) polarised light. As a consequence of the use of the prior art circular polarisers, hardly any non-RH polarised light is passed through the headlight cover (less than 1 % non-RH polarised light is passed), and essentially all of the RH polarised light received from the headlight is blocked by the windscreen (greater than 99% is blocked). Some ambient (unpolarised) light will be passed by the circular polariser in the windscreen, but with prior art circular polarisers (incorporating prior art linear polarisers) the amount of ambient light passed will not exceed 42%. As a consequence, insufficient ambient light may be passed in order for the driver of the vehicle to be properly aware of his or her surroundings. Additionally, the reduction in the intensity of the oncoming headlights caused by the use of this system may be too great.

The lower most portion of Figure 8 shows an arrangement of the invention, in which the headlight cover includes a RH circular polariser of the invention, and the windscreen of the oncoming vehicle includes a LH circular polariser of the invention. In one representative example, the circular polariser of the invention used in the headlight cover passes 20% non-polarised light, and the circular polariser of the invention used in the windscreen passes 65% ambient light . These performance values can be achieved by using a suitable linear polariser in combination with a quarter wave plate. For example, the circular polariser used with the headlight cover can be produced by incorporation of a linear polariser having perforations which correspond to 20% of its surface area. The circular polariser of the invention used with the windscreen can be produced by incorporating a linear polariser having perforations which correspond to 40% of its surface area in combination with a quarter wave plate. The skilled reader will appreciate that the use of a linear polariser having 40% perforations will also result in substantial transmission (40%) of RH polarised light. As a result, of the 40% perforations, which transmit ambient light with close to 100% transmittance, the amount of ambient light passed through the windscreen is increased, enabling the driver to be better aware of his/her surroundings. The intensity of oncoming headlights is still reduced compared to normal arrangements which do not employ polarisers.

In further embodiments, the polarising element of the rotating polariser fitted to the front of the headlights may be further extended in bandwidth to operate in the near-IR range. A rotating polariser of the opposite handedness with a polarising element also extending into the near-IR range can be placed in front of the image capturing portion of an IR imaging system, typically a multi- sensor array, fitted in an oncoming vehicle. This arrangement acts as an antidazzle system for the IR imaging system, and alleviates problems associated with 'headlight blooming'. Headlight blooming is a problem associated with some prior art IR night vision imaging systems.

Also, a near-IR rotating polariser can be fitted to the front of an IR lighting system which is used to project a beam of IR light ahead of vehicles fitted with some IR night vision imaging systems. IR LEDs may be used in order to project the beam of IR light. Again, the associated IR imaging system can be provided with a rotating polariser of the opposite handedness which operates in the IR region. This approach can be used to alleviate the problems associated with IR 'dazzle' caused by oncoming IR lighting. It will be apparent to the skilled reader that in these embodiments it is not necessary for the polarisers to transmit in the visible region.

It will be appreciated that many variations on the scheme shown in Figure 8 are possible. For example, the desired characteristics can be produced using other kinds of polarisers of the invention. The transmittance of the circular polarisers to unpolarised light (and to circularly polarised light of the "other" handedness) can be varied to achieve a desired effect. The properties of the polarisers of the invention used in both the headlight cover and the windscreen can be varied. However, it should be noted that it is not essential that the circular polariser used in the headlight cover is a polariser of the invention. Instead, it is possible to use conventional circular polarisers in the headlight cover in combination with a circular polariser of the invention in the windscreen.

Claims

1 . A circular or elliptical polariser including at least one partially polarising element having a structure configured so that the polariser i) transmits circularly or elliptically polarised light of a defined handedness with a transmittance which is close to 100%, ii) transmits circularly or elliptically polarised light of the opposite handedness with a transmittance which is substantially greater than 0%, and iii) transmits unpolarised light with a transmittance that exceeds 42% and preferably exceeds 50%.

2. A circular polariser according to claim 1 which the partially polarising element i) transmits circularly or elliptically polarised light of the defined handedness with a transmittance of (100 - x)%, where x is in the range 0 to 5%, and ii) transmits circularly or elliptically polarised light of the opposite handedness with a transmittance which substantially exceeds x%.

3. A polariser according to claim 2 in which the partially polarising element ii) transmits circularly or elliptically polarised light of the opposite handedness with a transmittance which exceeds 5%, and preferably exceeds 10%.

4. A polariser according to claim 2 in which the partially polarising element ii) transmits circularly or elliptically polarised light of the opposite handedness with a transmittance in the range 10 to 60%,

5. A polariser according to claim 2 in which the partially polarising element ii) transmits circularly or elliptically polarised light of the opposite handedness with a transmittance in the range 5 to 15%.

6. A polariser including at least one partially polarising element having a surface area and having a structure which gives rise to variable polarising properties across the surface area so that the partially polarising element is substantially transparent to unpolarised light incident on some portions of the surface area.

7. A polariser according to claim 6 in which the partially polarising element has one or more apertures formed therein which are substantially transparent to unpolarised light.

8. A polariser according to claim 7 in which the apertures are of a size less than 1000 microns, preferably in the range 50 to 500 microns.

9. A polariser according to claim 6 in which the partially polarising element includes a substrate having a coating thereon in a pattern, wherein the coating provides a polarising effect and the substrate is formed from a material which is substantially transparent to unpolarised light so that the partially polarising element is substantially transparent to unpolarised light incident on uncoated portions of the partially polarising element.

10. A polariser according to any one of claims 1 to 8 in which the partially polarising element includes a dopant which is present in a defined pattern so that a polarising effect is provided by doped regions of the polarising element in accordance with the defined pattern, and undoped regions of the polarising element do not provide a polarising effect.

1 1 . A polariser according to any previous claim in which the partially polarising element has an internal structure which provides a partially polarising effect.

12. A polariser according to claim 1 1 wherein the internal structure is produced by applying a magnetic and/or electric field to the partially polarising element during manufacture.

13. A polariser according to claim 1 1 wherein the internal effect is produced by controlling a stretching of the partially polarising element during manufacture.

14. A polariser according to claim 1 wherein the stretching is controlled to control the cross sectional thickness of the partially polarising element.

15. A polariser according to claim 13 or claim 14 wherein the internal effect is produced by halting the stretching of the partially polarising element during manufacture to achieve a desired polarising property.

16. A polariser according to claim 13 wherein the internal effect is produced by stretching one or more first regions of the partially polarising element and not stretching one or more second regions of the partially polarising element or stretching the second regions to a lesser extent than the first regions.

17. A polariser according to claim 1 1 in which the partially polarising element includes a dopant which is present in a concentration suitable to produce the partially polarising effect.

18. A polariser according to claim 17 in which the dopant is a dye.

19. A polariser according to any previous claim including a retarder and a linear polariser.

20. A polariser according to claim 19 in which the partially polarising element is the linear polariser.

21 . A polariser according to claim 20 in which the linear polariser is a polymeric film.

22. A polariser according to any one of claims 1 to 5 or any one of claims 7 to 21 when dependent on claim 1 in which the partially polarising element has a surface area and wherein the partially polarising element exhibits variable polarising properties across the surface area so that selected regions of the circular polariser transmit polarised light of the opposite handedness with a transmittance which is substantially greater than 0% and transmit unpolarised light with a transmittance that exceeds 42% and preferably exceeds 50%.

23. A polariser according to any previous claim which transmits polarised light in the visible and/or the IR regions of the electromagnetic spectrum.

24. A linear polariser having a structure configured so that the linear polariser i) transmits linearly polarised light polarised in a defined plane with a transmittance which is close to 100%, ii) transmits linearly polarised light polarised in an orthogonal plane with a transmittance which is substantially greater than 0%, and iii) transmits unpolarised light with a transmittance that exceeds 42% and preferably exceeds 50%.

25. A linear polariser having a surface area and having a structure which gives rise to variable polarising properties across the surface area so that the linear polariser is substantially transparent to unpolarised light incident on some portions of the surface area.

26. A polariser including at least one partially polarising element having a surface area and having a structure which gives rise to partially polarising properties so that unpolarised light incident on the surface is transmitted with a polarised component and a substantial unpolarised component, wherein preferably the substantial unpolarised component is transmitted with a transmittance which exceeds 5%, preferably is in the range 5-80%, and most preferably is in the range 5 - 60%.

27. A vehicle screen, preferably a windscreen, having a polariser according to any one of claims 1 to 23 attached or incorporated therein.

28. A visor having a polariser according to any one of claims 1 to 23 attached to or incorporated therein.

29. Spectacles having a polariser according to any one of claims 1 to 23 attached to or incorporated therein.

30. An IR imaging device having a polariser according to any one of claims 1 - 23 attached to or incorporated therein.

31 . A mirror having a polariser according to any one of claims 1 to 23 attached to or incorporated therein.

32. A screen, visor, spectacles or mirror according to any one of claims 27 to 31 in which the polariser has an inner region and an outer region, and the partially polarising element exhibits partially polarising properties so that the polariser transmits unpolarised light with a higher efficiency in the outer region.

33. A vehicle including:

a windscreen having a polariser according to any one of claims 1 to 23 attached to or incorporated therein which transmits light of a defined handedness incident on the windscreen exterior with a first efficiency, and transmits light of the opposite handedness incident on the windscreen exterior with a second efficiency, in which the second efficiency is substantially lower than the first efficiency; and, optionally, at least one headlight having a polariser attached to or incorporated therein so that the headlight outputs light of the opposite handedness.

34. A vehicle according to claim 33 further including at least one fog light having a linear or elliptical polariser attached to or incorporated therein, in which the linear or elliptical polariser is oriented so that light from the fog light which is incident on the windscreen after being reflected from water droplets in a fog is substantially blocked by the polariser of the windscreen.

35. A vehicle according to claim 34 in which the fog light has a linear polariser attached to or incorporated therein having an associated plane of polarisation, and polariser of the windscreen includes a linear polariser having an associated plane of polarisation wherein the plane of polarisation of the linear polariser attached to or incorporated in the fog light is orientated at about 45° to the plane of polarisation of the linear polariser of the windscreen.

36. A vehicle including:

an IR imaging device having a polariser according to any one of claims 1 to 23 attached to or incorporated therein which transmits IR light of a defined handedness incident thereon with a first efficiency, and transmits light of the opposite handedness incident thereon with a second efficiency, in which the second efficiency is substantially lower than the first efficiency; and, optionally, at least one headlight and/or IR projection device having a polariser attached to or incorporated therein so that the headlight and/or the IR projection device outputs IR light of the opposite handedness.

37. An IR imaging system including: an IR imaging device having a circular or elliptical IR polariser attached to or incorporated therein which transmits IR light of a defined handedness incident thereon with a first efficiency, and transmits light of the opposite handedness incident thereon with a second efficiency, in which the second efficiency is substantially lower than the first efficiency; and

38. A system according to claim 37 in which the IR imaging device comprises part of the vehicle.

39. A system according to claim 37 or claim 38 in which the circular or elliptical IR polarisers each include an IR retarder and an IR linear polariser.