WO2001040708A2

WO2001040708A2 - Light guide system with reflective optical mask

Info

Publication number: WO2001040708A2
Application number: PCT/US2000/029964
Authority: WO
Inventors: Richard C. Allen; Elisa M. Cross
Original assignee: 3M Innovative Properties Company
Priority date: 1999-12-02
Filing date: 2000-10-31
Publication date: 2001-06-07
Also published as: WO2001040708A3; TW463059B; AU1355301A

Abstract

A light guide assembly includes a layer of material having the property of transmitting a portion of electromagnetic radiation entering the layer of material. The layer of material includes an edge portion, with an optical mask covering at least part of the edge portion. The mask has a reflecting side for reflecting the entering electromagnetic radiation so as to reduce bright areas when the layer of material is viewed from the side opposite the reflecting side and to recycle light resulting from the bright areas.

Description

LIGHT GUIDE SYSTEM WITH REFLECTIVE OPTICAL MASK

Background of the Invention

The present invention relates to an optical film incorporating a reflective optical mask for use in a light guide system. The optical mask serves to hide from the viewer areas of the light guide that exhibit bright areas, particularly where a light source such as a cold cathode fluorescent tube meets the light guide. Backlights such as those in monitors and emissive electronic displays, particularly those used in notebook displays, used to be approximately 10 mm thick. Such backlights contain a light guide that totally internally reflects light. Light typically enters the light guide from a cold cathode fluorescent tube (CCFT). The light from the CCFT is received by a plurality of optical media including a reflector, an image generating device, at least 1 polarizer typically absorbing, and a variety of additional films, including diffusers, reflective polarizers, and other brightness enhancement films. The reflector is typically placed on or near one major surface of the light guide opposite the face where the viewable image is produced. A thin film transistor liquid crystal (TFT L display may serve as one type of image generating device. Backlights are now approaching 5 mm or less in thickness. As backlights have become thinner, design needs not adequately addressed by the prior art have been observed. For example, there exists a need to combine functionality between multiple films to help reduce the thickness. Further, it has been observed that as the light guide becomes thinner, backlights have become less efficient. Concomitantly, the lower efficiency causes some undesirable viewing characteristics, such as the ability of the viewer to perceive a bright area, particularly where the CCFT(s) or edge reflectors are coupled to the light guide.

Brightness enhancement films have served to restore some efficiency, but have not removed such undesirable bright areas. In some backlight configurations, such as that described in U.S. Patent No. 5,537,296, a portion of the housing or a reflector that partially surrounds the light source may extend beyond the light source to cover an edge of the light guide, thus minimizing any perceived bright spot. Alternately, manufacturers have introduced a black optical mask screen painted onto the diffuser or other films that are located on or between the back major surface of the image generating device and the front major surface of the light guide. Unfortunately, black optical masks only absorb light and do nothing to recycle light near the mask. The present invention addresses this need, by providing a reflective optical mask which can recycle light and increase the brightness of the display while removing undesirable bright areas.

Summary of the Invention

According to the present invention, an optical film includes a layer of material having the property of transmitting a portion of electromagnetic radiation entering the layer of material, wherein the layer of material includes a peripheral edge portion, with a reflective mask that covers at least part of the peripheral edge portion of the optical film. The mask has at least one reflecting side for reflecting the entering electromagnetic radiation along the edge portions that results from bright areas originating near the edge portion of the light guide, when the layer of material is viewed from the side opposite the reflecting side. In another aspect, the invention includes a light guide assembly including the optical film and a display device including the light guide assembly.

Brief Description of the Drawings Figure 1 illustrates an exploded perspective view of an example of a backlit display constructed in accordance with the present invention.

Figure 2 shows an expanded view of an example of components included in a light guide assembly constructed in accordance with the present invention.

Figure 3 shows an example of one type of optical layer including one example of a reflective optical mask.

Figure 4 shows a side view of one example of a reflective optical mask applied to the bottom of an optical layer.

Figure 5 shows a side view of one example of a reflective optical mask applied to the top of an optical layer. Figure 6A shows a side view of one example of reflective and non-reflecting optical masks applied to opposite sides of an optical layer. Figure 6B shows a side view of one example of reflective and non-reflecting optical masks applied to the same side of an optical layer.

Figure 7 shows an expanded side view of one example of reflective and non- reflecting optical masks applied to different optical layers of a light guide assembly.

Detailed Description

Figure 1 illustrates an exploded perspective view of an example of a backlit display 10 constructed in accordance with the present invention. The backlit display 10 includes a case 16 having an optical window 12 through which displayed information may be viewed. A light guide assembly 18 is secured within the case. A light source 14 is attached at one edge of the light guide assembly 18. In one embodiment of the invention, the light source 14 may be any known light source appropriate for use in a backlit display such as, for example, a cold cathode fluorescent tube. The light guide assembly 18 includes an edge portion 15 proximate the light source. In one embodiment of the invention, a reflective surface, such as a multilayer reflective mirror, may be coupled to edge portions of the light guide that are not proximate the light source. Additionally, more than one light source may be coupled to the light guide. For example, multiple light sources may be coupled to the same edge of the light guide, as shown, for example, in U.S. Pat. No. 5,537,296, or one or more light sources may be coupled to multiple edges of the light guide. Undesirable bright spots may result whenever the light source(s) or reflective surface(s) are coupled to an edge or other portion of the light guide. Generally, the optical mask of the present invention may be used to reduce undesirable viewing effects that occur whenever light is not effectively coupled into the light guide.

Figure 2 shows an expanded view of an example of components included in a light guide assembly 18 constructed in accordance with the present invention. As will be appreciated by those skilled in the art having the benefit of the disclosure herein, the light guide assembly 18 may be constructed in numerous combinations of optical layers. The example shown in Figure 2 illustrates a combination of a plurality of optical layers including a first optical layer through an nth optical layer 20, 22, 24, 26, 28 and 30 respectively. Each of the plurality of optical layers may themselves comprise a plurality of layers of optical films, such as is the case in multiple layer films. A reflective optical mask 110 covers at least part of the peripheral edge portion of at least one major surface of at least one of the optical layers.

In one useful embodiment, the optical layers 20, 22, 24, 26, 28 and 30 advantageously include a light guide, an image generating device such as a liquid crystal (e.g. TFT LC) display, at least 1 polarizer, typically absorbing, and a variety of additional films, including diffusers, reflective polarizers, and other brightness enhancement films arranged to produce a viewable display. For example, the first optical layer 20 may comprise a reflector, followed by a light guide, then diffusers, reflective polarizers, protective cover films, and other brightness enhancement films, and a light gating device such as a liquid crystal display, in accordance with known display design principles.

One purpose of the optical mask is to keep the viewer from perceiving brightened areas near the point or points where the light source, multiple light sources, or reflective surfaces couple to the light guide. Therefore, placement of the mask is made to mask areas that would otherwise exhibit unwanted brightness. The mask is preferably placed on the major surface nearest the light guide so that both polarizations of light in the vicinity of the mask are recycled. In other useful embodiments, the mask 110 may be placed on any optical component between the light guide and a light gating layer or image generating device, preferably on the major surface closest to the light guide.

Figure 3 shows an example of one type of optical layer 28 including one example of a reflective optical mask. A portion of the optical layer 28 is covered by a reflective optical mask 110 having an inner edge 101 and an outer edge 102. The outer edge 102 may be considered as that part of the mask farthest from the center of the optical layer 28. In one embodiment, the layer of material comprises an optical film of a selected geometric shape having a perimeter, wherein an edge portion covered by the mask coincides with at least part of the perimeter of the optical layer. The reflective optical mask may be of uniform density, or there may be a gradation in reflectivity from the outer edge 102 to the inner edge 101.

In one preferred embodiment of the invention, the optical layer 28 comprises a reflective polarizer. However, it will be understood that the optical layer 28 may comprise any suitable type of optical layer. Other optical materials on which the mask may be advantageously applied include prismatic films such as brightness enhancement films commercially marketed as BEF by 3M, St. Paul, Minnesota, surface or bulk diffusers, cover sheets which may be clear or lightly diffusing, and dichroic polarizers.

In the case of a reflective polarizer, any useful reflective polarizer elements may be used that transmits light of any desired polarization. Typically, the reflective polarizing elements transmit light of one polarization state and reflect light of a different polarization state. The materials and structures used to accomplish these functions can vary. Depending on the materials and structure of the optical film, the term "polarization state" can refer to, for example, linear, circular, and elliptical polarization states.

Examples of suitable reflective polarizing elements include multilayer reflective polarizers, continuous/disperse phase reflective polarizers, cholesteric reflective polarizers (which are optionally combined with a quarter wave plate), and wire grid polarizers. In general, multilayer reflective polarizers and cholesteric reflective polarizers are specular reflectors and continuous/disperse phase reflective polarizers are diffuse reflectors, although these characterizations are not universal (see, e.g., the diffuse multilayer reflective polarizers described in U.S. Patent No. 5,867,316). This list of illustrative reflective polarizing elements is not meant to be an exhaustive list of suitable reflective polarizing elements. Any reflective polarizer that preferentially transmits light having one polarization and preferentially reflects light having a second polarization can be used. Both multilayer reflective polarizers and continuous/disperse phase reflective polarizers rely on index of refraction differences between at least two different materials to selectively reflect light of one polarization orientation while transmitting light with an orthogonal polarization orientation. Suitable diffuse reflective polarizers include the continuous/disperse phase reflective polarizers described in U.S. Patent No. 5,825,543 and 5,783,120, incorporated herein by reference, as well as the diffusely reflecting multilayer polarizers described in U.S. Patent No. 5,867,316, incorporated herein by reference. Other suitable diffuse reflective polarizers are described in U.S. Patent No. 5,751,388, incorporated herein by reference.

Cholesteric reflective polarizers are described in, e.g., U.S. Patent No. 5,793,456, U.S. Patent No. 5,506,704, and U.S. Patent No. 5,691,789, all of which are incorporated herein by reference. One exemplary cholesteric reflective polarizer is marketed under the trademark TRANSMAX™ by E. Merck & Co. Wire grid polarizers are described in, for example, PCT Publication WO 94/11766, incorporated herein by reference. Illustrative multilayer reflective polarizers are described in, for example, PCT Publication Nos. WO95/17303; WO95/17692; WO95/17699; WO96/19347; and W099/36262, and U.S. Patent Application Serial No. 09/399,531, all of which are incorporated herein by reference. Other reflective multilayer polarizers are described, for example, in U.S. Patent Nos. 5,486,949, 5,612,820, 5,882,774, all of which are incorporated herein by reference. One commercially available form of a multilayer reflective polarizer is marketed as Dual Brightness Enhanced Film (DBEF) by 3M, St. Paul, Minnesota.

Reflective polarizers are used herein as an example to illustrate optical film structures and methods of making and using the optical films of the invention. The structures, methods, and techniques described herein can be adapted and applied to other types of optical films. The reflective polarizers or other optical films may include additional layers or coatings to tailor the optical properties of the film for desired end uses. For example, the reflective polarizer may include an absorptive polarizer layer, such as a dichroic polarizer layer, as described in WO95/17691 and WO 99/36813, herein incorporated by reference. Additionally, the reflective polarizer may include a diffusing layer as described in U.S. Pat. No. 5,825,542 and U.S. Patent Application Serial Number 09/399531, herein incorporated by reference. Other suitable layers and coatings are described in WO 97/01440, the contents of which are herein incorporated by reference. In one preferred embodiment of the present invention, the optical mask is applied directly to a reflective polarizer or other optical layer. The optical mask preferably has an optical density that attenuates transmission of visible light in the region of the mask to remove distracting bright spots. For example, the mask is preferably highly reflective to visible light. Alternatively, the optical mask may have a non-constant optical density. In one preferred embodiment, the outer edge 102 exhibits an optical transmission of less than about 25%, whereas the inner edge 101 of the mask preferably exhibits a transmission of greater than about 50% so as to remove the appearance of a distinct edge to a viewer.

Figure 4 shows a side view of one example of a reflective optical mask applied to the bottom of an optical layer. The optical layer 400 represents any of the plurality of optical layers between the light gating device and the light guide, with the top of the optical layer 400 positioned toward the viewable side of a display. In the example of figure 4, a reflective optical mask 410 is affixed to an edge 411 on the bottom of the optical layer 400. The reflective optical mask 410 includes a reflective surface 412 and a non-reflective surface 415. Arrows 420 indicate light emanating from the light source or other portions of the light guide assembly (not shown). Arrows 422 represent reflected light from the reflective surface 412. In the case of a film or foil being used for the optical mask, the non-reflective side 415 may include an adhesive compatible with the material of the optical layer being masked. The adhesive itself may be non-reflective, or it may be combined with a separate non-reflective layer opposite the reflective surface of the optical mask.

Figure 5 shows a side view of one example of a reflective optical mask applied to the top of an optical layer. The optical layer 500 represents any of the plurality of optical layers between the light gating device and the light guide, with the top of the optical layer 500 position toward the viewable side of a display. In the example of Figure 5, a reflective optical mask 510 is affixed to an edge 511 on the top of the optical layer 500. The reflective optical mask 510 includes a reflective surface 512 and a non-reflective surface 515. Arrows 520 indicate light emanating from the light source or other portions of the light guide assembly (not shown). Dashed-line arrows 522 represent reflected light from the reflective surface 512.

Now referring to Figure 6A, in another embodiment, an optical mask placed on a film layer 600 may have a reflective layer 601 on a side facing a light guide and non- reflective or absorptive layer 602 on the side facing the viewer. In this configuration, bright spots caused by reflection of ambient light off the reflective surface of the mask will be substantially eliminated. The reflective and non-reflective layers of the optical mask may be applied to the same film in a light guide assembly. The reflective and non- reflective layers of the optical mask may physically have the same dimensions, except that one is displaced vertically from the other, as would be the case if the two layers are coextensive with one another. Depending upon the exact nature of the of components included in a light guide assembly, the dimensions of one layer of the set of reflective and non-reflective layers be larger than the other(s) to preferentially reduce undesirable optical effects. Figure 6B shows a side view of an example of a reflective optical mask 612 and non-reflecting optical mask 614 applied to the same side of an optical layer 616. The reflective optical mask 612 faces the light guide, while the non-reflecting optical mask faces the viewer.

Now referring to Figure 7, an expanded side view of an alternate example of reflective and non-reflecting optical masks applied to different optical layers of a light guide assembly 700 are shown. A reflective optical mask layer 701 may be applied to one film 704 closer to the light guide 706 while a non-reflective or absorptive optical mask layer 708 is applied to another film 710 that is closer to the image generating device. For example, a non-reflective optical mask layer may be applied directly to one of the major surfaces of a dichroic absorptive polarizer that is typically adhered to the back surface of the image generating device, while a reflective optical mask layer may be applied to a diffuser film or reflective polarizer film that is in close proximity to the light guide.

When applied to the same film, both the reflective and non-reflective layers may be applied to the same major surface of an optical film layer, or they can be applied to opposite major surfaces of the optical film layer. Whether applied to the same major surface or to opposite major surfaces of the same optical film layer, the reflective layer is preferably placed in closer proximity to the light guide and the non-reflective layer is placed in closer proximity to the image generating device, and thus closer to the viewer and any source of ambient light. When the reflective layer and non-reflective layer of the optical mask are positioned on different optical film layers in the light guide assembly, they can be positioned on either major surface of their respective optical film layers, but preferably with the reflective layer placed in closer proximity to the light guide and the non-reflective layer in closer proximity to the viewer.

In some embodiments, the mask may not be applied along the entire circumference of the reflective polarizer or other optical layer, but rather only in the vicinity of the light source. Such placement still serves to reduce undesired effects on the luminance of the display. The width of the mask depends upon many parameters, most notably the perception of the light source. Where the light emerging from the light source is effectively coupled into the light guide, then the width of the mask may be very small, for example, less than about 1 mm. Where the coupling of the light source and light guide is less effective, then the width of the mask may be on the order of 1 mm or more in width.

For high-resolution displays, it is preferred that the optical mask exhibits no significant change in color. Therefore, in the regions where the mask transmits light, then the mask exhibits only shades of neutral gray. In CIE color, the absolute value of dx and dy should be less than about 0.1.

It is to be understood herein that the reflective side of the reflective optical mask may be constructed of any suitable specular or diffuse reflective material. Specular reflective materials advantageously include a metallized coating, a mirror coating, a metallized film, a multilayer mirror film, a metallized paint and metallized tape. Also useful are foils comprising metals like silver, aluminum, nickel and other known metals and alloys. Diffuse reflective materials also include a diffuse-coated reflective multilayer mirror film, white paint, micro-voided films, multi-phase films and equivalent diffuse reflective materials. The reflective optical mask may be applied using any conventional coating techniques such as laminating, sputtering, painting and the like, or it may be applied using a suitable adhesive. For some applications, it may be desirable to make the optical mask sufficiently thick so that it can act as a spacer layer between films in the light guide assembly to protect against abrasion or damage during handling or assembly and to eliminate Newton's rings and wet-out by introducing an air interface between optical film layers in the light guide assembly. When the optical mask is designed to function in this way, it may be preferable to select optical mask materials that are sufficiently rubbery or elastomeric that they can cushion relatively brittle films in the light guide assembly. With selected thickness and placement of the optical mask, it may be possible to eliminate one or more protective cover sheets and thus reduce the thickness of the light guide assembly. What is claimed is:

Claims

1. An optical film with a reflective mask comprising:

(a) a layer of material having the property of transmitting a portion of electromagnetic radiation entering the layer of material, wherein the layer of material includes a major surface having a peripheral edge portion; and

(b) a mask covering at least part of the peripheral edge portion so as to reduce bright areas from the entering electromagnetic radiation near the edge portion, wherein the mask has at least one major surface comprising a reflecting layer.

2. The optical film of claim 1 wherein the layer of material comprises a reflective polarizer.

3. The optical film of claim 2 wherein the reflective polarizer comprises a multilayer film of at least two different materials.

4. The optical film of claim 2 wherein the reflective polarizer comprises a continuous/disperse phase reflective polarizer.

5. The optical film of claim 2 wherein the reflective polarizer comprises a cholesteric reflective polarizer.

6. The optical film of claim 2, wherein the reflective polarizer comprises a wire grid polarizer.

7. The optical film of claim 1 wherein the layer of material comprises a film of a selected geometric shape having a perimeter, wherein the peripheral edge portion of the major surface coincides with at least part of the perimeter covered by the mask.

8. The optical film of claim 1 wherein the mask comprises a metallized coating.

9. The optical film of claim 1 wherein the mask comprises a mirror coating.

10. The optical film of claim 1 wherein the mask comprises a metallized film.

11. The optical film of claim 1 wherein the mask comprises a mirror film.

12. The optical film of claim 1 wherein the mask comprises a reflective multilayer film.

13. The optical film of claim 1 wherein the mask comprises a reflective paint.

14. The optical film of claim 1 wherein the mask comprises a micro-voided film.

15. The optical film of claim 1 wherein the layer of material comprises a diffuser.

16. The optical film of claim 1 wherein the layer of material comprises a brightness enhancement film.

17. The optical film of claim 1 wherein the mask further comprises a non-reflective layer opposite the reflective layer.

18. A light guide assembly comprising: (a) At least one light source;

(b) a light guide proximate the light source;

(c) a layer of material having the property of transmitting a portion of electromagnetic radiation entering the layer of material, wherein the layer of material includes a major surface having a peripheral edge portion; and (d) a mask covering at least part of the peripheral edge portion so as to reduce bright areas from the entering electromagnetic radiation near the edge portion, wherein the mask has at least one reflecting layer positioned to reflect the entering electromagnetic radiation back toward the light guide.

19. The light guide assembly of claim 18 wherein the layer of material comprises a reflective polarizer.

20. The light guide assembly of claim 19 wherein the reflective polarizer comprises a multilayer film of at least two different materials.

21. The light guide assembly of claim 19 wherein the reflective polarizer comprises a continuous/disperse phase reflective polarizer.

22. The light guide assembly of claim 19 wherein the reflective polarizer comprises a cholesteric reflective polarizer.

23. The light guide assembly of claim 19 wherein the reflective polarizer comprises a wire grid polarizer.

24. The light guide assembly of claim 18 wherein the layer of material comprises an optical film of a selected geometric shape having a perimeter, wherein the edge portion coincides with at least part of the perimeter covered by the mask.

25. The light guide assembly of claim 18 wherein the mask comprises a metallized coating.

26. The light guide assembly of claim 18 wherein the mask comprises a mirror coating.

27. The light guide assembly of claim 18 wherein the mask comprises a metallized film.

28. The light guide assembly of claim 18 wherein the mask comprises a mirror film.

29. The light guide assembly of claim 18 wherein the mask comprises a reflective multilayer film.

30. The light guide assembly of claim 18 wherein the mask comprises a reflective paint.

31. The light guide assembly of claim 18 wherein the layer of material comprises a diffuser.

32. The light guide assembly of claim 18 wherein the layer of material comprises a brightness enhancement film.

33. The light guide assembly of claim 18, further comprising a non-reflective optical mask layer positioned to absorb ambient light.

34. A backlit display comprising:

(a) a case having an optical window through which displayed information may be viewed;

(b) at least one light source for providing a backlight;

(c) a layer of material having the property of transmitting a portion of electromagnetic radiation entering the layer of material, wherein the layer of material is positioned to receive the backlight, and wherein the layer of material includes a major surface having a peripheral edge portion; and

(d) a mask covering at least part of the peripheral edge portion, wherein the mask reflects the entering backlight so as to recycle light originating from bright areas proximate the light source .

35. The backlit display of claim 34 wherein the layer of material comprises a reflective polarizer.

36. The backlit display of claim 35 wherein the reflective polarizer comprises a multilayer film of at least two different materials.

37. The backlit display of claim 35 wherein the reflective polarizer comprises a continuous/disperse phase reflective polarizer.

38. The backlit display of claim 35 wherein the reflective polarizer comprises a cholesteric reflective polarizer.

39. The backlit display of claim 35 wherein the reflective polarizer comprises a wire grid polarizer

40. The backlit display of claim 34 wherein the layer of material comprises an optical film of a selected geometric shape having a perimeter, wherein the peripheral edge portion of the major surface covered by the mask is proximate the light source.

41. The backlit display of claim 34 wherein the mask comprises a metallized coating.

42. The backlit display of claim 34 wherein the mask comprises a mirror coating.

43. The backlit display of claim 34 wherein the mask comprises a metallized film.

44. The backlit display of claim 34 wherein the mask comprises a mirror film.

45. The backlit display of claim 34 wherein the mask comprises a reflective multilayer film.

46. The backlit display of claim 34 wherein the layer of material comprises a diffuser.

47. The optical film of claim 34 wherein the mask comprises a micro-voided film.

48. A backlit display comprising:

(b) a light guide secured within the case;

(c) at least one light source for providing a backlight, the light source being coupled to an edge of the light guide;

(d) a reflective polarizer positioned over the light guide, the reflective polarizer including a major surface having a peripheral edge portion coextensive with the area where the light source couples to the light guide; and

(e) a mask covering the peripheral edge portion of the reflective polarizer, wherein the mask has the property of reflecting the backlight.

49. The backlit display of claim 48 wherein the reflective polarizer comprises a multilayer film of at least two different materials.

50. The backlit display of claim 48 wherein the mask comprises a metallized coating.

51. The backlit display of claim 48 wherein the mask comprises a mirror coating.

52. The backlit display of claim 48 wherein the mask comprises a metallized film.

53. The backlit display of claim 48 wherein the mask comprises a mirror film.

54. The backlit display of claim 48 wherein the mask comprises a reflective multilayer film.

55. The backlit display of claim 48, further comprising a non-reflective optical mask layer selectively positioned to absorb ambient light.

56. The optical film of claim 48 wherein the mask comprises a micro-voided film.