WO2006120638A2

WO2006120638A2 - Polarized backlight device

Info

Publication number: WO2006120638A2
Application number: PCT/IB2006/051450
Authority: WO
Inventors: Hugo J. Cornelissen; Dirk K. G. De Boer; Martin J. J. Jak; Dirk J. Broer
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-05-13
Filing date: 2006-05-09
Publication date: 2006-11-16
Also published as: TW200643568A; WO2006120638A3

Abstract

A backlight device for use in display devices, comprising: a light-guiding portion (2) connectable to a light source (3) and having a top face (4) arranged to allow light of a predetermined nature to exit via said top face (4); and an optical structure (5,6,7,8,9,15,20,23) arranged along said top face (4) in said light-guiding portion (2) to angularly separate a light beam into light beams having mutually opposite directions of polarization; and to angularly separate a light beam into a multiple of light beams having different coloring wavelengths, said structure (5,6,7,8,9,15,20,23) being arranged in such a way that, for a predetermined direction of polarization of said light having passed said structure, said different coloring wavelengths are separated into distinct angular regions.

Description

Polarized backlight device

The invention relates to a backlight device for use in display devices.

Several devices are known to provide polarized light while keeping an optimum yield of light through "recycling" the directions of polarization that do not have a proper orientation. Reference is made to US2002/0075427 and US2003/0058386. Several of these devices use birefringent materials or form birefringence, so as to angularly separate two distinct directions of polarization. In addition to a form of birefringent grating, US2003/0058386 shows a macroscopic structure for coupling out a predetermined proper orientation of polarized light. A disadvantage of said reference is that color separation does not take place. Such a feature is nowadays desirable for colored display devices, to which the invention relates.

In the other reference US2003/0058386, color separation is provided through a microstructure. However, the exiting light is not of a polarized nature, which means that the use of the device is limited or that other arrangements must be made to block or redirect the polarization of light having a certain undesired direction of polarization.

It is an object of the invention to provide a backlight device while conveniently dealing with the shortcomings of the prior art. In particular, it is an object to provide a backlight device wherein color separation and polarization separation are realized in an easy and cost-effective manner. To this end, the invention proposes a backlight device as defined in claim 1.

In particular, the invention provides a backlight device comprising an optical structure arranged along said top face in said light-guiding portion so as to angularly separate a light beam into light beams having mutually opposite directions of polarization; and to angularly separate a light beam into a multiple of light beams having different coloring wavelengths, said structure being arranged in such a way that, for a predetermined direction of polarization of said light having passed said structure, said different coloring wavelengths are separated into distinct angular regions.

Specifically, said optical structure can realize a predetermined desired angular separation of a beam having a predetermined polarization into distinct coloring wavelengths. Distinct coloring wavelengths are understood to be at least wavelengths that differ significantly in the human perception. More specifically, these wavelengths comprise red, green and blue wavelengths. Distinct angular regions are understood to mean that, although small overlapping regions may be possible, a majority of the light having a predetermined color and/or polarization is separated from the remaining light.

Further features and benefits of the invention will be elucidated with reference to the drawings.

Figure 1 shows schematically a first embodiment of the invention having two interface layers; Figure 2 shows schematically a second embodiment of the invention with the two interface layers combined;

Figure 3 shows another embodiment of the invention with a third interface layer;

Figure 4 shows yet another embodiment of the invention with first, second and third interfaces combined;

Figure 5 shows a backlight device having a birefringent layer and a grating;

Figure 6 shows a diffraction efficiency- versus-angle for s-polarized light for the embodiment shown in Figure 5;

Figure 7 shows another example of a backlight device having a grating and a birefringent layer;

Figure 8 shows the experimental results for the backlight of Figure 7;

Figure 9 shows yet another example of a backlight device having a grating and a birefringent layer;

Figure 10 shows a first example of a grating showing color separation and focusing; and

Figure 11 shows a second example of a grating showing color separation and focusing.

Figure 1 shows schematically a first embodiment of an optical structure according to the invention. Figure 1 shows a backlight device 1 comprising a light-guiding portion 2. The light-guiding portion 2 is an optically transparent plate of possibly more than one layered material for illuminating a rear side of a display screen such as will be described below. Light can be provided to the light-guiding portion by a light source 3 which can be connected to a side of the light-guiding portion 2 or (not shown, but shown in, for instance, WO2004/027466) to its bottom face.

The top face 4 of the light-guiding portion 2 is designed to allow light to exit via said top face 4. Typically, only light of a predetermined polarization may exit the top face. Alternatively, light of the "wrong" polarization may be reflected back or absorbed in optical materials, which are known in the art.

Figure 1 further shows a first interface 5 arranged along the top face 4 in said light-guiding portion 2 so as to angularly separate a light beam into light beams having mutually opposite directions of polarization. Through birefringence of the material between interlaces 5 and 6, light of mutually opposite polarizations is refracted differently, typically p-light (parallel to the plane of incidence) has a diffractive index no; and s-light has a diffractive index ne > no.

This difference of refraction has the effect that light of different directions of polarization is refracted at different angles, so that angular separation between these directions of polarization is realized. In the drawing, this is shown by different orientations of s and p-polarizations, wherein, by way of illustration, the p-polarized beam almost travels unhindered through the interface 5, whereas the s-polarized beam is refracted more strongly.

Hence, in the embodiment shown, the p-polarized beam is refracted through a smaller angle with respect to interface 5 than the s-polarized beam. The refracted p-light can be absorbed in a subsequent layer or can be reflected, for instance, by total internal reflection by an adequate choice of refractive indices.

The s-polarized beam refracts through a steeper angle and encounters a second interlace 6 arranged along said top face 4 in said light-guiding portion 2 so as to angularly separate a light beam into a multiple of light beams having different coloring wavelengths (R₅G₅B). This can be done by a grating having a period ranging from 200 to 2000 nm. In practice, values of 300 to 600 nm and a modulation depth ranging from 50 to 700 nm can be applied. The first-order diffraction for various wavelengths is then angularly separated.

By selective choice of the mentioned diffractive parameters, said different coloring wavelengths are separated into distinct angular regions after having passed both interlace layers.

Figure 2 shows schematically a second embodiment of the invention with the two interface layers 5, 6 described hereinbefore with reference to Figure 1 integrated in a single interface layer 7 so as to angularly separate a light beam into light beams having mutually opposite directions of polarization; and to angularly separate a light beam into a multiple of light beams having different coloring wavelengths. This embodiment provides an efficient integration of polarization and color separation by combining the sub-structures of the optical structure disclosed in the embodiment of Figure 1. In particular, when the refractive indices are chosen to be such that the no of the birefringent material matches an nl of an isotropic optical material facing the birefringent material, the interface is virtually

"invisible" for p-polarized beams and the p-beam travels on unhindered from the nl region to the no region (assuming that the isotropic material is situated below the interface 7). For s-polarized beams, however, an interface is visible with different refractive indices: ne versus nl. This effect could be present due to a mismatch of roughly smaller than 2% between the refractive constants.

Figure 3 shows a further embodiment of the invention, wherein the angularly separated beams are converged by another optical structure 8 into beams that are spatially separated. Such an interface can be formed, for instance, by a micro-optics layer or a layer as further described with reference to Figures 10 and 11. The result of this convergence is that a micro-pixel in a display device may be illuminated by three substantially parallel different distinct color beams extending along lines normal to the plane of the drawing.

Figure 4 shows yet a further embodiment wherein the optical structures described hereinbefore are merged into a single interface layer 9. As will be shown in the paragraphs below, the effect of separating polarization, color separation and focusing can be achieved by using birefringence, the diffractive grating and a micro-optics structure as illustrated in Figures 10 and 11.

Figure 5 shows a realistic example of a first type illustrating the inventive aspects. The Figure shows a light-guiding body 10 having a reflective layer 11 on the bottom face 12 to reflect light traveling downwards. In the case of straight angles, a substrate consisting of a material having a refractive index of more than Λ/2 is known to act as a light guide for light incident on one of the sides. A grating of (a film of) material 13 is applied on top of the substrate and is coated with a film of material 14 that follows the grating on one side and is flat on the other. One of the materials 13 or 14 is birefringent with no in the plane of the drawing and ne perpendicular to it. The other material as well as the substrate material is isotropic, with refractive indices as close as possible to no. This implies that light with its electric vector in the plane of the drawing (p-polarized) does not encounter an optical interlace and remains in the light guide by total internal reflection. Light with its electric vector perpendicular to the plane of the drawing (s-polarized), however, will be diffracted by the grating. Figure 6 shows a calculated diffraction efficiency of the arrangement of Figure 5. In this case, the refractive index of both the substrate and the material 13 is 1.49, whereas material 14 has no = 1.49, ne = 1.86. The grating has a period of 400 nm and a modulation depth of 125 nm. The incident light (coming from the left in Figure 5) is assumed to have an isotropic angular distribution. The calculation assumes a long light guide and takes into account multiple reflections, while part of the light is reflected (OR in Figure 5) at each reflection. In Figure 6, the diffraction efficiency is shown for light of three different wavelengths as a function of the diffraction angle outside the light guide (θ_iτ in Figure 5). The solid lines show the contribution of transmitted light (indicated by -IT in Figure 5), whereas the broken lines show the contribution of diffracted reflected light (indicated by -IR in Figure 5). In a possible embodiment, this light is reflected back by an additional mirror into the same direction as the transmitted light (this mirror may also be placed at the same side as the grating). It can be seen that color separation indeed takes place. For blue light, some of the intensity is in the 2^nd-order diffraction. If material 13 is birefringent and material 14 and the substrate material are isotropic, a similar behavior is found, with diffraction efficiencies for both transmission and reflection of the order of 0.5 in the same angular range. (Note that the angular range for each wavelength depends only on the refractive index of the substrate material.)

The substrate material may be PMMA (n = 1.49) or glass (n = 1.5). The birefringent material may be PEN (no = 1.56, ne = 1.86) or PET (no = 1.53, ne = 1.71), which are materials that can be embossed with a grating structure. The material may also be a liquid crystal polymer that is polymerized in the right shape. The isotropic film may be polycarbonate (n = 1.56) or silicon dioxide (n = 1.4-1.5), possibly applied by means of a sol- gel technology. To apply the polarized backlight with color separation in a display, the light should be directed towards the sub-pixels corresponding to the different colors. This can be done by using a lens array. Alternatively, each group of (three) sub-pixels may have a grating directing the light to the right sub-pixels. This will be further explained with reference to Figures 10 and 11. It is clear from Figure 6 that color separation is not perfect. This means that it may be desired in practice to use color filters so as to absorb light of undesired wavelengths. Note, however, that the absorption will be much smaller than without color separation.

The above calculations were made for light rays in the plane of the drawing. For skew rays, the separation between s and p-polarized light will be less complete. As in the case of other polarized backlights a clean-up polarizer may be used to obtain the desired contrast.

Figure 7 shows a set-up in which liquid crystals (LC) can be switched from the state with the optical axis perpendicular to the plane of the drawing to a state with the optical axis in the plane of the drawing. In particular, a grating structure 15 is provided on a face of an optically isotropic material 16. Liquid crystal layer 17 is provided as an interface for said grating structure so as to align with said grating structure for forming a birefringent region. The grating in the polycarbonate layer has a period of about 400 nm (more generally: between 200 and 2000 nm; color separation can take place in this range). Through alignment by the grating structure, the LC layer 17 is arranged with its optical axis aligned perpendicularly to the plane of the drawing. The total layer stack acts as a light guide for p- polarized light, because the ordinary refractive index of the LC layer 17 is matched to the refractive index of the polycarbonate layer 16. For s-polarized light, the structured layer acts as a grating 15, leading to outcoupling of the light at angles determined by the wavelength. Figure 8 shows pictures for experimental values of displayed brightness and color of s and p-polarized light, as well as for the s/p contrast ratio (CR). The LC material used was TL 213 (Merck) with no = 1.53, ne = 1.77.

When using liquid crystals, as in Figure 7, one is able to switch from a state in which s-polarized light is coupled out towards a state in which some p-polarized light and no s-polarized light is coupled out. With an additional polarizer on top of the display, it is possible to block the p-polarized light. This additional polarizer may generally be already present to enhance the contrast. In this way, a color-separating polarized backlight is obtained, whose amount of outcoupling can be controlled. A rubbed polymer layer 18 is provided for aligning the liquid crystals in combination with the grating 15. The LC layer 17 may have a twist. This twist can be provided by providing a rubbed polymer layer having a rubbing direction which is oriented at an angle relative to the grating direction. Such a twist can be used beneficially in order to provide an output polarization in the same direction as the desired input polarization of the display, for instance, if one has to match a desired polarization orientation of an input device that is to be coupled with the backlight device 1. It is noted that, instead of the grating 15 controlling the alignment of the crystals, another alignment-controlling layer may generally be provided, particularly another rubbed layer 18. The invention also encompasses such embodiments. Said layer 18 may be provided with electrodes 19 so as to selectively align a predetermined portion of said crystals. Alternatively, the electrodes 19 may be provided in the optically isotropic material 16. In a preferred embodiment, the electrode structure consists of strip electrodes 19 separated by a thin dielectric layer from a sheet electrode (cf. FFS structure as used by Boe-Hydis: Kim et al., IDW '03, p. 85). In this case, the directors can be switched to a nearly vertical position, causing little outcoupling of p-polarized light. Hence, local dimming and highlighting is possible by providing a macroscopic structure in the LC layer. Indeed, by locally blocking light exiting the top surface by changing the orientation of the liquid crystals, more light can be directed to the regions where exiting is permitted (highlighting).

It might even be possible to use the structured LC layer as a display (LCD) without an additional LCD, although in that case additional means (e.g. color-sequential backlighting) should be present to modulate the intensities for the three color sub-pixels.

Figure 9 shows yet another example of a backlight device having a grating and a birefringent layer. Here, a grating 20 is provided in an upper isotropic layer. Just below it, a birefringent layer 21 is present, having no <ne, with no being roughly equal to the refractive index of the upper isotropic layer. Using this arrangement, a p-polarized beam reflects back by total internal reflection, whereas the s-polarized beam is coupled out and enters the grating structure 20. Color separation is provided for the s-polarized beam by first-order diffraction.

Figure 10 shows an alternative for the third interface shown in Figure 3. Although a lenticular array can be used to provide focusing of the colored beams, the grating can be preferably adapted to provide a focusing effect. Figures 10 and 11 show two different embodiments for providing such a focusing effect. In the embodiment of Figure 10, it is shown schematically how a grating can be combined with a curved surface so as to focus the light of different colors onto the corresponding sub-pixels 22. In this example, there is a curved surface 23 with a grating for every three sub-pixels. The curved shape is close to spherical or parabolic in the cross-section shown. It may be slightly curved (e.g. elliptical) in the perpendicular direction so as to direct the rays as satisfactorily as possible to the sub- pixels.

Figure 11 shows schematically a grating with a period gradient 24 for every three sub-pixels. Diffraction is governed by the grating law: mλ/p = ns sinθs - nl sinθm, where m is the diffraction order (and m = -1 is used), λ is the wavelength of the light, p is the grating period, ns and nl are the refractive indices of substrate and outside medium (e.g. air with nl =1), respectively, θs is the incident angle in the substrate and θm is the angle of the diffracted beam. Since the period is larger at the left side, the light is diffracted at larger angles at that side. This causes an effective focusing, as indicated in the Figure. A linear decrease of the period from about 310 nm on the left to about 540 nm on the right would be appropriate. The period may change slightly in the direction perpendicular to the plane of the drawing so as to direct the rays as satisfactorily as possible to the sub-pixels.

If a rectangular light guide is used in combination with an additional mirror at the bottom of the light guide, the diffracted reflected light (not shown in the Figures) will be reflected by the mirror into the same direction as the diffracted transmitted light. This remains true in the case of a graded grating (Figure 11), whereas this is no longer true in the case of a curved grating (Figure 10). The grating structures can be produced in several ways. In a preferred embodiment, they are embossed in a suitable substrate material, e.g. polycarbonate. The master may be made by lithographic means or by a combination of mechanical and lithographic means.

Although the invention has been described with reference to examples of embodiments, the invention is not limited thereto. Furthermore, the diffractive gratings shown with reference to Figures 10 and 11 are preferably used in conjunction with the birefringent materials shown in the preceding examples. However, the invention also relates to embodiments in which such birefringent materials are not used and only a color separation and focusing is provided. The skilled person will appreciate that variations and modifications of these embodiments are possible without departing from the scope of the invention. This scope is defined by the appended claims.

Claims

CLAIMS:

1. A backlight device (1) for use in display devices, comprising: a light-guiding portion (2) connectable to a light source (3) and having a top face (4) arranged to allow light of a predetermined nature to exit via said top face (4); and an optical structure (5,6,7,8,9,15,20,23) arranged along said top face (4) in said light-guiding portion (2) to angularly separate a light beam into light beams having mutually opposite directions of polarization; and to angularly separate a light beam into a multiple of light beams having different coloring wavelengths, said structure (5,6,7,8,9,15,20,23) being arranged in such a way that, for a predetermined direction of polarization of said light having passed said structure, said different coloring wavelengths are separated into distinct angular regions.

2. A backlight device according to claim 1, wherein said optical structure includes an interface between a birefringent material (17) and another optically transmissive material (16) having refractive properties which differ from at least one of the refractive indices of said birefringent material.

3. A backlight device according to claim 1 or 2, wherein the birefringent material (17) is arranged to provide a twist in the polarization orientation of light transmitted through said birefringent material.

4. A backlight device according to claim 2, wherein said optically transmissive material (16) has an index of refraction closely matching one of the indices of refraction of the birefringent material (17).

5. A backlight device according to any one of claims 1 to 4, wherein said optical structure comprises a grating (23) having a grating constant which is suitable for separating a multiple of coloring wavelengths.

6. A backlight device according to claim 5, wherein said grating (15, 20, 23) has a period ranging from 300 to 600 nm and a modulation depth ranging from 50 to 700 nm.

7. A backlight device according to claim 1, wherein said optical structure includes a grating (15) forming an interface between a birefringent material (17) and another optically transmissive material (16) having refractive properties which differ from at least one of the refractive indices of said birefringent material (17), said grating (15) having a grating constant adapted to separate a multiple of coloring wavelengths.

8. A backlight device according to claim 7 in conjunction with claims 2 and 5, wherein the grating is provided on a face of an optically isotropic material, and wherein liquid crystals (17) are provided as an interface for said grating (15) so as to align with a principal orientation of said grating for forming a birefringent region.

9. A backlight device according to claim 8, wherein the liquid crystals (17) are twisted in order to provide an output polarization in the same direction as the desired input polarization of the display.

10. A backlight device according to claim 8 or 9, wherein a further optically transmissive material (18) is provided for alignment of the liquid crystals.

11. A backlight device according to claim 10, wherein said further optically transmissive material (18) comprises a rubbed polymer material.

12. A backlight device according to claim 11 in conjunction with claims 8 and 9, wherein said rubbed polymer material is rubbed in an orientation different from the principal orientation of the grating.

13. A backlight device according to any one of claims 8 to 11, wherein electrodes (19) are provided in a layer neighboring the liquid crystals so as to selectively align a predetermined portion of said liquid crystals.

14. A backlight device according to claim 13, wherein said electrode structure (19) consists of strip electrodes separated by a thin dielectric layer from a sheet electrode.

15. A backlight device according to claim 13 or 14, wherein said electrode structure (19) has a macroscopic structure for highlighting a predetermined area of the backlight device.

16. A backlight device according to any one of the preceding claims, wherein the angular region of the light having a direction of polarization opposite to said predetermined direction of polarization is chosen to be reflected in the backlight device through total internal reflection.

17. A backlight device according to any one of the preceding claims, wherein said optical structure (23, 24) is arranged to converge said light of different coloring wavelengths to be substantially spatially separated.

18. A backlight device according to claim 17, wherein said optical structure comprises a grating (24) having a varying grating constant, said variation being chosen to converge an incoming light beam.

19. A backlight device according to claim 17, wherein said optical structure comprises a grating (23) having a curved surface shape.

20. A backlight device according to claim 19, wherein said curved surface shape is close to spherical or parabolic.

21. A display device, comprising:

- a display panel including a plurality of picture elements for selectively modulating incident light, and

- a backlight device according to any one of the preceding claims, for providing illumination to the display panel.

22. A display device according to claim 21, wherein the display panel is a transmissive liquid crystal display panel.