WO2019104035A1

WO2019104035A1 - Backlight apparatuses for displays

Info

Publication number: WO2019104035A1
Application number: PCT/US2018/062006
Authority: WO
Inventors: Byung Yun Joo
Original assignee: Corning Incorporated
Priority date: 2017-11-21
Filing date: 2018-11-20
Publication date: 2019-05-31
Also published as: TW201926312A

Abstract

Backlight apparatuses for use in displays are disclosed. According to one embodiment, a backlight apparatus for use in a display may include a light guide plate and a white light source. The white light source may be oriented to direct white light into a light inlet face of the light guide plate. The white light may comprise at least a first light color and a second light color. An angular range over which the second light color is emitted from the white light source may be less than an angular range over which the first light color is emitted from the white light source.

Description

BACKLIGHT APPARATUSES FOR DISPLAYS

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims benefit of priority of U.S. Provisional Application Serial No. 62/589,204 filed on November 21, 2017 the contents of which are relied upon and

incorporated herein by reference in their entirety as if fully set forth below.

BACKGROUND

FIELD

[0002] The present specification generally relates to backlight apparatuses for displays and, more particularly, to backlight apparatuses including a glass light guide plate and a light source emitting at least two wavelengths of light.

Technical Background

[0003] LED displays typically include a light guide plate and a light source that directs light onto the light guide plate. Polymer plates, such as polymethylmethacrylate (RMMA) plates, are most commonly used for light guide plates because of the high optical transmittance of the RMMA material and relatively low manufacturing cost. However, RMMA plates lack the dimensional stability required for ultra-slim displays.

[0004] Glass light guide plates have better dimensional stability than RMMA light guide plates and, as such, are better suited for use in ultra-slim displays. However, the optical characteristics of glass may cause a perceptible color shift in the light emitted from the light guide plate, which is undesirable.

[0005] Accordingly, alternative backlight apparatuses with glass light guide plates are desired. SUMMARY

[0006] According to one embodiment, a backlight apparatus for use in a display may include a light guide plate and a light source. The light source may be oriented to direct light into a light inlet face of the light guide plate. The light may comprise at least a first light color and a second light color. The light source may be, for example, a white light source (e.g., a source that emits white light). An angular range over which the second light color is emitted from the light source may be less than an angular range over which the first light color is emitted from the light source.

[0007] According to another embodiment, a display may comprise a light guide comprising glass, and a light source. The light source may be oriented to direct light into a light inlet face of the light guide plate. The light may comprise at least blue light and yellow light. An emission cone of the blue light may be less than an emission cone of the yellow light.

[0008] According to another embodiment, a method for adjusting color shift in a light guide plate may include directing, by a light source, light toward a light inlet face of the light guide plate such that the light propagates through the light guide plate and is emitted from a light emission surface of the light guide plate. The light may comprise yellow light and blue light. An emission cone of the blue light emitted from the light source may be less than an emission cone of the yellow light emitted from the light source.

[0009] Additional features and advantages will be set forth in the detailed description that follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the embodiments described herein, including the detailed description which follows, the claims, as well as the appended drawings.

[0010] It is to be understood that both the foregoing general description and the following detailed description describe various embodiments and are intended to provide an overview or framework for understanding the nature and character of the claimed subject matter. The accompanying drawings are included to provide a further understanding of the various embodiments, and are incorporated into and constitute a part of this specification. The drawings illustrate the various embodiments described herein, and together with the description serve to explain the principles and operations of the claimed subject matter. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 schematically depicts the structure of a backlight apparatus comprising a light guide plate and a white light source;

[0012] FIG. 2 graphically depicts the optical transmittance (left y-axis) as a function of wavelength (x-axis) for PMMA and glass in addition to the normalized intensity (right y- axis) of a phosphor-based white LED as a function of wavelength;

[0013] FIG. 3 A schematically depicts the structure of a phosphor-based white LED;

[0014] FIG. 3B graphically depicts the normalized light intensity profile (y-axis) for yellow light and blue light emitted from a phosphor-based white LED as a function of viewing angle (x-axis), according to one or more embodiments shown and described herein;

[0015] FIG. 3C graphically depicts the CIE 1931 Y color coordinate (y-axis) of the phosphor-based white LED of FIG. 3 A as a function of viewing angle (x-axis), according to one or more embodiments shown and described herein;

[0016] FIG. 4A schematically depicts a comparative backlight apparatus including a glass light guide plate and a phosphor-based white LED in which the light intensity profile of the yellow light emitted from the phosphor-based white LED and the light intensity profile of the blue light emitted from the phosphor-based white LED are substantially the same over a range of viewing angles;

[0017] FIG. 4B schematically depicts a backlight apparatus including a glass light guide plate and a phosphor-based white LED in which the light intensity profile of the blue light emitted from the phosphor-based white LED is narrower than the light intensity profile of the yellow light emitted from the phosphor-based white LED, according to one or more embodiments shown and described herein;

[0018] FIG. 5 graphically depicts the normalized yellow light intensity profiles for yellow light and blue light emitted from the light source of FIG. 4B as a function of viewing angle Q on polar coordinates, according to one or more embodiments shown and described herein; [0019] FIG. 6 A graphically depicts the CIE 1931 Y color coordinate (y-axis) for LEDs with different light intensity profiles as a function of viewing angle (x-axis), according to one or more embodiments shown and described herein;

[0020] FIG. 6B graphically depicts the CIE 1931 Y color coordinate (y-axis) of white light emitted from a glass light guide plate for a 55-inch display as a function of distance from the light source (x-axis) in the light propagation direction of the glass light guide plate, according to one or more embodiments shown and described herein; and

[0021] FIG. 7 graphically depicts the color shift for displays with various dimensions that include backlight apparatuses according to one or more embodiments shown and described herein.

DETAILED DESCRIPTION

[0022] Reference will now be made in detail to embodiments of backlight apparatuses with glass light guide plates, examples of which are illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts. One embodiment of a backlight apparatus is schematically depicted in FIG. 4B and generally includes a light guide plate and a light source oriented to direct light toward the light guide plate. The light may comprise at least a first light color and a second light color. The light may be, for example, white light. An angular range over which the second light color is emitted from the light source may be less than an angular range over which the first light color is emitted from the white light source. Various embodiments of backlight apparatuses will be described in further detail herein with specific reference to the appended drawings.

[0023] Ranges can be expressed herein as from“about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent“about,” it will be understood that the particular value forms another embodiment. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint. [0024] Directional terms as used herein - for example up, down, right, left, front, back, top, bottom - are made only with reference to the figures as drawn and are not intended to imply absolute orientation.

[0025] Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order, nor that with any apparatus specific orientations be required. Accordingly, where a method claim does not actually recite an order to be followed by its steps, or that any apparatus claim does not actually recite an order or orientation to individual components, or it is not otherwise specifically stated in the claims or description that the steps are to be limited to a specific order, or that a specific order or orientation to components of an apparatus is not recited, it is in no way intended that an order or orientation be inferred, in any respect. This holds for any possible non-express basis for interpretation, including: matters of logic with respect to arrangement of steps, operational flow, order of components, or orientation of components; plain meaning derived from grammatical organization or punctuation, and; the number or type of embodiments described in the specification.

[0026] As used herein, the singular forms“a,”“an” and“the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to“a” component includes aspects having two or more such components, unless the context clearly indicates otherwise.

[0027] In the present specification, the“normalized light intensity profile” of a light source is defined as the intensity of the light at one or more wavelengths as a function of the viewing angle of the light source emitting the light. Referring to FIG. 3A by way of example, the viewing angle may range from about -90 degrees to about +90 degrees. For reference, a viewing angle of zero degrees is perpendicular to the plane on which the light source is positioned, as depicted in FIG. 3A. For example, in FIG. 3A, the light source 110 is located on a y-z plane, and the viewing angle of zero degrees is perpendicular to the y-z plane. Generally, the intensity of light for any wavelength of light emitted from a light source is greatest at a viewing angle of 0 degrees and decreases as the viewing angle increases or decreases, as shown in FIG. 3B.

[0028] Backlight apparatuses used in display devices, such as televisions, computer monitors, tablets, handheld devices and the like, may utilize a light source, such as a white LED. The light output of the LED is directed into a light guide plate. When the light guide plate is made from commercially available display glasses (for example and without limitation EAGLE XG^®, Lotus™, Willow^®, iris™, and Gorilla^® glasses from Corning incorporated), it has been determined that the white light experiences attenuation as it propagates through the light guide plate. For example, blue light from a phosphor-based white LED may attenuate more than yellow light from the phosphor-based white LED resulting in a color shift in the light emitted from the light guide plate as the light travels from one end of the light guide plate to the other end of the light guide plate.

[0029] The embodiments of the backlight apparatuses described herein mitigate or eliminate the aforementioned color shift in the light emitted from backlight apparatuses comprising glass light guide plates. In particular, the embodiments of the backlight apparatuses described herein utilize a light source in which the light intensity profile of a first light color (e.g., yellow light) is broader than the light intensity profile of a second color (e.g., blue light). That is, the light from the light source comprises a second light color (e.g., blue light) that is emitted over an angular range that is less than the angular range of the first light color (e.g., yellow light). Said differently, the solid angle of the blue light emitted from the light source is less than the solid angle of the yellow light emitted from the light source.

[0030] Specifically referring to FIG. 1, a backlight apparatus 100 is schematically depicted. The backlight apparatus 100 may include a light source 110, a light guide plate 120, light extraction features 140, and a reflection film 130.

[0031] The light guide plate 120 may be formed from a glass that is substantially transparent in the visible spectrum. As used herein, the term "substantially transparent" denotes that the light guide plate has a transmittance of greater than about 85% in the visible region of the electromagnetic spectrum (i.e., at wavelengths from about 400 nanometers (nm) to about 700 nm). For example, a suitable light guide plate may have greater than about 85% transmittance in the visible spectrum, such as greater than about 90%, greater than about 95%, or even greater than about 99% transmittance.

[0032] The light guide plate 120 may, for example, comprise a thickness in the +/- y direction of the coordinate depicted in FIG. 1 in a range from about 0.1 millimeters (mm) to about 2.5 mm. In embodiments, the thickness of the light guide plate 120 may be from about 0.3 mm to about 2 mm, from about 0.5 mm to about 1.5 mm, or even from about 0.7 mm to about 1 mm, including all ranges and subranges therebetween.

[0033] In embodiments, the light guide plate 120 may, for example, be formed from a glass suitable for use in display applications, such as, for example and without limitation, aluminosilicate glasses, borosilicate glasses, aluminoborosilicate glasses, soda lime glasses, or other glasses suitable for display applications. Specific examples of glasses suitable for display applications include, without limitation, EAGLE XG\ Lotus™, Willow^®, Iris™, and Gorilla^® glasses from Coming Incorporated.

[0034] The light guide plate 120 may include a light inlet face 121, a light emission surface 122, a back surface 124, and a distal face 123. The light guide plate 120 includes a length extending from the light inlet face 121 to the distal face 123 in the propagation direction 150 of the light guide plate 120. The propagation direction 150 of the light guide plate 120, as used herein, defines the general direction that light from the light source 110 travels through the light guide plate 120 from the light inlet face 121 towards the distal face 123 and not necessarily the propagation direction of the light itself, which may be parallel with the propagation direction 150 or non-parallel with the propagation direction 150. The light emission surface 122 is generally planar and parallel to the propagation direction 150 of the light guide plate 120. That is, the vector normal to the light emission surface 122 (which is parallel to the +y direction of the coordinate axes depicted in FIG. 1) is generally orthogonal to the propagation direction 150. The back surface 124 of the light guide plate 120 is opposite the light emission surface 122,

[0035] In the embodiments described herein, the light extraction features 140 are formed on or in the back surface 124 of the light guide plate 120. The light extraction features 140 may be arranged in a predetermined pattern such that, when light propagating through the light guide plate 120 encounters the light extraction features 140, the light extraction features 140 scatter and redirect the light towards either the light emission surface 122 or the back surface 124 of the light guide plate 120.

[0036] The reflection film 130 is positioned proximate to the back surface 124 of the light guide plate 120 such that the light extraction features 140 are disposed between the back surface 124 of the light guide plate 120 and the reflection film, as depicted in FIG. 1. The reflection film 130 assists in reflecting light emitted from the back surface 124 of the light guide plate 120 back into the light guide plate 120 such that the light can be emitted from the light emission surface 122 or, alternatively, further internally reflected within the light guide plate 120 and scattered hack towards the light emission surface 122 by the light extraction features 140 such that the light is emitted from the light emission surface 122.

[0037] The light source 110 is positioned adjacent to the light inlet face 121 of the light guide plate 120 and is oriented to direct light, for example white light, into the light inlet face 121. In some embodiments described herein, the light source 110 is a phosphorous-based white LED. For example, FIG 3A schematically depicts the structure of a light source 110 where the light source 110 is a phosphorous-based white LED. As depicted in FIG. 3 A, in this embodiment, the light source 110 comprises a blue dye layer 320 positioned over an emitter 310 of the LED and a yellow phosphor layer 330 coated over the blue dye layer 320 such that light from the emitter 310 passes through both the blue dye later 320 and the yellow phosphor layer 330. The blue dye layer 320 and the yellow phosphor layer 330 act in concert with one another such that the light emitted from the LED appears“white”. In particular, light emitted from the emitter 310 of the LED passes through the blue dye layer 320 generating blue or ultraviolet (UV) photons. The blue or UV photons may then travel through the yellow phosphor layer 330. In areas where the yellow phosphor layer 330 has a greater thickness, blue or UV photons may excite the emission of yellow photons in the yellow phosphor layer 330. Conversely, in areas where the yellow phosphor layer 330 is thinner, blue or UV photons may pass through yellow phosphor layer 330 without exciting the emission of yellow photons in the yellow phosphor layer 330. The blue and yellow photons combine with photons of other wavelengths generated by the emitter 310 to emit light that appears white.

[0038] While the light source 110 has been described as being, in at least one embodiment, a phosphorous-based LED, it should be understood that other light sources are contemplated and possible. For example, in an alternative embodiment, the light source 110 may be a red- green-blue (RGB) based LED, in which white light is produced from the combination of light from a red LED, a green LED, and a blue LED.

[0039] When light from the light source 110 is directed into the light inlet face 121, the light propagates through the light guide plate 120 in the propagation direction. Because the light from the light source comprises divergent rays of light, some light propagates parallel to the propagation direction 150 of the light guide plate 120, while other rays are non-parallel with the propagation direction 150 of the light guide plate 120. The divergent light rays may be internally reflected within the light guide plate 120 along the propagation direction 150 of the light guide plate 120. A portion of these divergent rays may be emitted from the light emission surface 122 of the light guide plate 120. Another portion of the divergent light rays may be incident on the light extraction features 140 that scatter the light from these divergent rays towards the light emission surface 122 such that the scattered light is emitted from the light emission surface 122. Portions of the divergent light rays not directly incident on the light extraction features 140 may be reflected by the reflection film 130 back towards and emitted from the light emission surface 122 (or internally reflected and subsequently scattered by the light extraction features 140). The light emitted from the light emission surface 122 is the illumination light from the backlight apparatus 100 and may be used, for example, to illuminate a display panel.

[0040] For a light source 110 emitting multiple wavelengths of light, certain wavelengths of light from the light source are attenuated by the glass more than others as the light travels through the light guide plate 120 in the propagation direction 150. For example, for white light sources 110 in which the various wavelengths of light within the emitted white light have the same or similar light intensity profiles over the entire range of viewing angles, white light is emitted from the light emission surface 122 of the light guide plate 120 closest to the light source 110 while yellowish-white light is emitted from the light emission surface 122 of the light guide plate 120 farthest from the light source 110. The resultant color shift is attributable to the attenuation of blue light in the glass light guide plate 120 as the light propagates through the light guide plate 120 in the propagation direction 150.

[0041] Specifically, when a white light source 110 in which the various wavelengths of light within the emitted white light have the same or similar light intensity profiles over a broad range of viewing angles is used in the backlight apparatus 100, the light guide plate 120 emits Light A from the light emission surface 122 proximate to the light inlet face 121 (i.e., closest to the light source 110) and Light B from the light emission surface 122 proximate the distal face 123 (i.e. farthest from the light source 110). Both Light A and Light B are white light that includes both blue light and yellow light. Light A is well-balanced white light. The phrase“well-balanced white light,” as used herein, means that the proportions of blue light and yellow light in the white light are balanced such that the light does not exhibit a color shift towards either blue or yellow. However, while Light A is well-balanced, light B is unbalanced and exhibits a color shift toward yellow. This is because the blue light emitted from the light source 110 is attenuated by the light guide plate 120 as the blue light travels through the light guide plate 120 with more blue light being attenuated by the light guide plate 120 with increasing distance from the light source 110. Thus, the light emitted from the light guide plate 120 closest to the light source 110 (i.e., the“Short Path Length” light in FIG. 1) is more well balanced than the light emitted farthest from the light source 110 (i.e., the “Long Path Length” light in FIG. 1) that has a pronounced shift toward yellow. This is due to the optical transmittance of the light guide plate 120 as well as the light intensity profiles as a function of viewing angle of the wavelengths of light making up the white light emitted from the white light source 110.

[0042] Specifically referring to FIG. 2 by way of further explanation and example, the optical transmittance (left vertical ordinate) as a function of wavelength (horizontal ordinate) for a PMMA light guide plate (curve 220) and a glass light guide plate (curve 230) formed from a transparent display glass are graphically depicted. In addition, FIG. 2 graphically depicts the normalized light intensity profile of a phosphor-based white LED (right vertical ordinate) as a function of wavelength (horizontal ordinate). While FIG. 2 depicts an illustrative optical transmittance for a glass light guide plate, it should be understood that different glass compositions may produce slightly different optical transmittance curves. As depicted in FIG. 2, with respect to the PMMA optical transmittance curve 220, the optical transmittance of blue light (i.e., light having a wavelength from about 450 nanometers (nm) to about 495 nm) is about 98%, and the optical transmittance of yellow light (i.e., light having a wavelength from about 570 nm to about 590 nm) is about 99%. With respect to the glass optical transmittance curve 230, the optical transmittance of blue light is from about 88% to about 92% and the optical transmittance of yellow light is from about 92% to about 94%. Thus, in general terms, the optical transmittance of the PMMA light guide plate is generally greater than the optical transmittance of the glass light guide plate for most wavelengths within the visible spectrum. Moreover, the glass light guide plate generally attenuates wavelengths of visible light corresponding to blue light greater than wavelengths of light corresponding to yellow light.

[0043] Still referring to FIG. 2, the normalized light intensity profile curve 210 of the phosphor-based white LED has a narrow emission peak at a wavelength of about 450 nm (i.e., in the blue light portion of the visible spectrum). Thus, even a slight attenuation of light with a wavelength of 450 nm from the phosphor-based white LED will cause a color shift in the white light emitted from the phosphor-based LED. This color shift may manifest as a yellowish-white color on a display utilizing a backlight apparatus including the phosphor- based white LED. Because glass has a lower optical transmittance than PMMA with respect to light with a wavelength of about 450 nm, the light guide plate made of glass shows a greater color shift than the light guide plate made of the PMMA. However, it should be understood that light guide plates formed from PMMA may exhibit some color shift, albeit less pronounced than light guide plates formed from glass.

[0044] FIG. 3B graphically depicts a normalized yellow light intensity profile 340 and a normalized blue light intensity profile 350 of a conventional phosphor-based white LED as a function of viewing angle. The x-axis (i.e., the horizontal ordinate in FIG. 3B) of the graph corresponds to the viewing angle of the conventional phosphor-based white LED relative to the normal direction (i.e., the zero degree viewing angle of FIG. 3A) of the conventional phosphor-based white LED. The y-axis (i.e., the vertical ordinate) of the graph in FIG. 3B indicates the normalized intensity of the emitted light. As shown in FIG. 3B, the normalized yellow light intensity profile 340 and the normalized blue light intensity profile 350 of the conventional phosphor-based white LED are substantially the same over the viewing angle range from about -90 degrees to about + 90 degrees. As such, the conventional phosphor- based white LED is regarded as having good color uniformity over the range of viewing angles.

[0045] Referring now to FIG. 3C, FIG. 3C depicts the International Commission on Illumination (CIE) 1931 Y color coordinate on the y-axis (i.e., the vertical ordinate in FIG. 3C) for the conventional phosphor-based white LED of FIG. 3A as a function of viewing angle on the x-axis (i.e., the horizontal ordinate in FIG. 3C). The CIE 1931 Y color coordinate equates to the luminance or luminous intensity of the specified light. The value of the CIE 1931 Y color coordinate at different viewing angles can be used to assess the color uniformity of the light as a function of viewing angle. As shown in FIG. 3C, the value of the CIE 1931 Y color coordinate for a conventional phosphor-based white LED is a minimum at a viewing angle of 0 degrees, and generally increases as the viewing angle increases or decreases from a viewing angle of 0 degrees. This generally indicates that the color uniformity of the phosphor-based white LED decreases slightly as the viewing angle increases or decreases from a viewing angle of 0 degrees. The trends of FIG. 3C generally comport with the light intensity trends evident from FIG. 3B. That is, FIG. 3 generally indicates that, while the intensity of blue light and the intensity of the yellow light emitted from the phosphor-based white LED are substantially the same over the range of viewing angles, the intensity of blue light decreases slightly faster than the intensity of the yellow light with increasing or decreasing viewing angle. FIGS. 3B and 3C, taken together, indicate that the CIE 1931 Y color coordinate may be used as a measure for assessing color uniformity due to variations in light intensity profiles.

[0046] LEDs with uniform normalized light intensity profiles for a range of wavelengths, such as phosphor-based white LEDs, are generally regarded as suitable light sources for LED displays where PMMA light guide plates are used. In contrast to the PMMA light guide plates, when the light guide plates are made of glass, LEDs exhibiting uniform normalized light intensity profiles over a range of wavelengths are not ideal light sources because of the color shift which occurs as the light propagates through the glass due to the attenuation of blue light in the glass.

[0047] Referring now to FIGS. 4 A and 4B, FIG. 4 A depicts a backlight apparatus 100 including a conventional white light source 110A exhibiting a uniform light intensity profile over the viewing angle range from -90 degrees (i.e., -y direction in FIG. 4A) to +90 degrees (i.e., +y direction in FIG. 4A) for the wavelengths of light within the white light emitted from the conventional white light source. FIG. 4B depicts a backlight apparatus 100 including a light source 110B in which different wavelengths of light within the emitted white light may have different (i.e., narrower or broader) light intensity profiles over the viewing angle range from -90 degrees (i.e., -y direction in FIG. 4B) to +90 degrees (i.e., +y direction in FIG. 4B). The backlight apparatuses in FIGS. 4A and 4B are generally constructed the same and comprise the same components as the backlight apparatus 100 depicted in FIG. 1 and described herein.

[0048] Referring to FIG. 4A, the light source 110A is a conventional light source which emits white light that comprises a first light color (e.g., yellow light 410) and a second light color (e.g., blue light 420). The yellow light 410 is emitted from the light source 110A over an angular range (i.e., solid angle) that is substantially the same as the angular range (i.e., solid angle) of the blue light 420 such that the light source 110 exhibits good uniformity in the light intensity profile for viewing angles ranging from -90 degrees to + 90 degrees relative to the normal direction of the light source 110, as described hereinabove with respect to FIG. 3B. [0049] As described above with respect to FIG. 1, light from the light source 110 A, including the yellow light 410 and the blue light 420, is directed into the light inlet face 121 of the light guide plate 120. The yellow light 410 and the blue light 420 propagate within the light guide plate 120 along the propagation direction 150 toward the distal face 123 of the light guide plate 120. As the yellow light 410 and the blue light 420 propagate, the yellow light 410 and the blue light 420 attenuate according to the optical transmittance of the light guide plate 120. As the light propagates through the light guide plate 120, the yellow light 410 and the blue light 420 are scattered by the light extraction features 140 and/or reflected by the reflection film (not shown) and emitted from the light emission surface 122 of the light guide plate 120 as shown in FIG. 4A.

[0050] At a first portion 440 of the light emission surface 122 of the light guide plate 120 located closest to the light source 110A, a combination of yellow light 410-1 and blue light 420-1 is emitted from the light emission surface 122 proximate the light inlet face 121. This combination of yellow light 410-1 and blue light 420-1 exhibits a well-balanced white color (i.e., Color A) because the attenuation of the blue light 420-1 is minimal due to the short travel path within the light guide plate 120 with respect to the light source 110 A.

[0051] However, on a second portion 450 of the light emission surface 122 of the light guide plate 120 farther from the light source 110A than the first portion 440, the combination of yellow light 410-2 and blue light 420-2 emitted from the light emission surface 122 exhibits a yellowish-white color (i.e., Color B) due to attenuation of the blue light 420-2 as it travels through the light guide plate 120. Specifically, because the light guide plate 120 has a lower optical transmittance for blue light than for yellow light, the blue light 420-2 attenuates more than the yellow light 410-2 while traveling from the light source 110A to the second portion 450 of the light guide plate 120. Thus, a yellow color shift occurs in the light emitted from the light emission surface at the second portion 450 of the light guide plate 120.

[0052] By way of contrast, FIG. 4B depicts a backlight apparatus 100 comprising a light source 110B according to one or more embodiments shown and described herein. Specifically, the backlight apparatus 100 includes a light guide plate 120, a reflection film (not depicted), and light extraction features 140, as described herein with respect to FIG. 1. The backlight apparatus 100 also includes a light source 110B that assists in mitigating the color shift in the light emitted from the backlight apparatus 100. Specifically, the light source 110B may be a phosphor-based white LED. A normal direction of the light source 110B is parallel with the x-axis of FIG. 4B. That is, the propagation direction 150 of the light guide plate 120 is parallel with the x-axis of the coordinate axes depicted in FIG. 4B.

[0053] In the embodiment of the backlight apparatus 100 depicted in FIG. 4B, the light source 110B emits white light that comprises a first light color (e.g., yellow light 410) and a second light color (e.g., blue light 430). However, in this embodiment, the second light color is emitted over an angular range F2 (i.e., a solid angle) that is less than the angular range Fi (i.e., a solid angle) of the first light color. More specifically, in the embodiment depicted in FIG. 4B, the blue light 430 is emitted over an angular range FB that is less than the angular range Fg of the yellow light 410. That is, the light intensity profile of the blue light 430 is narrower than the light intensity profile of the yellow light 410. For example and without limitation, rays of the blue light 430 may have a normalized intensity greater than zero over a viewing angle range from about -50 degrees to about +50 degrees relative to the normal direction of the light source 110B, and a normalized intensity of approximately zero for viewing angles of less than -50 and greater than +50 degrees. In contrast, rays of the yellow light 410 may have a normalized intensity greater than zero over a viewing angle range from about -90 degrees to about +90 degrees relative to the normal direction of the light source 110B. As such, the emission cone of the yellow light 410 is greater than the emission cone of the blue light 430. The phrase“emission cone,” as used herein, is the cone of light rays circumscribed by rotating the angular range about a normal direction of the light source that bisects the angular range.

[0054] More particularly, FIG. 5 graphically depicts a normalized yellow light intensity profile 510 of the yellow light 410 of FIG. 4B and a normalized blue light intensity profile 520 of the blue light 430 of FIG. 4B on polar coordinates. The normalized blue light intensity profile varies as a function of cos Q to the power of N (cos^N Q), where N is a value greater than 1 (for example, 1.7), and Q is a viewing angle of the light source 110B relative to the normal direction of the light source 110B, as described herein with respect to FIG. 3A. The normalized yellow light intensity profile varies as function of cos Q, where Q is a viewing angle of the light source 110B relative to the normal direction of the light source 110B. As shown in FIG. 5, the normalized blue light intensity profile 520 of the blue light 430 is narrower than the normalized yellow light intensity profile 510 of the yellow light 410 over the viewing angle range. That is, rays of the blue light 430 are emitted from the light source over an angular range (i.e., solid angle) that is less than the angular range (i.e., solid angle) of the yellow light 410. As the value of N for the normalized blue light intensity increases from 1, the normalized blue light intensity profile 520 becomes narrower (i.e., the angular range over which the blue light is emitted becomes narrower).

[0055] The variation in the distribution of yellow light and blue light emitted from the light source 110B may be achieved, for example, by varying the thickness of the blue dye layer and/or the yellow phosphor layer on the LED device to achieve the desired distribution of light.

[0056] Referring again to FIG. 4B, light from the light source 110B, including the yellow light 410 and the blue light 430, is directed toward the light inlet face 121 of the light guide plate 120. The yellow light 410 and the blue light 430 propagate within the light guide plate 120 along the propagation direction 150 toward the distal face 123 of the light guide plate 120. As the light propagates through the light guide plate 120, the yellow light 410 and the blue light 420 are scattered by the light extraction features 140 and/or reflected by the reflection film (not shown) and emitted from the light emission surface 122 of the light guide plate 120 as shown in FIG. 4B.

[0057] At a first portion 440 of the light emission surface 122 of the light guide plate 120 located closest to the light source 110B, a combination of the yellow light 410-1 and the blue light 430-1 exhibits a slightly yellowish white color (Color C) compared to the combination of the yellow light 410-1 and the blue light 420-1 (Color A) in FIG. 4A. This is because more blue light is directed toward the second portion 450 of the light emission surface 122 of the light guide plate 120 and less blue light is scattered by the light extraction features 140 proximate to the first portion 440 of the light guide plate 120, compared to blue light 420-1 in FIG. 4A. At the second portion 450 of the light emission surface 122 of the light guide plate 120, the combination of yellow light 410-2 and blue light 430-2 has a reduced yellow color compared to the combination of yellow light 410-2 and the blue light 420-2 (Color B) in FIG. 4 A. Particularly, while the blue light 430-2 attenuates due to the low optical transmittance of the light guide plate 120 for blue light, more blue light 430-2 is directed toward the second portion 450 of the light emission surface 122 of the light guide plate 120 compared to the blue light 420-2 in FIG. 4A because the blue light 430 of FIG. 4B is emitted over an angular range (i.e., solid angle) that is less than the angular range (i.e., solid angle) of the blue light 420 in FIG. 4 A. Thus, the intensity of the blue light 430-2 emitted from the second portion 450 of the light emission surface 122 of the light guide plate 120 in FIG. 4B is greater than the intensity of the blue light 420-2 in FIG. 4A.

[0058] Thus, by using a light source 110B in which the blue light 430 is emitted over an angular range (i.e., a solid angle) that is less than the angular range (i.e., solid angle) of the yellow light 410, as described above with respect to FIG. 4B, a color shift in the light emitted from the second portion 450 of the light guide plate 120 is mitigated and, as a result, the color of light emitted from the light guide plate 120 is more uniform over the length of the light guide plate 120 in the propagation direction 150.

[0059] FIG. 6A graphically depicts a CIE 1931 Y color coordinate for light sources having various light intensity profiles. The y-axis (i.e., the vertical ordinate) of the graph is the CIE 1931 Y color coordinate and the x-axis (i.e., the horizontal ordinate) of the graph is the viewing angle. Line 610 represents a CIE 1931 Y color coordinate value of a white light emitted from the light source 110B, as described above with respect to FIGS. 4A, as a function of viewing angle (Q) where the normalized blue light intensity profile of the light source 110B varies as a function of cos Q and the normalized yellow light intensity profile of the light source 110B varies as a function of cos Q. In this case, the CIE 1931 Y color coordinate value is constant across viewing angles ranging from -90 degrees to +90 degrees.

[0060] Line 620 depicts a CIE 1931 Y color coordinate value of white light emitted from the light source 110B as a function of the viewing angle (Q) where the normalized yellow light intensity profile of the light source 110B varies as a function of cos Q and the normalized blue light intensity profile of the light source 110B varies as a function of cos^{1 1} Q. Line 630 depicts a CIE 1931 Y color coordinate value of white light emitted from the light source 110B as a function of the viewing angle (Q) where the normalized yellow light intensity profile of the light source 110B varies as a function of cos Q and the normalized blue light intensity profile of the light source 110B varies as a function of cos^{1 3} Q. Line 640 depicts a CIE 1931 Y color coordinate value of white light emitted from the light source 110B as a function of viewing angle (Q) where the normalized yellow light intensity profile of the light source 110B varies as a function of cos Q and the normalized blue light intensity profile of the light source 110B varies as a function of cos^{1 5} Q. Line 650 depicts a CIE 1931 Y color coordinate value of white light emitted from the light source 110B as a function of viewing angle (Q) when the normalized yellow light intensity profile of the light source 110B varies as a function of cos Q and the normalized blue light intensity profile varies as a function of cos^{1 7} Q. As shown in FIG. 6A, as the value of N is increased from 1 to 1.7, the CIE 1931 Y color coordinate deviates from the line 610, particularly at viewing angles greater than about +50 degrees and less than about -50 degrees. This change in the CIE 1931 Y color coordinate generally indicates a decrease in the uniformity of the light intensity profile of the light source for increasing values of N.

[0061] FIG. 6B depicts the CIE 1931 Y color coordinate of white light emitted from a light guide plate 120 (e.g., white light including blue light 430-1 and yellow light 410-1) coupled to a light source 110B, as depicted in FIG. 4B, in a 55 inch display having a width of about 1220 millimeters and a height of about 680 millimeters. Thus, the light guide plate 120 of the display has a length in the propagation direction 150 of about 680 millimeters. The x-axis (i.e., the horizontal ordinate) indicates the distance from the light source 110B to a point in the light guide plate 120 along the propagation direction 150. The y-axis (i.e., the vertical ordinate) indicates the CIE 1931 Y color coordinate of white light emitted from the light guide plate 120. Line 612 depicts the CIE 1931 Y color coordinate value of white light emitted from the light guide plate 120 where the normalized blue light intensity profile of the light source 110B varies as a function of cos Q and the normalized yellow light intensity profile of the light source 110B varies as a function of cos Q. As shown in FIG. 6B, the CIE 1931 Y color coordinate value generally increases as the distance from the light source 110B increases. Particularly, the white light emitted from the light guide plate 120 becomes more yellowish, as the distance from the light source 110B increases as shown in FIG. 6B.

[0062] Line 622 depicts the CIE 1931 Y color coordinate of white light emitted from the light guide plate 120 where the normalized yellow light intensity profile of the light source 110B varies as a function of cos Q and the normalized blue light intensity profile of the light source 110B varies as a function of cos^{1 1} Q. Line 632 depicts the CIE 1931 Y color coordinate of white light emitted from the light guide plate 120 where the normalized yellow light intensity profile of the light source 110B varies as a function of cos Q and the normalized blue light intensity profile of the light source 110B varies as a function of cos^{1 3} Q. Line 642 depicts the CIE 1931 Y color coordinate value of white light emitted from the light guide plate 120 where the normalized yellow light intensity profile of the light source 110B varies as a function of cos Q and the normalized blue light intensity profile of the light source 110B varies as a function of cos^{1 5} Q. Line 652 depicts the CIE 1931 Y color coordinate value of white light emitted from the light guide plate 120 where the normalized yellow light intensity profile of the light source 110B varies as a function of cos Q and the normalized blue light intensity profile of the light source 110B varies as a function of cos^{1 7} Q.

[0063] As depicted in FIG. 6B, line 652 shows the least variation in the CIE 1931 Y color coordinate value among the five lines 612, 622, 632, 642, and 652. That is, when the value N for the normalized blue light intensity profile is set as 1.7 in a 55-inch display, the light guide plate 120 outputs white color light with a reduced yellow color shift along the propagation direction 150 although the light source 110B itself emits white light with less color uniformity as indicated by the line 650 in FIG. 6A. Table 1 below reports the color simulation results for different values of N for the normalized blue light intensity profile with respect to a 55 -inch display.

Table 1 - 55 inch display

[0064] As shown in Table 1, when the normalized blue light intensity profile varies as a function of cos Q, the phosphor-based light source (e.g., a phosphor-based white LED) has the greatest color uniformity among the five simulations. As the value N of the normalized blue light intensity profile increases, the uniformity of the light intensity profile for different wavelengths emit by the light source 110B decreases. However, when the value N is set as 1, the light guide plate 120 emits white light with the greatest variation in the CIE 1931 Y color coordinate value (Cy) (i.e., the difference between the Cy value at one edge of the light guide plate 120 farthest from the light source 110B and the Cy value at the other edge of the light guide plate 120 closest to the light source 110B (Cy Far - Cy Near) is 0.0206) among the five simulations. In contrast, when the normalized blue light intensity profile varies as a function of cos^{1 7} Q, the phosphor-based light source 110B has the lowest color uniformity (i.e., the light intensity profile varies the most over the range of viewing angles for different wavelengths) among the five simulations. However, the light guide plate 120 emits white light with the smallest variation in the CIE 1931 Y color coordinate value (e.g., 0.0006) among the five simulations. In this regard, by intentionally varying the uniformity of the light intensity profile of the light source 11 OB for different wavelengths, the white light emitted from the light guide plate 120 exhibits a reduced yellow color shift in the light propagation direction of the light guide plate 120.

[0065] In embodiments, the optimal normalized blue light intensity profile to reduce the variation in the CIE 1931 Y color coordinate value may be different based on the dimensions of a display, particularly the dimension in the light propagation direction of the light guide plate 120. Tables 2 through 5 report color simulation results for a 42-inch display, a 47-inch display, a 60-inch display, and a 70-inch display, respectively.

Table 2 - Simulation results for 42 inch display

Table 3 - Simulation results for 47 inch display

Table 4 - Simulation results for 60 inch display

Table 5 - Simulation results for 70 inch display

[0067] FIG. 7 graphically depicts the variation in Cy value for displays with various dimensions in the light propagation direction of the light guide plate. As noted above, the variation in Cy value is the difference between the Cy value at one edge of the light guide plate 120 farthest from the light source 110B and the Cy value at the other edge of the light guide plate 120 closest to the light source 110B. The x-axis (i.e., the horizontal ordinate) of the graph is the value of N for the normalized blue light intensity profile, and the y-axis (i.e., the vertical ordinate) of the graph is the variation in the Cy value. Line 710 represents the variation in Cy value for a 42-inch display, line 720 represents the variation in Cy value for a 47-inch display, line 730 represents the variation in Cy value for a 55-inch display, line 740 represents the variation in Cy value for a 60-inch display, and line 750 represents the variation in Cy value for a 70-inch display.

[0068] As shown in FIG. 7, as the size of the display increases, the value of N corresponding to a minimum variation in Cy value generally increases. For example, with respect to a 42- inch display, the value of N corresponding to the minimum variation in Cy value is about 1.5. With respect to a 47-inch display, the value of N corresponding to the minimum variation in Cy value is about 1.6. With respect to a 55-inch display, the value of N corresponding to the minimum variation in Cy value is 1.7. With respect to a 70-inch display, the value of N corresponding to a minimum variation in Cy value is about 1.74.

[0069] While the foregoing description generally describes using a phosphor-based white LED for a backlight apparatus and adjusting the normalized blue light intensity profile to minimize color shift, it should be understood that, in some embodiments, an RGB based white LED may be used for a backlight apparatus, and the light intensity profile of one of the red, green, and/or blue colors may be adjusted to mitigate color shift. For example, if one edge of the light guide plate 120 emits greenish white color, the normalized red light intensity profile and the normalized blue light intensity profile may be made narrower than the normalized green light intensity profile such that more red light and blue light reach the edge of the light guide plate 120. Hence, in the embodiments described herein, the white light source may be configured such that the white light emitted from the light source comprises at least a first light color and a second light color, and the angular range over which the second light color is emitted may be smaller than the angular range over which the first light color is emitted.

[0070] According to one or more embodiments of the present disclosure, by intentionally decreasing the uniformity of the light intensity profile for wavelengths of light emitted from the light source 110B, the white light emitted from the light guide plate 120 shows a reduced color shift. Particularly, in some embodiments, the normalized blue light intensity profile from the light source 110B is adjusted by adjusting the angular range over which the blue light is emitted is less than the angular range over which the yellow light is emitted. In embodiments, the normalized blue light intensity profile may vary as a function of cos Q to the power of N (i.e., cos^N Q) where N is greater than 1. The value of N may be varied depending on the dimension of a display in the light propagation direction, including dimensions the light guide plate. Thus, the backlight apparatus may be used to reduce the color shift in the backlight apparatus that occurs when the light guide plate is made of glass.

[0071] While specific embodiments of the backlight apparatuses are described herein as utilizing a light guide plate formed from glass, it should be understood that the light sources and techniques for mitigating color shift described herein may be utilized in conjunction with light guide plates made from other materials, particularly when the material from which the light guide plate is formed has a tendency to attenuate certain wavelengths of light more than others.

[0072] It will be apparent to those skilled in the art that various modifications and variations can be made to the embodiments described herein without departing from the spirit and scope of the claimed subject matter. Thus it is intended that the specification cover the modifications and variations of the various embodiments described herein provided such modification and variations come within the scope of the appended claims and their equivalents.

Claims

What is claimed is:

1. A backlight apparatus comprising:

a light guide plate; and

a light source oriented to direct light into a light inlet face of the light guide plate, the light comprising at least a first light color and a second light color,

and an angular range over which the second light color is emitted from the light source is less than an angular range over which the first light color is emitted from the light source.

2. The backlight apparatus of claim 1, wherein the light source comprises a phosphor- based white LED.

3. The backlight apparatus of claim 2, wherein the phosphor-based white LED comprises an emitter, a blue dye layer positioned over the emitter and a yellow phosphor layer positioned over the blue dye layer.

4. The backlight apparatus of claim 1, wherein the first light color is blue light and the second light color is yellow light.

5. The backlight apparatus of claim 1, wherein the light guide plate comprises glass.

6. The backlight apparatus of claim 1, wherein a normalized light intensity of the second light color is greater than zero within the angular range over which the second light color is emitted and the normalized light intensity is zero outside the angular range over which the second light color is emitted.

7. The backlight apparatus of claim 1, wherein a normalized light intensity of the second light color is greater than zero for viewing angles from about -50 degrees to about +50 degrees and the normalized light intensity of the second light color is zero for viewing angles less than -50 degrees and greater than +50 degrees, wherein a viewing angle of 0 degrees is normal to the light source and the viewing angles are relative to the normal.

8. The backlight apparatus of claim 7, wherein a normalized light intensity of the first light color is greater than zero for viewing angles from about -90 degrees to about +90 degrees.

9. The backlight apparatus of claim 1, wherein:

a normalized intensity profile of the first light color varies as a function of cos Q; and

a normalized intensity profile of the second light color varies as a function of cos^N Q, where Q is a viewing angle of the light source and N is greater than 1.

10. The backlight apparatus of claim 1, wherein the light source comprises an RGB based white light source.

11. A display comprising:

a light guide plate comprising glass; and

a light source oriented to direct light into a light inlet face of the light guide plate, the light comprising at least blue light and yellow light,

wherein an emission cone of the blue light is less than an emission cone of the yellow light.

12. The display of claim 11, wherein the light source comprises a phosphor-based white LED.

13. The display of claim 11, wherein a normalized light intensity of the blue light is greater than zero for viewing angles from about -50 degrees to about +50 degrees and the normalized light intensity of the blue light is zero for viewing angles less than -50 degrees and greater than +50 degrees, wherein a viewing angle of 0 degrees is normal to the light source and the viewing angles are relative to the normal.

14. The display of claim 13, wherein a normalized light intensity of the yellow light is greater than zero for viewing angles from about -90 degrees to about +90 degrees.

15. The display of claim 11, wherein:

a normalized intensity profile of the yellow light varies as a function of cos Q; and a normalized intensity profile of the blue light varies as a function of cos^N Q, where Q is a viewing angle of the light source and N is greater than 1.

16. A method for adjusting color shift in a light guide plate comprising:

directing, by a light source, light comprising yellow light and blue light toward a light inlet face of the light guide plate such that the white light propagates through the light guide plate and is emitted from a light emission surface of the light guide plate,

and an emission cone of the blue light emitted from the light source is less than an emission cone of the yellow light emitted from the light source.

17. The method of claim 16, wherein the light source comprises a phosphor-based white LED.

18. The method of claim 16, wherein the light guide plate comprises glass.

19. The method of claim 16, wherein an optical transmittance of the light guide plate is greater for the yellow light than for the blue light.

20. The method of claim 16, wherein: