Composite diffuser plates and direct-lit liquid crystal displays using same Number:7,436,469 from the United States Patent and Trademark Office (PTO) owispatent

Title: Composite diffuser plates and direct-lit liquid crystal displays using same

Abstract: In a directly-illuminated liquid crystal display (LCD), for example an LCD monitor or an LCD-TV, a number of light management layers lie between the light source and the LCD panel to provide bright, uniform illumination. The light management layers, including, for example, a diffuser, a reflective polarizer and a brightness enhancing layer, are contained in a light management unit that is formed from two subassemblies. The two subassemblies each contain a substrate and are attached together so as to leave a gap between the two subassemblies. The diffuser is located in one of the subassemblies, and the other light management layers may be in either of the subassemblies, or may be disposed in the gap between the subassemblies.

Patent Number: 7,436,469 Issued on 10/14/2008 to Gehlsen,   et al.

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Inventors:	Gehlsen; Mark D. (Eagan, MN), Chen; Chingwen (Taoyuan, TW), Ko; Byungsoo (Hwasugn, KR), Emmons; Robert M. (St. Paul, MN), Laumer; James W. (White Bear Lake, MN), Fabick; Ryan T. (St. Paul, MN), Rivard; Linda M. (Stillwater, MN), Epstein; Kenneth A. (St. Paul, MN), Park; Youngsoo (Suwon, KR), Kim; Chideuk (Sungnam, KR), Stevenson; James A. (St. Paul, MN)
Assignee:	3M Innovative Properties Company (St. Paul, MN)
Appl. No.:	10/965,937
Filed:	October 15, 2004

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Current U.S. Class:	349/62 ; 349/63; 349/64; 349/65
Current International Class:	G02F 1/1335 (20060101)
Field of Search:	349/62-65

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References Cited [Referenced By]

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Primary Examiner: Nelms; David C.
Assistant Examiner: Vu; Phu
Attorney, Agent or Firm: Pralle; Jay R.

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Claims

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We claim:

1. A light management unit for use between a light source and a liquid crystal display panel, comprising: a first optical subassembly comprising at least a first substrate, the first optical subassembly also comprising one or more diffusing elements; a second optical subassembly comprising at least a second substrate; and a spacer positioned between the first and second subassemblies, the spacer spacing the first and second subassemblies apart to define the gap, the second optical subassembly being mounted to the first subassembly, wherein the spacer is formed of an adhesive foam tape adhering to the first and second subassemblies.

2. A unit as recited in claim 1, wherein the one or more diffusing elements comprise at least a first diffuser layer.

3. A unit as recited in claim 2, wherein the first diffuser layer is attached to the first substrate via an adhesive layer disposed between the first diffuser layer and the first substrate.

4. A unit as recited in claim 2, wherein the first diffuser layer is provided on a side of the first substrate facing towards the second subassembly, and the first subassembly comprises a second diffuser layer provided to a side of the first substrate facing away from the second subassembly.

5. A unit as recited in claim 1, wherein the first optical subassembly provides a diffusion characteristic that is uniform across the width of the first optical subassembly.

6. A unit as recited in claim 1, wherein the one or more diffusing elements comprises the first substrate, the first substrate being a bulk diffuser plate.

7. A unit as recited in claim 1, at least one of a reflective polarizer and a brightness enhancing layer being attached to either the first or second optical subassembly or being disposed in the gap between the first and second optical subassemblies.

8. A unit as recited in claim 7, wherein the second optical subassembly comprises the brightness enhancing layer.

9. A unit as recited in claim 8, wherein the second optical subassembly comprises the reflective polarizer.

10. A unit as recited in claim 7, wherein the second optical subassembly comprises the reflective polarizer.

11. A unit as recited in claim 1, wherein the single pass optical transmission through the one or more diffusing elements is in the range from about 72%-95%.

12. A unit as recited in claim 11, wherein the range is from about 75%-90%.

13. A unit as recited in claim 1, wherein the spacer is positioned along peripheral edges of respective surfaces of the first and second subassemblies.

14. A display system, comprising: a backlight; a liquid crystal display (LCD) panel comprising upper and lower plates and a liquid crystal layer disposed between the upper and lower plates; and a light management unit disposed between the backlight and the LCD panel, the light management unit having a first optical subassembly comprising a first substrate and a second optical subassembly comprising a second substrate; and a spacer positioned between the first and second subassemblies, the spacer spacing the first and second subassemblies apart to define the gap, the second optical subassembly being mounted to the first subassembly, wherein the spacer is formed of an adhesive foam tape adhering to the first and second subassemblies, the light management unit diffusing light passing from the backlight to the LCD panel.

15. A system as recited in claim 14, wherein the backlight comprises a plurality of light sources disposed between a reflector and the first optical subassembly.

16. A system as recited in claim 15, wherein the light sources comprise fluorescent lamps.

17. A system as recited in claim 14, wherein the liquid crystal display panel comprises first and second absorbing polarizers on respective first and second sides.

18. A system as recited in claim 14, wherein the first light management unit comprises at least a first diffuser layer.

19. A system as recited in claim 18, wherein the first diffuser layer is attached to the first substrate via an adhesive layer disposed between the first diffuser layer and the first substrate.

20. A system as recited in claim 18, wherein the first diffuser layer is provided on one side of the first substrate and the first optical subassembly comprises a second diffuser layer provided on a second side of the first substrate.

21. A system as recited in claim 14, wherein the first substrate comprises a bulk diffuser plate.

22. A system as recited in claim 14, wherein the light management unit provides a diffusion characteristic that is substantially uniform across its width.

23. A system as recited in claim 14, wherein the first optical subassembly comprises a brightness enhancing layer.

24. A system as recited in claim 14, wherein the second optical subassembly comprises a brightness enhancing layer.

25. A system as recited in claim 14, wherein the second optical subassembly comprises a reflective polarizer.

26. A system as recited in claim 14, wherein the light management unit further comprises a brightness enhancing layer and a reflective polarizer.

27. A system as recited in claim 26, wherein the brightness enhancing layer and the reflective polarizer are in the same optical subassembly.

28. A system as recited in claim 26, wherein the brightness enhancing layer and the reflective polarizer are in different. optical subassemblies.

29. A system as recited in claim 14, wherein the light management unit comprises at least one diffusing element to diffuse light passing from the backlight to the LCD panel, the single pass optical transmission through the at least one diffusing element is in the range from about 72%-95%.

30. A system as recited in claim 29, wherein the range is from about 75%-90%.

31. A system as recited in claim 14, further comprising a spacer positioned between the first and second optical subassemblies, the spacer spacing the first and second subassemblies apart to define the gap.

32. A system as recited in claim 31, wherein the spacer adhesively holds the first and second subassemblies together.

33. A system as recited in claim 31, wherein the spacer is positioned along peripheral edges of respective surfaces of the first and second subassemblies.

34. A system as recited in claim 14, wherein a value of .sigma./I for light between the light management unit and the LCD panel is less than 1.5%, where I is the level of illumination light passing from the light management unit to the LCD panel, averaged across the LCD panel, and a is the root mean square deviation in the level of illumination light entering the LCD panel.

35. A system as recited in claim 34, wherein the value of .sigma./I is less than 1.3%.

36. A system as recited in claim 14, further comprising a controller coupled to control an image displayed by the LCD panel.

37. A system as recited in claim 36, wherein the controller comprises a computer.

38. A system as recited in claim 36, wherein the controller comprises a television controller.

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Description

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FIELD OF THE INVENTION

The invention relates to optical displays, and more particularly to liquid crystal displays (LCDs) that are directly illuminated by light sources from behind, such as may be used in LCD monitors and LCD televisions.

BACKGROUND

Liquid crystal displays (LCDs) are optical displays used in devices such as laptop computers, hand-held calculators, digital watches and televisions. Some LCDs include a light source that is located to the side of the display, with a light guide positioned to guide the light from the light source to the back of the LCD panel. Other LCDs, for example some LCD monitors and LCD televisions (LCD-TVs) are directly illuminated using a number of light sources positioned behind the LCD panel. This arrangement is increasingly common with larger displays, because the light power requirements, to achieve a certain level of display brightness, increase with the square of the display size, whereas the available real estate for locating light sources along the side of the display only increases linearly with display size. In addition, some LCD applications, such as LCD-TVs, require that the display be bright enough to be viewed from a greater distance than other applications, and the viewing angle requirements for LCD-TVs are generally different from those for LCD monitors and hand-held devices.

Some LCD monitors and most LCD-TVs are commonly illuminated from behind by a number of cold cathode fluorescent lamps (CCFLs). These light sources are linear and stretch across the full width of the display, with the result that the back of the display is illuminated by a series of bright stripes separated by darker regions. Such an illumination profile is not desirable, and so a diffuser plate is used to smooth the illumination profile at the back of the LCD device.

Currently, LCD-TV diffuser plates employ a polymeric matrix of polymethyl methacrylate (PMMA) with a variety of dispersed phases that include glass, polystyrene beads, and CaCO.sub.3 particles, or blends thereof. These plates often deform or warp after exposure to the elevated temperatures of the lamps. In addition, some diffusion plates are provided with a diffusion characteristic that varies across its width, in an attempt to make the illumination profile at the back of the LCD panel more uniform. Such non-uniform diffusers are sometimes referred to as printed pattern diffusers. They are expensive to manufacture, since the diffusing pattern must be registered to the illumination source. In addition, the diffusion plates require customized extrusion compounding to distribute the diffusing particles uniformly throughout the polymer matrix, which further increases costs.

SUMMARY OF THE INVENTION

One embodiment of the invention is directed to a light management unit for use between a light source and a liquid crystal display panel. The light management unit has a first optical subassembly and a second optical subassembly. The first optical subassembly comprises at least a first substrate and one or more diffusing elements. The second optical subassembly comprises at least a second substrate and is mounted to the first subassembly in such a manner as to produce a gap between the first and second optical subassemblies.

Another embodiment of the invention is directed to a display system having a backlight and a liquid crystal display (LCD) panel comprising upper and lower plates and a liquid crystal layer disposed between the upper and lower plates. A light management unit is disposed between the backlight and the LCD panel and has a first optical subassembly comprising a first substrate and a second optical subassembly comprising a second substrate. The second optical subassembly is mounted to the first subassembly in such a manner as to produce a gap between the first and second optical subassemblies. The light management unit diffuses light passing from the backlight to the LCD panel.

The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:

FIG. 1 schematically illustrates a back-lit liquid crystal display device that is capable of using a diffuser plate according to principles of the present invention;

FIGS. 2A-2E schematically illustrate embodiments of light management units according to principles of the present invention;

FIGS. 3A-3I schematically illustrate embodiments of light management units that incorporate a brightness enhancing layer according to principles of the present invention;

FIGS. 4A-4G schematically illustrate embodiments of light management units that incorporate a reflecting polarizer according to principles of the present invention;

FIGS. 5A-5F schematically illustrate embodiments of light management units that incorporate a reflecting polarizer and a brightness enhancing layer according to principles of the present invention;

FIGS. 6A-6C schematically illustrate embodiments of light management units attached to a flat fluorescent light source, according to principles of the present invention;

FIG. 7A schematically illustrates an experimental set up used for optically testing sample light management units;

FIG. 7B schematically illustrates a construction of a composite light management unit according to embodiments of the present invention;

FIG. 8A presents a graph showing brightness uniformity plotted against overall brightness for control samples and example light management units fabricated in accordance with principles of the present invention;

FIG. 8B presents a graph showing axial gain plotted against integrated gain control samples and sample light management units;

FIG. 9 presents a graph showing luminance as a function of position across a screen for two control samples and sample light management units S2, S8, S26 and S27;

FIGS. 10A and 10B schematically show the structure of samples S28 and S29 respectively;

FIG. 11 presents a graph showing luminance as a function of position across a screen for two control samples and sample light management units S28, S29 and S30;

FIG. 12 schematically shows the structure of samples S31, S33, S34 and S35;

FIG. 13 presents a graph showing luminance as a function of position across a screen for two control samples and sample light management units S31, S33, S34 and S35;

FIG. 14 schematically shows the structure of samples S32, S36, and S38;

FIG. 15 presents a graph showing luminance as a function of position across a screen for two control samples and sample light management units S32, S36, and S38;

FIG. 16 schematically shows the structure of samples S39-2 and S39-3;

FIG. 17 presents a graph showing luminance as a function of position across a screen for two control samples and sample light management units S39-2 and S39-3;

FIG. 18 presents a summary list of process steps for manufacturing a light management unit according to principles of the present invention;

FIGS. 19A and 19B schematically present one embodiment of an arrangement for fabricating a subassembly according to principles of the present invention;

FIGS. 20A and 20B schematically present another embodiment of an arrangement for fabricating a subassembly according to the present invention;

FIG. 21 schematically presents another embodiment of an arrangement for fabricating a subassembly according to principles of the present invention;

FIG. 22 schematically illustrates an embodiment of an arrangement for assembling a light management unit from pre-assembled subassemblies, according principles of the present invention;

FIGS. 23A and 23B schematically present other embodiments of arrangements for fabricating a subassembly according to principles of the present invention; and

FIG. 24 presents a graph showing brightness uniformity plotted as a function of single pass transmission through the diffuser plate for several sample uniform light management units and for a printed diffuser plate.

While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

The present invention is applicable to liquid crystal displays (LCDs, or LC displays), and is particularly applicable to LCDs that are directly illuminated from behind, for example as are used in LCD monitors and LCD televisions (LCD-TVs).

The diffuser plates currently used in LCD-TVs are based on a polymeric matrix, for example polymethyl methacrylate (PMMA), polycarbonate (PC), or cyclo-olefins, formed as a rigid sheet. The sheet contains diffusing particles, for example, organic particles, inorganic particles or voids (bubbles). These plates often deform or warp after exposure to the elevated temperatures of the light sources used to illuminate the display. These plates also are expensive to manufacture and to assemble in the final display device.

The invention is directed to a directly-illuminated LCD device that has an arrangement of light management layers positioned between the LCD panel and the light source. In some embodiments, one or more of the light management layers are included in a light management unit that is formed from two optical subassemblies attached together with a gap therebetween. Each optical subassembly includes a supporting layer, often referred to as a substrate, may also include one or more light management layers. The substrates may be organic or inorganic substrates. The light management units are used to provide various optical functions, such as diffusion, polarization and gain (on-axis brightness enhancement), to light that passes from the light source to the LCD panel.

Light management units of the present invention are simple to manufacture and provide a high degree of flexibility in the materials and processes used in manufacturing. Combining the various functions of the light management unit into a single, integrated optical unit allows for superior optical designs. In contrast, the conventional approach is to provide different light management functions in different elements: optimization of each of these separate elements doe not often lead to the best overall system design.

In a light management unit according to some embodiments of the present invention, the structural and optical requirements may be separated: the substrate provides structural performance and one or more attached diffuser layers provide the optical performance. By separating these functions, the cost advantages of using common transparent materials and common diffuser sheets can be exploited, to reduce overall costs. This also permits the introduction of warp resistant plates, for example glass plates, at low cost. In addition, it is easier to control the diffusion properties more precisely when the diffuser is contained in a layer separate from the plate. Also, patterned diffuser films can be used at significantly less expense than with patterned, rigid, bulk diffuser plates.

A schematic exploded view of an exemplary embodiment of a direct-lit LC display device 100 is presented in FIG. 1. Such a display device 100 may be used, for example, in an LCD monitor or LCD-TV. The display device 100 is based on the use of an LC panel 102, which typically comprises a layer of LC 104 disposed between panel plates 106. The plates 106 are often formed of glass, and may include electrode structures and alignment layers on their inner surfaces for controlling the orientation of the liquid crystals in the LC layer 104. The electrode structures are commonly arranged so as to define LC panel pixels, areas of the LC layer where the orientation of the liquid crystals can be controlled independently of adjacent areas. A color filter may also be included with one or more of the plates 106 for imposing color on the image displayed.

An upper absorbing polarizer 108 is positioned above the LC layer 104 and a lower absorbing polarizer 110 is positioned below the LC layer 104. In the illustrated embodiment, the upper and lower absorbing polarizers are located outside the LC panel 102. The absorbing polarizers 108, 110 and the LC panel 102 in combination control the transmission of light from the backlight 112 through the display 100 to the viewer. In some LC displays, the absorbing polarizers 108, 110 may be arranged with their transmission axes perpendicular. When a pixel of the LC layer 104 is not activated, it may not change the polarization of light passing therethrough. Accordingly, light that passes through the lower absorbing polarizer 110 is absorbed by the upper absorbing polarizer 108, when the absorbing polarizers 108, 110 are aligned perpendicularly. When the pixel is activated, on the other, hand, the polarization of the light passing therethrough is rotated, so that at least some of the light that is transmitted through the lower absorbing polarizer 110 is also transmitted through the upper absorbing polarizer 108. Selective activation of the different pixels of the LC layer 104, for example by a controller 114, results in the light passing out of the display at certain desired locations, thus forming an image seen by the viewer. The controller may include, for example, a computer or a television controller that receives and displays television images. One or more optional layers 109 may be provided over the upper absorbing polarizer 108, for example to provide mechanical and/or environmental protection to the display surface. In one exemplary embodiment, the layer 109 may include a hardcoat over the absorbing polarizer 108.

It will be appreciated that some type of LC displays may operate in a manner different from that described above. For example, the absorbing polarizers may be aligned parallel and the LC panel may rotate the polarization of the light when in an unactivated state. Regardless, the basic structure of such displays remains similar to that described above.

The backlight 112 includes a number of light sources 116 that generate the light that illuminates the LC panel 102. The light sources 116 used in a LCD-TV or LCD monitor are often linear, cold cathode, fluorescent tubes that extend across the display device 100. Other types of light sources may be used, however, such as filament or arc lamps, light emitting diodes (LEDs), flat fluorescent panels or external fluorescent lamps. This list of light sources is not intended to be limiting or exhaustive, but only exemplary.

The backlight 112 may also include a reflector 118 for reflecting light propagating downwards from the light sources 116, in a direction away from the LC panel 102. The reflector 118 may also be useful for recycling light within the display device 100, as is explained below. The reflector 118 may be a specular reflector or may be a diffuse reflector. One example of a specular reflector that may be used as the reflector 118 is Vikuiti.TM. Enhanced Specular Reflection (ESR) film available from 3M Company, St. Paul, Minn. Examples of suitable diffuse reflectors include polymers, such as PET, PC, PP, PS loaded with diffusely reflective particles, such as titanium dioxide, barium sulphate, calcium carbonate or the like. Other examples of diffuse reflectors, including microporous materials and fibril-containing materials, are discussed in co-owned U.S. Patent Application Publication 2003/0118805 A1, incorporated herein by reference.

An arrangement 120 of light management layers is positioned between the backlight 112 and the LC panel 102. The light management layers affect the light propagating from backlight 112 so as to improve the operation of the display device 100. For example, the arrangement 120 of light management layers may include a diffuser 122. The diffuser 122 is used to diffuse the light received from the light sources, which results in an increase in the uniformity of the illumination light incident on the LC panel 102. Consequently, this results in an image perceived by the viewer that is more uniformly bright.

The arrangement 120 of light management layers may also include a reflective polarizer 124. The light sources 116 typically produce unpolarized light but the lower absorbing polarizer 110 only transmits a single polarization state, and so about half of the light generated by the light sources 116 is not transmitted through to the LC layer 104. The reflecting polarizer 124, however, may be used to reflect the light that would otherwise be absorbed in the lower absorbing polarizer, and so this light may be recycled by reflection between the reflecting polarizer 124 and the reflector 118. At least some of the light reflected by the reflecting polarizer 124 may be depolarized, and subsequently returned to the reflecting polarizer 124 in a polarization state that is transmitted through the reflecting polarizer 124 and the lower absorbing polarizer 110 to the LC layer 104. In this manner, the reflecting polarizer 124 may be used to increase the fraction of light emitted by the light sources 116 that reaches the LC layer 104, and so the image produced by the display device 100 is brighter.

Any suitable type of reflective polarizer may be used, for example, multilayer optical film (MOF) reflective polarizers; diffusely reflective polarizing film (DRPF), such as continuous/disperse phase polarizers, wire grid reflective polarizers or cholesteric reflective polarizers.

Both the MOF and continuous/disperse phase reflective polarizers rely on the difference in refractive index between at least two materials, usually polymeric materials, to selectively reflect light of one polarization state while transmitting light in an orthogonal polarization state. Some examples of MOF reflective polarizers are described in co-owned U.S. Pat. No. 5,882,774, incorporated herein by reference. Commercially available examples of a MOF reflective polarizers include Vikuiti.TM. DBEF-D200 and DBEF-D440 multilayer reflective polarizers that include diffusive surfaces, available from 3M Company, St. Paul, Minn.

Examples of DRPF useful in connection with the present invention include continuous/disperse phase reflective polarizers as described in co-owned U.S. Pat. No. 5,825,543, incorporated herein by reference, and diffusely reflecting multilayer polarizers as described in e.g. co-owned U.S. Pat. No. 5,867,316, also incorporated herein by reference. Other suitable types of DRPF are described in U.S. Pat. No. 5,751,388.

Some examples of wire grid polarizers useful in connection with the present invention include those described in U.S. Pat. No. 6,122,103. Wire grid polarizers are commercially available from, inter alia, Moxtek Inc., Orem, Utah.

Some examples of cholesteric polarizer useful in connection with the present invention include those described in, for example, U.S. Pat. No. 5,793,456, and U.S. Patent Publication No. 2002/0159019. Cholesteric polarizers are often provided along with a quarter wave retarding layer on the output side, so that the light transmitted through the cholesteric polarizer is converted to linear polarization.

The arrangement 120 of light management layers may also include a brightness enhancing layer 128. A brightness enhancing layer is one that includes a surface structure that redirects off-axis light in a direction closer to the axis of the display. This increases the amount of light propagating on-axis through the LC layer 104, thus increasing the brightness of the image seen by the viewer. One example is a prismatic brightness enhancing layer, which has a number of prismatic ridges that redirect the illumination light, through refraction and reflection. Examples of prismatic brightness enhancing layers that may be used in the display device include the Vikuiti.TM. BEFII and BEFIII family of prismatic films available from 3M Company, St. Paul, Minn., including BEFII 90/24, BEFII 90/50, BEFIIIM 90/50, and BEFIIIT.

The diffuser and one or more other light management layers may be included in a light management unit disposed between the backlight and the LCD panel. The light management unit comprises a stack of attached layers and provides a stable structure for holding the diffuser and the one or other light management layers. The structure is less prone to warping than conventional diffuser plates. Also, the ability to supply a display manufacturer with a light management unit that contains a diffuser plate and one or more other light management layers as a single integrated unit results in simplified assembly of the display.

Several different exemplary embodiments of light management unit are schematically shown in cross-sectional views in FIGS. 2A-2E. In FIG. 2A, a light management unit 200 comprises a first optical subassembly 202 and a second optical subassembly 204 separated by a gap 206. In the illustrated embodiment, a spacer 208 is disposed between the first optical subassembly 202 and the second optical subassembly 204 to separate the first and second optical subassemblies 202 and 204, resulting in the gap 206. In some exemplary embodiments, the spacer 208 is disposed around the edge of the unit 200, so that the light from the backlight passes through the gap 206 rather than the spacer 208. In addition, in some exemplary embodiments, the spacer 208 may act as a seal around the gap 206, to avoid the ingress of dust and the like into the gap 206.

A number of different optical layers, including a diffuser, a brightness enhancing layer and a reflective polarizer may be included in the first and/or second subassemblies. The following description discusses a number of different embodiments of light management unit in which the diffuser, brightness enhancing layer and/or reflective polarizer are located at different positions in the first or second subassemblies.

An optical subassembly comprises at least one optical layer, and the optical layers are attached together where there are two or more layers. The first subassembly 202 itself may include a number of different layers or optical sheets attached together, such as a diffuser, a brightness enhancing layer and/or a reflective polarizer layer. In the exemplary embodiment illustrated in FIG. 2A, the first subassembly 202 comprises a diffusive substrate plate 210, sometimes referred to as a diffuser plate. The diffusive substrate 210 may be a bulk diffuser plate formed from a polymer material that incorporates diffusing particles throughout its thickness. The polymer material may be any suitable polymer, such as those listed below. The diffusing particles may be any type of particle useful for diffusing light, for example transparent particles whose refractive index is different from the surrounding polymer matrix, diffusely reflective particles, or voids or bubbles in the matrix. Examples of suitable diffusely reflecting particles include particles of titanium dioxide (TiO.sub.2), calcium carbonate (CaCO.sub.3), barium sulphate (BaSO.sub.4) and the like. The diffusing particles may be distributed with uniform or graded concentration throughout the plate, or may be patterned, for example, to provide greater diffusion above a light source and less diffusion between light sources, for improved uniformity.

The second subassembly 204 contains one or more layers attached together. In one embodiment, the second subassembly 204 includes a substrate 212. The attachment of one subassembly to another results in an I-beam structure that is relatively strong and resistant to bending. This structure also provides an insulating layer of air that may assist in lowering the cavity temperature on the opposite side of the bulb surface.

Another configuration of first subassembly 202 includes a substantially transparent substrate 216 with an attached diffuser layer 218, as is schematically illustrated in FIG. 2B. The substrates 212, 216 may be made of any material that is substantially transparent to visible light, for example, organic or inorganic materials, including glasses and polymers. The substrates 212, 216 of the different subassemblies need not be made of the same material. Suitable glasses include float glasses, i.e. glasses made using a float process, or LCD quality glasses, referred as LCD glass, whose characteristic properties, such as thickness and purity, are better controlled than float glass. Suitable polymer materials may be amorphous or semi-crystalline, and may include homopolymer, copolymer or blends thereof. Polymer foams may also be used. Example polymer materials include, but are not limited to, amorphous polymers such as poly(carbonate) (PC); poly(styrene) (PS); acrylates, for example acrylic sheets as supplied under the ACRYLITE.RTM. brand by Cyro Industries, Rockaway, N.J.; acrylic copolymers such as isooctyl acrylate/acrylic acid; poly(methylmethacrylate) (PMMA); PMMA copolymers; cycloolefins and cycoolefin copolymers; acrylonitrile butadiene styrene (ABS); styrene acrylonitrile copolymers (SAN); epoxies; poly(vinylcyclohexane); PMMA/poly(vinylfluoride) blends; atactic poly(propylene); poly(phenylene oxide) alloys; styrenic block copolymers; polyimide; polysulfone; poly(vinyl chloride); poly(dimethyl siloxane) (PDMS); polyurethanes; poly(carbonate)/aliphatic PET blends; and semicrystalline polymers such as poly(ethylene); poly(propylene); poly(ethylene terephalate) (PET); poly(ethylene naphthalate)(PEN); polyamide; ionomers; vinyl acetate/polyethylene copolymers; cellulose acetate; cellulose acetate butyrate; fluoropolymers; poly(styrene)-poly(ethylene) copolymers; and PET and PEN copolymers.

A substrate is a sheet of material that is self-supporting, and is used to provide support to the layers to which it is attached. While each of the layers in stack of attached layers contributes to the stiffness of the stack, the substrate is the layer that contributes most to the stiffness, i.e. provides more resistance to bending than any of the other layers of the stack. A substrate does not significantly deform under its own weight, although it may sag to a certain extent. The substrate may be, for example, up to a few mm thick, depending on the size of the display and the type of material used. In one exemplary embodiment, a 30'' LCD-TV has a 2 mm thick PMMA bulk diffuser plate. In another exemplary embodiment, a 40'' LCD-TV has a 3 mm thick PMMA bulk diffuser plate.

One or both sides of one or more of the layers in the light management unit, for example, the diffuser layer, the substrate, polarizer or brightness enhancing layer may be provided with a matte finish.

Exemplary embodiments of the diffuser layer include a polymer matrix containing diffusing particles. The polymer matrix may be any suitable type of polymer that is substantially transparent to visible light, for example any of the polymer materials listed above.

The diffusing particles may be any type of particle useful for diffusing light, for example transparent particles whose refractive index is different from the surrounding polymer matrix, diffusely reflective particles, or voids or bubbles in the matrix. Examples of suitable transparent particles include solid or hollow inorganic particles, for example glass beads or glass shells, solid or hollow polymeric particles, for example solid polymeric spheres or hollow polymeric spheres. Examples of suitable diffusely reflecting particles include particles of titanium dioxide (TiO.sub.2), calcium carbonate (CaCO.sub.3), barium sulphate (BaSO.sub.4), magnesium sulphate (MgSO.sub.4) and the like. In addition, voids in the polymer matrix may be used for diffusing the light. Such voids may be filled with a gas, for example air or carbon dioxide. Commercially available materials suitable for use in a diffuser layer include 3M.TM. Scotchcal.TM. Diffuser Film, type 3635-70 and 3635-30, and 3M.TM. Scotchcal.TM. ElectroCut.TM. Graphic Film, type 7725-314, available from 3M Company, St. Paul, Minn. Other commercially available diffusers include acrylic foam tapes, such as 3M.TM. VHB.TM. Acrylic Foam Tape No. 4920.

The diffuser layer 218 may itself be a diffuse adhesive layer, in which case the diffuser layer 218 may be attached directly to the substrate 216, for example, by lamination. Adhesive diffusive layers are discussed in greater detail in International (PCT) Patent Publications WO99/56158 and WO97/01610, incorporated herein by reference. Adhesive diffusive layers may be used in any of the embodiments of light management unit discussed herein. In some exemplary embodiments, the diffuser layer 204 has a diffusion characteristic that is uniform across its width, in other words the amount of diffusion experienced by light is the same for points across the width of the diffuser layer.

In other exemplary embodiments, the diffuser layer 218 may be attached to the surface of the substrate 216 using an adhesive layer 220, as is schematically illustrated in FIG. 2C. In some exemplary embodiments, the adhesive layer 220 may be an optically clear adhesive, a diffusive adhesive, or an acrylic foam tape either with or without optical diffusion.

In the exemplary embodiments illustrated in FIGS. 2B and 2C, the first subassembly 202 is shown with the diffuser layer 218 lying closer to the gap 206 than the substrate 216. This need not be the case, and the substrate 216 may lie closer to the gap 206 than the diffuser layer 218.

The diffuser layer 218 may optionally be supplemented with an additional patterned diffuser 218a. The patterned diffuser 218a may include, for example, a patterned diffusing surface or a printed layer of diffuser, such as particles of titanium dioxide (TiO.sub.2). The patterned layer 218a may lie on the substrate 216, between the diffuser layer 218 and the substrate 216, or above the diffuser layer 218. The patterned diffuser 218a may be, for example, printed onto the diffuser layer 218, as illustrated in FIG. 2B, or onto a sheet that lies above the diffuser layer 218.

In another exemplary embodiment, schematically illustrated in FIG. 2D, the diffuser plate 230 may be double-sided, having a first diffuser layer 232 on one side of the substrate 236 and a second diffuser layer 234 on another side of the substrate 236. The first and second diffuser layers 232, 234 may each be applied directly to the respective surface of the substrate 236, as illustrated, or may be attached using respective adhesive layers.

The double-sided diffuser plate 230 may be symmetrical, with the two diffuser layers 232, 234, having the same diffusion properties, or may be asymmetric, with the diffuser layers 232, 234 having different diffusing properties. For example, the first diffuser layer 232 may possess a different transmission or haze level from the second diffuser layer 234, or may be of a different thickness.

Another exemplary embodiment of light management unit 250 is schematically illustrated in FIG. 2E. In this embodiment, the substrate 212 of the second subassembly 204 also forms the lower panel plate of the LC display panel 102. Other light management layers, not illustrated, may be included in the second subassembly 204, for example between the substrate 212 and the gap 206. Some exemplary configurations of second subassembly 204, in which the uppermost layer of the second subassembly is a substrate that may also constitute the lower panel plate of an LCD panel, are described further below.

The first subassembly 202 is not restricted to including only a diffuser plate and may include other optical layers. Some exemplary embodiments of other layers being included in the first subassembly 202 are discussed below.

The second subassembly 204 may be formed from a single sheet of material or may include a number of different layers. The second subassembly 204 may be formed from a substrate alone. The second subassembly 204 may also include a diffuser and/or other layers, as will become apparent in the discussion below.

In some exemplary embodiments, the spacer 208 has a thickness selected to create the gap 206 between the first and second subassemblies 202, 204. The spacer 208 may be formed, for example, using an adhesive tape, a pressure sensitive adhesive (PSA) or other suitable forms of adhesive. For example, the spacer 208 may be formed using a hook and loop-type of attachment, with the hook and loop layers attached to either of the first and second subassemblies via a single-sided adhesive. Another approach includes using a sealant. Another approach includes structuring the edges of at least one of the first and second subassemblies with a raised portion to provide the gap. In another approach, a plate, such as an injection molded plate, could be used as the spacer. The spacer may optionally include tabs that extend laterally beyond the edges of the different layers of the first and second subassemblies: these tabs may be used for additional mounting supports in the backlight of the LCD device.

The light management unit may be provided with protection from ultraviolet (UV) light, for example by including UV absorbing material or material in one of the layers that is resistant to the effects of UV light. In particular, one or more of the layers may include a UV absorbing material, or may include a separate layer of UV absorbing material. Suitable UV absorbing compounds are available commercially, including, e.g., Cyasorb.TM. UV-1164, available from Cytec Technology Corporation of Wilmington, Del., and Tinuvin.TM. 1577, available from Ciba Specialty Chemicals of Tarrytown, N.Y. The diffuser plate may also include brightness enhancing phosphors that convert UV light into visible light.

One or more of the layers of the light management unit may also include other materials to provide additional protection to UV light. One example of such a material is a hindered amine light stabilizing composition (HALS). Generally, the most useful HALS are those derived from a tetramethyl piperidine, and those that can be considered polymeric tertiary amines. Suitable HALS compositions are available commercially, for example, under the "Tinuvin" tradename from Ciba Specialty Chemicals Corporation of Tarrytown, N.Y. One such useful HALS composition is Tinuvin 622. UV absorbing materials and HALS are further described in co-owned U.S. Pat. No. 6,613,619, incorporated herein by reference.

Other exemplary embodiments of light management units may incorporate additional light management layers. For example, a light management unit may include a brightness enhancing layer in either the first or second subassemblies or in the gap between the subassemblies. One exemplary embodiment of light management unit 300, schematically illustrated in FIG. 3A, includes a first subassembly 302 separated from a second subassembly 304 by a gap 306. A spacer 308 between the subassemblies 302, 304 may be used to define the gap 306. In the illustrated embodiment, the first subassembly 302 includes a diffuser plate formed with a substrate 310 and a diffuser layer 312. The first subassembly 302 may include different layers, for example, as shown in FIGS. 2A-2D, as well as other layers.

In the exemplary embodiments discussed here, the second subassembly 304 includes at least a semi-rigid substrate 320, although other layers may also be present. A brightness enhancing layer 322 is attached to the substrate 320. Examples of suitable brightness enhancing layers include the Vikuiti.TM. BEFII and BEFIII family of prismatic films available from 3M Company, St. Paul, Minn., such as BEFII 90/24, BEFII 90/50, BEFIIIM 90/50, BEFIII-T, T-BEF, R-BEF, W-BEF and PC-BEF (a prismatic coating on a non-birefringent polymer).

The brightness enhancing layer 322 may be attached directly to the adjacent layer in the second subassembly 304, or may be attached through the use of one or more adhesive layers.

In some exemplary embodiments, it may be desirable for at least some of the light to enter the brightness enhancing layer 322 through an air interface or an interface having an increased refractive index difference. Therefore, a layer of low index material, for example a fluorinated polymer, may be placed between the brightness enhancing layer 322 and the next layer below the brightness enhancing layer, in this case the substrate 320.

In other exemplary embodiments, an air gap may be provided between the brightness enhancing layer 322 and the layer below the brightness enhancing layer 322, so that diffused light enters the brightness enhancing layer 322 from air. One approach to providing the air gap is to include a structure on one or both of the opposing faces of the brightness enhancing layer 322 and the adjacent layer. In the illustrated embodiment, the lower surface 330 of the brightness enhancing layer 322 is structured with protrusions 332 that contact a layer of adhesive 334 on the substrate 320. Voids 336 are thus formed between the protrusions 332, with the result that light enters into the brightness enhancing layer 322 from air, at those regions between the protrusions 332.

Other approaches to forming voids, and thus providing an air interface to light entering the brightness enhancing layer, may be used. For example, the brightness enhancing layer 322 may have a flat lower surface 330, with the adhesive 334 being structured with protrusions. In another exemplary embodiment, either the unstructured surface of the brightness enhancing layer, or the surface to which it is attached, or both surfaces, may be roughened, for example with a matte finish, to provide pockets of air between the two surfaces. Additional approaches are discussed in co-owned U.S. Patent Publication No. 2003/0223216 A1, incorporated herein by reference. Any of the embodiments of light management unit discussed herein may be adapted to provide an air interface for light entering the brightness enhancing layer.

The brightness enhancing layer 322 may be located at different positions within the light management unit 300. For example, the brightness enhancing layer 322 may be positioned within the second sub-assembly 304 closer to the gap 306 than the substrate 320, as is schematically illustrated in FIG. 3B. In this embodiment, the brightness enhancing layer 322 contacts the spacer 308 and defines a boundary with the gap 306. In such a configuration, the apexes of the structure members of the brightness enhancing layer 322 may be adhered to the substrate 320 using a thin layer of adhesive. Approaches to attaching the surface of a brightness enhancing layer to another layer are discussed more fully in co-owned U.S. patent application Ser. No. 10/439,450, incorporated herein by reference.

In another exemplary embodiment, schematically illustrated in FIG. 3C, the order of the layers is the same as in FIG. 3B, but the lateral extent of the brightness enhancing layer 322 may be reduced, so as to fit within the volume defined by the spacer 308. In such a configuration, the spacer 308 may contact a layer of the second subassembly 304 that is not the lowest layer of second subassembly 304. In the exemplary embodiment illustrated in FIG. 3C, the spacer 308 contacts the substrate 320, while the lowest layer of the second subassembly 304, the brightness enhancing layer 322, is located within the volume formed by the spacer 308. A gap 306, however small, may still exist between the first and second subassemblies 302, 304.

A gap is considered to exist even if the brightness enhancing layer 322 touches the first subassembly 302 because, structurally, the first subassembly 302 is connected to the second subassembly 304 via the spacer, and the two subassemblies 302, 304 are directly connected around their edges in a manner that provides mechanical rigidity to the light management unit 300. Also, either the lower surface of the brightness enhancing layer or the uppermost surface of the first subassembly 302, or both, may be provided with a matte or anti-wet-out finish, which results in much of the light passing from the first subassembly 302 into the brightness enhancing layer 322 through air. In such a case, the layers of the different subassemblies may contact each other at various points, with an air gap present between the points of contact. Such a gap may be as small as around one micron.

In other exemplary embodiments, the brightness enhancing layer 322 may be included with the first subassembly 302, rather than the second subassembly 304. For example, as is schematically illustrated in FIG. 3D, the brightness enhancing layer 322 may be the uppermost layer in the first subassembly 302, closest to the gap 306. In the illustrated embodiment, the lateral extent of the brightness enhancing layer 322 is set so as to fit within the space defined by the spacer 308. The brightness enhancing layer 322 may be attached directly to the next lower layer in the first subassembly, in this case the diffuser layer 312, or may be attached to the diffuser layer 312 via an adhesive layer (not shown).

In addition, the brightness enhancing layer 322 may be free-standing within the gap 306 without being attached to either subassembly 302, 304. The uppermost surface of the first subassembly 302 may be provided with a matte or an anti-wet-out finish, resulting in the light that propagates upwards from the first subassembly 302 passing into air before entering the brightness enhancing layer 322.

The brightness enhancing layer 322 may be positioned at other locations within the first subassembly 302. In one exemplary embodiment, the brightness enhancing layer 322 may be positioned between the uppermost and lowermost layers of the first subassembly 302, for example, between the diffuser layer 312 and substrate 310, as is schematically illustrated in FIG. 3E.

In other exemplary embodiments, there may be two prismatic brightness enhancing layers, with the prismatic structures of one of the layers oriented perpendicular to the prismatic structures of the other layer. Such an arrangement is referred to as crossed brightness enhancing layers, and provides control of the viewing angle in two dimensions. For example, one of the brightness enhancing layers affects the horizontal viewing angle for light emitted by an LCD-TV or LCD monitor, while the crossed brightness enhancing layer affects the vertical viewing angle of the light. An example of such an arrangement is schematically illustrated in FIG. 3F, in which the second subassembly 304 includes two brightness enhancing layers 322 and 324. The two brightness enhancing layers 322 and 324 may be located in the first subassembly 302 or in the second subassembly 304. The two brightness enhancing layers 322 and 324 may be located adjacent to each other, but need not be adjacent, and may even be located in the different subassemblies.

Other approaches to forming a subassembly having a layer of air between the lower surface of the brightness enhancing layer and the layer below are now discussed with referen