GB2530725A

GB2530725A - Active Retroreflector

Info

Publication number: GB2530725A
Application number: GB1416851.2A
Authority: GB
Inventors: John Rudin; Stephen Kitson; Timothy Taphouse
Original assignee: Folium Optics Ltd
Current assignee: Folium Optics Ltd
Priority date: 2014-09-24
Filing date: 2014-09-24
Publication date: 2016-04-06
Also published as: GB201416851D0; WO2016046285A1; EP3198333A1

Abstract

An active retroreflector 100 comprising a retroreflective layer 102 and a switchable electro-optic absorption layer 104, arranged to cover at least part of the retroreflective layer 102, wherein the switchable electro-optic absorption layer 104 is a guest host liquid crystal layer switchable between a low absorption state wherein incident light is retroreflected and a high absorption state wherein fluorescent dichroic dye particles fluoresce. A polymer dispersed liquid crystal layer (412 fig 4) may also be provided which is switchable between high and low scattering states. The present arrangement allows the visibility of the active retroreflector to be adapted to different lighting conditions.

Description

ACTIVE RETROREFLECTOR

Technical Field

The present invention relates to an active retroreflector,

Background to the Invention

Tt is known to use retroreflectors as markings on vehicles, people or other objects to make them more visible in the dark, Retroreflectors are used because they reflect light in approximately the direction of the source so that a substantial proportion of the light from the source is reflected directly back towards the source.

Example applications for retroreflectors include high visibility clothing for cyclists or road workers, markings on wide or emergency vehicles, bicycles and road signs, high visibility markings on safety equipment at sea such as life rafts or life jackets and in emergency signage, for example, for drawing attention to fire exits. In the example of a retroreflector fitted to a bicycle, a substantial proportion of light from a following vehicles headlights will be reflected towards the eyes of the driver of the following vehicle, thereby making the bicycle more conspicuous to the driver. However, there is a desire to make retroreflectors and the objects to which they are applied even more conspicuous.

One option is to use a light source, such as one or more light emitting diodes (LEDs) or other lights, in addition to retroreflectors, This approach is often used in bicycle light systems, which often employ lights in addition to retroreflectors. However, this approach consumes significant power and the lights can be so bright that they are dazzling, or not bright enough so as to be easily washed out by nearby retroreflectors when illuminated by bright lights such as car headlamps.

A need therefore exists in the art for a power-efficient way of increasing the conspicuousness of reirorellectors.

Summary of Invention

According to a first aspect of the invention there is provided an active retroreflector comprising a retroreflective layer and a switchable electro-optic absorption layer, arranged to cover at least part of the retroreflective layer, wherein the switchable electro-optic absorption layer comprises a guest-host liquid crystal (G1-ILC) layer.

The reflectivity of the retroreflector can be modulated by switching the GHLC layer between a low absorption state and a high absorption state. This has the effect of simulating motion, which makes the retroreflector more eye-catching because human vision is particularly sensitive to motion. Because the active retroreflector does not emit any light itself, it is less dazzling and more power efficient than known flashing light solutions.

Using a GHLC layer gives rise to an advantage of improved transmission through the GHLC layer in the low absorption state compared with liquid crystal layers that use polarisers, It also enables a colour to be imparted to the active retroreflector in one or more of the modulation states, without needing to use a colour filter (although in some embodiments a colour filter may be used advantageously in addition to the GI-ILC layer).

According to a second aspect of the invention there is provided an active retroreflector comprising a retroreflector, an electro-optic switchable absorption layer and an electro-optic switchable scattering layer, wherein the electro-optic switchable layers are arranged to cover at least part of the retroreflector.

The active retroreflector of the second aspect can switch between a diffuse reflecting state and a retroreflecting state. As a diffuse reflector may be more visible than a retroref'lector under certain lighting conditions, such as in diffuse daylight, this enables the active retroreflector to be switched between different states so that it is effective in differeni lighting conditions and therefore more versatile.

One or both of the electro-optic switchable absorption layer and the electro-optic switchable scattering layer may comprise a plurality of portions wherein one of the portions is arranged to be in a low absorption state when another of the portions is in a high absorption state.

This enables the active retroreflector to simulate animation when the active retroreflector is modulated by switching the portions of the electro-optic layer, thereby making the active retroreflector more conspicuous.

The active retroreflector may comprise an energy harvesting device for powering the switchable electro-optic layer or layers.

The energy-harvesting device enables the active retroreflector to be run continuously without the need for an external power source or to change batteries, The energy harvesting device may be a photovoltaic cell, for example.

The active retroreflector may comprise a light sensor for detecting an ambient light level in the vicinity of the active retroreflector.

The active retroreflector may have a plurality of operational modes and the operational mode of the active retroreflector may depend on the detected ambient light level.

Thus the active retroreflector may behave differently in different light conditions to improve visibility of the active retrorefi ector.

For example, the active retroreflector may predominantly scatter incident light in high ambieni light levels and may predonunantly reirorefleci incident light in low ambient light levels.

The active retroreflector may comprise a motion sensor.

The operational mode of the active retroreflector may depend on the motion detected by the motion sensor.

For example, the active retroreflector may move into a different operational mode once motion is detected.

The different operational modes may be for example a powered up and a powered down state, or different switching modes that may produce different animations or different colours.

The electro-optic switchable absorbing layer may be configured to absorb light in a given waveband or wavebands.

The given waveband may be in the visible range of the spectrum.

Thus the active retroreflector may appear to be coloured, Different colours may be used for different applications and may also he used to increase visihility of the active retroreflector.

The given waveband may be in the infra-red range of the spectrum.

This may be used for signage or signalling in the infra-red region of the spectrum, for example for identification of friend or foe (1FF) systems.

The active retroreflector according to the first aspect may also include a switchable scallering layer.

The switchable scattering layer may be a polymer dispersed liquid crystal.

The switchable scattering layer may be positioned between the retroreflective layer and the switchable absorption layer.

The switchable absorbing electro-optic layer according to the second aspect may be a guest-host liquid crystal layer.

The guest-host liquid crystal layer of the first or second aspects may include a fluorescent dichroic dye.

Including a fluorescent dichroic dye in the GHLC layer means that when the GHLC layer is in a high absorption state, the fluorescent dichroic dye will absorb incident light and fluoresce, so the active retroreflector will appear bright. When the GHLC layer is in a low absorption state, the dye will absorb less incident light so the fluorescence will be weak, Thus the conspicuousness of the active retroreflector may be enhanced under certain light conditions such as in daylight conditions.

The plurality of portions of the switchable electro-optic layer may be produced by patterning electrodes of the switchable electro-optic layer The plurality of portions of the switchable electro-optic layer may be produced by patterning an alignment layer of the switchable electro-optic layer.

The active retroreflector may comprise a second switchable guest-host liquid crystal layer.

Brief Description of the Drawings

Embodiments of the invention will now be described, strictly by way of example only, with reference to the accompanying drawings, of which: Figure 1 is a schematic representation of a cross-section through an active retroreflector, with a switchable absorption layer in a low absorption state; Figure 2 is a schematic representation of a cross-section through the active retroreflector of Figure 1 when the switchable absorption layer is in a high absorption state; Figure 3 is a schematic representation of a cross-section through a switchable absorption layer of the active retroreflector of Figures 1 and 2; Figure 4 is a schematic representation of a cross-section through an alternative active retroref'lector; and Figure 5 is a schematic representation of a cross-section through a further alternative active retroreflector,

Description of the Embodiments

Referring first to Figure 1, an active retroreflector is shown generally at 100. The active retroreflector includes a retroreflective layer 102 which has a retroreflective surface 103 on one face thereof The retroreflective layer 102 may be a standard retroreflective tape (for example 3M Scotchlite 3MW680-10). The retroreflective surface 103 is covered by a switchable absorption electro-optic layer which is a switchable guest-host liquid crystal (GHLC) layer 104 in the embodiment shown in Figure I The switchable guest-host liquid crystal layer 104 may be laminated onto the retroreflective layer 102 using a transparent pressure sensitive adhesive, although alternative attachment means could be used. The GFIILC layer I 04 is switchable between a high absorption state in which light incident on the GHLC layer 104 is absorbed, and a low absorption state in which light incident on the GI-ILC layer 104 is transmitted to the retroreflective layer 102. For example, in the low absorption state the GFILC layer 104 may have a transmittance of 50% and in the high absorption state the GHLC layer may have a transmittance of 4%. In another example, the GHLC layer may have a transmittance of 60% in the low absorption state and a transmittance of 10% in the high absorption state.

In use, light 106 from a light source 108 (for example, a vehicle headlight) is incident on the GHLC layer 104. In Figure 1, the GHLC layer 104 is shown in a low absorption state.

In the low absorption state, the incident light 106 passes through the G1-ILC layer 104, is retroreflected by the retroreflective layer 102, and the retroreflected light 112 is transmitted back through the GHLC layer 104 towards the light source 108. A viewer 110 (e.g. a driver of the vehicle) positioned close to the light source 108 is therefore able to see the retroreflected light 112. In the active retroreflector shown in Figure 1, when the GHLC layer 104 is in the low absorption state the liquid crystal molecules and the molecules of dichroic dye that are present in the GHLC are aligned substantially perpendicular to the plane of the GHLC layer 104 so that they are end-on to the viewer 110. This alignment minimises the absorbance of light by the GHLC layer 104 so that the layer 104 transmits light.

Figure 2 shows the active retroreflector of Figure 1 after the G1-ILC layer 104 has been switched into the high absorption state, In the active retroreflector shown in Figure 1, when the GHLC layer 104 is in the high absorption state, the liquid crystal molecules are aligned parallel to the plane of the GHLC layer 104 so that they are side-on to the viewer 110. This side-on alignment maximises the absorbance of light by the GHLC layer 104.

As shown in Figure 2, as light 106 emitted from light source 108 enters the GHLC layer

S

104, a proportion of the light 106 is absorbed, The remaining light that is not absorbed by the GHLC layer 1 04 is retroreflected by the retroreflective layer I 02 and then re-enters the GHLC layer 104 where a proportion of the remaining light is absorbed Tn this way, the amount of light 106 that reaches the viewer 110 is minimised when the GHLC layer 104 is in tile lugh absorption stale.

The G1-]ILC layer may include a fluorescent dichroic dye. in that case, when the GI-ILC layer 104 is in a high absorption state, the fluorescent dichroic dye will absorb incident light and fluoresce, so the active retroreflector 100 will appear bright. When the GHLC layer 104 is in a low absorption state, the dye will absorb less incident light so the fluorescence will be weak. Thus, under certain lighting conditions, such as under direct illumination from a vehicle's headlights, the active retroreflector will appear brighter in the low absorption state as in this case the incident light will be retroreflected by the retroreflective layer 103. Under certain other lighting conditions, such as in daylight, the active retroreflector 100 will appear more visible in the high absorption state. The active retroreflector 1 00 may be switchable between the high and low absorption states in response to different lighting conditions to enhance the conspicuousness of the active retroreflector 100.

It will be appreciated that in alternative active retroreflectors, different liquid crystal phases and alignments could be used, for example the molecules may alternatively have a twisted configuration to ensure absorption of both polarisations of light.

In order to switch the GHLC layer 104 between the low and high absorption states, a voltage is applied to transparent electrodes in the Gi-ILC layer 104 by appropriate low power control electronics (which will not be discussed in detail here, as they are not the subject of the present application).

It will be appreciated that the GHLC layer 104 may also be switched to a plurality of intermediate states, each having an absorption between the high and low absorption states, by applying a voltage to the transparent electrodes at a level between the voltage required for the low absorption state and the voltage required for the high absorption state.

Referring now to Figure 3, a GI-ILC layer is shown generally aL 300. The G}ILC layer 300 comprises a GHLC mixture 302 sandwiched between two transparent substrates 304 which may be polymer sheets (for example Dupont Teijin 51506 PET of SOum thickness).

Each of the transparent substrates 304 is coated with a transparent conductor (for example, indium tin oxide (110)) on its inward facing surface to form a pair of transparent electrodes 306, Each of the transparent electrodes 306 is coated on its inward surface with an alignment layer 308. The alignment layers 308 determine the zero-field alignment directions of the GHLC 302. In sonic embodiments, the liquid crystal molecules may have a planar alignment, in which they align parallel to the substrate 304, when zero voltage is applied to the &ectrodes 306. In other embodiments the liquid crystal molecules may have a homeotropic alignment in which they are aligned perpendicular to the substrate 304 when zero voltage is applied to the electrodes 306. In order to induce a planar alignment the alignment layer 308 may be a polyimide (for example Nissan Chemical SE 130). Tn order to induce a homeotropic zero-field alignment, the alignment layer 308 may be another type of polyimide (for example Nissan Chemical 5E481 1).

The GHLC mixture 302 includes one or more dichroic dyes which impart a colour to the absorption layer 300, The specific colour or waveband of the spectrum of the GHLC layer 300 can be chosen by using an appropriate selection of dichroic dyes in the GHLC mixture 302.

The GHLC layer 300 may be configured to absorb and/or transmit light in a visible waveband. Tn that case, the colour of the dichroic dyes may be chosen to match the application. For example, for an application in which the active retroreflector will be applied or attached to a vehicle or cyclist, red or orange may be used. For an application in winch the active retroreflector will be applied or attached to an emergency vehicle or an emergency responder's uniform, the colour might be chosen to represent the respective emergency service. For example, blue may be used to represent the police and green may be used to represent paramedics. The dyes could also be chosen to modulate the reflectivity in the near infrared (NTR) for applications such as 1FF (identification of friend or foe) systems.

The GII-ILC layer 300 shown in Figure 3 also includes one or more spacers 310 between the substrates 304. The spacers 310 may be, for example, rigid polymer or glass spacer beads. Alternatively the spacers 310 may be photopatterned onto the inward facing surface of one of the substrates 304 on top of the transparent electrode 306. The substrate 304 may have, for example, an array of photopatterned, fixed height spacer structures fabricated onto the electrode surface, formed by coating the substrate with a liv curable epoxy resin (e.g. MicroChem STJ8-2005), exposing with TJV light through a mask, baking and removing the uncured resin (e.g. with Microposit EC solvent), and baking to fully cure prior to coating the substrate 304 with the alignment layer 308.

The G1-ILC layer 300 shown in Figure 3 is assembled by bonding the two substrates 304 together using a UV curable glue seal. The spacing between the two substrates will typically be 3 to 10 microns. The liquid crystal mixture 302 (for example Merck v1LC- 2037 with 1-5% Merck S811 chiral dopant and 1-10% of a dichroic dye such as Hayashibara G-472) is introduced between the substrates 304 by vacuum filling (although other known techniques could be used, such as continuous filling or one-drop' filling).

Referring now to Figure 4, an active retroreflector is shown generally at 400. The active retroreflector 400 includes a retroreflective layer 402 which has a retroreflective surface 403. The retroreflective surface 403 of the retroreflective layer 402 is covered by a switchable absorption electro-optic layer 404 which may be a GHLC layer or another type of swiichable absorption electro-oplic layer such as an electrowetling or electrophoretic device. In the embodiment shown in Figure 4, the active retroreflector 400 further comprises a switchable scattering electro-optic layer 412. The switchable scattering electro-optic layer 412 may be placed directly on top of the switchable absorption layer 404, Alternatively, as shown in Figure 4, the switchable scattering layer 412 may be placed between the retroreflective layer 402 and the switchable absorption layer 404. The switchable scattering electro-optic layer 412 may be any suitable electro-optic layer that can be switched between a transparent state and a scattering state. For example, the switchable scattering layer 412 may be a polymer dispersed liquid crystal (PDLC) layer.

The PDLC layer may be fabricated in a similar manner as the GHLC layer 300 shown in Figure 3, although in the PDLC layer the liquid crystal mixture inside the layer is chosen to include a phase separating polymer (e.g. a mixture of 50% Norland Optical Adhesive NOA6S, and 50% neniatic LC Merck E-7) and the layer is then cured under a liv lamp after filling to induce permanent phase separation and formation of scattering domains at

zero field.

The switchable scattering electro-optic layer 412 may be fixed in position between the switchable absorption layer 404 and the retroreflective layer 402 by lamination using a transparent pressure sensitive adhesive.

The switchable scattering layer 412 is operable to switch between a transparent state and a scattering state. In the transparent state incident light will be transmitted by this layer 412 without any deviation, and will therefore be retroreflected normally by the retroreflective layer 402. If the layer 412 is switched into the scattering state, incident light will be deviated randomly by the scattering layer 412, and thus the active retroref'lector 400 will act as a diffuse reflector.

When light is incident on the active retroreflector 400 in low light conditions, when the switchable scattering layer 4t2 is in the scattering stale, the active reirorefleclor 400 will appear darker than when the scattering layer is in the transparent state, when viewed from the direction of the incident light. On the other hand, switching the scattering layer to its transparent state in such conditions will cause more light to be retroreflected. Thus, switching the scattering layer 412 provides an additional way of modulating the visibility of the active retroreflector 400.

In daylight conditions, when the switchable scattering layer 412 is in the scattering state the active retroreflector 400 will appear brighter than when the switchable scattering layer 412 is in the transparent state. This is because when the switchable scattering layer 412 is in the transparent state, light incident on the active retroreflector 400 will be retroref'lected (i.e. only light that originated from a direction that is close to the viewed direction will be sent back to the viewer). In daylight conditions the light can be coming from many directions, especially on a cloudy day, or from a specific direction not in line with the observer, such as bright sunlight. Under these conditions, a diffuse reflector (which is how the active retroreflector 400 operates with the layer 41 2 in the scattering state) will return more light to the viewer and will therefore appear brighter.

Therefore, in daylight conditions, operating the active retroreflector 400 as a diffuse reflector in this way will make the active retroreflector 400 more conspicuous.

Modulating the absorption electro-optic layer 404 by switching it between a low absorption state and a high absorption state will make the active retroreflector 400 even more conspicuous in daylight when the switchable scattering layer 412 is in the scattering state. Thus, the switchable scattering electro-optic layer 412 can be switched to a scattering state during daylight or bright light conditions and can be switched to a transparent state during low light conditions so that the active retroreflector 400 is conspicuous in both types of light condition, Thus, the active retroreflector 400 has at least two operational modes, a diffuse reflector mode and a retroreflecting mode.

Tn Figure 5, an active retroreflector is shown generally at 500. As in the active reiroreflector too shown in Figure 1, the active retroreflecior 500 comprises a retroreflective layer 502 and an electro-optic switchable absorption layer 504. The switchable absorption layer 504 shown in Figure 5 is a G1-ILC layer. However, in other examples, an alternative electro-optic switchable absorption layer may be used such as a conventional twisted nematic liquid crystal display. The electro-optic switchable absorption layer 504 in the example illustrated in Figure 5 includes first and second portions 506 and 508 respectively, although it is to be understood that the active retroreflector 500 may have more than two portions. Each of the portions 506, 508 may be switchable between a high absorption state, in which light incident on the portion 506, 508 is absorbed, and a low absorption state, in which light incident on the portion 506, 508 is transmitted through that portion 506, 508 of the electro-optic layer 504. In the arrangement shown in Figure 5, the first portion 506 is shown in a low absorption state and the second portion 508 is shown in a high absorption state.

The different portions may be formed by patterning the alignment layer of the GHLC layer so that at zero field the two portions have different zero field alignments. For example, the first portion 506 may a homeotropic zero field alignment and the second portion 508 may have a planar zero field alignment.

Therefore, when the electric field across the G1-]ILC layer is switched to a non-zero field, one of the portions will switch to a different alignment and the other portion will remain in the same alignment as at zero field. For example, if the liquid crystal molecules have positive dielectric anisotropy and therefore align with the electric field, the liquid crystal molecules that have a planar alignment at zero field in portion 508 will be switched to a homeotropic alignment when a non-zero field is applied across the GI-ILC layer.

However as the liquid crystal molecules in portion 506 have a homeotropic alignment at zero field, changing the electric field across the layer to a non-zero field will result in the alignment of the liquid crystal molecules in portion 506 remaining constant.

Alternatively the transparent electrodes of the GHLC layer may be patterned so that the first porLion 506 is at zero field whilsi the second porlion is at non-zero field and vice versa.

In that case, the first and second portions 506, 508 are arranged to be in different states when the active retroreflector 500 is in use. Switching the active retroreflector 500 of Figure 5 by changing the electric field across the electro-optic absorption layer 504 will cause the first portion 506 to move to a high absorption state and the second portion 508 to move to a low absorption state.

The first and second portions 506, 508 may be formed by forming a pattern in transparent electrodes of the electro-optic absorption layer 504 to define the first and second portions 506, 508. The pattern niay by means of standard photolithography and etch techniques.

For example, ITO electrodes niay be coated with a photoresist (e.g. Rohm&Haas SI 813).

The electrodes are then exposed to TJV light through a mask, and the image formed on the TTO is then developed (e.g. using Microposit MF-26A). The ITO is then etched (e.g. with dilute Hydrochloric Acid) and the resist layer is removed to leave a patterning of the TTO, Alternatively, the first and second portions 506, 508 may be separate liquid crystal cells that may be independently switchable.

In some arrangements that include a switchable scattering layer, the switchable scattering layer may also be pattemed in a similar maimer such that it has portions that may be switchable between a transparent state and a scattering state. The patterning may be the same as or different from the patterning on the switchable absorption layer.

Tn sonic arrangements in which the switchable absorption layer and/or switchable scattering layer has two or more portions, the portions may be arranged such that rapidly switching the switchable electro-optic layer produces a simple animation that may give the impression of movement, which may make the active retroreflector more conspicuous or could be used to convey information.

The active retroreflecior 100, 400, 500 may include Iwo or more GHLC layers slacked on top of each other that each include different dichroic dyes so that the active retroreflector can be switched between a plurality of different colours. Including a second switchable guest-host liquid crystal layer may enable the active retroreflector to modulate in more than one waveband to produce different colours. Additionally or alternatively the second G1-ILC layer may be used to produce different patterns, for example one set of patterns on one G1-ILC layer may be produced in the infra-red and a different set of patterns may be produced on a second GHLC layer in a visible waveband.

The active retroreflector 100, 400, 500 may include a motion sensor. The motion sensor may be used to switch on the modulation of the active retroreflector 100, 400, 500 when it detects motion, For example, if the active retroreflector 100, 400, 500 were located on a bicycle, it may automatically tum on when the bicycle starts moving. Further, the active retroreflector 100, 400, 500 may have a plurality of operational modes in which the nature or speed of the switching of the electro-optic layers may vary, The operational mode of the active retro-reflector 100, 400, 500 at a given time may depend on the speed or direction of motion detected by the motion detector.

The active retroreflector 100, 400, 500 may include an ambient light sensor. In an active retroreflector 400 with a scattering layer 412, the ambient light sensor may, for example, be used to switch the active retroreflector between different operational modes, for example by switching scattering layer 412 from its scattering state to its transparent state, when it detects that the ambient light level has fallen to the point where a retroreflecting mode would make the active retroreflector 400 more conspicuous than a diffuse reflecting mode, Alternatively the active retroreflector may switch between other operational modes in response to the ambient light condition such as different animation modes, different colours or between a powered up and a powered down state.

The active retroreflector 100, 400, 500 may be attached to the side of an emergency velucle or may be posilioned Lo inghuight tile Outline of an emergency vehicle.

An advantage of a non-emissive retroreflecting system is very low power consumption.

The active retroreflector 100, 400, 500 may incorporate energy harvesting (e.g. by use of a small area of photovoltaic converter) such that energy could be gathered and stored through normal daytime use, and continuous operation in day/night conditions could be effected without the need for external power sources or replacement of batteries. In alternative embodiments, the active retroreflector 100, 400, 500 may be battery powered, for example by a small button cell battery or the like.

The active retroreflector 100, 400, 500 may additionally include a light source to thrther increase visibility of the active retroreflector I 00, 400, 500. The light source may be, for example an electroluminescent panel or an LED.

Preferably, the active retroreflector IOU, 400, 500 will be formed using thin plastic substrates. For example the thickness of the active retroreflector 100, 400, 500 may be preferably less than 2mm, or more preferably less than 0.75mm. The active retroreflector 100, 400, 500 may be flat or alternatively it may be curved. Tn sonic embodiments the active retroreflector 100, 400, 500 may be a flexible sheet.

The active retroreflector 100, 400, 500 may include attachment formations to permit the active retroreflector 100, 400, 500 to be releasably attached to an object. For example the active retroreflector 100, 400, 500 may include a hook and eye mechanism.

The active retroreflector 100, 400, 500 may include an optical compensation film to improve the optical performance.

Claims

CLAIMS1. An aclive retroreflecior comprising a retrorefleclive layer and a switchable eleciro-optic absorption layer, arranged to cover at least part of the retroreflective layer, wherein the switchable electro-optic absorption layer comprises a guest-host liquid crystal (G1-ILC) layer.
2. An active retroreflector comprising a retroreflector, an electro-optic switchable absorption layer and an electro-optic switchable scattering layer, wherein the electro-optic switchable layers are arranged to cover at least part of the retroreflector,
3. An active retroreflector according to claim 1 or 2 wherein one or both of the electro-optic switchable absorption layer and the electro-optic switchable scattering layer comprises a plurality of portions and wherein one of the portions is arranged to be in a low absorption state when another of the portions is in a high absorption state.
4. An active retroreflector according to any preceding claim further comprising an energy harvesting device for powering the switchable electro-optic layer or layers.
5, An active retroreflector according to claim 4 wherein the energy harvesting device is a photovoltaic cell.
6. An active retroreflector according to any preceding claim, further comprising a light sensor for detecting an ambient light level in the vicinity of the active retroreflector,
7. An active retroreflector according to claim 6, wherein the active retroreflector has a plurality of operational modes and the operational mode of the active retroreflector depends on the detected ambient light level.
8. An active retroreflector according to any preceding claim further comprising a motion sensor.
9. An active retroreflector according to claim 8 wherein the active retroreflector has a plurality of operational modes and the operational mode of the active retroreflector depends on motion detected by the motion sensor,
10. An active retroreflector according to any preceding claim wherein the electro-optic switchable absorbing layer is configured to absorb and/or transmit light in a given waveband,
11. An active retroreflector according to claim 10 wherein the given waveband is in the visible range of the spectrum.
12. An active retroreflector according to claim 10 wherein the given waveband is in the infra-red range of the spectrum.
13. An active retroreflector according to any of claims 2 or 3 to 12, where dependent on claim 2, wherein the switchable absorbing electro-optic layer is a guest-host liquid crystal (GHLC) layer.
14. An active retroreflector according to claim 1 or claim 13 wherein the GHLC layer includes a fluorescent dichroic dye.IS. An active retroreflector according to claim 1, or any of claims 3 to 12 where dependent on claim I, further comprising a switchable scattering electro-optic layer.16. An active retroreflector according to claim 2, claim 15 or any one of claims 3 to 13 where dependent on claim 2, wherein the switchable scattering layer is a polymer dispersed liquid crystal layer.17. An active retroreflector according to claim 2, claim IS or any one of claims 3 to 13 where dependent on claim 2, wherein the switchable scattering layer is positioned between the retroreflective layer and the switchable absorption layer.18. An active retroreflector according to claim 3, or any of claims 4 to 17 where dependent on claim 3, wherein the plurality of portions of the switchable electro-optic layer are produced by patterning electrodes of the switchable electro-optic layer.19. An active retroreflector according to claim 3, or any of claims 4 to 17 where dependent on claim 3, wherein the plurality of portions of the switchable electro- optic layer are produced by patterning an alignment layer of the switchable electro-optic layer.20. An active retroreflector according to claim 3 or any of claims 4 to 19 when dependent on claim 3, wherein the plurality of portions are arranged such that switching the active retroreflector produces an animation.21. An active retroreflector according to any preceding claim comprising two or more switchable guest-host liquid crystal (GHLC) layers.22. An active retroreflector according to any preceding claim further comprising a light source.23. An active retroreflector substantially as hereinbefore described with reference to the accompanying drawings.