The invention relates to an illumination system comprising a fluorescent lamp and a broadband, cholesteπc polarizer. The invention also relates to a linear polarizer which can suitably be used in such an illumination system The invention further relates to a display device comprising such an illumination system An illumination system of the type mentioned in the opening paragraph is known per se from European Patent Application EP-A 606 940, in the name of the current Applicant. In Fig 8 of said publication, a description is given of an illumination system in the form of a housing comprising three fluorescent lamps and a broadband, cholesteπc polarizer Fluorescent lamps are provided with a layer of a fluorescent material, which is generally composed ot three different fluorescent compounds which each have a specific fluorescence wavelength In general, mixtures of fluorescent compounds are used which have the most important fluorescence bands at approximately 435 inn (blue), approximately 545 n (green) and approximately 610 inn (red) The bandwidth of the emission spectrum of fluorescent lamps is much smaller than that of incandescent lamps or standard sunlight The polarizer used in the known illumination system comprises a layer of a polymeric material having a cholesteπc order, the so-called cholesteπc layer. By means of this layer, the polarization state of light passing through the polarizer is influenced The polymeric material of the cholesteπc layer is ordered in such a way that a molecular helix can be distinguished, the axis of this helix extending at right angles to said cholesteπc layer The above-described polarizer has a large bandwidth because the pitch ot the molecular helix increases from a minimum value at a first surface ot the cholesteπc layer to a maximum value at the second surface ot the layer In the case ol narrow -band polarizers, the pitch length of the molecular helix is substantially constant throughout the layer.

In experiments it has been established that the known illumination system with the fluorescent lamps has an important drawback regarding the viewing-angle dependence of the system. It has been found that under many circumstances there is a relatively large variation in the lightness and color of the transmitted light when the viewing direction relative to the system is changed. This drawback is problematic, in particular, if such an illumination system is used ιr a display device

It is an object of the to overcome the above-mentioned drawback. The invention more particularly aims at providing an illumination system and a display device having a small viewing-angle dependence ot the lightness and the color. The invention further aims at providing a linear polarizer, which enables these objects to be achieved in a display device and in an illumination system

These and other objects are achieved by means ot an illumination system which comprises a fluorescent lamp and a broadband, cholesteπc polarizer with a layer of a polymeric material having a cholesteπc order, said material being ordered in such a manner that the axis of the molecular helix extends at right angles to the layer, and the pitch of the molecular helix increasing from a minimum value at a first surface of the cholesteπc layer to a maximum value at the second surface of the layer, characterized in that the polarizer is positioned in the illumination system so that the cholesteπc layer faces the radiation source with its first surface and that the value ot the product p _mri n _e of the maximum value of the pitch in the cholesieric layer p _ιτwχ and of the extraordinary refractive index n _e ranges between 0 61 micron and 0 76 micron

The invention is inter alia based on the experimentally gained insight that the bandwidth ot the cholesteπc polarizer should be propei ly adapted to the bandwidth of the emission spectrum ot the fluorescent lamp(s) used in the case ot a properly adapted polarizer, the view ing-angle dependence ol the illumination system is found to be surprisingly lov, as regards the intensity and discoloration ot the obliquely transmitted light This applies even if the acceptance angle is 30° or more relative to the normal to the cholesteπc layer

It has been found that the product p _X n ot the cholesteπc layer must be properly attuned to the bandwidth ot the fluorescent lamps used On the one hand, the value of this product should be maximally 0 76 micron At larger values the viewing-angle dependence of the intensity ot the transmitted light assumes unacceptably large values On the other hand, the value ot this product should be minimally 0 61 micron At smaller values, discoloration in the case of obliquely incident light with an acceptance angle, for example, of more than 30 ^c assumes undesirably large values Optimum adaptation of the cholesteπc layer to conventional fluorescent lamps is obtained it the above-mentioned product ranges between 0 63 micron and 0 74 micron Conventional fluorescent lamps are to be understood to mean herein lamps whose most important fluorescence bands lie, approximately, at 435 nni, 545 nin and 610 mil

In the illumination system in accordance with the invention the

broadband, cholesteπc polarizer should be positioned relative to the radiation source in such a manner that the surface of the polarizer with the smallest pitch is directed towards the fluorescent lamp. It has been found that, in this configuration, undesirable coloration of the light emanating at an angle is considerably less than in the configuration in which the radiation source is situated on the other side of the polarizer. Said coloration occurs if the system is viewed under a (polar) angle which is larger than or equal to a specific, minimum value. In addition, experiments have revealed that the configuration in accordance with the invention has a much smaller viewing-angle dependence of the intensity of the light passed than the configuration in which the surface of the polarizer with the largest pitch is directed towards the fluorescent lamp.

It is noted that in the International Patent Application WO 96/02016, in the name of the current Applicant, a description is given of an illumination system comprising a radiation source and a broadband, cholesteπc polarizer. In this system, the surface of the polarizer having the maximum pitch length is directed towards the radiation source.

An advantageous embodiment of the illumination system accordance with the invention is characterized in that the illumination system also comprises a quarter- lambda plate which is situated at the side of the polarizer facing away from the light source. By virtue of the presence of this quarter-lambda plate, the light emanating from the illumination system is linearly polarized In the absence of such a quarter-lambda plate, circularly polarized light is obtained. Particularly, illumination systems producing linearly polarized light are suitable for use in commercially available display devices. It is noted that a quarter-lambda plate is to be understood to mean herein a birefnngent layer, which may or may not be laminated, whose optical retardation ranges from 125 to 150 nm at a wavelength of approximately 550 nm.

A further advantageous embodiment ol the illumination system in accordance with the invention is characterized in that the quarter-lambda plate is situated on the second surface of the broadband polarizer. By virtue ot the fact that the quarter-lambda plate and the polarizer are provided on each other so as to be in direct contact with each other, the illumination system in accordance with this embodiment ot the invention exhibits lower reflection losses.

A further interesting embodiment of the illumination system in accordance with the invention is characterized in that the quarter-lambda plate is made of a foil of oriented polymeric material In theory, use can be made of quarter-lambda plates which are

made from inorganic materials, such as calcite er, the difference between the ordinary and the extraordinary refractive index ot this material is relatively large As a result, the thickness of the quarter-lambda plate must be approximately 0 8 micrometer in order to be suitable for an illumination system tor use the visible portion of the spectrum In practice, however, such a small thickness for plates having a surface area of 10 cm ² or more can hardly, or perhaps not at all , be realized In addition, quarter-lambda plates based on oriented foils are much cheaper than quarter-lambda plates made from inorganic materials. Such oriented foils can be obtained by polymerizing specific monomer mixtures on an oπented surface to form a toil. Such a surface can be oriented by rubbing it in a specific direction The manufacture of such foils usually takes place by stretching a finished foil in a specific direction Very good results were obtained by using a stretched foil of substituted or unsubstituted polystyrene or a copolymei thereot Satistactoi y results were also obtained by using a foil of polycarbonate

In a further favourable embodiment, the inventive illumination system also comprises a dichroic polarizer which is situated on the side ot the quarter-lambda plate facing away from the broadband, cholesteπc polaπzei The presence ot such a polaπzer leads to a higher contrast of the relevant illumination system This dichroic polarizer is preferably situated on the quarter-lambda plate This configuration leads to lower reflection losses of the incident light The invention also relates to a linear polarizer tor use in an illumination system This linear polarizer is characterized in accordance with the invention in that it comprises a broadband, cholesteπc polarizer as well as a quarter-lambda plate, said polaπzer including a layer of a polymeric material having a cholesteπc order, said material being ordered in such a manner that the axis ot the molecular helix is directed at right angles to the layer, and the pitch of the molecular helix increasing liom a minimum value at a first surface of the cholesteπc layer to a maximum value at the second sui tace ot the layer, the quarter- lambda plate being situated on the second surface ol the polaπzei , and the value ot the product p _mrtX n _e ot the maximum value ol the pitch in the cholesteπc layer p _ιndλ and of the extraordinary refractive index n _e ranging between 0 61 micron and 0 76 micron In the linear polarizer in accordance with the invention, the broadband, cholesteπc polarizer is positioned relative to the quarter-lambda plate in such a manner that the surface of the polarizer having the largest pitch physically contacts said plate. The use of a linear polarizer of this configuration in an illumination system comprising a conventional fluorescent lamp ensures that the undesirable discoloration of the emanating light as a

function of the viewing angle is considerably less than in the configuration in which the cholesteπc polarizer physically contacts the quartei -lambda plate with its opposite surface In addition, experiments have revealed that in the inventive configuration of the linear polarizer, the viewing-angle dependence of the intensity of the transmitted light of the illumination system is much smaller than in the other configuration This small viewing-angle dependence can be attributed, to a large extent, to the fact that the bandwidth ot the polarizer is adapted to the bandwidth of the fluorescent lamp used

Another favorable embodiment of the linear polarizer in accordance with the invention is characterized in that the quarter-lambda plate is made of a foil of an oriented polymeric material The use of such synthetic resin foils enhances the ease of manufacture because this type of foils can be processed relatively easily Moreover, such foils are relatively cheap A linear polarizer which functions very satisfactorily is obtained if stretched foils of substituted or unsubstituted polystyrene or copolymers thereof are used Good results can also be obtained by using polycarbonate Yet another advantageous embodiment ot the linear polarizer in accordance with the invention is characterized in that a dichroic polarizer is present on the surface of the quarter lambda plate facing awa> from the broadband, cholesieric polarizer. The use of a linear polaπzer in accordance with this embodiment in an illumination system leads to an increase in contrast The invention also relates to a display device This display device comprises an illumination system as described hereinabove as well as a display panel. Said display panel includes two transparent substrates between which a liquid-crystalline material is sandwiched, an electrode pattern and a driving means tor these electrodes An image is formed by locally applying electric fields to the liquid-crystalline material The display device in accordance with the invention may be ot the ferroelectric, anti-ferroelectric, untwisted nematic, twisted nematic or supertwistcd nematic type

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

In the drawings

Fig 1 is a schematic, sectional view ot an embodiment of an illumination system in accordance with the invention,

Fig. 2 shows structural formulas of compounds which can be used in the manufacture of the linear polarizer in accordance with the invention,

Fig. 3 shows a graph in which the pitch P and the reflection wavelength R of a cholesteπc polarizer are plotted as a function of the mixing ratio ot the nematic and cholesteπc monomers used,

Fig. 4 shows a graph in which the minimum reflection and the minimum transmission are plotted as a function of the product p, _llil n _e ot a c olesteric polarizer,

Fig 5 is a sectional view ot two embodiments ot a linear polarizer in accordance with the invention, Fig. 6 is a schematic, sectional view of a display device in accordance with the invention.

It is noted that, for clarity, the parts of the Figures are not drawn to scale Fig 1 is a sectional, schematic view of an embodiment ot the illumination system in accordance with the invention Said system comprises a radiation source 1 and a broadband, cholesteric polarizer 2 The radiation source includes five light sources 3 in the form of fluorescent lamps, a reflector 4, and a ditfuser 5. It is noted that, instead of a number of separate fluorescent lamps, use can alternatively be made of a single fluorescent tube, in particular of the meander-shaped type. The unpolaπzed light generated via the light source(s) 3 is guided, if necessary via reflection from reflector 4, via the diftuser in the direction ot the broadband polarizer Said reflector 4 can be made ot rubber filled with a white pigment such as titanium dioxide or barium sulphate Said reflector may also consist of a metal film The diftuser 5 may consist ot a scattei ing toil T e radiation source 1 described herein causes the unpolaπzed light to be incident on the cholesteric polarizer 2 in a very uniform manner. It is noted that the expression "radiation source" is to be interpreted in a broad sense. This expression, tor example, also comprises a light-guiding layer which reflects radiation, generated by means of a fluorescent lamp situated next to the illumination system, through the broadband polarizer Said construction is also referred to as "side- hghting" .

The broadband polarizer 2 comprises a layer ot a polymeric material having a cholesteric order, in w hich the material is ordered so that the axis of the molecular helix is directed at right angles to the cholesteπc layer The pitch ot the molecular helix increases from a minimum value at a first surlace ot the cholesteπc layer to a maximum value at the second surface of the layer In this case, the polaπzer is positioned in the illumination system in such a manner that the cholesteric layer faces the radiation source with

its first surface. Further, the maximum value of the pitch is selected so that it is optimally adapted to conventional fluorescent lamps. This means that the value of the product p _ιn n _e of the maximum value of the pitch in the cholesteric layer p _|1Mχ and of the extraordinary refractive index ι ranges between 0.61 micron and 0 76 micron In general, the extraordinary refractive index n _e ot cholesteric layers ranges from 1 .6 to 1 .7.

European Patent Application EP-A 606 940 describes a number of ways of manufacturing a broadband, cholesteric polarizer. In the present case, use was made of a mixture of a cholesteric diacrylate Cr70Ch921 whose structure is in accordance with formula (1) of Fig. 2, and a nematic monoacrylate CrlOON Ϊ75I whose structure is in accordance with formula (2) of Fig. 2. A small quantity of a dye was added to this mixture. The chemical structure of this dye is referenced as (3) in Fig 2. This dye has a maximum extinction of 31524 1/mol.cm at a wavelength of 334 nm. Also a small quantity of a photomitiator (formula (4) of Fig. 2) was added The mixture was sandwiched between two parallel substrates and subsequently cured by means ot actinic radiation During curing, a radiation profile was provided on the layer to be polymerized By virtue thereof, a continuous variation in the pitch of the molecular helix ot the cholesteric material was obtained. During curing, a three-dimensional polymeric network is formed The molecular helix having a variable pitch is fixed by the presence ot this network

The maximum and the minimum pitch ot (he cholesteric layer can be adapted by changing the mixing ratio ot the cholesteric and nematic monomers Fig. 3 shows the results of experiments in which the mixing ratio ol the monomers ( 1 ) and (2) is varied In this Figure, the pitch P is plotted as a function ot the quantity of monomer ( 1 ) relative to the overall quantity of monomers ( 1) and (2) This figuie also shows the relevant reflection wavelength R. For clarity, no radiation profile was provided in this experiment during curing of the cholesteric layer. Using the data given in said Patent document EP-A 606.940, a cholesteric layer whose product p _πκ(X n _e ranges between 0 61 micron and 0 76 micron, can be manufactured in a simple manner by those skilled in the art

The viewing-angle dependence ot an illumination system having the above-mentioned structure was checked. Said view g-angle dependence was found to be much smaller than that ot a system in which the broadband, cholesteric polarizer faces the radiation source with the surface having the largest pilch

The illumination system in accordance with the invention preferably also comprises a quarter-lambda plate as well as a dichroic polarizer The presence of these elements, however, is not essential for the operation ot the invention The quarter-lambda

o plate 6 and the dichroic polarizer 7 are also shown in Fig. 1 They are preferably incorporated in the illumination system as a three-layer structure. Said three optical components can, however, alternatively be incorporated in the system as separate elements. An illumination system of the above-mentioned structure was subjected to a number of measurements. The broadband polarizers used tor this purpose comprise a 20 micron thick layer of a cholesteric material whose structure is indicated hereinabove In each polarizer the pitch of the molecular helix at the surface ot ihc layer facing the light source was 0.22 micron (minimum value) At the surface facing away from the light source, the pitch of the various polarizers varied from 0 60 to 0.80 micrometers (maximum value). On this other surface of the broadband polarizers, there were provided , in succession, a quarter- lambda plate of polycarbonate and a dichroic polarizer

In Fig 4, the lightness ot the linear polarizers described m the preceding paragraph is determined. The lightness L was determined in transmission (L,) and in reflection (L _r ) as a function ot the product P (nm) ot the maximum pitch in the cholesteric layer (p _{m x} ) and the extraordinary refractive index (n _e ) ot this layer This product is equal to the maximum wavelength in the reflected spectrum tor perpendicularly incident light and can thus be measured tor any cholesteric reflector Foi the light source, use is made of a conventional fluorescent lamp ot the above-def ined type

The lightness (L) is given as psychometric lightness This measure of lightness is constructed in such a way that v ariations in this quantity correspond accurately to variations as they are perceived as relevant by the human visual system There is a non¬ linear relation between the psychometric lightness and the light intensity , because the sensitivity of the human visual system to variations in intensity is not linear

Fig 4 shows that the minimum ot lightness ot the transmitted light L _t decreases as the product p _{ll X} n _e increases This is caused by changes in the state of polarization ot the light travelling through the cholesteric layer In this respect it is noted that the value of n _e is constant tor the same layer II the product p _ιi n _e exceeds 0 76 micron, the lightness assumes an unacceptably low level Preterably, this product is chosen to be smaller than 0.74 micron It has been found that a diminution ot the product p _lll/iX .n_. is accompanied by an increase of the viewing-angle dependence of the coloration of the reflected light L _r . If the value of this product is too small , the division ol the unpolaπzed light into two beams with complementary polarization states does not take place tor the red portion of the incident light. As a result, the re component of the light is not reflected and, consequently, the light

which is reflected is less bright and more colored. The reflected light is depolarized and reflected again in the diffuser. Subsequently, this light is supplied again to the cholesteric layer. Now, a portion of this light is passed by the cholesteric polarizer. In this manner, an increase in coloration in the initially reflected beam becomes visible to the user of the device. It has been found that this coloration becomes unacceptable if the product p _max .n _e is below 0.61 micron. Preferably, this product is chosen to be above 0.63 micron.

Fig. 5 shows a number of linear polarizers in accordance with the invention. The polarizer 10, shown in Fig. 5a, comprises a broadband, cholesteric polarizer 1 1 as well as a quarter-lambda plate 12. The broadband, cholesteric polarizer 1 1 includes a layer of a polymeric material having a cholesteric order. The material is ordered in such a way that the axis of the molecular helix extends at right angles to the layer. Moreover, the pitch of the molecular helix increases from a minimum value at a first surface 13 of the cholesteric layer to a maximum value at the second surface 14 of said layer. The quarter- lambda plate 12 is situated on the second surface of the polarizer. Fig. 5-b shows another embodiment of the linear polarizer in accordance with the invention. In addition to a broadband, cholesteric polarizer 1 1 and a quarter-lambda plate 12, this polarizer 16 comprises a dichroic polarizer 15. This dichroic polarizer is situated on the surface of the quarter-lambda plate facing away from the cholesteric polarizer. The linear polarizers shown in Fig. 5 can be manufactured by bonding together separate foils of a broadband, cholesteric polarizer, a quarter-lambda plate and, if necessary, a dichroic polarizer. If necessary, the cholesteric polarizer may be situated on a transparent substrate. The polarizer may alternatively be embedded between two substrates. However, it is alternatively possible to use self-supporting cholesteric polarizers. The dichroic polarizer is composed, for example, of a layer of pυlyvinyl alcohol (PVA), which is situated between two substrates of cellulose acetate. Said PVA layer is stretched to approximately 6 times its original length. Subsequently , it is impregnated with an iodide complex or an organic dye. The stretched PVA and the additions together form a layer which absorbs light having a polarization parallel to the direction of stretching and which passes light having a polarization which is at right angles to said direction. In the linear polarizer, the main axis of the quarter-lambda plate should enclose an angle of 45 ° with the direction of light transmission of the dichroic polarizer.

Fig. 6 shows a display device in accordance with the invention. Said display device comprises an illumination system 20, as described in greater detail in Fig. 1 .

The various components of this illumination system bear the same reference numerals as those of Fig. 1. The illumination system of the display device comprises a radiation source 3, in the form of a meander-shaped fluorescent lamp, a reflector 4 and a diffuser 5. The system also includes a broadband, cholesteric polarizer 2, a quarter-lambda plate 6 and a dichroic polarizer 7. In this case, they are provided as a stack on a substrate 8.

The display device also comprises a display panel 30. This panel is composed of two transparent substrates 2 1 , which are each provided with an array of electrodes 22 and an orientation layer 23. The electrodes consist of a transparent, electroconductive material such as indium-tin oxide (ITO). The orientation layers may be made of a rubbed polymeric material such as polyimide or PVA, or of obliquely sputtered silicon oxide. Finally, a layer 24 of smectic or nematic liquid crystalline material is situated between the substrates. The order of this material can be influenced by electric fields which can be generated locally by means of electrodes. The display device also includes an electronic driving means for these electrodes. For clarity, this driving means is not shown in the Figure.

It is of essential importance for the effectiveness of the invention that the display device comprises a broadband, cholesteric polarizer, which is properly oriented. The surface of the polarizer where the pitch of the cholesteric material is smallest must be directed towards the fluorescent lamp(s). The maximum and the minimum value of the product p _maχ .n _e of the maximum pitch p _ll and of the extraordinary refractive index n _e should be at least 0.61 micron and maximally 0.76 micron. Polarizers which meet this requirement provide the display device with a surprisingly low viewing-angle dependence of the lightness and the color.