The invention relates to an optical element and a picture display apparatus provided with said optical element.

WO 98/30923 (PHN 16165) describes an optical element comprising a substrate provided with a transparent coating which contains a photochromic material, with the light transmission of said transparent coating in the visible wavelength range being dependent on the intensity of light which is locally incident on the transparent coating. The transmission of the selectively transparent coating decreases automatically as the intensity of the incident radiation increases.

The transparent coating comprises an inorganic network of silicon oxide and an organic polymer which is chemically bonded to the inorganic network via Si-C bonds. In general, such optical elements comprising a photochromic material are manufactured via a process known as the wet-chemical sol-gel route. A sol-gel process is a process in which, by the (controlled) addition of water, a solution of alkoxysilane in alcohol, in succession, hydrolyzes (homogeneously) and (poly) condenses, thereby forming a (porous) inorganic network of silicon (di) oxide. Subjecting it to a thermal treatment, in which the formation of the silicon oxide is completed, yields a dense inorganic network.

In a preferred embodiment, the transparent coating is provided on a display window of a display device. Alternatively, the transparent coating may be used on sunglasses.

Such photochromic coatings provide a compromise between the switching behavior and the scratch-resistance. It is a drawback that this prior-art optical element is not optimized for a quick coloration/decoloration of the photochromic material used in the transparent coating.

It is, inter alia, an object of the invention to provide an optical element having an improved performance with respect to the coloration/decoloration rate of the optical element.

To this end, a first aspect of the invention provides an optical element, which is characterized in that the photochromic material is at least partially surrounded by an organic polymer which has a glass transition temperature Tg below 40 degrees Celsius. A second aspect of the invention provides a picture display apparatus comprising such an optical element. Advantageous embodiments are defined in the dependent claims.

As described in WO 98/30923, the polychromic material in the known optical element is incorporated in a (purely) organic network, and in WO 98/30923, the photochromic material in the optical element is incorporated in a hybrid network comprising a silicon oxide and polymers.

It was found that a special class of polymers, namely organic polymers having a Tg below 40 degrees Celsius, yielded an excellent switching behavior of the optical element when said polymers at least partially surrounded the photochromic material.

The crux of the invention is that the direct environment of the photochromic material is sufficiently mobile to ensure a quick coloration/decoloration of the photochromic material. An approach to obtaining such a mobile direct environment of the photochromic material is presented in the previous paragraph. It should be noted that the photochromic material may be a photochromic dye dispersed on a molecular level, such that the photochromic dye is at least partially surrounded by an organic polymer having a Tg below 40 degrees Celsius. Moreover, the organic polymer may be a part of a (purely) organic network or it may be a part of a hybrid network as described in WO 98/30923.

The glass transition temperature Tg is the temperature marking the transition between a glassy state and a rubbery state of the organic polymer. Below Tg, there is no motion of the polymer chains because the average energy of the polymer chains is too low to overcome the rational energy barrier of the polymer backbone, so that they are essentially frozen into a solid condition known as the glassy state. As the polymer is heated, however, the average energy of the polymer chains increases and will become high enough to overcome the rational energy barrier of the polymer backbone, resulting in freedom of motion of the polymer, and a transition occurs from the glassy state to a rubbery state. The onset of the rubbery state is indicated by a marked increase in volume, caused by the increased molecular motion. The temperature at which this occurs is called the glass transition temperature. Above Tg, the polymer is in the rubbery state and the freedom of motion of the polymer yields a sufficiently mobile environment of the photochromic material which is dispersed on a molecular level in the polymer in the rubbery state to allow a quick coloration/decoloration of the photochromic material.

The organic polymer preferably has a glass transition temperature Tg below 10 degrees Celsius. This measure ensures that the organic polymer is in the rubbery state at room temperature. The freedom of motion of the organic polymer chains above Tg yields a large mobility of the photochromic material and facilitates a quick coloration/decoloration of the photochromic material.

In an embodiment as defined in claim 3, the organic polymer is a polymer formed from the group of (meth) acrylates, di (meth) acrylates, styrene derivatives, vinyl ethers, urethanes and epoxides. The properties of the final photochromic coating, such as the Tg, can be tuned by the chemical composition of the polymer backbone and/or side chains.

In an embodiment as defined in claim 4, the transparent coating comprises core-shell particles having a core comprising the organic polymer and the photochromic material, and having a shell of cross-linked polymer for protecting the core. The core is optimized to improve the mobility of the photochromic material by using an organic polymer which has a glass transition temperature Tg below 40 degrees Celsius, preferably below 10 degrees Celsius. The main function of the shell is to maintain the integrity of the particles, to protect the photochromic material against reactive species that may occur in the transparent coating, particularly during processing of the transparent coating, and to make the particles compatible with the transparent coating. The shell preferably consists of a polymer which has a high Tg. A core-shell latex is preferably used. The core-shell morphology is described in a publication by D. J. Williams, published in Polymer Science and Engineering (Prentice-Hall Inc., Englewood Cliffs, New Jersey, 1971).

The crux of this embodiment is to create, in a matrix that is rigid (for example a polymer with a Tg above 50 degrees Celsius), micro-environments for the photochromic material, which micro-environments have a sufficient mobility to ensure a quick coloration/decoloration of the photochromic material. These micro-environments may be provided while using the core-shell particles as described above, which are dispersed in a rigid matrix.

The transparent coating must be non-scattering for light in the ultra-violet (UV) and visible (VIS) wavelength range. Therefore, the particles should be either small (below approximately 300 nm, preferably below 100 nm) or have a refractive index close to that of the other materials of the transparent coating (difference in refractive index between the particles and the other materials of the transparent coating is less than 0.1, preferably less than 0.01).

Furthermore, the particles should be compatible with the other materials of the transparent coating, which means that the surface of the particles should be rather polar when the transparent coating is manufactured while using sol-gel type systems, and should be rather a-polar when the transparent coating is manufactured while using organic reactive monomers, such as (meth) acrylates.

Another way of ensuring the local mobility of a photochromic material in an otherwise rigid matrix is to incorporate the photochromic material in the cavities of dendrimers or hyperbranched polymers before mixing it with the monomer mixture and subsequent polymerization as in the embodiment defined in claim 5. The branches of dendrimers and hyperbranched polymers have freedom of motion at the practical temperatures of approximately-10 degrees Celsius and higher, and therefore dendrimers and hyperbranched polymers are flexible, a situation that is comparable to a polymer in the rubbery state. This is described in detail in a publication by Jansen et. al., published in Science (J. F. G. A. Jansen, E. M. M. de Brabander-van den Berg, E. W. Meijer, Science, Vol.

266,18 November 1994,"Encapsulation of Guest molecules into a Dendritic Box"). In the resulting (possibly rigid) polymer matrix, the photochromic material is in a mobile micro- environment due to the fact that it is encapsulated by the flexible dendrimer or hyperbranched polymer.

The optical element as defined in claim 6 arranges the transparent coating between the substrate and a protective layer for protecting the transparent coating against abrasion. For some applications of the optical element, where the transparent coating may be subjected to environmental influences, for example to abrasion, scratching or scraping, the transparent coating of the invention must be protected. Especially when a substantial fraction of the transparent coating consists of the organic polymer, the transparent coating may have a poor mechanical strength with respect to hardness, resistance to wear and scratch resistance.

The protective layer preferably comprises a silicon-oxide coating or a glass plate.

Alternatively, the transparent coating further comprises an inorganic network of silicon-oxide, as in the optical element defined in claim 8. The photochromic material, which is at least partially surrounded by the organic polymer of the invention, is incorporated, in the transparent coating, in an inorganic network Alternatively, the inorganic network of silicon-oxide may form a combined inorganic/organic network with the organic polymer as described in WO 98/30923. This yields an optical element having a transparent coating which demonstrates a good switching behavior and is abrasion and scratch-resistant.

The picture display apparatus as defined in claim 9 uses the optical element as defined in claim 1 wherein the transmission of the optical element decreases as the intensity of the incident radiation increases, so as to improve the contrast of the image produced. The optical element of the invention influences the intensity of both the reflected ambient light and the light originating from the internal light source (s) of the picture display apparatus, for example from the phosphors in a cathode ray tube (CRT), from the plasma discharge in a plasma-display panel (PDP) or from the electroluminescent material in a LED display panel, such as an organic LED display panel. The incident ambient light passes through the optical element and is reflected, for example at the CRT phosphors or color filters, whereafter the reflected light again traverses the optical element. If the transmission of the optical element is T, the intensity of the reflected ambient light decreases by a factor of T2. However, the light originating from the internal light source (s) traverses the optical element only once, so that the intensity of this light decreases only by a factor of T. The combination of these effects causes an increase of the contrast by a factor of T'1.

A picture display apparatus as described above may be manufactured by means of a lamination process as described in the International Application PCT/IB00/11385 (PHN 17.765) yielding a display window and, in front of the display window, an auxiliary transparent plate (preferably a glass plate), which display window and auxiliary plate are attached to each other by a curable material. The photochromic property can be obtained by mixing the photochromic material with the resin needed in the lamination process. The resin in the lamination process is preferably an organic polymer with a Tg below 40 degrees Celsius, preferably below 10 degrees Celsius. Alternatively, the resin in the lamination process forms a rigid matrix comprising micro-environments for the photochromic material having a sufficient mobility to ensure quick coloration/decoloration of the photochromic material, for example by using core-shell particles, dendrimers or hyperbranched polymers, as described above. Because of the auxiliary plate, the display window has a very good scratch-resistance. When the display apparatus is a Cathode Ray Tube (CRT), an intrinsically implosion-safe CRT tube is obtained when using the lamination process.

The picture display apparatus as defined in claim 10 has an optical element wherein the transmission is dependent on incident radiation having a wavelength outside the emission rage of the display panel. As a result, the transmission of the optical element is not influenced by light transmitted by the picture display apparatus itself.

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

In the drawings: Fig. 1 shows an optical element provided with a transparent coating in accordance with the invention, which optical element may be provided at the viewing side of a display panel, Fig. 2 shows the interconversion of the closed (CF) and open (OF) form of 2,2-diphenyl-2H-benzo [h] chromene, in which the open form can be represented by two mesomeric structures: a dipolar zwitterionic form with localized charges and a less polar polyenic or quinonic form, Fig. 3 shows the time-resolved absorption spectrum of the open form of a naphthopyran upon switching on and off, Fig. 4 shows the chemical structure of PolyEthyleneGlycolDiMethacrylate, Fig. 5 shows a core-shell particle, Fig. 6 shows a fifth generation poly (propylene imine) dendrimer, Fig. 7 shows an embodiment of the invention where the transparent coating is provided with a protective layer, Fig. 8 shows a display device, in this case a CRT, having a separate optical element of the invention in the light path of the display device, Fig. 9 shows a display device, in this case a CRT, having an optical element of the invention laminated onto the display panel, Fig. 10 is a schematic representation of a laminated CRT screen; due to the non-flatness of the CRT screen, the thickness of the polymer layer is not constant, Fig. 11 shows the absorption spectrum of a laminate consisting of two 1 mm thick glass plates and a 1 mm thick poly (2-ethylhexyl methacrylate) (poly (EHMA)) layer containing 0.11% by weight of Photosol7-114, Fig. 12 shows the absorption spectra of Photosol7-114 in a 100 % EHMA matrix (I) and in a copolymer matrix comprising 20 wt. % MAA and 80 wt. % EHMA (II), and Fig. 13 shows the absorption spectra of Photosol7-114 in the colored form in a 100 % EHMA matrix (A) and in an EHMA/Bisphenol-A (90/10) matrix (B).

The Figures are purely schematic and not drawn to scale. In particular for clarity, some dimensions are exaggerated strongly. In the Figures, like reference numerals refer to like parts, whenever possible.

Optical elements for varying the transmission of light are used to influence the transmission and/or reflection of (visible) light, for example, of lamps, rear view mirrors and sunroofs in cars, or of windows for buildings ("smart windows") or of spectacle lenses.

Said optical elements are also used at the viewing side of display windows of (flat) display devices, such as cathode ray tubes (CRTs), plasma-display panels (PDPs), electroluminescent display panels, such as organic LED display panels, and liquid-crystal display devices (LCDs, LC-TV's or plasma-addressed LCDs) to improve the contrast of the image reproduced.

Fig. 1 shows an optical element comprising a substrate 1 provided with a transparent coating 2 in accordance with the invention, which optical element may be provided at the viewing side of a display panel. A light spot 3 originating from the surroundings of the display device is incident on a part of the optical element. Radiation Ls, which causes the light spot 3, may originate, for example, from sunlight entering directly or indirectly through one or more window glasses or otherwise, and being incident on a part of the optical element. Said light spot 3 may also originate from another radiation source in the vicinity of the optical element, for example a lamp. In Fig. 1, a circular light spot 3 is shown.

Said light spot 3, however, may have any shape and be incident on a part of the optical element or on the entire optical element. The light spot 3 may also comprise a number of light spots. In particular, the intensity of light spot 3 may be different at different locations on the optical element.

In the case of an optical element provided at the viewing side of a display panel, the contrast of the image reproduced on the display window of the display device is reduced substantially at the location of the light spot 3, due to the intensity of the light spot 3.

One of the properties of the transparent coating 2 is that the transmission of the coating decreases automatically at the location of the light spot 3 when the intensity of the light in the light spot 3 increases, so that the contrast of the display device is increased at the location of said light spot 3. Another property of the transparent coating 2 is that the transmission of the coating increases automatically (again) at the location of the light spot 3 when the intensity of the light of the light spot 3 decreases (again) or when the light spot on the display window disappears. In general, the transparent coating reacts in a reversible manner to variations in the light intensity of (ambient) light incident on the coating.

For using the optical element at the viewing side of a display panel, the dynamic glass transmission range of the optical element should preferably be between 30 % and 70 %, depending on the amount of daylight illumination. In general, the display panel is

placed in an environment which provides a more or less uniform illumination of the optical element, resulting in a coloration/decoloration of the whole optical and element yielding a high-quality image.

Switching times of less than 10 minutes are preferred. The photochromic material should preferably respond only to light with wavelengths between 320 nm and 400 nm. These boundaries are imposed by the fact that a front glass panel is not transparent below 320 nm, and that the optical element should not respond to the light emitted by the blue light emitted by the display panel (-400 nu). In terms of sensitivity, the optical element should preferably respond to daylight with illumination levels of 300 lux or more. The optical element should preferably become neutrally colored, preferably gray, since a gray film will not change the colors of the light emitted by the display panel. Moreover, the optical element should have a long lifetime. The service life should preferably exceed 5 years under indoor illumination conditions.

The color of the optical element is very important, because it affects the colors of the picture on the display panel. The color of the optical element can be expressed in two coordinates x and y, referred to as the color point in the C. I. E. chromaticity diagram. For a given light source, the color point can be calculated from the transmission spectrum of the optical element. For the observed color of a bright segment on the display panel, the light emitted from the light sources in the display panel, for example the phosphors in a CRT, has to pass through the optical element only once. For the observed color of a'black'segment, the light source is the ambient light that has to pass through the optical element twice.

Therefore, for the color of a bright segment, the color points for single transmission of the optical element are important, whereas for the color of a black segment, the color points for double transmission are important. For using the optical element at the viewing side of a display panel, the desired color of the optical element is approximately gray, i. e. the optical element more or less equally absorbs the light between 380 and 780 nm. The color points of such a gray optical element are between 0.30 and 0.37 for x and y, and preferably between 0.31 and 0.35 for x and y with respect to white E-light, and for single and double transmission. A coating having such a color point has little influence on the colors of the picture displayed by the display panel.

Photochromic materials are compounds that, upon irradiation, can (reversibly) change from one state to another state with a distinguishably different absorption spectrum There is a convention that the initial state of a photochromic material absorbs at a shorter wavelength than the photoproduct. Since the inducing radiation as well as the changes in the

absorption spectra are often in the UV-VIS region, photochromism is usually visible as a color change of the compound. For our application we are interested in photochromic materials that turn from a colorless to a colored state. Whereas the forward reaction (coloration) is by definition induced by light, the reverse reaction (decoloration) can be induced by light or heat, or a combination of both.

A large number of photochromic materials are known from literature, which materials can be divided into several classes. Especially for use in display devices, the optical element may comprise a material from the class of spiropyranes, spiro-oxazines, fulgides or naphthopyrans. An example of a photochromic material from the class of spiropyranes is 6- nitro-8-methoxy-1', 3', 3'-trimethyl-spiro [2H-l] benzopyrane-2,2'-indoline. An example of a photochromic material from the class of spiro-oxazines is 1, 3,3-trimethylspiro [indoline-2,3'- [3H] napth [2,1-b] [1, 4] oxazine]. An example of a photochromic material from the class of naphthopyrans is 2,2-diphenyl-2H-benzo [h] chromene, which is given in Fig. 2. Because of their higher photostability, spiro-oxazines and naphthopyrans are more suitable for practical applications such as photochromic coatings than spiropyranes.

The absorption of UV light by such a naphthopyran molecule in solution leads to cleavage of the indicated carbon-oxygen bond. In this open form, conjugation is possible between the two previously isolated s-systems. This leads to the appearance of a broad absorption band in the visible region (in addition to the absorption bands in the UV region) and a concomitant color change of the molecule. The structure of the so-called open form (OF) is similar to that of a merocyanine and therefore the open form is referred to as the photomerocyanine. The open form can be represented by two mesomeric forms: a dipolar zwitterionic form with localized charges and a less polar polyenic or quinonic form. It is therefore not surprising that the open form of naphthopyrans is highly solvatochromic, i. e. the absorption spectrum depends on the polarity of the solvent. Since the closed form is thermodynamically more stable than the open form, the ring closing reaction is a thermally activated process and occurs spontaneously. An example of a time-resolved absorption spectrum of the open form is given and a model for the description of the coloration and decoloration process is shown in Fig. 3. The closed form (CF) of the naphthopyran is excited by UV light (CF*) and this may either induce the cleavage of the carbon-oxygen bond resulting in the open form (OF) or a competing deactivation process back to the ground state (CF). Due to the spontaneously occurring reverse reaction, an equilibrium between the colorless closed and the colored open form is reached after a certain time, which is referred to as the photostationary state. At this equilibrium, the ring-opening rate is equal to the ring-

closing rate. When the irradiation is switched off again, the photomerocyanine starts to decolor. The OF can exist in different conformational and configurational isomers. The presence of different isomers of the open form is believed to be responsible for its relatively broad absorption band.

In order to obtain an optical element according to the invention, the photophysical properties of the photochromic material must be preserved during the application of the transparent coating. The photochromic material is, preferably, at least partially surrounded by an organic polymer. In a particular embodiment, the organic polymer is the main ingredient of the transparent coating. It was found that with an increasing Tg of the organic polymer and thus an increasing rigidity of the organic polymer, the coloration and decoloration reaction of the photochromic material become slower. Thus the physical properties, particularly the Tg, of the organic polymer surrounding the photochromic material are important for the preservation of the photophysical properties of the photochromic material.

For example, the half-time of the decoloration of Photosol7-114 in poly (poly (ethyleneglycol) dimethacrylate) 330 (the monomer PEGDMA330 has an average molecular weight of 330 g/mol) which has a Tg of 123 degrees Celsius is more than 10 minutes. In contrast, the half-time of the decoloration of Photosol7-114 in poly (poly (ethyleneglycol) dimethacrylate) 550 (the monomer PEGDMA550 has an average molecular weight of 550 g/mol) which has a Tg of 13 degrees Celsius is less than 2 minutes.

The chemical structure of the monomers PEGDMA is depicted in Fig. 4. With reference to Fig. 4, the difference between PEGDMA330 and PEGDMA550 is the chain length of the monomers; for PEGDMA330, n is 4 on average, and for PEGDMA550, n is 9 on average.

The term Tg is defined here as the temperature of maximum mechanical loss (tan timaX) obtained from Dynamical Mechanical Thermal Analysis (DMTA) measurements using, for example, a Rheometric Scientific DMTA apparatus. The measurements were preferably performed in the 3-point bending-mode with a heating rate of 2° C/min at a frequency of 1 Hz. The dimensions of the sample were 40x 10x 1 mm.

The organic polymer is, preferably, an optically clear polymer material, such as poly (meth) acrylates, poly (di (meth) acrylates), poly (styrene) derivatives, poly (vinyl ethers), poly (urethanes), poly (epoxides). The polymerization reaction can be carried out by a radical polymerization process in which the radical species are formed photochemically, thermally or by means of an e-beam.

Photopolymerization is favorable because it is a fast and versatile method. For photopolymerization in the presence of the photochromic dye, the polymer system and polymerization conditions have to be compatible with the dye or else the photochromic properties will be (partially) lost. An example is the photopolymerization of acrylates in the presence of the naphtopyran Photosol7-114, which was provided by PPG Industries. It was found that, in general, the photopolymerization conditions of acrylates are not compatible with the naphtopyran Photosol7-114 : the acrylate radicals generated in the polymerization process are very mobile and therefore too reactive and destroy the photochromic material during polymerization. Furthermore, it was found that in its open form, Photosol7-114 is much more susceptible to attack by radicals than in its closed form. Using UV light with wavelengths below 380 nm to induce the generation of radicals often results in the loss of the desired photochromic properties. Therefore, photoinitiators with an extinction coefficient at wavelengths above 400 nm should be used. Much better results, i. e. less loss of photochromic activity, have been obtained with the photopolymerization of methacrylates and when an irradiation wavelength was used that leaves the photochromic material in its (more stable) closed form. For example, for Photosol7-114 this is a wavelength above 405 nm.

As mentioned before, for fast switching times, polymer systems with moderate to low Tg's are required. However, it was found that upon photopolymerization more photochromic activity was generally lost in low Tg systems than in higher Tg systems. This was caused by the higher mobility of the generated radicals in low Tg systems compared to high Tg systems.

By incorporating the photochromic dye in a dendrimer or hyperbranched polymer or core-shell particle prior to the polymerization process, the photochromic dye is shielded from the radical species generated in the polymerization process, which results in the conservation of the photochromic properties.

Alternatively, the photochromic material, which is at least partially surrounded by an organic polymer, may be introduced in the transparent coating in the form of polymer particles. Polymer particles, which can be fine-tuned in properties such as size, viscosity and chemical composition, can be loaded with one or more materials with functional properties such as photochromic materials, optical materials (to fine-tune properties such as color, refractive index, luminescence), and photostabilizers. The materials thus obtained can be introduced into the transparent coating by making use of polymer particles as carriers for the functional materials. The environment of these compounds can be optimized with minimal interference with the properties of the transparent coating. Hence, it may be possible to

realize dynamic processes even in coatings with a good scratch resistance, like"hard"sol-gel based coatings. Moreover, the stability of the functional materials can be improved, for example, by protecting them from reactive species like radicals present in polymerization reactions and reactive species like radicals generated by photo or thermal processes, as would be required when the photochromic materials are introduced in reactive monomer mixtures.

In a preferred embodiment of the invention, the transparent coating comprises core-shell particles comprising a core of organic polymer and photochromic material, and a shell of cross-linked polymer for protecting the core (see Fig. 5). More particularly, the photochromic material is applied as dopant in the core of a core-shell latex.

For example, a core of poly (2-ethylhexyl acrylate) (PEHA) is prepared with non ionic surfactants like PS-PEO block-copolymers. This latex is filtered and dialyzed to remove all aggregated latex particles and the excess of surfactants. Subsequently, the photochromic material is incorporated in this core by a swelling procedure. To this end, the photochromic material is dissolved in an appropriate organic solvent (with our without surfactants) and this solution is then added to the latex. The resulting mixture is stirred for a few days. Subsequently, the latex is filtered, dialyzed and filtered again to remove all the precipitated photochromic material, aggregated latex particles and the excess of surfactants (if present). After the incorporation of the photochromic material in the core, a shell of cross- linked polystyrene (PS) is prepared at an appropriate pH and without the addition of extra surfactants. The core-shell particle thus prepared containing the photochromic material is filtered again and then freeze-dried. After freeze-drying, the core-shell particles are dried in an oven under reduced pressure at 50 degrees Celsius. The core-shell particles can be easily re-dispersed in organic solvents or liquid meth (acrylic) monomers when non-ionic surfactants are added to the core-shell latex and appropriate pH adjustment.

In another example, a core of PEHA is prepared with non-ionic surfactants like PS-PEO block-copolymers. The core-latex is filtered and dialyzed to remove the aggregated latex particles and the excess of surfactants. Subsequently, a shell of cross-linked PS is prepared without any added surfactants and at an appropriate pH. The core-shell latex is filtered. Afterwards, the photochromic material is incorporated in the core-shell latex by a swelling procedure. To this end, the photochromic material is dissolved in an appropriate organic solvent with or without surfactants, and this solution is added to the core-shell latex.

The resulting mixture was stirred for a few days. The core-shell particles thus prepared, containing the photochromic material are filtered again and then frieze-dried. After freeze- drying, the core-shell particles are dried in an oven under reduced pressure at 50 degrees

Celsius. Again, these core-shell particles can be easily re-dispersed in organic solvents or liquid (meth) acrylic monomers when non-ionic surfactants are added to the core-shell latex and appropriate pH adjustment.

Another alternative to surrounding the photochromic material by an organic polymer is to encapsulate the photochromic dye in dendrimers or hyperbranched polymers that are flexible and ensure the local mobility of the dye. An example of a dendrimer is shown in Fig. 6. An example of the incorporation of dyes in dendrimers is described in a publication by J. F. G. A. Jansen et al. ("Encapsulation of guest molecules into a dendritic box", Science, Vol. 266,18 November 1994, pages 1226-1229). An example of the incorporation of dyes in a hyperbranched polymer is described in a publication by Schmaljohann et al. ("Blends of amphiphilic, hyperbrached polyesters and different polyolefins", Macromolecules, Vol. 32,1999, pages 6333-6339).

A method of manufacturing the optical element of the invention is to use a reactive mixture comprising monomers with two or more functionalities such as, for example, (meth) acrylic resins, styrene derivatives and vinyl ethers, and the photochromic material (as such, or incorporated in core-shell particles or in hyperbranched polymers). This mixture is applied as a thin layer (a few microns thick) onto the substrate, for example a glass panel, after which the polymerization reaction can be carried out by using photochemical processes, thermal processes or by means of an e-beam.

Alternatively, the photochromic material is incorporated in a polymer foil, for example, by using a swelling procedure. Examples of suitable materials for the polymer foil are PET, poly (carbonate), poly (acrylate), or poly (vinylbutyral). The foil is subsequently applied on the surface of the substrate, using a pressure-sensitive adhesive, UV-curing resins or thermal curing resins.

For example, a poly (vinylbutyral) (PVB) foil is swelled, using a saturated solution of Photosol7-114 in ethanol. Subsequently the foil is dried in air. The polymer foil comprising the photochromic material is laminated onto the substrate, using a method as described in US patent application 2001-0006732-A1 (PHN 17854). Alternatively, the foil is placed between a glass substrate 1 and a glass protective layer 4. This stack (see Fig. 7) is subsequently pressed together for one hour at a pressure of 1 bar and at a temperature of 60 degrees Celsius.

In the case of using the optical element at the viewing side of a display panel, the optical element may form a separate panel which is arranged in the light path at the outer side of the display screen, as shown in Fig. 8. Alternatively, the optical element may be

laminated onto the display panel, or the display panel itself may be used as a substrate of the optical element, as shown in Fig. 9.

In a particular embodiment, a glass plate is laminated onto the display screen as described in International Application PCT/IB00/11385 (PHN 17765). To this end, the glass plate 4 is positioned at some distance from the display screen 1 and the volume between the display screen 1 and the glass plate 4 is filled with a curable (via polymerization and/or cross-linking) material, which curable material yields the transparent coating 2 after curing.

Thus, the glass plate of the previous paragraph may be provided with a transparent coating containing a photochromic material, or the photochromic material (as such, or incorporated in core-shell particles or in hyperbranched polymers) can be incorporated in the resin used for the lamination process. The resin, such as (meth) acrylates, styrene derivatives or vinyl ethers, may be polymerized in situ, for example by using radical polymerization.

For CRTs laminated with a polymer layer or a transparent coating containing a photochromic material and a front glass panel, the thickness of the polymer layer may not be constant due to the non-flatness of the CRT screen (see Fig. 10). When the photochromic material is in its colored form, this variation in polymer layer thickness may lead to a varying absorption (more absorption where the layer is thick), which is not acceptable. A solution to this problem is to incorporate such a large amount of the photochromic material in the polymer layer that the penetration depth of UV light between 320-400 nm (i. e. the depth at which 90% of the UV light has been absorbed by the photochromic molecules) is less than the polymer layer thickness. By incorporating the proper amount of dye, 90% of the UV light will be absorbed in, for example the first 0.5 mm of the polymer layer, whereas the polymer layer has a thickness of, for example, 1 mm to 3 mm. In this way, only the photochromic molecules in this'active'layer can become colored upon LJV irradiation, resulting in a uniformly colored photochromic laminate.

Example: A photochromic laminate based on Photosol7-114 and poly (EHMA). The absorption spectrum of a photochromic laminate consisting of two 1 mm thick glass plates and a 1 mm poly (EHMA) layer containing 0.11 % by weight of Photosol7- 114 is depicted in Fig. 11. By incorporating 0.11 % by weight of the photochromic material Photosol7-114 in the poly (EHMA) layer, 90% of the light with a wavelength of 360 nm is absorbed within the first 0.5 mm and 90% of the light with a wavelength of 400 nm is absorbed within the first 1 mm of the polymer layer (the absorption is wavelength-dependent due to the fact that the absorption spectrum of the dye is not uniform). In other words, the

penetration depth of UV light between 320 nm and 400 in the polymer layer is between 0.5 mm and 1 mm. For a photochromic laminate on a CRT, this particular concentration of the dye is sufficient, provided that the polymer layer in the laminate is more than 1 mm thick.

From the wide range of photochromic materials available, a photochromic material with a more or less suitable color can be selected. However, when incorporated in the organic polymer of the invention, the color of the colored, low-transmission form of the photochromic material may not be totally satisfactory. It was found that when changing the polarity of the organic polymer, the color point of the optical element can be fine-tuned. For example, a purple transparent coating can be transferred into a blue/grayish transparent coating. Furthermore, the lifetime of the photochromic material may be increased by using a more polar environment. A polarity change may be achieved by one of the following two approaches: A. The monomer of the organic polymer is copolymerized with another monomer with a different polarity. An example of such an approach is the copolymerization of 2- ethylhexyl-methacrylate (forming the organic polymer) with methacrylic acid.

B. A soluble, non-volatile, low molecular weight, non-reactive compound which has a different polarity is added to the monomer of the organic polymer. For example, bisphenol-A is added to 2-ethylhexyl methacrylate.

An example of approach A is the following: A mixture of 80 parts by weight of 2-ethylhexyl methacrylate (EHMA), 20 parts by weight of methacrylic acid (MAA), 0.5 parts by weight of LTPO, and 0.1 parts by weight of Photosol7-114 is poured into the volume between the display screen and the glass plate as described in International Application PCT/IB00/11385 (PHN 17765). Subsequently, the mixture is cured by irradiation with light having a wavelength of 436 nm, and using an irradiation intensity of 10 mW/cm2 for about 80 minutes.

After the photo-polymerization process, a photochromic laminate is obtained with fast switching times (coloration/decoloration in less than 5 minutes). Upon illumination with UV light, such a transparent laminate, when having a thickness of 0.75 mm, has a transmission at 570 nm as low as 5 % depending on the illumination intensity and temperature. Without illumination with UV light, this transparent laminate has a transmission of about 96 % at 570 nm, irrespective of the temperature. When compared with a transparent laminate which is made of EHMA alone, the color of the transparent laminate has changed (see Fig. 12). Curve I in Fig. 12 shows the absorption spectrum of Photosol7-114 in 100 % poly (EHMA), and curve II in Fig. 12 shows the absorption spectrum of Photosol7-114 in a

copolymer comprising 20 wt% MAA and 80 wt% EHMA. The shift of the absorption spectrum due to the MAA is obvious, and results in a shift of the color point of the optical element.

Such a comparable transparent laminate without MAA can be made as follows: A mixture of 100 parts by weight of EHMA, 0.5 parts by weight of LTPO and 0.1 parts by weight of Photosol7-114 is poured into the volume between the display screen and the glass plate as described in International Application PCT/IB00/11385 (PHN 17765).

An example of approach B is the following: A mixture of 90 parts by weight of EHMA, 10 parts by weight of Bisphenol-A, 0.5 parts by weight of Lucirine 2,4,6- trimethylbenzoyl diphenylphosphineoxide (LTPO, which is a photoinitiator as supplied by BASF) and 0.1 parts by weight of Photosol7-114 is poured into the volume between the display screen and the glass plate as described in International Application PCT/IB00/11385 (PHN 17765). Subsequently, the mixture is cured by irradiation with light having a wavelength of 436 nm, and using an irradiation intensity of 10 mW/cm2 for about 80 minutes. After the photo-polymerization process, a photochromic laminate is obtained with fast switching times (coloration/decoloration in less than 5 minutes). Upon illumination with W light, such a transparent laminate, when having a thickness of 0.75 mm, has a transmission at 570 nm as low as 5 % depending on the illumination intensity and temperature. Without illumination with UV light, this transparent laminate has a transmission of about 96 % at 570 nm, irrespective of the temperature.

When compared with a transparent laminate which is made without using Bisphenol-A, the color or the transparent laminate is shifted. Fig. 13 shows the absorption spectra of Photosol7-114 in the colored form in a 100 % EHMA matrix (A) and in an EHMA/Bisphenol-A (90/10) matrix (B). Without Bisphenol-A, the colored form of Photosol7-114 has absorption maxima at 435 nm and 565 nm. The addition of 10 parts by weight of Bisphenol-A leads to a shift of the absorption maxima to 440 nm and 575 nm, respectively. It can clearly be seen from Fig. 13 that the addition of 10 parts by weight of Bisphenol-A to EHMA changes the position of the absorption maxima and thus of the color point of the photochromic laminate.

It may be obvious that many variations within the scope of the invention are possible to those skilled in the art.

For example, the photochromic layer may be augmented with layers having an antireflective or antiglare effect or with layers having an antistatic effect. Stabilizers may also

be added to the organic polymer which surrounds the photochromic material to improve the resistance of the photochromic material against radicals or other reactive species. Moreover, a desired mixture of photochromic materials may be applied, for example, to fine-tune the optical properties of the optical element such as color purity or switching speed of the optical element.

In general, the invention relates to an optical element comprising a substrate which is provided with a transparent coating comprising a photochromic material, in which the transmission of said optical element varies in the visible wavelength range in response to a variation of light which is locally incident on the transparent coating. In order to obtain an optical element having an improved performance with respect to a quick coloration/decoloration of the optical element, the photochromic material is at least partially surrounded by an organic polymer which has a glass transition temperature below 40 degrees Celsius.