WO2006025023A1

WO2006025023A1 - A collimator for use in a backlight liquid crystal display system

Info

Publication number: WO2006025023A1
Application number: PCT/IB2005/052839
Authority: WO
Inventors: Bianca M. I. Van Der Zande; Bart-Hendrik Huisman; Emile J. K. Verstegen; Johan Lub; Edwin M. Wolterink; Johan G. Kloosterboer
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-09-03
Filing date: 2005-08-30
Publication date: 2006-03-09

Abstract

The present invention relates to a system for lighting a display. The system comprises a light source (102), a collimator (120), and a liquid crystal display configured to receive a collimated light beam (126) and adapted to modulate the collimated light beam (126). The system further comprises a reflective sheet (110) having one or more pin holes (112) therein and having a reflective surface (116) facing the light source (102) and the other surface facing the collimator (120). The reflective sheet (110) is configured to transmit a part (114) of a light beam (108) to the collimator (120) and configured to reflect to a reflector (106) other parts (118) of the light beam (108).

Description

A collimator for use in a backlight liquid crystal display system

Field of invention

This invention relates to a collimator for use in a backlight liquid crystal display (LCD) system.

Background of invention

A backlight LCD generally consists of a light source for providing light to illuminate through a collimator (e.g. Selfoc lens) to a liquid crystal array and a diffuser screen. The light must be projected on the liquid crystal array orthogonally to the plane of the liquid crystal array having a maximum allowable deviation of +/- 30° in both horizontal and vertical directions. Currently, this collimation is achieved by positioning a matrix of hollow cylinders in front of the light source and light that exceeds the 30° limit is absorbed in the cylinder walls. Only light within the 30° limit is transmitted through the collimator.

International patent application no.: WO 01/77745, which is hereby incorporated by reference, discloses a use of gradient index (GRIN) technique for forming of a two-element micro-prism array plate for collimating and focusing incident white light on sub-pixels of a backlight liquid crystal display system.

The prior art technology is disadvantageous because it is fragile and has an unsatisfactory light efficiency. The collimator lenses are easily damaged upon lamination on top of the display surface.

Summary of the invention

An object of the present invention is to provide a backlight LCD system having a high light efficiency.

It is a further object of the present invention to provide a collimator ensuring the high light efficiency while providing a robust structure.

The above objects together with numerous other objects, advantages and features, which will become evident from below detailed description, are obtained according to a first aspect of the present invention by a system for lighting a display and comprising: a light source adapted to generate a light beam and having a reflector adapted to reflect light into said light beam, a collimator configured to receive said light beam and adapted to collimate said light into a collimated light beam, a liquid crystal display configured to receive said collimated light beam and adapted to modulate said light beam, and a reflective sheet having one or more pin holes therein and having a reflective surface facing said light source and the other surface facing said collimator, and said one or more pin holes being configured to transmit a part of said light beam to said collimator and said reflective surface being configured to reflect to said reflector other parts of said light beam.

The part of the light beam hitting the one or more pin holes is transmitted through the pin holes and onto the collimator, while the other parts of the light beam are reflected by the reflective surface. The reflected light beam is directed to the reflector, which may subsequently reflect the light back towards the reflecting sheet. Sooner or later the light reflected forth and back between the reflector and the reflecting sheet will enter the one or more pin holes and be transmitted to the collimator. Hence the light efficiency may be significantly increased as a function of the number of reflections and the reflection coefficient.

The light source is for example an array of light emitting diodes, or one or a plurality of fluorescent lamps associated with a light guide. The light beam and collimated light beam have a diameter substantially corresponding to that of the screen of the liquid crystal display. The liquid crystal display comprises a plurality of individually addressable picture elements (pixels), each pixel being arranged to modulate a corresponding part of the light beam, so that an image can be displayed on the screen.

The collimator according to the first aspect of the present invention may comprise a gradient index (GRIN) lens matrix. The GRIN lens matrix may comprise a first material having a first refractive index and a second material having a second refractive index. The reflective sheet may be positioned so that the one or more pin holes are placed before the first material.

According to a second aspect of the present invention, an optical element for collimating light in a backlighting system is provided, which comprises: a collimator configured to receive a light beam and adapted to collimate said light into a collimated light beam, and a reflective sheet having one or more pin holes therein and having a reflective surface facing outward and the other surface facing said collimator, and said one or more pin holes being configured to transmit a part of said light beam to said collimator and said reflective surface being configured to reflect other parts of said light beam.

The optical component according to the second aspect of the present invention may advantageously be applied in a backlighting system for a liquid crystal display. The collimator may further advantageously be implemented as a GRTN lens matrix as described above with reference to the system according to the first aspect of the present invention.

The present invention further relates to a method for manufacturing the collimator and reflection sheet according to the first aspect of the present invention and the optical component according to the second aspect of the present invention.

The method includes spin-coating of a mixture of reactive mesogens on an alignment; layer of a substrate. The molecules form a nematic phase with a corresponding birefringence, which is a function of the order parameter of the molecules. The advantage of using these molecules is that the order may be decreased locally.

The decrease may be achieved by irradiation, whereupon the birefringence is decreased locally with an amount depending on irradiation dose. The orientation of the molecules coated on to the optical component according to the second aspect of the present invention may then be fixed by ultraviolet (UV) polymerization.

This manufacturing method according to the third aspect of the present invention advantageously yields flat optical components with a gradient in birefringence (or alternating values of birefringence such that the introduced aberration is compensated). The flat nature of the optical components facilitate the stacking in a light path.

Further, the optical components may be made polarization insensitive by stacking two optical components on top of each other. Alternatively, the optical components may be made polarization insensitive by using homeotropically aligned liquid crystal phases.

Brief description of the drawings

The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non- limiting detailed description of preferred embodiments of the present invention, with reference to the appended drawing, wherein:

Fig. Ia, shows a first cross sectional view of the optical component according to a first embodiment of the present invention; Fig. Ib, shows a second cross sectional view of a collimator in the optical component according to the first embodiment of the present invention; and

Figs. 2a through 2d show the method for the manufacture of the optical component according to the first embodiment of the present invention.

Detailed description of preferred embodiments

In the following description of the various embodiments, reference is made to the accompanying figures, which show by way of illustration various embodiments. It is to be understood that other embodiments may be utilized and structural and functional modifications may be made without departing from the scope of the present invention. Figure Ia, shows an optical component according to a first embodiment of the present invention and designated in entirety by reference numeral 100. Figure Ia further shows a light source designated in entirety by reference numeral 102 and comprising one or more light generating units 104 positioned by a reflector 106. The light source 102 provides a light beam, indicated by arrows 108a and 108b and generally referred to in the text as 108, which is projected towards the optical component 100.

The optical unit 100 comprises a reflective sheet 110 having one or more pin holes 112 transmitting a part 114 of the light beam 108 through the reflective sheet 110 and having a reflective surface 116 reflecting the parts 118 of the light beam 108 not hitting the one or more pin holes 112. The reflective surface 116 reflects the light back toward the reflector 106, which again reflects the light back toward the optical component 100. The light efficiency of a backlighting system utilizing this optical component 100 and the light source 102 is thereby increased significantly. Since the reflected parts 118 of the light beam 108 are not absorbed in the optical component 100 but rather reflected back to the light source 102 for further use. The optical component 100 further comprises a collimator 120 having two materials with different refractive indexes, such as a gradient index (GRIN) matrix lens. The collimator 120 thus comprises a first material 122 having a first refractive index and a second material 124 having a second refractive index. The first refractive index is larger than the second refractive index, thus allowing light 114 projected onto the collimator 120 through the second material 124 while refracting light 114 projected onto first material 122 of the collimator 120.

The collimator 120 is positioned relative to the reflecting sheet 110, so that the one or more pin holes 112 transmitting part 114 of the light beam is substantially positioned by the second material 124 of the collimator 120. Hence the part 114 of the light 108 is output from the optical component 100 as a collimated light beam 126, which may be projected onto a LCD for further processing.

The reflecting sheet 110 is shown in figure Ia as being a specific distance from the collimator 120. This is shown only for illustrative purposes, since the reflective sheet in fact may be a layer in contact with the collimator 120. In addition, the light source 102 may also be in contact with the reflective sheet 110 thereby providing a stacked optical component generating collimated light. The flat nature of the optical component(s) facilitates the stacking of a light path.

The optical component 100 is birefringent and therefore polarization sensitive. However, by stacking two optical components 100 on top of one another a polarization insensitive optical composition is achieved. Alternatively, the optical component 100 may be made polarization insensitive by using homeotropically aligned liquid crystal phases. In this case a mask with reversed shading is used for obtaining the desired effect due to different orientations of the liquid crystal molecules. Figure Ib, shows a cross sectional view of the collimator 120 taken along the line A-A. The border between the first and second materials 122 and 124 respectively is not to be construed as a hard border, but may be understood as a soft border where the change in refractive index from the first to the second material 122 and 124 is a gradual change. Figures 2a through 2d illustrate a particularly efficient method for the manufacture of the optical component according to the first embodiment of the present invention. The method is largely similar to the method described earlier in the applicant's unpublished international patent application PCT/IB2004/050280.

A substrate 200, such as glass, plastic or the like, is provided with an alignment layer 202. The alignment layer 202 comprises a surface 204 enabling monodomain formation. That is, a surface which is rubbed (or photo-aligned), plasma-treated, or ion beam- treated polyimide, or an inorganic layer. The surface 204 is spin-coated with a mixture of photo-isomerizable compounds, nematic compounds and a photo-initiator at a rate ensuring that a retardation film 206 is provided. A pixelated mask 208 with black pixels (T=O) 210 and white pixels (T=IOO) 212 is placed above the retardation film 206 and, subsequently, the masked retardation film 206 receives a gradient UV radiation 214, while being exposed to air. The retardation film 206 covered by the black pixels is not isomerized, while the retardation film 206 covered by the white pixels is isomerized. The total exposure of UV radiation 214 is selected so as to achieve that the retardation film 206 below the white pixels of the mask 208 will decrease its birefringence thereby becoming substantially isotropic.

The retardation film 206 is annealed and, subsequently, subjected to an atmosphere of nitrogen while being flooded by further UV radiation 216. This initiates the photo-polymerization of the retardation film 206.

Subsequent to the flooding the substrate 200 with further UV radiation 216 the substrate 200 is subjected to thermal polymerization at 150°C for 2 hours in order to obtain a high conversion of the polymerization reaction. Any changes in the optical properties of the retardation film 206 due to polymerization may be corrected by changing the composition of monomers, the total UV radiation used for the isomerization reaction and the gray level in the mask 208.

A reactive mesogen that forms a nematic phase comprises of a rigid group connected to two polymerizable groups, such as an acrylate, through oligomethylene spacers. The polymerizable groups allow crosslinking induced by UV light. When the birefringent material is spin-coated upon an alignment layer, commonly a rubbed polyimide, a planarly aligned monodomain is created. The retardation of the spin-coated film is the product of the thickness of the film and the birefringence. DOEs demand isotropic domains (i.e. Δn= 0) next to birefringent domains, whereas non-periodic phase structures require domains with a different birefringence. Both demands can be fulfilled by changing the Δn through irradiation. In order to change the Δn of a film made of a nematic mixture keeping the total thickness of the film constant, the anisotropy of polarizability of one of the chemical compounds has to be changed, by changing the order, preferably by irradiation. If one of the chemical compounds contains an isomerizable olefmic group as for example in derivatives of stilbene or cinnamic acid, E-Z photo-isomerization will lower the Δn due to the fact that the anisotropy of polarizability of the Z isomer is lower than that of the E isomer. Furthermore, the liquid crystalline properties of the Z isomer will be such that the isotropic transition of the mixture will decrease upon irradiation. Hence, in addition to the change in anisotropy of polarizability, the order of the system decreases upon isothermal isomerization as the order parameter is a function of the isotropic transition. The invention in this embodiment includes, thus, two driving forces for a decrease in Δn: a change in anisotropy of polarizability and a change in order parameter. Also other photochemical processes such as cyclo-addition or fragmentation can be used or may occur simultaneously with the isomerization process.

In order to stabilize the properties of the optical film, the individual components should contain polymerizable groups which may be photo-polymerized after tuning the optical properties.

The advantage of photo-polymerization over thermal polymerization is the fact that the photochemical process can be performed isothermally, avoiding changes in the optical properties due to temperature changes before or during polymerization. The isomerization process should be performed in air to avoid interference of both photochemical processes (i.e. isomerization and polymerization). The presence of oxygen will inhibit polymerization in the case of radical initiated polymerization allowing for the occurrence of isomerization as the only process.

After changing the atmosphere to an inert one (nitrogen or a noble gas) photo- polymerization takes place upon UV exposure. In order to prepare a thermally, mechanically and chemically stable film, it should comprise at least 20% of tetrafunctional chemical compounds that form a crosslinked polymeric network.

Due to the fast polymerization and crosslinking, the UV light has no or only a minor effect on the optical properties. The decoupling between the isomerization and polymerization processes can also be made by using different wavelengths of irradiation.

This for example is necessary for the use of photo-isomerizable species reacting cationically. These compounds are not sensitive to the presence of oxygen, and can therefore not be used in a combination with the former process, where oxygen acts as an inhibitor, preventing the polymerization reaction. To adjust the birefringence locally, the irradiation to isomerize the components should be performed with the aid of a gray scale mask. The exposure on each domain determines the birefringence, while a flat film is maintained.

Claims

CLAIMS:

1. A system for backlighting a display, comprising: a light source (102) adapted to generate a light beam (108) and having a reflector (106) adapted to reflect light into said light beam (108), a collimator (120) configured to receive a part (114) light beam (108) and adapted to collimate said part (114) into a collimated light beam (126), a liquid crystal display configured to receive said collimated light beam (126) and adapted to modulate said collimated light beam (126), and a reflective sheet (110) having one or more pin holes (112) therein and having a reflective surface (116) facing said light source (102) and the other surface facing said collimator (120), and said one or more pin holes (112) being configured to transmit said part (114) of said light beam (108) to said collimator (120) and said reflective surface (116) being configured to reflect to said reflector (106) other parts (118) of said light beam (108).

2. A system according to claim 1, wherein said collimator (120) comprises a gradient index (GRIN) lens matrix.

3. A system according to claim 2, wherein the GRIN lens matrix comprises a first material (122) having a first refractive index and a second material (124) having a second refractive index.

4. A system according to claims 3, wherein said reflective sheet (110) is positioned so that said one or more pin holes (112) are placed before said second material (124).

5. An optical component (100) for collimating light in a backlighting system, and comprising: a collimator (120) configured to receive a light beam (108) and adapted to collimate a part (114) of said light (108) into a collimated light beam (126), and a reflective sheet (110) having one or more pin holes (112) therein and having a reflective surface (116) facing outward and the other surface facing said collimator (120), and said one or more pin holes (112) being configured to transmit said part (114) of said light beam (108) to said collimator (120) and said reflective surface (116) being configured to reflect other parts (118) of said light beam (108).

6. A liquid crystal display, including a backlighting system according to any of claims 1 to 4.

7. A method for manufacturing said collimator and reflection sheet according to any of claims 1 to 4 and the optical component according claim 5 and comprising: spin-coating of a mixture of reactive compounds on an alignment layer (202) of a substrate (200) thereby establishing a retardation film (206); masking said retardation film (206) with a pixelated mask (208) and, subsequently, irradiating the masked retardation film (206); annealing said retardation film (206) and, subsequently, subjecting said retardation film (206) to an atmosphere of nitrogen while being further irradiated; and baking said substrate 200 by further irradiation thereby obtaining a high conversion of a polymerization reaction.