Backlight unit for backlit displays

Abstract

The BLU includes a 2D array of LEDs that produce light with a nominal angular distribution of ±90 degrees around the center. A layer of transparent solid material is positioned with its bottom surface spanning the 2D array and is configured to convert the light produced by the 2D array into light with an angular distribution of less than ±90 degrees. A diffuser film is positioned across the upper surface of the layer of transparent solid material and is configured to diffuse light and produce diffused light on the upper surface of the diffuser film. A BEF is positioned across the upper surface of the diffuser film and comprises multiple prism microstructures, at least some of which have an apex angle of about 90 degrees. The BEF is configured to narrow the angular optical distribution of the diffused light produced on the upper surface of the diffuser film.

Description

Technical Field

[0001] The headings of the sections used in this specification are for organizational purposes only and should not be construed as limiting the subject matter described in this application in any way. (Cross - Reference to Related Applications)

[0002] This application is a non - provisional application of U.S. Provisional Patent Application No. 63 / 503,517, filed on May 22, 2023, entitled "Back Light Unit for Backlit Displays". The entire content of U.S. Provisional Patent Application No. 63 / 503,517 is incorporated herein by reference.

Background Art

[0003] The present teachings generally relate to techniques for backlight units used in backlit displays. Display technology is evolving rapidly in an attempt to improve display image quality. In particular, liquid crystal displays (LCDs) are increasingly deploying backlight unit architectures to improve image quality. It is highly desirable to have improved backlight units for this and other applications. For example, micro - LEDs are emerging as an important backlight technology for new high - dynamic range (HDR) LCD displays, including monitors, mobile phones, and tablets. The advantages of these displays and associated backlight units (BLUs) are local dimming, where individual LEDs within the BLU can be turned on and off to produce a higher contrast ratio, which is required to support HDR, for example, compared to edge - lit LEDs. Such systems provide local dimming for more vivid contrast, darker blacks, lower power, ultra - high brightness, high reliability, and no burn - in, compared to, for example, OLED displays. Applications for these displays include, for example, automotive displays, laptops, desktops, tablets, and special custom displays.

Brief Description of the Drawings

[0004] In preferred exemplary embodiments, this teaching, along with its further advantages, will be described more specifically in the following detailed description, which will be considered in conjunction with the accompanying drawings. Those skilled in the art will understand that the drawings described below are for illustrative purposes only. The drawings are not necessarily to scale and instead, the emphasis is generally on illustrating the principles of this teaching. The drawings are not intended to limit the scope of the applicant's teaching in any way.

[0005] [Figure 1A] Figure 1A illustrates a schematic diagram of one embodiment of a backlight unit architecture, including an array of light-emitting diodes (LEDs) for a liquid crystal display (LCD) according to this teaching.

[0006] [Figure 1B] Figure 1B illustrates a schematic diagram of another embodiment of the backlight unit architecture, including an array of light-emitting diodes (LEDs) for the liquid crystal display (LCD) of this teaching.

[0007] [Figure 2A] Figure 2A illustrates a three-dimensional plot of the light distribution generated by a known LED, as measured by a light distribution meter, as a function of angle.

[0008] [Figure 2B] Figure 2B illustrates the measured light distribution of the illumination pattern, which is described in relation to Figure 2A, and is represented in two dimensions.

[0009] [Figure 3] Figure 3 illustrates a schematic diagram of a two-dimensional array of LEDs that may be used in the embodiment of the narrow backlight unit described herein.

[0010] [Figure 4A]Figure 4A illustrates a table relating to an embodiment of the diffuser layer including a single-sided coating according to this instruction.

[0011] [Figure 4B] Figure 4B illustrates a table relating to an embodiment of the diffuser layer including double-sided coating according to this instruction.

[0012] [Figure 4C] Figure 4C illustrates a table relating to embodiments of diffuser layers, including a volumetric diffuser layer having strong opacity or LED smoothing properties.

[0013] [Figure 5] Figure 5 illustrates a graph of the measured angular intensity distribution of an embodiment of a highly invisible microstructure bifacial diffuser layer of this teaching.

[0014] [Figure 6A] Figure 6A illustrates a schematic diagram of the layer structure of an embodiment of a backlight unit having a double-sided diffuser layer and voids according to this teaching.

[0015] [Figure 6B] Figure 6B illustrates a visual image of one embodiment of the backlight unit shown in Figure 6A, which has a double-sided diffuser layer and a thin void thickness.

[0016] [Figure 6C] Figure 6C illustrates a visual image of one embodiment of the backlight unit shown in Figure 6A, which has a double-sided diffuser layer and a thicker void thickness.

[0017] [Figure 7A] Figure 7A illustrates a schematic diagram of the layer structure of an embodiment of a backlight unit according to this teaching, which has a double-sided diffuser layer, a second diffuser layer, and a transparent material layer.

[0018] [Figure 7B]FIG. 7B illustrates a visual image of an embodiment of the backlight unit of FIG. 7A having a double-sided diffuser layer and a thin transparent material layer thickness.

[0019] [Figure 7C] FIG. 7C illustrates a schematic diagram of a layer structure relating to an embodiment of a backlight unit having a double-sided diffuser layer of the present disclosure and a thicker transparent material layer thickness.

[0020] [Figure 7D] FIG. 7D illustrates a visual image of an embodiment of the backlight unit of FIG. 7C.

[0021] [Figure 7E] FIG. 7E illustrates a schematic diagram of a layer structure relating to an embodiment of a backlight unit having a double-sided diffuser layer of the present disclosure and an even thicker transparent material layer thickness.

[0022] [Figure 7F] FIG. 7F illustrates a visual image of an embodiment of the backlight unit of FIG. 7E.

[0023] [Figure 7G] FIG. 7G illustrates a visual image of an embodiment of a backlight unit having a double-sided diffuser layer across a 20 mm thick transparent material layer.

[0024] [Figure 7H] FIG. 7H illustrates a visual image of an embodiment of a backlight unit having a double-sided diffuser layer across a 24 mm thick transparent material layer.

[0025] [Figure 7I] FIG. 7I illustrates a visual image of an embodiment of a backlight unit having a double-sided diffuser layer across a 28 mm thick transparent material layer.

[0026] [Figure 8A]Figure 8A illustrates a schematic diagram of the layer structure of one embodiment of a backlight unit, which has a double-sided diffuser layer spanning the gaps, an additional pair of high refractive index cross-brightness enhancing films above the double-sided diffuser layer, and a polarizer reflector film.

[0027] [Figure 8B] Figure 8B illustrates a visual image of one embodiment of the backlight unit shown in Figure 8A, with a gap thickness of 6 mm.

[0028] [Figure 8C] Figure 8C illustrates a visual image of one embodiment of the backlight unit shown in Figure 8A, with a gap thickness of 7 mm.

[0029] [Figure 9A] Figure 9A illustrates a schematic diagram of the layer structure of an embodiment of a backlight unit according to this teaching, which includes a double-sided diffuser layer, a second diffuser layer, a transparent material layer, a pair of high refractive index cross-brightness enhancing films, and a polarizer reflector film.

[0030] [Figure 9B] Figure 9B illustrates a visual image of one embodiment of the backlight unit shown in Figure 9A.

[0031] [Figure 9C] Figure 9C illustrates a schematic diagram of the layer structure of one embodiment of a backlight unit, which has a double-sided diffuser layer spanning the void, and above the double-sided diffuser layer, an additional pair of high refractive index cross-brightness enhancing films and a polarizing reflector film.

[0032] [Figure 9D] Figure 9D illustrates a visual image of one embodiment of the backlight unit shown in Figure 9C.

[0033] [Figure 10] Figure 10 illustrates a graph of the point spread function of a single LED in relation to an embodiment of the backlight unit of this teaching.

[0034] [Figure 11A] Figure 11A illustrates a schematic diagram of the layer structure of an embodiment of a backlight unit having a double-sided diffuser layer spanning a transparent material layer, with an additional pair of high refractive index cross-brightness enhancing films above the double-sided diffuser layer.

[0035] [Figure 11B] Figure 11B illustrates a visual image of one embodiment of the backlight unit shown in Figure 11A.

[0036] [Figure 11C] Figure 11C illustrates a schematic diagram of the layer structure of an embodiment of a backlight unit having a bifacial diffuser layer spanning the void, with an additional pair of high refractive index cross-brightness enhancing films above the bifacial diffuser layer.

[0037] [Figure 11D] Figure 11D illustrates a visual image of one embodiment of the backlight unit shown in Figure 11C.

[0038] [Figure 12A] Figure 12A illustrates a schematic diagram of the layer structure of an embodiment of a backlight unit having a volumetric diffuser layer spanning a transparent material layer, with an additional pair of high refractive index cross-brightness enhancing films above the volumetric diffuser layer.

[0039] [Figure 12B] Figure 12B illustrates a visual image of one embodiment of the backlight unit shown in Figure 12A.

[0040] [Figure 12C] Figure 12C illustrates a schematic diagram of the layer structure of an embodiment of a backlight unit having a volumetric diffuser layer spanning the voids, with an additional pair of high refractive index cross-brightness-enhancing films above the volumetric diffuser layer.

[0041] [Figure 12D] Figure 12D illustrates a visual image of one embodiment of the backlight unit shown in Figure 12C.

[0042] [Figure 13A] Figure 13A illustrates a schematic diagram of the layer structure of one embodiment of a backlight unit, which has a volumetric diffuser layer spanning a transparent material layer, a second volumetric diffuser layer, and above the second volumetric diffuser layer, an additional high refractive index cross-brightness enhancing film and a polarizer reflector layer.

[0043] [Figure 13B] Figure 13B illustrates a visual image of one embodiment of the backlight unit shown in Figure 13A.

[0044] [Figure 13C] Figure 13C illustrates a schematic diagram of the layer structure of one embodiment of a backlight unit, which has a volumetric diffuser layer spanning the void, and above the volumetric diffuser layer, an additional pair of high refractive index cross-brightness enhancing films and a polarizing reflector film.

[0045] [Figure 13D] Figure 13D illustrates a visual image of one embodiment of the backlight unit shown in Figure 13C.

[0046] [Figure 14A-1] Figure 14A illustrates graphs of intensity versus horizontal divergence angle, graphs of intensity versus vertical divergence angle, a three-dimensional plot of light distribution, and a projection of the three-dimensional plot of light distribution generated by known LEDs passing through a pair of high refractive index cross-brightness-enhancing films and a polarizing reflector film used in embodiments of this teaching. [Figure 14A-2]Figure 14A illustrates graphs of intensity versus horizontal divergence angle, graphs of intensity versus vertical divergence angle, a three-dimensional plot of light distribution, and a projection of the three-dimensional plot of light distribution generated by known LEDs passing through a pair of high refractive index cross-brightness-enhancing films and a polarizing reflector film used in embodiments of this teaching.

[0047] [Figure 14B-1] Figure 14B illustrates the intensity-to-horizontal divergence graph, intensity-to-vertical divergence graph, and three-dimensional plot of the light distribution, as well as a projection of the three-dimensional plot of the light distribution, generated by a known LED passing through a single high refractive index brightness-enhancing film and polarizing reflector film used in embodiments of this teaching. [Figure 14B-2] Figure 14B illustrates the intensity-to-horizontal divergence graph, intensity-to-vertical divergence graph, and three-dimensional plot of the light distribution, as well as a projection of the three-dimensional plot of the light distribution, generated by a known LED passing through a single high refractive index brightness-enhancing film and polarizing reflector film used in embodiments of this teaching. [Modes for carrying out the invention]

[0048] Description of various embodiments This teaching will be described in more detail here with reference to exemplary embodiments, such as those shown in the accompanying drawings. While this teaching will be described in conjunction with various embodiments and examples, it is not intended to be limited to such embodiments. Rather, this teaching includes various alternatives, modifications, and equivalents, as will be understood by those skilled in the art. Those skilled in the art who have access to the teachings herein will recognize additional implementations, modifications, and embodiments, as well as other fields of use, that fall within the scope of this disclosure as described herein.

[0049] In this specification, any reference to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in relation to that embodiment is included in at least one embodiment of this teaching. The phrase "in one embodiment" in various places within this specification does not necessarily refer to the same embodiment.

[0050] It should be understood that the individual steps of the method described herein may be performed in any order and / or simultaneously, as long as the instruction remains operational. Furthermore, it should be understood that the apparatus and method described herein may include any number or all of the embodiments described herein, as long as the instruction remains operational.

[0051] In pursuit of improved image quality, various thin displays, including liquid crystal displays (LCDs), are increasingly using backlight unit architectures. This teaching relates to a backlight unit for a backlit display having a two-dimensional array of light-emitting diode (LED) light sources. Figures 1A and 1B show schematic diagrams of a backlight unit including an array of LEDs for a liquid crystal display (LCD). Figure 1A illustrates a schematic diagram of one embodiment of the backlight unit architecture including an array of LEDs for an LCD according to this teaching.

[0052] Referring to Figure 1A, the backlight unit architecture 100 may include an array 110 of individual LEDs 112, which may be, for example, blue LEDs, white LEDs, and / or red / green / blue LED clusters. Any variety of known LEDs can be used in connection with this teaching. Figure 2A illustrates a three-dimensional plot of the light distribution produced by known LEDs as a function of angle, as measured by a photometer. Figure 2B illustrates the measured light distribution of an illumination pattern, described in connection with Figure 2A, represented in two dimensions. Figures 2A and 2B illustrate the typical intensity distribution of light emitted from a single LED as a function of angle, as measured by a photometer. As illustrated, the LED source approximates a Lambertian source. A Lambertian source is a source that emits a light distribution that is approximately symmetrical with respect to the nadir. This corresponds to the highest intensity of light present at the nadir. Generally, the light emission angle of a Lambertian source is 180 degrees, i.e., ±90 degrees around the center.

[0053] Returning to Figure 1A, a series of films, also referred to as layers, may be used to disperse or diffuse the light emitted from the LED 112 so that the backlight unit architecture 100 can deliver more uniform light to the LCD module 180, which contains liquid crystal located above the backlight unit. Some embodiments of the backlight unit in this teaching do not use the LCD module 180. Some embodiments of this teaching use different lighting modules instead of the LCD module 180. In some embodiments, the backlight unit architecture 100 includes a diffuser film 120. Various known diffuser films 120 can be used. The diffuser film 120 may also be referred to as a diffuser layer or diffuser. For example, the diffuser film 120 can be a single-sided diffuser layer, a double-sided diffuser layer, a volumetric diffuser layer, and / or a circular diffuser. A color conversion layer 130 may be used, for example, when the LED 112 is emitting blue light. The color conversion layer 130 can use either quantum dots or phosphor material to convert, for example, a portion of the blue light emitted by the LED 112 into green and red light. A second diffuser film 140, which may be a volumetric diffuser or a circular diffuser resulting from, for example, a randomly textured surface, is configured to disperse or diffuse the light emitted from the color conversion layer 130. Two brightness-enhancing films (BEFs) 150, 160 are present, positioned across the second diffuser film 140. In some embodiments, only one brightness-enhancing film 150, 160 is used. In some embodiments, the two brightness-enhancing films 150, 160 are two prism films rotated approximately 90 degrees relative to each other. Both of these 90-degree rotated prism films are also known as a pair of crossed brightness-enhancing films. A polarizer reflector 170, also referred to as a DBEF, is positioned across the brightness-enhancing films 150, 160. In some embodiments, the backlight unit architecture 100 may include an additional film (not shown) used to improve the overall uniformity and brightness of the light delivered to the LCD panel.

[0054] In some embodiments, the DBEF170 film (which reflects the polarizer) is incorporated either with the LCD module 180 or as a separate film below the LCD module 180. In some backlight units, white LEDs may be used without the color conversion layer 130. By using an array of LEDs 110, a better contrast ratio can be achieved by adjusting the brightness of individual LEDs 112, often referred to as local dimming.

[0055] Generally, consumers prefer electronic devices, including LCDs, to evolve to become thinner and thinner. Consequently, there is a need for the backlight units of such displays to also become thinner and thinner. This presents the challenge of managing the light emitted by LEDs in an effective manner that ensures uniformity and brightness in thinner backlight units.

[0056] Figure 1B illustrates a schematic diagram of another embodiment of the backlight unit architecture 190, which includes an array of light-emitting diodes (LEDs) for a liquid crystal display (LCD) according to this teaching. Referring, for example, to the embodiment of the LCD display with the backlight unit architecture 190 of Figure 1B, when the diffuser film 120 is placed over the array 110 of LEDs 112, the individual points of light emitted by the LEDs are diffused so that light with less intensity from adjacent LEDs 112 begins to overlap, creating an area of ​​light with higher intensity between the LEDs 112. If the thickness of the diffuser film 120 is increased, for example, which may be undesirable with respect to thinner backlight units, the individual points of light may be dispersed further apart, providing better uniformity of light, but there may still be brighter and darker areas.

[0057] In one embodiment, the LED 112 is a blue LED formed within the array 110, and the backlight unit of this teaching delivers bright, uniform light to the LCD module 180 in a thin layered form between the array 110 and the module 180. These layers may include a pair of cross-shaped brightness-enhancing films 150, 160 placed across a diffuser film 120, and a polarizer reflector 170 placed across the pair of cross-shaped brightness-enhancing films 150, 160. This layered structure effectively obscures the individual LEDs 112 when viewed from the module 180 side of the backlight unit architecture 190 and provides a thinner backlight unit form than the backlight unit architecture 100 described in association with Figure 1A. Various layered structures of this teaching, including different layer types, layer thicknesses, and layer positions, can be used to provide one or more display performance objectives, as will be further described herein. These performance objectives include, for example, producing bright and uniform light while maintaining the desired backlight unit thickness, obscuring individual elements, and / or supporting dimming operations with a better contrast ratio with the display.

[0058] The LEDs 112 can be arranged within the array 110. Figure 3 illustrates a schematic diagram of a two-dimensional array 300 of LEDs 302 that may be used in an embodiment of the narrow backlight unit of this teaching. The LED elements 302 of the array 300 are positioned with nominally uniform spacing X304 in one dimension and nominally uniform spacing Y306 in a second dimension. The LED spacings X304 and Y306 may be the same or they may be different. In some embodiments of the LED array 300 of this teaching, the LED spacing X304 is 8 mm and the LED spacing Y306 is 8 mm. Other LED spacings may also be used. The spacing X304 may also be referred to as the column spacing, and the spacing Y306 may be referred to as the row spacing. Embodiments of the array 300 have 7 rows and 7 columns. The number of rows and columns may also vary in various configurations. The number of rows can be the same as the number of columns, as shown in array 300. In some embodiments, the first dimension has fewer LED elements than the second dimension. For example, the narrow dimensions of the backlight unit may consist of fewer than 15 LED rows or even fewer than 10 LED rows. For example, the wider dimensions may have more columns than rows.

[0059] Referring to all of Figures 1A, 1B, and 3, when the LEDs 112, 302 are configured in arrays 110, 300, it is desirable to make the individual LEDs 112, 302 invisible and to present a bright, uniform light to the LCD module 180. As described above, one approach to achieving this goal is to include one or more diffuser films 120, 140 within the backlight units 100, 190 to diffuse, scatter, or blur the beam of light emitted by the LEDs 112, 302. In some embodiments, there is a desire to keep the backlight unit architecture 100, 190 as thin as possible, and a series of MLA films are used.

[0060] In some other embodiments, the thickness of the backlight unit is not a significant constraint, and the backlight unit layer thickness can be equal to or greater than the LED spacing 304, 306. In these embodiments, the challenges may be somewhat similar to those faced in LED-based lighting. Often, a single diffuser film can be used above the LEDs, with a gap beneath it. As an example, Tables 400, 430, and 450, described in relation to Figures 4A–C below, provide guidelines regarding the relative size of the gap to the LED spacing for achieving uniform light distribution for these embodiments with thicker backlight unit dimensions. These Tables 400, 430, and 450 refer to various commercially available diffuser layers produced by Brightview Technologies, the assignee of this application.

[0061] One feature of the backlight unit in this teaching is its ability to produce very uniform light output from an array of LEDs, while being thin, efficient, and cost-effective. Rather than using conventional diffuser films that rely on light scattering, some embodiments of this teaching use microlens arrays, i.e., MLAs, diffuser films, to provide improved performance. Instead of scattering, MLAs rely on refraction and total internal reflection to manage the optics, resulting in numerous advantages, including better light uniformity and local dimming performance, thinner film stacks, improved efficiency, and the ability to work with color-transition films to minimize crosstalk.

[0062] The diffuser films used in the embodiments of this teaching may include surface microstructures that can be produced using many techniques known in the art. For example, in some embodiments, the diffuser films of this teaching may be fabricated with photo-revision microstructures having a desired shape cast onto a substrate using a suitable master mold and a thermosetting polymer or ultraviolet (UV) photocurable polymer. The microstructure shape may also be pressed into a thermoplastic substrate through compression molding or other molding. The shape may also be produced simultaneously with the substrate using extrusion embossing or injection molding. The microstructure may be produced by replicating a master. For example, an optical film may be produced by replicating a master containing the desired shape, as described in U.S. Patent No. 7,190,387 B2 by Rinehart et al., titled "Systems and Methods for Fabricating Optical Microstructures Using a Cylindrical Platform and a Rastered Radiation Beam," U.S. Patent No. 7,867,695 B2 by Freese et al., titled "Methods for Mastering Microstructures Through a Substrate Using Negative Photoresist," and U.S. Patent No. 7,192,692 B2 by Wood et al., titled "Methods for Fabricating Microstructures by Imaging a Radiation Sensitive Layer Sandwiched Between Outer Layers" (all disclosures of these patents are incorporated herein by reference as a whole). Masters themselves may be processed using the laser scanning techniques described in these patents, and may also be replicated to provide microstructures using the replication techniques described in these patents.

[0063] In some embodiments, projection or contact photolithography, such as techniques used in semiconductors, displays, circuit boards, and other common technologies, which are known in the art, can be used to expose microstructures into photosensitive materials. In some embodiments, laser ablation, which is either using a mask or using a focused or modulated laser beam, can be used to generate microstructures containing marks within a material. In some embodiments, micromachining (particularly diamond micromachining) can be used to generate desired microstructures from solid materials. In some embodiments, additive manufacturing, such as 3D printing, can be used to generate desired microstructures within solid materials.

[0064] In some embodiments of this teaching, the diffuser film can also be a volumetric diffuser in which particles of a certain concentration are embedded in a matrix of different refractive indices.

[0065] Diffuser films can be characterized by their so-called full width at half maximum (FWHM). FWHM is a measure of the dispersion of the light distribution emitted from the film when illuminated by a collimated beam. FWHM can be expressed as an angle in degrees. Diffusers with a larger FWHM are more powerful diffusers than those with a smaller FWHM. Some embodiments of this teaching utilize diffuser films with an FWHM greater than 60 degrees.

[0066] Figure 4A illustrates Table 400 relating to embodiments of diffuser layers including single-sided coating according to this teaching. Table 400 shows the approximate ratios of LED spacing and voids required to achieve uniform spatial intensity and angular distribution between the top of the LED array and the bottom surface of the diffuser layer. Diffusers directed towards the top in Table 400 have relatively weak opacity or LED smoothing and require larger voids to achieve smooth and uniform brightness. Diffusers directed towards the bottom in Table 400 are used when strong and smooth opacity of the LED source is desired along with smaller voids.

[0067] Figure 4B illustrates Table 430 relating to embodiments of diffuser layers, including double-sided coatings, according to this teaching. Table 430 shows the approximate ratios of LED spacing and voids for achieving uniform spatial intensity and angular distribution. Several embodiments of diffuser layers include double-sided coatings, which may provide stronger LED opacity and higher efficiency than the embodiments of diffuser layers described in connection with Figure 4A. Diffuser layers towards the top of Table 430 have relatively weak opacity or LED smoothing and require larger voids to achieve smooth and uniform brightness. Diffuser layers towards the bottom of Table 430 are used when strong and smooth opacity of the LED source is desired along with smaller voids. The greater the intensity of diffusion, the more LED opacity is produced with smaller voids.

[0068] Figure 4C illustrates Table 450 relating to embodiments of diffuser layers containing volumetric diffusers having strong opacity or LED smoothing properties. Table 450 shows approximate ratios of LED spacing to voids for achieving uniform spatial intensity and angular distribution. The volumetric diffuser films shown in Table 450 provide smooth and uniform brightness with small voids.

[0069] Figure 5 illustrates graph 500 of the measured angular intensity distribution of a high-invisibility microstructure bifacial diffuser layer in one embodiment of the teaching. High or high invisibility means producing a uniform output from multiple LEDs that does not make the individual LED elements visible at the output. Lower invisibility will still smooth the light at the output and make it more uniform, but there will be more variation in the light at the center of the LEDs compared to the area adjacent to the LED center, i.e., at the top surface of the backlight unit, i.e., at the output. The bifacial diffuser layer under measurement is the diffuser associated with the last entry in Table 430, which will be described in relation to the description in Figure 4B. It is the diffuser of product number C-HH90, which has a distance to achieve complete LED invisibility, such as an LED spacing-to-gap ratio of 1:0.85.

[0070] In some embodiments of the backlight units described herein, the LEDs typically have a light-emitting area exceeding several hundred microns to millimeters. A large light-emitting area is necessary to achieve high brightness from a relatively small number of LEDs. As with known lighting applications, it is desirable that the backlight units described herein achieve uniformity of intensity and angular distribution while maximizing brightness. In embodiments using very small LED arrays, it is also desirable to minimize the point spread function of light from a single LED to facilitate local dimming.

[0071] The various descriptions of embodiments in this specification refer to the use of bifacial diffuser layers. However, different diffuser layers, including, for example, single-sided diffuser layers, volumetric diffuser layers, and circular diffuser layers, can also be used.

[0072] Figure 6A illustrates a schematic diagram of a layer structure 600 relating to one embodiment of a backlight unit having a double-sided diffuser layer and a void of the teaching. The diffuser film 602 is positioned over an LED array 604, which comprises individual LEDs 612 spaced apart with a spacing x605. A void 606 is formed between the top of the LED array 604 and the bottom of the film 602. The void has a thickness d607. Light emerges from the top of the LEDs 612 with an angular distribution characterized by an angle 608. The distribution from the LEDs 612 can be a Lambertsian distribution into air, having an angular distribution with a spreading angle 608 equal to 180 degrees (±90 degrees). With respect to a fixed angle 608 that characterizes the light emitted from the LED, it can be seen that the thicker the void, the larger the size of the light distribution on the bottom surface of the diffuser film 602. Therefore, in order to produce smooth and uniform brightness from the upper surface of the layer structure 600, less opacity or LED smoothing is required from the film 602.

[0073] Figure 6B illustrates a visual image 630 of an embodiment of the backlight unit of Figure 6A having a double-sided diffuser layer and a thin void thickness. Specifically, Figure 6B is a visual image 630 of an approximately 8x8 array of LEDs spaced 8mm apart, with a 200-micron thick HH90 diffuser positioned 8mm above the LEDs and with a void between the LEDs and the HH90 diffuser. Thus, x=8mm and d=8mm. In this specification, HH90 refers to a diffuser having product number C-HH90, as illustrated in Table 430 of Figure 4B.

[0074] Figure 6C illustrates a visual image 650 of a certain embodiment of the backlight unit of Figure 6A, having a double-sided diffuser layer and a thicker gap thickness. Specifically, Figure 6C is a visual image 650 of an approximately 8 × 10 array of LEDs spaced 8 mm apart, with a 200 micron thick HH90 diffuser placed 12 mm above (d=12 mm) the LEDs, with a gap between the LEDs and the HH90 diffuser. The additional smoothing of illumination caused by the thicker gap thickness d607 is evident, even when using the same diffuser film 602. Referring to both Figures 6B and 6C, from the visual image 630 of Figure 6B, a significant spatial heterogeneity with intensity exists in the 8 mm gap, but as shown in the visual image 650 of Figure 6C, with a 12 mm gap, the intensity can be seen to be very uniform. Therefore, the embodiments of the backlight unit of this teaching can provide the desired uniformity and / or invisibility of individual LEDs by using a specific ratio of array spacing and void thickness between the array and the diffuser layer.

[0075] Figure 7A illustrates a schematic diagram of the layer structure of one embodiment of a backlight unit according to this teaching, having a double-sided diffuser layer, a second diffuser layer, and a transparent material layer. The first diffuser film 702 is positioned over an LED array 704, which comprises individual LEDs spaced apart with a spacing of x705. In one embodiment, the first diffuser film 702 comprises an upper surface diffuser layer, a bottom surface diffuser layer, and a thin transparent material layer between the upper surface diffuser layer and the bottom surface diffuser layer. Such a film is sometimes referred to as a double-sided diffuser layer. In some configurations, a second diffuser film 702' is positioned over the first diffuser film 702.

[0076] A layer 706 of transparent material having a thickness d707 is positioned between the LED array 704 and the diffuser film 702. In various embodiments, the transparent material layer 706 comprises one or more films occupying the region from the top of the LED array 704 to the bottom of the diffuser film 702. The transparent material layer 706 generally has a higher refractive index than air. In some embodiments, the transparent material layer 706 is one or more films of polymethyl methacrylate (PMMA). Light emerges from the top of the LEDs in the array 704 in a Lambertsian distribution, which is transformed into a narrower angular distribution characterized by an angle 708 within the transparent material layer 706. The angle 708 characterizing the angular distribution of light within the transparent material layer 706 is smaller than the angle 608 with respect to light in the gap 606, which is described in relation to Figure 6A, because the refractive index is greater than 1. The transformation is described by Snell's law, as further described below. For example, the Lambertsian distribution from an LED emitting light in a material with a refractive index of 1.5 produces an angular distribution with an angle 708 equal to 79.2 degrees (±39.6 degrees). In general, the transparent material layer 706 converts the angle of the light distribution from an exposed LED emitting light into air to an angle less than ±90 degrees around the center.

[0077] Figure 7B illustrates a visual image 710 of one embodiment of the backlight unit of Figure 7A, having a double-sided diffuser layer and a thin transparent material layer. Specifically, Figure 7B is a visual image 710 of an approximately 8 × 10 array of LEDs, with two 4 mm thick PMMA layers between the LEDs and the HH90 diffusers, and a 200 micron thick HH90 diffuser positioned 8 mm above the LEDs and spaced 8 mm apart.

[0078] Figure 7C illustrates a schematic diagram of a layer structure 720 relating to one embodiment of a backlight unit having a double-sided diffuser layer and a thicker transparent material layer of the teaching. The diffuser film 722 is positioned over an LED array 724, which comprises individual LEDs spaced apart with a spacing of x725. A transparent material layer 726 having a thickness d727 is positioned between the LED array 724 and the diffuser film 722. Light emerges from the top of the LEDs in the array 724 with an angular distribution characterized by an angle 728.

[0079] Figure 7D illustrates a visual image 730 of one embodiment of the backlight unit shown in Figure 7C. Specifically, the visual image 730 is from an array of approximately 8 × 10 LEDs spaced 8 mm apart, with a 200 micron thick HH90 diffuser positioned 12 mm above the LEDs, and with three 4 mm thick PMMA layers between the LEDs and the HH90 diffuser. Compared to the visual image 710 described in relation to Figure 7B, for example, the further smoothing of illumination from the individual LEDs in the array, caused by the thicker transparent material layer, is evident from this visual image 730.

[0080] Figure 7E illustrates a schematic diagram of a layer structure 740 relating to one embodiment of a backlight unit having a double-sided diffuser layer and an even thicker transparent material layer. Like the layer structures 700 and 720 described in relation to Figures 7A and 7C, the layer structure 740 also has a diffuser layer 742, an LED array 744 with spacing x745, a transparent material layer 746, and an angular distribution angle 748. The thickness d747 is thicker in the layer structure 740 compared to the layer structures 700 and 720. The thicker transparent material layer 746 leads to better invisibility of the individual LEDs in the array 744 with respect to the same diffuser film 742 properties.

[0081] Figure 7F illustrates a visual image 750 of one embodiment of the backlight unit shown in Figure 7E. Specifically, the visual image 750 is from an array of approximately 8 × 10 LEDs spaced 8 mm apart, with a 200 micron thick HH90 diffuser positioned 16 mm above the LEDs, and with four 4 mm thick PMMA layers between the LEDs and the HH90 diffuser. The better opacity of the thicker transparent material layers is evident.

[0082] Figure 7G illustrates a visual image 760 of an embodiment of a backlight unit having a double-sided diffuser layer spanning a 20 mm thick transparent material layer. Specifically, the visual image 760 is from an array of approximately 8 × 10 LEDs spaced 8 mm apart, with a 200 micron thick HH90 diffuser positioned 20 mm above the LEDs, with five 4 mm thick PMMA layers between the LEDs and the HH90 diffuser. The visual image 760 also shows illumination from some LEDs directly from the LED array without the film. Better opacity when the thickness of the transparent material layer is increased is evident.

[0083] Figure 7H illustrates a visual image 770 of an embodiment of a backlight unit having a double-sided diffuser layer spanning a 24 mm thick transparent material layer. Specifically, the visual image 770 is from an array of approximately 8 × 10 LEDs spaced 8 mm apart, with a 200 micron thick HH90 diffuser positioned 24 mm above the LEDs, with six 4 mm thick PMMA layers between the LEDs and the HH90 diffuser. Better opacity when the thickness of the transparent material layer is increased is evident.

[0084] Figure 7I illustrates a visual image 780 of one embodiment of a backlight unit having a double-sided diffuser layer spanning a 28 mm thick transparent material layer. Specifically, the visual image 780 shows an array of approximately 8 × 10 LEDs spaced 8 mm apart, with a 200 micron thick HH90 diffuser positioned 28 mm above the LEDs, and seven 4 mm thick PMMA layers between the LEDs and the HH90 diffuser. Better opacity when the transparent material layer thickness is increased is evident. The embodiments presented in Figures 7A-I illustrate how embodiments of the backlight unit of this teaching can provide desired uniformity and / or opacity of individual LEDs by using a specific ratio of array spacing and transparent material layer thickness between the array and the diffuser layer. The refractive index of the transparent layer can also be selected as a parameter to provide desired uniformity.

[0085] Comparing the backlight unit embodiment described in relation to Figures 6A-C with that described in relation to Figures 7A-I, it is clear that when the air gap is replaced with PMMA with a refractive index of 1.5, the visual uniformity deteriorates significantly when the air gap thickness and the transparent material layer thickness are the same or similar. For example, comparing visual image 650 in Figure 6C, which has 12 mm of air between the diffuser and the LED, with visual image 730 in Figure 7D, which has 12 mm of PMMA between the diffuser and the LED, it can be seen that 12 mm of air provides excellent uniformity, while 12 mm of PMMA results in very poor uniformity. In fact, it is found that when using a transparent material layer with PMMA, a thickness of 24-28 mm is required to achieve the excellent uniformity evident in visual image 770 in Figure 7H and visual image 780 in Figure 7I.

[0086] In air, the Lambertsian distribution has rays ranging from -90 to +90 degrees, allowing light to rapidly diverge laterally. Therefore, a thicker thickness requirement is expected, involving a material with a higher refractive index in the space between the LED array and the bottom of the diffuser layer. In contrast, once the Lambertsian distribution from the LEDs is incident on a transparent material layer made of, for example, PMMA, it is confined to within ±41.8 degrees due to Snell's equation, namely n1 × sin(Θ1) = n2 × sin(Θ2) (where n1 = 1 for air and n2 = 1.5 for PMMA). For polycarbonate material with a higher refractive index of 1.57, the light will be confined to within ±39.6 degrees. If the refractive index of the material between the LEDs and the diffuser is 1.75, the light will be confined to within ±34.8 degrees. For example, some polymer materials can have high refractive indices. When using PMMA or other high refractive index transparent materials between the LED and the diffuser, the increased thickness can be disadvantageous due to the significant increase in thickness, weight, and material. Therefore, some embodiments of the backlight unit in this teaching utilize additional layers in the layered structure architecture of the backlight unit, as further described below.

[0087] For example, some embodiments of this teaching use a backlight unit architecture that includes a pair of brightness-enhancing films (e.g., using vertex-angle prisms of about 90 degrees) which are oriented substantially perpendicular to each other and are also DBEF films that are reflective polarizers. The intersecting brightness-enhancing films narrow the optical distribution and enhance on-axial brightness. The DBEF films also enhance brightness outward from the LCD module by transmitting only one polarization and reusing the others.

[0088] Figure 8A illustrates a schematic diagram of a layer structure 800 relating to one embodiment of a backlight unit having a double-sided diffuser layer 802 spanning a gap 806, an additional pair of high refractive index cross-brightness-enhancing films 810, 812 and a polarizer-reflector film 814 above the double-sided diffuser layer 802. The diffuser film 802 is positioned across an LED array 804, which comprises individual LEDs spaced apart with a spacing x805. The gap 806 is formed between the top of the LED array 804 and the bottom of the film 802. The gap has a thickness d807. Light emerges from the top of the LEDs in the array 804, with an angular distribution characterized by an angle 808. The pair of brightness-enhancing films 810, 812 are positioned across the diffuser film 802, and the polarizer-reflector (DBEF) film 814 is positioned across the brightness-enhancing film pair 810, 812.

[0089] Figure 8B illustrates a visual image 820 of an embodiment of the backlight unit of Figure 8A with a gap thickness of 6 mm. Specifically, the visual image 820 is from an array of approximately 8 × 10 LEDs spaced 8 mm apart, with a 200-micron thick HH90 diffuser positioned 6 mm above the LEDs, with air between the LEDs and the HH90 diffuser. In this embodiment, above the HH90 diffuser are additional pairs of high refractive index cross-brightness enhancing films and DBEF films, each film having a thickness of approximately 200 microns.

[0090] Figure 8C illustrates a visual image 840 of an embodiment of the backlight unit of Figure 8A with a gap thickness of 7 mm. Specifically, the visual image 840 is from an array of approximately 8 × 10 LEDs spaced 8 mm apart, with a 200-micron thick HH90 diffuser positioned 7 mm above the LEDs, with air between the LEDs and the HH90 diffuser. In this embodiment, above the HH90 diffuser are additional pairs of high refractive index cross-brightness enhancing films and DBEF films, each film having a thickness of approximately 200 microns.

[0091] Comparing Figures 6A-C and 8A-C illustrates the benefits of including two brightness-enhancing films 810, 812 and a polarizer reflector 814 across the diffuser film 802 and the gap 806. Uniform illumination is achieved with relatively smaller gaps when additional films are used. Visual image 840 in Figure 8C shows very good uniformity achieved with a gap of approximately 7 mm. This is comparable to values ​​approaching 10 or 12 mm without the three additional films above the HH90 diffuser. For example, visual image 650 in Figure 6C has a very uniform distribution with a 12 mm gap, relating to the architectural layer structure 600 shown in Figure 6A, without additional films above the diffuser film 602.

[0092] Figure 9A illustrates a schematic diagram of a layer structure 900 relating to one embodiment of a backlight unit according to this teaching, having a double-sided diffuser layer 902, a second diffuser layer 902', a transparent material layer 906, a pair of high refractive index cross-brightness enhancing films 910, 912, and a polarizer reflector film 914. The first diffuser film 902 is positioned over one or more transparent material layers 906. The first diffuser film 902 can be a double-sided diffuser layer comprising an upper surface diffuser layer, a bottom surface diffuser layer, and a thin transparent material layer between the upper surface diffuser layer and the bottom surface diffuser layer (structural details are not shown). In other embodiments, the first diffuser film 902 is a single surface diffuser layer.

[0093] In some configurations, the second diffuser film 902' is positioned across the first diffuser film 902. An LED array 904 with an element spacing of x905 is positioned directly beneath the transparent material layer 906. The transparent material layer 906 has a thickness d907. Each individual LED has an angular distribution of light characterized by an angle 908 within the transparent material layer 906. A pair of high refractive index cross-brightness enhancing films 910, 912 are positioned across the second diffuser film 902'. A polarizer reflector film 914 is positioned above the double-sided diffuser layer.

[0094] Figure 9B illustrates a visual image 920 of one embodiment of the backlight unit of Figure 9A. Specifically, the visual image 920 is from an array of approximately 8 × 10 LEDs spaced 8 mm apart, with a 200-micron thick HH90 diffuser positioned 7 mm above the LEDs and with 4 mm and 3 mm PMMA layers between the LEDs and the HH90 diffuser. In this embodiment, above the HH90 diffuser 902 are additional pairs of high refractive index crossed BEF films 910, 912 and a DBEF film 914, each film having a thickness of approximately 200 microns.

[0095] Figure 9C illustrates a schematic diagram of a layer structure 930 relating to one embodiment of a backlight unit, having a double-sided diffuser layer 932 spanning a gap 936, with an additional pair of high refractive index cross-brightness enhancing films 940, 942 and a polarizer reflector film 944 above the double-sided diffuser layer 932. An LED array 934 with an element spacing of x935 is positioned directly below the gap 936. The gap 935 has a thickness d937. Each individual LED has a light angular distribution characterized by an angle 938.

[0096] Figure 9D illustrates a visual image 950 of one embodiment of the backlight unit in Figure 9C. Specifically, the visual image 950 is from an array of approximately 8 × 10 LEDs spaced 8 mm apart, with a 200-micron thick HH90 diffuser positioned 7 mm above the LEDs, with air between the LEDs and the HH90 diffuser. In this embodiment, above the HH90 diffuser are additional pairs of high refractive index crossed BEF films 940, 942 and a DBEF 944 film, each film having a thickness of approximately 200 microns.

[0097] Surprisingly, the uniformity shown in the visual image 920 of Figure 9B, which has a 4mm PMMA layer and a 3mm PMMA layer beneath the HH90 diffuser, can be seen to be equivalent to or better than the visual image 950 of Figure 9D, which has a 7mm void beneath the HH90 diffuser. This is comparable to obtaining similar uniformity with 24-28mm PMMA without the three extra films and with a void of the same thickness as discussed in relation to Figures 7H and 7I.

[0098] In some embodiments, the thickness of the solid transparent material layer is greater than or equal to half the thickness measured from the top of the two-dimensional array of LEDs to the top of the backlight unit. For example, referring to one embodiment including three films spanning the transparent material layer, such as one embodiment in Figure 9A, the thickness d907 of the transparent material layer 906 is half the thickness measured from the top of the array 904 to the top of the polarizer reflector 914. This thickness is the sum of the thickness d907, the thickness of the diffuser film 902, the thickness of the brightness-enhancing film 910, the thickness of the brightness-enhancing film 912, and the thickness of the polarizer reflector film 914. In various embodiments, this thickness is generally the thickness of the transparent material layer and the thickness of any additional layers on top of the transparent material layer. In some embodiments, the thickness of the solid transparent material layer is greater than or equal to 0.7 times the thickness measured from the top of the two-dimensional array of LEDs to the top of the backlight unit. In various embodiments, this thickness is the thickness of the transparent material layer and the thickness of any additional layers on top of the transparent material layer.

[0099] Figure 10 illustrates graph 1000 of the point spread function of a single LED in an embodiment of the backlight unit layer structure of this teaching. The point spread function can be considered as characterizing the degree of dispersion of the point source image produced by the optical system. Graph 1000 shows the luminance distribution when a single LED is on, as a function of position (m), for either 7.5 mm PMMA (dashed line) or air (solid line) between the LED and a film stack consisting of an HH90 diffuser, a pair of crossed BEF films, and a DBEF film. The outlines are normalized by peak intensity. In the case of 7.5 mm PMMA, the peak intensity is 67% higher than in the case of air. Correspondingly, in the case of PMMA, the luminance is relatively lower in the tail. This results in a better point spread function than air for the case of solid material (in this case, PMMA) in the transparent material layer between the LED and the film stack. A better point spread function facilitates local dimming. The higher peak intensity and lower energy in the wing section are due to the narrower localization of light within the PMMA compared to the Lambertsian distribution in air. This, in turn, often results in a higher overall average brightness. For example, with an LED spacing of 8 mm and a total dimension of approximately 90 mm in the narrow direction, the average brightness is about 10% higher, which is highly desirable.

[0100] Figure 11A illustrates a schematic diagram of a layer structure 1100 relating to one embodiment of a backlight unit, having a double-sided diffuser layer 1102 spanning a transparent material layer 1106, with an additional pair of high refractive index cross-brightness enhancing films 1110, 1112 above the double-sided diffuser layer 1102. An LED array 1104 with an element spacing of x 1105 is positioned directly beneath the transparent material layer 1106. The transparent material layer 1106 has a thickness d 1107. Each individual LED has a light angular distribution characterized by an angle 1108.

[0101] Figure 11B illustrates a visual image 1120 of one embodiment of the backlight unit of Figure 11A. Specifically, the visual image 1120 is from an array of approximately 8 × 10 LEDs spaced 8 mm apart, with 200 micron thick HH90 diffusers positioned 7 mm above the LEDs and 3 mm PMMA layers between the LEDs and HH90 diffusers 1102. In this embodiment, an additional pair of high refractive index cross-brightness enhancing films 1110, 1112 are present above the HH90 diffusers 1102, each film having a thickness of approximately 200 microns.

[0102] Figure 11C illustrates a schematic diagram of a layer structure 1130 relating to an embodiment of a backlight unit having a double-sided diffuser layer 1132 spanning a gap 1136, with an additional pair of high refractive index cross-brightness enhancing films 1140, 1142 above the double-sided diffuser layer 1132. An LED array 1134 with an element spacing of x 1135 is positioned directly below the gap 1136. The gap 1136 has a thickness d 1137. Each LED has a light angular distribution characterized by an angle 1138.

[0103] Figure 11D illustrates a visual image 1150 of one embodiment of the backlight unit in Figure 11C. Specifically, the visual image 1150 is from an array of approximately 8 × 10 LEDs, spaced 8 mm apart, with a 200-micron thick HH90 diffuser 1132 positioned 7 mm above the LEDs, with air between the LEDs and the HH90 diffuser 1132. In this embodiment, additional pairs of high refractive index cross-brightness enhancing films 1140, 1142 are present, each film having a thickness of approximately 200 microns.

[0104] Figures 11B and 11D correspond to Figures 8B and 8D, except that the DBEF has been removed. In Figures 11B and 11D, it can be observed that the uniformity is worse for both the PMMA and void cases. This is because the DBEF reflects about 50 percent of the light, and this additional recirculated light improves the uniformity. The less uniformity without the DBEF can be compensated for by increasing the thickness or opacity of the diffuser. For example, a volumetric diffuser has a greater opacity than a bifacial diffuser.

[0105] One feature of this teaching is that a diffuser with a stronger opacity intensity can provide uniformity and / or dimming behavior within a very thin backlight unit. For example, Figures 12A–D illustrate an embodiment of a backlight unit using a volumetric diffuser layer with a total thickness at half maximum of 105 degrees. The volumetric diffuser layer has a higher opacity intensity compared to, for example, a double-sided or single-sided diffuser layer.

[0106] Figure 12A illustrates a schematic diagram of a layer structure 1200 relating to one embodiment of a backlight unit, having a volumetric diffuser layer 1202 spanning a transparent material layer 1206, with an additional pair of high refractive index cross-brightness enhancing films 1210, 1212 above the volumetric diffuser layer 1202. An LED array 1204 with an element spacing of x 1205 is positioned directly beneath the transparent material layer 1206. The transparent material layer 1206 has a thickness d 1207. Each individual LED has a light angular distribution characterized by an angle 1208.

[0107] Figure 12B illustrates a visual image 1220 of one embodiment of the backlight unit in Figure 12A. Specifically, the visual image 1220 is from an array of approximately 8 × 10 LEDs spaced 8 mm apart, with 200 micron thick VH105 diffusers 1202 positioned 8 mm above the LEDs and 4 mm PMMA layers between the LEDs and the VH105 diffusers 1202. In this embodiment, an additional pair of high refractive index cross-brightness enhancing films 1210, 1212 are present above the VH105 diffusers 1202, each film having a thickness of approximately 200 microns.

[0108] Figure 12C illustrates a schematic diagram of a layer structure 1230 relating to one embodiment of a backlight unit, having a volumetric diffuser layer 1232 spanning a gap 1236, with an additional pair of high refractive index cross-brightness enhancing films 1240, 1242 above the volumetric diffuser layer. An LED array 1234 with an element spacing of x 1235 is positioned directly below the gap 1236. The gap 1236 has a thickness d 1237. Each LED has a light angular distribution characterized by an angle 1238.

[0109] Figure 12D illustrates a visual image 1250 of an embodiment of the backlight unit in Figure 12C. Specifically, the visual image 1250 is from an array of approximately 8 × 10 LEDs, spaced 8 mm apart, with a 200-micron thick VH105 diffuser 1232 positioned 8 mm above the LEDs, with air between the LEDs and the VH105 diffuser 1232. In this embodiment, additional pairs of high refractive index cross-brightness enhancing films 1240, 1242 are present, each film having a thickness of approximately 200 microns. The visual image 1220 in Figure 12B with PMMA is visually brighter than the visual image 1250 in Figure 12D with air gaps.

[0110] In some applications, it is preferable to use one BEF film in combination with a DBEF film instead of two cross BEF films. This results in greater unidirectional light dispersion, as will be explained in connection with the description in Figures 13A-D. Figure 13A illustrates a schematic diagram of a layer structure 1300 relating to one embodiment of a backlight unit, having a volumetric diffuser layer 1302 and a second volumetric diffuser layer 1302' over a transparent material layer 1306, with an additional high refractive index cross brightness-enhancing film 1310 and a polarizer reflector layer 1314 above the second volumetric diffuser layer 1302'. An LED array 1304 with an element spacing of x1305 is positioned directly below the transparent material layer 1306. The transparent material layer 1306 has a thickness d1307. Each individual LED has a light angular distribution characterized by an angle 1308.

[0111] Figure 13B illustrates a visual image 1320 of one embodiment of the backlight unit of Figure 13A. Specifically, the visual image 1320 is from an array of approximately 8 × 10 LEDs spaced 8 mm apart, with a 200-micron thick VH105 diffuser 1302 positioned 8 mm above the LEDs and with 4 mm and 4 mm PMMA layers between the LEDs and the VH105 diffuser 1302. In this embodiment, additional high refractive index cross-brightness enhancing films 1310 and DBEF films 1314 are present above the VH105 diffuser 1302, each film having a thickness of approximately 200 microns.

[0112] Figure 13C illustrates a schematic diagram of a layer structure 1330 relating to one embodiment of a backlight unit, having a volumetric diffuser layer 1332 spanning a gap 1336, with an additional high refractive index cross-brightness enhancing film 1340 and a polarizing reflector film 1344 above the volumetric diffuser layer 1332. An LED array 1334 with an element spacing of x 1335 is positioned directly below the gap 1336. The gap 1336 has a thickness d 1337. Each LED has a light angular distribution characterized by an angle 1338.

[0113] Figure 13D illustrates a visual image 1350 of one embodiment of the backlight unit in Figure 13C. Specifically, the visual image 1350 is from an array of approximately 8 × 10 LEDs spaced 8 mm apart, with a 200-micron thick VH105 diffuser 1332 positioned 8 mm above the LEDs, with air between the LEDs and the VH105 diffuser 1332. In this embodiment, additional high refractive index cross-brightness enhancing films 1340 and DBEF films 1344 are present, each film having a thickness of approximately 200 microns. Compared to Figures 12A-D and 13A-D, it can be observed that uniformity is slightly worse when one BEF film is replaced with a DBEF film.

[0114] Figure 14A illustrates a graph 1400 of intensity versus horizontal divergence angle, a graph 1410 of intensity versus vertical divergence angle, a three-dimensional plot 1420 of light distribution, and a projection 1430 of the three-dimensional plot of light distribution, generated by known LEDs passing through a pair of high refractive index cross-brightness-enhancing films and a polarizing reflector film used in embodiments of this teaching.

[0115] Figure 14B illustrates a graph 1450 of intensity versus horizontal divergence angle, a graph 1460 of intensity versus vertical divergence angle, a three-dimensional plot 1470 of the light distribution, and a projection 1480 of the three-dimensional plot of the light distribution, generated by a known LED passing through a single high refractive index brightness-enhancing film and polarizing reflector film used in embodiments of this teaching.

[0116] A key feature of this teaching is that the increase in brightness associated with the use of a transparent material layer is particularly pronounced when the number of rows of LEDs in the narrow direction is less than 10 or 15. In some cases, it may be desirable to tilt the optical distribution or to widen it in one direction. This can be achieved by using a prism angle bending film above a crossing BEF film to tilt the distribution, or by using an elliptical diffuser such as BVT E40-01 to widen the distribution in one direction.

[0117] In some display applications, thickness is not a major constraint, but it is desirable to use very small LEDs to achieve higher brightness and to use local dimming. One option is to use a technique similar to lighting where a gap is used between the LED and a strong diffuser to achieve excellent uniformity. One feature of this teaching is the recognition that it is advantageous to use a solid transparent material with a refractive index of 1.4–1.7 instead of a gap. When used in combination with two crossing brightness-enhancing films (e.g., a prism with an apex angle of approximately 90 degrees, referred to as a BEF) and a DBEF film (reflective polarizer), a backlight unit with excellent uniformity, brightness, and a good point spread function for local dimming is produced.

[0118] In some embodiments, the backlight unit includes, namely, an array of micro LEDs, a layer of transparent solid material such as PMMA or polycarbonate, a strong diffuser (with full width at half maximum when measured with a collimated light angle of >60 degrees, better but 80 or 100 degrees), a pair of high refractive index crossed BEF films, and a DBEF film. In some embodiments, the solid transparent material occupies at least 50%, preferably >70%, of the height from the top of the LEDs to the top film in the BLU. In some embodiments, an additional film may be present above the crossed BEF film to further shape or redirect the beam. In some embodiments, either the BEF film or the DBEF is omitted. Also, in some embodiments, the narrow dimensions of the backlight unit may consist of fewer than 15 LED rows or even fewer than 10 LED rows. Equal portions

[0119] The applicant's teachings are described in conjunction with various embodiments, but the applicant's teachings are not intended to be limited to such embodiments. Rather, the applicant's teachings include various substitutions, modifications, and equivalents that can be made within the spirit and scope of the teachings, as will be understood by those skilled in the art.

Claims

1. It is a backlight unit, a) A two-dimensional array of light-emitting diodes (LEDs), wherein at least one LED in the two-dimensional array of LEDs produces light with a nominal angular distribution of ±90 degrees around the center of the at least one LED, b) A layer of transparent solid material positioned such that its bottom surface spans the two-dimensional array of LEDs, wherein the layer of transparent solid material is configured to convert the light produced by the two-dimensional array of LEDs into light having a nominal angular distribution of less than ±90 degrees around the center of at least one LED on the upper surface of the layer of transparent solid material, c) A diffuser film positioned across the upper surface of the layer of the transparent solid material, wherein the diffuser film is configured to provide a full width at half maximum greater than 60 degrees, and the diffuser film is configured to diffuse the light produced on the upper surface of the layer of the transparent solid material and to produce diffused light on the upper surface of the diffuser film, d) A brightness-enhancing film positioned across the upper surface of the diffuser film and comprising a plurality of prism microstructures on at least one surface, wherein at least some of the prism microstructures have an apex angle of about 90 degrees, and the brightness-enhancing film is configured to narrow the angular optical distribution of the diffused light produced on the upper surface of the diffuser film, thereby improving the uniformity of the light output from the backlight unit. A backlight unit equipped with this feature.

2. The backlight unit according to claim 1, further comprising a second diffuser film positioned between the diffuser film and the brightness enhancer.

3. The backlight unit according to claim 1, wherein the two-dimensional array of LEDs has a row spacing of 8 mm and a column spacing of 8 mm.

4. The backlight unit according to claim 1, wherein the two-dimensional array of LEDs comprises 10 rows and 8 columns.

5. The backlight unit according to claim 1, wherein the two-dimensional array of LEDs comprises rows of fewer than 15 or equal to 15 LEDs and columns of more than 15 LEDs or equal to 15 LEDs.

6. The backlight unit according to claim 1, wherein the two-dimensional array of LEDs comprises rows of fewer than 10 or equal to 10 LEDs and columns of more than 10 LEDs or equal to 10 LEDs.

7. The backlight unit according to claim 1, wherein the transparent solid material comprises a polymethyl methacrylate material.

8. The backlight unit according to claim 1, wherein the transparent solid material comprises a polycarbonate material.

9. The backlight unit according to claim 1, wherein the transparent solid material comprises a material having a refractive index of 1.

75.

10. The backlight unit according to claim 1, wherein the thickness of the layer of the transparent solid transparent material is greater than or equal to half of the thickness measured from the upper surface of the two-dimensional array of LEDs to the upper surface of the backlight unit.

11. The backlight unit according to claim 1, wherein the thickness of the layer of the transparent solid transparent material is greater than or equal to 0.7 times the thickness measured from the upper surface of the two-dimensional array of LEDs to the upper surface of the backlight unit.

12. The backlight unit according to claim 1, wherein the diffuser film includes the full width at half maximum which exceeds 80 degrees.

13. The backlight unit according to claim 1, wherein the diffuser film includes the full width at half maximum which exceeds 100 degrees.

14. The backlight unit according to claim 1, further comprising at least one additional film positioned above the brightness-enhancing film.

15. The backlight unit according to claim 14, wherein the at least one additional film is configured to further shape the light from the backlight unit.

16. The backlight unit according to claim 14, wherein the at least one additional film is configured to redirect the light from the backlight unit.

17. The backlight unit according to claim 1, further comprising a polarizing reflector film positioned across the brightness-enhancing film.

18. The backlight unit according to claim 1, further comprising a second brightness-enhancing film positioned across the brightness-enhancing film.

19. The backlight unit according to claim 18, further comprising a polarizing reflector film positioned across the second brightness-enhancing film.

20. The backlight unit according to claim 19, further comprising at least one additional film positioned above the polarizer reflector film.

21. The backlight unit according to claim 20, wherein the at least one additional film is configured to further shape the light from the backlight unit.

22. The backlight unit according to claim 20, wherein the at least one additional film is configured to redirect the light from the backlight unit.

23. It is a backlight unit, a) A two-dimensional array of light-emitting diodes (LEDs), wherein the number of rows in the two-dimensional array is less than the number of columns in the two-dimensional array, and at least one LED in the two-dimensional array produces light with a nominal angular distribution of ±90 degrees around the center of the at least one LED, b) A layer of transparent solid material positioned such that its bottom surface spans the two-dimensional array of LEDs, wherein the layer of transparent solid material is configured to convert the light produced by the two-dimensional array of LEDs into light having a nominal angular distribution of less than ±90 degrees around the center of at least one LED on the upper surface of the layer of transparent solid material, c) A diffuser film positioned across the upper surface of the layer of the transparent solid material, wherein the diffuser film is configured to provide a full width at half maximum greater than 60 degrees, and the diffuser film is configured to diffuse the light produced on the upper surface of the layer of the transparent solid material and to produce diffused light on the upper surface of the diffuser film, d) A pair of cross-brightness-enhancing films positioned across the upper surface of the diffuser film and having a plurality of prism microstructures on at least one surface, wherein at least some of the prism microstructures have an apex angle of about 90 degrees, e) A polarizer reflector film positioned across the pair of cross-brightness enhancing films, wherein the polarizer reflector film is configured to transmit one polarization and reflect another polarization, thereby improving the uniformity of the light output from the backlight unit. A backlight unit equipped with this feature.

24. The backlight unit according to claim 23, further comprising a second diffuser film positioned between the diffuser film and the pair of cross-brightness enhancing films.

25. The backlight unit according to claim 23, further comprising a prism angle bending film positioned across the polarizer reflector film and configured to tilt the light distribution from the output of the backlight unit.

26. The backlight unit according to claim 23, further comprising an elliptical diffuser film positioned across the polarizer reflector film and configured to expand the light distribution for the output of the backlight unit in one direction.

27. The backlight unit according to claim 23, wherein the number of rows in the plurality of rows is less than 15 or equal to 15.

Images