GB2489670A

GB2489670A - Temperature dependent smart window

Info

Publication number: GB2489670A
Application number: GB1105147.1A
Authority: GB
Inventors: Philip Mark Shryane Roberts; Allan Evans; Martin David Tillin
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 2011-03-28
Filing date: 2011-03-28
Publication date: 2012-10-10
Also published as: GB201105147D0

Abstract

A window having temperature-dependent spectral characteristics comprises a first liquid crystal layer including a first liquid crystal material (7a, 11, 14) that, at a temperature below a transition temperature, is in a first crystal or liquid crystal state and transmits visible light 1 and infra-red light 2 and that, at a temperature higher than the transition temperature, is in a second liquid crystal state and transmits visible radiation 1 and at least partially reflects infra-red radiation 2. The transition temperature is between 10Â°C and 40Â°C. As the ambient temperature increases the window becomes at least partially reflective to infra-red radiation, and the heat load transmitted through the window falls. No operator action is required as the change in spectral characteristics of the window arises automatically owing to a change in state of the liquid crystal material that is induced by the rising temperature.

Description

Smart Window

TECHNICAL FIELD

This invention relates to materials that control heat flow in buildings and structures. In particular, it relates to a Smart Window' that controls the flow of solar radiant heat into a building, in order to improve thermal comfort and reduce heating/air conditioning costs. Further, it relates to a method to make such a window.

BACKGROUND ART

There is a need to better control solar and internally generated heat in buildings and structures. Transmission of solar radiation through windows provides a significant component of the solar heat load on a building. Controlling how the windows transmit and reflect different spectral components of the incident solar radiation can be used to affect both the light level in the building, and the solar heat load upon it.

Conventional glass is designed to be transparent in the visible part of the electromagnetic spectrum (400-700nm) to allow visible light to pass through it. It also displays significant transmission in the near infra-red (NIR) part of the electromagnetic spectrum (700-2500nm). Solar radiation occurs across this entire range (400-2500nm) so conventional windows can transmit both visible light and near-infra-red heat' from the sun.

In some situations it is desirable to reduce both the amount of solar visible and solar NIR radiation passing through a glazing unit. This has been achieved in the past literature using tinted or mirror' glass. Such materials absorb or reflect a fixed fraction of visible and near infra-red radiation in order to reduce glare and heating from the sun. An example of this type of product is the Reflective Stainless Steel VS glass products available from Viracon.

In other situations it is desirable to pass only the visible part of the incident solar light. Such materials are also described in the prior art and are referred to as "spectrally selective materials". They have a fixed, wavelength dependant

I

transmission. Such materials are usually transparent in the visible range (400-TOOnm) and reflective in the near infra-red range (700-2500nm). An example of this type of product is V-Kool (http://www.vkooLcom) which uses multilayers of silver and conductive oxide to achieve high NIR reflectivity.

Other prior art describes spectrally selective coatings that produce high NIR reflectivity using cholesteric liquid crystals (CLC). US200701 09673 Infrared light reflecting film, 3M is an example of a spectrally selective device that uses an IR reflecting CLC film with low haze (<3%). It includes 2 and 3-layer embodiments to reflect multiple wavelength ranges and half-wave retarder in between layers to reflect both polarisations of the incident light. The wavelength range that this prior art reflects is 880-1 O6Onm or 1300-1 640nm. U56800337 Thermal Insulation coating, Siemensmeyer, BASF, Oct 5 2004 describes an IR cholesteric blocking film to block IR from a light source. It also uses many layers and A/2 plates to reflect both polarisations at multiple wavelengths. It is based on polymerised cholesterics to lock-in structure so provides a fixed response. U57736532 Composition to reduce the transmission of NIR radiation, Silverman Du Pont, Jun 2010 describes non-micellar twisted nematic liquid crystals to reflect NIR, combined with NIR absorbing dyes & nanoparticles. The application for the technology is described as solar control windows although the response is fixed.

U5200201 21625 Temperature measurement and temperature controlled switching based on helical sense dependent liquid crystal phases, Green, New York Polytechnic university, Sept 5 2002 describes a method using chiral dopants with different temperature responses to control pitch of the CLC which is used to sense temperature difference from a pre-selected value. It is used to make an optical switch' which provides visual indication of temperature difference (hotter or colder). U520040201 816 Method and structure for broadening cholesteric liquid crystals spectrum, Chen, Taiwan, Oct 14 2004 describes a method and structure for broadening the reflection spectrum of cholesteric liquid crystals. An electrode structure is added on a side of cholesteric liquid crystals to produce a fringe field.

Still other situations require that the spectral transmission can be modified according to the requirements of the user of the building. Such adaptive techniques are widely described in the past literature, based on electrochromic windows, suspended particle displays and gaschromic devices. However, these adaptive techniques can be power hungry, have limited optical adaptability or are difficult to implement.

Other techniques have also been described. US200301 93709 Switchable electro-optical laminates, Mallya, US, October 16 2003 describes an electrically switchable laminate construction for applications including smart windows and other uses in which light management is desired. The response of the material is determined by the strength of the applied field. US6630974 Super-wide-angle cholesteric liquid crystal based reflective broadband polarizing films, Gallabova, Reveo Inc, Oct 7 2003 describes another electrically controllable laminate, which incorporates a cholesteric liquid crystal film that uses varying pitch helix structures aligned perpendicular to the surface of the film for broadband reflection and transmission of circularly polarized light. One proposed example of a "smart window" contains a switchable ii phase shifter placed between two cholesteric liquid crystal films and, depending on the state of the ii phase shifter, either reflects 100% of the incident light or transmits 50% and reflects 50% of the incident light.

SUMMARY OF INVENTION

Prior art technologies therefore provide fixed or electrically controllable, spectrally selective performance. Fixed performance cannot adapt to changing conditions, and electrically controllable devices require complicated wiring and control circuits. Furthermore, some of the aforementioned prior art provides control of radiation over only a limited wavelength range.

A first aspect of the invention provides a window having temperature-dependent spectral characteristics, the window comprising a first liquid crystal layer including a first liquid crystal material that, at a temperature below a transition temperature, is in a first crystal or liquid crystal state and transmits visible light and infra-red light and that, at a second temperature higher than the transition temperature, is in a second liquid crystal state and transmits visible radiation and at least partially reflects infra-red radiation. The transition temperature is between 10°C and 40°C.

Thus, as the temperature increases above the transition temperature the window becomes reflective, at least partially, to infra-red radiation, and the heat load transmitted through the window falls. No operator action is required as the change in spectral characteristics of the window arises automatically owing to the change in liquid crystal state induced by the rising temperature. Conversely, if the temperature then falls back below the transition temperature, the liquid crystal material returns to the first crystal or liquid crystal state, and the heat load transmitted through the window increases.

The first liquid crystal material may reflect, when in the second liquid crystal state, infra-red radiation having a first polarisation and transmits infra-red light having a second polarisation different from the first polarisation. For example, the first liquid crystal material may reflect, when in the second liquid crystal state, infra-red radiation of one circular polarisation while transmitting IR radiation of the other circular polarisation, eg may reflect right-hand circularly-polarised IR radiation and transmit left-hand circularly-polarised IR radiation, The window may further comprise a second liquid crystal layer for, at a temperature above the transition temperature, reflecting infra-red radiation transmitted by the first liquid crystal material in its second state. This minimizes the amount of IR radiation transmitted by the window at temperatures above the transition temperature.

The window may comprises a retarder disposed between the first liquid crystal layer and the second liquid crystal layer, the second liquid crystal layer containing a liquid crystal material that reflects infra-red radiation of the first polarisation. The first liquid crystal layer reflects infra-red radiation of the first polarisation and transmits infra-red radiation of the second polarisation. The retarder changes the polarisation of the transmitted infra-red radiation, so that it will then be reflected by the second liquid crystal layer. In an embodiment where the liquid crystal material reflects, when in the second liquid crystal state, infra-red radiation of one circular polarisation while transmitting IR radiation of the other circular polarisation, the retarder preferably converts light of one circular polarisation to light of the other circular polarisation.

The second liquid crystal layer may contain the first liquid crystal material. This allows the first and second liquid crystal layers to be the same as one another, simplifying the manufacture of the window.

Alternatively, the first liquid crystal layer may comprise a second liquid crystal material for reflecting, at a temperature above the transition temperature, infra-red radiation of the second polarisation. This allows infra-red radiation of the first and second polarisations to be reflected by a single liquid crystal layer.

The window may comprise a third liquid crystal material for, at a temperature above the transition temperature, reflecting infra-red radiation, the spectral characteristics of the third liquid crystal material at the temperature above the transition temperature being different to the spectral characteristics at that temperature of the first LC material. In general, the reflectivity for infra-red radiation of a liquid crystal material will be wavelength-dependent, and a liquid crystal material will typically not have a high reflectivity across the entire near infra-red wavelength range (usually taken as 700nm to 2500nm). Providing two (or more) different liquid crystal materials which reflect infra-red radiation but have different spectral characteristics to one another increases the wavelength range over which good reflection of infra-red radiation occurs, thereby reducing still further the heat load transmitted by the window. (If the third liquid crystal material reflects radiation of only one polarisation a further liquid crystal layer, or further liquid crystal material, may be provided to reflect radiation in the same wavelength range, but of another polarisation, as reflected by the third liquid crystal material.) The wavelength of peak reflectivity of the third liquid crystal material at the temperature above the transition temperature may be different to the wavelength of peak reflectivity at the temperature above the transition temperature of the first LC material (but in both cases within the infra-red wavelength range).

The first liquid crystal layer may comprise the first liquid crystal material encapsulated in a host material.

The first liquid crystal layer may comprise the first and second liquid crystal materials encapsulated in the host material.

The first liquid crystal layer may further comprise the third liquid crystal material encapsulated in the host material.

The or each liquid crystal material may be a cholesteric liquid crystal material.

The first crystal or liquid crystal state may be a smectic state and the second liquid crystal state may be a cholesteric state.

The first crystal liquid crystal state may be a crystal state and the second liquid crystal state may be a cholesteric state.

The transition temperature of first liquid material from the first liquid crystal to the second liquid crystal state may optionally be in the range 15-30°C, and for example may be in the range 20-25°C or 20-30°C.

A second aspect of the invention provides a use of a liquid crystal material to control radiation entering a structure on the basis of the ambient temperature, the use comprising providing a layer of liquid crystal material in a path of radiation into the structure, the liquid crystal material, when in a first crystal or liquid crystal state, transmitting visible radiation and infra-red radiation and, when in a second liquid crystal state, transmitting visible radiation and at least partially reflecting infra-red radiation, the liquid crystal material being in the first crystal or liquid crystal state or the second liquid crystal depending on whether or not the temperature of the liquid crystal material is below a transition temperature at which the liquid crystal material switches from the first crystal or liquid crystal state to the second liquid crystal state.

The transition temperature at which the liquid crystal material switches from the first crystal or liquid crystal state to the second liquid crystal state may for example be between 10°C and 40°C, or may be between 15°C and 30°C, or may be between 20°C and 25°C.

The invention thus makes it possible for the radiation entering a structure (for example a building), and thus the heatload on the structure, to be controlled on the basis of the temperature, by making use of the property that the optical properties of a liquid crystal are temperature dependent. As the ambient temperature increases the temperature of the liquid crystal layer will also increase, and, once the temperature of the liquid crystal material reaches the transition temperature, the liquid crystal layer will at least partially reflect infra-red radiation thereby reducing the heatload on the structure. However there is little or no change in the transmissivity of the liquid crystal material for visible light.

The focus of this invention is a glazing unit which can respond to the changing conditions according to its internal construction and choice of constituent materials. It should be able to transmit or reflect NIR radiation from the sun in the range 800 to lSOOnm, preferably in the range 700 to 2500nm, according to pre-determined ambient conditions. Within this wavelength range the unit has neither fixed spectral transmission properties nor electrically switchable spectral transmission properties. The responsive properties of the glazing unit are determined by the inherent phase behaviour of its constituent materials.

Referring to Figures 1(a) and 1(b), the constituents of the current invention are chosen in order that the window: 1. Transmits solar visible light I under all ambient conditions 2. Transmits solar NIR 2 when the room temperature is low (figure 1(a)), and rejects (reflects) it when the temperature is above a pre-determined (comfort) level (figure 1(b)).

Moreover, optionally (but advantageously) the window also: 3. Reflects room ambient heat 3 back into the room This invention is related to coatings containing CLCs. One aspect of the invention relates to thermochromic CLCs, another aspect relates to temperature-insensitive CLCs. In either case, the LCs are chosen such that they have a sufficiently long pitch to reflect NIR radiation, but be transparent to visible light. The cholesteric phase transition should occur at predetermined temperatures, for example 20°C.

Below the cholesteric phase the LOs display a smectic phase which is transparent to both visible and NIR radiation. In the cholesteric phase, the LCs reflect NIR radiation to produce the responsive, spectrally selective properties. The film could be used in conjunction with double glazing, more preferably "K GLASS" (TM) (manufactured by Pilkington Group) which reflects radiant heat generated within the room, back into the room. Solar radiation at ground level occurs across the approximate range 400-2500nm, therefore the cholesteric, NIR reflecting smart window is required to reflect in the range 800-l500nm, preferably 700-2500nm.

Aspects relating to point 3 above, the reflection of ambient heat radiation (radiation of wavelength >3 microns) are not specifically addressed by this invention. Techniques for reflection of room ambient heat, ie for reflection of radiation with a wavelength >3 microns, are known, and any suitable technique for reflection of room ambient heat may optionally be applied to a window of the invention Such windows can be used to improve thermal comfort, reduce heat load and reduce requirements for air conditioning in any structure which contains glass or plastic-based glazing units including, but not limited to, buildings, greenhouses, conservatories etc. The location of the active component of the Smart Window glazing unit is important since it affects how the unit responds to ambient conditions. Positioning the CLC film on the interior surface of the exterior pane of a double glazed unit, as shown in Figure 2(a), would provide control linked to the outside temperature, which may be preferred where the Smart Window is used in environments employing air conditioning. By attaching the CLO to the interior glass surface, it is insulated from UV, moisture, temperature cycling etc. Positioning the CLC film on the exterior surface of the interior pane of a double glazed unit, as shown in Figure 2(b), would provide control linked to the internal room temperature, which may be preferred in environments where the internal room temperature is not controlled using other techniques, for example in greenhouses or other buildings without air conditioning.

Since an individual CLC species can only reflect one handedness of circular polarised light, i.e. right hand (RH) or left hand (LH), the final active layer can employ A/2 waveplates to reflect both polarisations of light.

Furthermore, since individual CLC species can only reflect a relatively narrow range of wavelengths, the final active layer can employ numerous layers with different compositions or mixtures of encapsulated LC droplets to display broadband reflection as shown in Figure 3.

To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.

BRIEF DESCRIPTION OF DRAWINGS

In the annexed drawings, like references indicate like parts or features: Figures 1(a) and 1(b): Concept of the Smart Window: In winter (or when outside temperature is less than a pre-determined level, for example 20°C) ALL solar radiation is allowed to pass -visible I and NIR 2; in summer (or when outside temperature is greater than a pre-determined level, for example 20°C) only visible light I is allowed to pass -NIR 2 is reflected. (Optionally in both cases internal ambient heat 3 can be reflected through use of suitable glazing products such as K-GLASS.) Figures 2(a) and 2(b) Location of the active layer of a Smart Window according to this invention. Location of the active layer 5a on the inside of the external pane 4a of the double glazing unit produces a response closely linked to the outside temperature. Location of the active layer Sb on the external side of the inside pane of the double glazing unit 4b produces a response closely linked to the inside temperature Figure 3 Schematic diagram of the construction of a Smart Window according to this invention using multilayer construction and waveplates.

Figure 4 Schematic plot of the desirable temperature response of a thermochromic CLC according to this invention Figure 5 Schematic diagram of the construction of a Smart Window according to this invention using a dispersion of RH reflectors 11 and 12 which reflect wavelengths A1 and A2 (>A1) separated by a retarder plate 13 Figure 6 Schematic diagram of the construction of a Smart Window according to this invention using a dispersion of RH reflectors of wavelength A1 14, LH reflectors of wavelength A1 15, RH reflectors of wavelength A2 16 and LH reflectors of wavelength A2 17.

Figure 7 Peak reflection of Hallcrest UNR28C3W with 4.3wt% SlOll, as a function of temperature, showing NIR reflection in the range 22 to 45°C

DETAILED DESCRIPTION OF INVENTION

The spectral transmission and reflection requirements of the glass according to this invention are shown schematically in Figure 1. In more detail, the glass is required to remain transparent to the whole solar spectrum when the temperature is below the pre-determined level, and become reflective for wavelengths >800nm when the temperature rises above the pre-determined level. The solar spectrum at ground level spans the approximate range 300-2500nm, thus the smart window should ideally reflect in the range 800-2500nm when the temperature rises above the pre-determined level.

This can be achieved using two principal methods which are described in this invention.

The first method uses thermochromic CLCs. The LCs are chosen such that they have a sufficiently long pitch to reflect NIR radiation, but be transparent to visible light. The cholesteric to smectic phase transition should occur at a predetermined temperature, below which the CLCs display a smectic phase which is transparent to both visible and NIR radiation. Final coatings can employ A/2 waveplates to reflect both polarisations of light. They could also employ numerous layers with different compositions or mixtures of encapsulated LC droplets to display broadband reflection as shown in Figure 3. While Figure 3 shows only two layers of reflectors, it is implicit in this invention that any number of layers can be used to produce sufficiently broadband reflection. As an illustrative example, Figure 3 shows a glazing unit 6 that is coated with two pairs of active layers 7a,7b & 8a,8 b and half wave retarder 9. Active layer 7a reflects a first wavelength A1 with RH polarisation back through the glazing unit 6. Retarder 9a converts the transmitted A1 with LH polarisation to RH polarisation, which is reflected by active layer 7b.

Retarder 9a converts the reflected RH back to LH polarisation which is transmitted by active layer 7a and out through the glazing unit 6. Active layers 7a and 7b are transparent to a second wavelength A2 (>Ai), active layers 8a and 8b are reflective to A2, and are used to reflect both polarisations of A2 in a similar manner. Layers 8 are joined to layers 7 directly, or with an additional adhesive layer 10. As discussed above, many layers can be built up to produce broadband reflection.

Figure 4 shows a schematic plot of the desirable performance of one such layer.

At low temperatures the CLC layer displays a smectic phase and is thus transparent to visible and NIR radiation. As the temperature increases, the CLC displays a cholesteric phase and as the temperature is increased through the cholesteric phase the pitch of the cholesteric structure decreases. This results in the LC displaying a shorter wavelength of reflection as the temperatures is increased. It is desirable that away from the smectic to cholesteric phase transition, the temperature dependence of the pitch is as insensitive to temperature as possible. Only one polarisation is reflected from this individual layer, so many layers are used, in conjunction with retarders to produce broadband reflection of both polarisations.

In a variant of the first method, it is desirable to produce the coating in a single layer, rather than by building up many layers. This is achieved using encapsulated CLCs, which can be dispersed in a suitable host polymer. This composite material can then be applied as an ink-like coating or adhered to the windows as a self-supporting film. Figure 5 show an example of this approach which uses a dispersion of RH reflectors 11 and 12 which reflect wavelengths A1 and A2 (>Ai) separated by a retarder plate 13. Figure 6 shows a more desirable approach using a dispersion of RH reflectors of wavelength A1 14, LH reflectors of wavelength A1 15, RH reflectors of wavelength A2 16 and LH reflectors of wavelength A2 17. In both cases, additional encapsulated CLC can be used, with or without additional retarders, to produce broadband reflection of both polarisations.

Example properties of the LC component are -Smectic to cholesteric phase transition in the range 15-30°C, more preferably 20-25°C -Cholesteric to isotropic transition Ch-l at a high temperature which is unlikely to be reached in the normal operating conditions of the window, e.g., >50°C CLCs can be chosen from any cholesteric material which shows the phase behaviour described above. This is usually achieved using mixtures of individual cholesteric species chosen from, but not limited to, fatty acids, esters of cholesterol such as cholesteryl acetate, cholesteryl benzoate, cholesteryl butyrate, cholesteryl caprinate, cholesteryl caprylate, cholesteryl 4-carbaxomethoxybenzoate, cholesteryl chloride, cholesteryl cm namate, cholesteryl 4-cyano cinnamate, cholesteryl decanoate, cholesteryl 3,4-diethoxy benzoate, cholesteryl heptanoate, cholesteryl hexanoate, cholesteryl laurate, cholesteryl myristate, cholesteryl octanoate, cholesteryl oleate, cholesteryl perlaganonate, cholesteryl pentanoate, cholesteryl 3-phenyl propionate, cholesteryl propionate, cholesteryl undecylate, cholesteryl valerate, cholesteryl ceratrate etc. An example mixture of 41.5% cholesteryl nonanoate, 41.5% cholesteryl chloride and 17% cholesteryl isostearyl carbonate shows a reflection at 8SOnm. Other materials are commercially available such as Hallcrest's UNR28C3W, UNR35C3W Spectral properties are modified either through selection of mixtures or through addition of a chiral additive. Chiral additives are chosen from commercially available additives such as SlOll or other as described in the prior art. Other examples of chiral dopants that can be used to induce the desired helical structure are S811 for a left hand' twist, and RIOlI, R811, CBI5 and C15 (all from E Merck) for a right hand' twist. Many other materials could be used to achieve the desired effect and would be well known to anyone skilled in the art.

Ink-like films can be made by dispersing encapsulated LC materials in a suitable transparent polymer host. (An "ink-like" film is a film that could be applied using a suitable printing method such as, for example, gravure printing, offset printing, screen printing or other known printing methods (including ink-jet printing) or that could be applied using other coating methods such as spray coating, dip coating, spin coating, roller coating, doctor blade coating or another known coating method.) The polymer host should be stable to UV, moisture and temperature cycling. Many suitable polymers are suitable for this application including a wide range of solvent-based and water-based systems such as acrylics, polyesters, epoxies, and urethanes. An exemplary example of this approach uses an aerospace grade, UV-stable 2-pack polyurethane coating such as Bayer's Desmophen 850 2-pack polyurethane. Other water-based binders can be used, for example the lncorez W2000 range of polyurethane-acrylic polymers available from Industrial Copolymers Ltd. Free-standing films can be made by dispersing the encapsulated materials in a suitable transparent polymer host. The polymer should also be stable to UV, moisture and temperature cycling and can be chosen from the common thermoplastics such as acrylics, polyesters, epoxies, urethanes, polystyrene acrylonitrile butyl styrene and other polyoefins polymers such as polyethylene, polypropylene or cyclic olefin copolymers.

The second method uses "temperature insensitive" cholesterics, i.e. materials with crystal -cholesteric transition rather than smectic -cholesteric transition. These show single colour reflection, irrespective of temperature. Example properties of this type of LC component are: -X-Ch (crystal to cholesteric) phase transition in the range 15-30°C, more preferably 20-25°C -Ch-l (cholesteric to isotropic phase transition) at a high temperature which is unlikely to be reached in the normal operating conditions of the window, e.g., >50°C This second method can also use multilayer and single layer techniques or use numerous encapsulated CLCs dispersed in a suitable polymer host. Materials are selected from those described above or from other formulations which produce temperature insensitive performance, such as the additives described in US200201 87281 Chiral Additives for Cholesteric Displays, Doane, December 2002. A commercially available example of a temperature-insensitive CLC is Hallcrest BNR5OC.

Both first and second methods can apply the CLCs in their neat, as-synthesized/mixed form, contained within a lower substrate and an upper superstrate, or applied to a substrate without the containing superstrate. They can also be dispersed into a host carrier, which can take the form of a liquid solvent or solid binder'.

Alternatively, the CLCs can be first encapsulated before being dispersed in the host carrier. This encapsulation process is well known in the prior art and is described for example in patent US2800457.

Although the invention has been shown and described with respect to a certain embodiment or embodiments, equivalent alterations and modifications may occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a "means") used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.

Advantages The advantages of this invention over the prior art are -No power is required to be supplied to the unit to change the spectral properties so no wiring etc is required. The coating is therefore easy to implement in the form of a coating or window film -The spectral response of the active layer is chosen to minimise additional heating and/or cooling of the building since it allows heat to pass in cold conditions, and reflects it in hot conditions -There is no absorption to reject NIR, the rejected NIR is reflected. This will improve longevity of the device Embodiment I This embodiment of the invention uses CLC available from Hallcrest, product name UNR28C3W. It is mixed with chiral dopant SlOll at a concentration 4.3% w/w. This formulation is capillary-filled into a 10 micron thick glass cell. It displays a smectic to cholesteric transition at 24°C and a cholesteric to isotropic transition at 42°C. Above 24°C, the peak reflection shifts from 11 5Onm (24°C) to 730nm (40°C) as shown in Figure 7. This material reflects only one polarisation of incident light Embodiment 2 This embodiment of the invention uses Hallcrest UNR28C3W with 4.3% SlOil.

It is filled into 2 separate cells which are separated by a A12 plate. This material has the same temperature response of Embodiment 1, but reflects both polarisations of incident light Embodiment 3 This embodiment of the invention uses 2 mixtures based on Hallcrest UNR28C3W with 4.3 and 5.5% 51011. Two layers are applied with a A/2 plate to reflect both polarisations. The additional layer containing 5.5% Si Oil broadens the reflection band of the material such that it reflects at i600nm & iiSOnm (at 24°C) and ii OOnm & 730nm (at 40°C) Embodiment 4 This embodiment uses encapsulated, temperature-insensitive LCs. LH and RH mixtures are chosen, for example Hallcrest BNR5OC modified using the methods described in, for example, U520020187281 to produce the required handedness and phase behaviour. The CLCs are then microencapsulated according to the process described in U S2800457. The resulting encapsulated CLCs are then dispersed in a suitable host binder' such as Industrial Copolymers Incorez W2000 water-based polyurethane-acrylic by gentle mixing. The mixture is then draw-coated onto a glass substrate and the binder is allowed to dry to produce a Smart Window film.

INDUSTRIAL APPLICABILITY

This invention is relevant to any structure containing large areas of glazing, e.g. office blocks, houses and greenhouses. It is very relevant to use in greenhouses since demands for optical clarity are lower.

Claims

CLAIMS1. A window having temperature-dependent spectral characteristics, the window comprising a first liquid crystal layer including a first liquid crystal material that, below a transition temperature, is in a first crystal or liquid crystal state and transmits visible light and infra-red light and that, above the transition temperature, is in a second liquid crystal state and transmits visible radiation and at least partially reflects infra-red radiation; wherein the transition temperature is between 10°C and 40°C.
2. A window as claimed in claim 1 wherein the first liquid crystal material reflects, when in the second liquid crystal state, infra-red radiation having a first polarisation and transmits infra-red light having a second polarisation different from the first polarisation.
3. A window as claimed in claim 1 or 2 and comprising a second liquid crystal layer for, at a temperature above the transition temperature, reflecting infra-red radiation transmitted by the first liquid crystal material in its second state.
4. A window as claimed in claim 3 wherein the window comprises a retarder disposed between the first liquid crystal layer and the second liquid crystal layer, the second liquid crystal layer containing a liquid crystal material that reflects infra-red radiation of the first polarisation.
5. A window as claimed in claim 4 wherein the second liquid crystal layer contains the first liquid crystal material.
6. A window as claimed in claim 2 wherein the first liquid crystal layer comprises a second liquid crystal material for reflecting, at a temperature above the transition temperature, infra-red radiation of the second polarisation.
7. A window as claimed in any preceding claim and comprising a third liquid crystal material for, at a temperature above the transition temperature, at least partially reflecting infra-red radiation, the spectral characteristics of the third liquid crystal material at the temperature above the transition temperature being different to the spectral characteristics at the temperature above the transition temperature of the first LC material.
8. A window as claimed in claim 7 wherein the wavelength of peak reflectivity of the third liquid crystal material at the temperature above the transition temperature is different to the wavelength of peak reflectivity at the temperature above the transition temperature of the first LC material.
9. A window as claimed in any preceding claim wherein the first liquid crystal layer comprises the first liquid crystal material encapsulated in a host material.
A window as claimed in claim 9 wherein the first liquid crystal layer comprises the first and second liquid crystal materials encapsulated in the host material.
11 A window as claimed in claim 9 or 1 0 wherein the first liquid crystal layer further comprises the third liquid crystal material encapsulated in the host material.
12. A window as claimed in any preceding claim wherein the or each liquid crystal material is a cholesteric liquid crystal material.
13. A window as claimed in claim 12 wherein the first crystal or liquid crystal state is a smectic state and the second liquid crystal state is a cholesteric state.
14. A window as claimed in claim 12 wherein the first crystal or liquid crystal state is a crystal state and the second liquid crystal state is a cholesteric state.
A window as claimed in any preceding claim wherein the transition temperature is in the range 15-30°C.
16. A window as claimed in claim 1 5 wherein the transition temperature is in the range 20-25°C.
17. Use of a liquid crystal material to control radiation entering a structure on the basis of the ambient temperature, the use comprising providing a layer of liquid crystal material in a path of radiation into the structure, the liquid crystal material, when in a first crystal or liquid crystal state, transmitting visible radiation and infra-red radiation and, when in a second liquid crystal state, transmitting visible radiation and at least partially reflecting infra-red radiation, the liquid crystal material being in the first crystal or liquid crystal state or the second liquid crystal depending on whether or not the temperature of the liquid crystal material is below a transition temperature at which the liquid crystal material switches from the first liquid crystal state to the second liquid crystal state.