Backlight unit for backlight display

Abstract

A backlight unit includes a plurality of light emitting diodes and a plurality of planar lenses positioned on a substrate. A respective one of the plurality of planar lenses is positioned a distance above a respective one of the plurality of light emitting diodes. Each planar lens of the plurality of planar lenses includes a plurality of annular segments having a common center. Each of the plurality of annular segments includes a prism-shaped microstructure having a first facet configured to refract light in a desired direction and a second facet oriented in a plane parallel to a plane of an emission surface of a respective one of the plurality of light emitting diodes.

Description

[0001] Cross - Reference to Related Applications

[0002] This application claims the benefit of the following patent applications: U.S. Provisional Patent Application Serial No. 63 / 413,030, filed October 4, 2022, titled "Back Light Unit for Backlit Displays"; U.S. Provisional Patent Application Serial No. 63 / 416,633, filed October 17, 2022, titled "Back Light Unit for Backlit Displays"; U.S. Provisional Patent Application Serial No. 63 / 418,826, filed October 24, 2022, titled "Back Light Unit for Backlit Displays"; and U.S. Provisional Patent Application Serial No. 63 / 425,293, filed November 14, 2022, titled "Back Light Unit for Backlit Displays". The entire contents of U.S. Provisional Patent Application Serial Nos. 63 / 413,030, 63 / 416,633, 63 / 418,826, and 63 / 425,293 are incorporated herein by reference. Technical Field

[0003] The present invention generally relates to a backlight unit (BLU) for a backlit display, particularly for a backlit display having a two - dimensional array of light - emitting diode (LED) light sources with an array of collimating lenses. In the pursuit of improved image quality, liquid crystal displays (LCDs) increasingly use the backlight unit architecture 100 schematically illustrated in Figure 1 which includes an array of individual (blue LED, white LED, or red - green - blue LED clusters) LEDs 112. Figure 2A And Figure 2B illustrates the relationship of the typical intensity distribution of light emitted by a single LED measured by a goniophotometer with the angle. As shown in these figures, the LED source approximately emits a Lambertian source of light distribution that is substantially symmetric with respect to the lowest point, where the light intensity is the highest. Background Art

[0004] Referring back to Figure 1, a series of films can be used to scatter or diffuse the light emitted from the LED 112, so that the backlight unit 100 delivers more uniform light to the LCD panel (180) containing liquid crystal located above the backlight unit 100. The backlight unit 100 generally includes a diffuser film 120, and the diffuser film 120 can be a volume diffuser or a circular diffuser. If the LED emits blue light, for example, if the LED uses either quantum dots or phosphorescent materials to convert some of the blue light emitted by the LED 110 into green light and red light, then a color conversion layer 130 can be used. The diffuser film 140 can be a volume diffuser or a circular diffuser produced by a random texture surface. The diffuser film 140 can be configured to scatter or diffuse the light leaving the color conversion layer 130. Two brightness enhancement films (BEF) 150, 160 are often two prism films that are rotated approximately 90 degrees relative to each other. Additional films can be present in the backlight unit 100 to improve the overall uniformity and brightness of the light delivered to the LCD panel. Generally, a reflective polarizer film 170, such as a dual brightness enhancement film (DBEF), is used, which is either combined with the LCD or located below the LCD as a separate film. In some backlight units, white LEDs are used without using a color conversion layer. By using an array of LEDs, better contrast can be achieved by adjusting the brightness of each LED, which is often referred to as local dimming.

[0005] When the LEDs 112 are arranged in an array, such as Figure 3 the array shown in, it is desirable to hide each LED 112 and present bright and uniform light to the LCD panel. As described above, one way to achieve this goal is to include one or more diffusers, such as the diffuser film 120, in the backlight unit 100 to diffuse, scatter or blur the light beams emitted by the LEDs 112. In other cases, a series of microlens array (MLA) films are used, such as those described in U.S. Patent No. 11,181,776 titled "Back Light Unit for Backlit Displays", which is assigned to the present assignee.

[0006] There is often a desire to have a narrow light distribution in one or two directions. FIG. 4a shows the output distribution from two high refractive index crossed BEF films. It can be seen that there are relatively narrow peaks in both directions, with a full width at half maximum (FWHM) of approximately 36 degrees. However, there are relatively wide tails in the distribution that are not desired, such that >50% of the total light has an angle >50 degrees. A narrow distribution is achieved because the crossed BEF films only transmit a relatively small fraction of the light (light approaching at very specific angles) and reflect the rest. The percentage of light reflected from the two crossed BEF films can be >75%. This reflected light is recycled and exits in subsequent passes. But each time the light is recycled, some of it is lost due to absorption and is also scattered laterally, thus weakening the effect of local dimming of the LED. In some applications, it is desired to narrow the light in only one direction, which can be achieved using a single BEF film, as shown in FIG. 4b. However, while the shape of the output using a single BEF film is elliptical, even the narrow direction is much wider than the example with two crossed BEF films. Alternatively, another film can be added after the crossed BEF to scatter the light in one direction while having a minimal effect in the other direction. Additionally, it may be desired to tilt the light distribution, which can also be achieved by adding a directional turning film (DTF). However, each time a film with a significant back reflectivity is added, in addition to increasing cost, it also has a negative impact on brightness. Adding a film with back reflectivity also results in greater crosstalk between dimming zones. There are applications where it is possible to have a thicker backlight unit. This allows for some new designs for mini LED backlight units (BLUs). SUMMARY OF THE INVENTION

[0007] According to an embodiment of the present invention, there is provided a display light unit including an array of light emitting diodes and an array of planar lenses having a series of refractive annular segments that are not necessarily circular and are sometimes referred to as microlenses located at a certain distance above the LEDs. These planar lenses can more generally be referred to as collimating lenses. The collimating lenses can be characterized by their collimation, which is measured, for example, as the full width at half maximum (FWHM) in degrees of the light generated by the LEDs and emitted from the top surface of each lens. Each planar lens includes a plurality of annular microstructures having a common center. Each of these annular microstructures has a facet that is substantially parallel to the surface of the LED array, a facet that points towards the LED emission surface, and a facet that refracts light in a desired direction. In some embodiments, the planar lens is a Fresnel lens. In some embodiments, the planar lens includes an annular microstructure having a facet coplanar with the LED and an exit facet that varies continuously from being almost horizontal at the center of the lens to being almost vertical at the outermost annular microstructure of the lens.

[0008] In some embodiments, each of the plurality of planar lenses is located above a respective LED and has an output collimation with a FWHM of <40 degrees in one or two directions, where the light in the long tails is reduced and the crosstalk between local dimming zones is less. The planar lens assembly includes a relatively thick layer having a thickness of about 50% of the LED grid pitch which is the spacing distance between individual LED elements, and a refractive index much greater than air. The planar lens is fabricated on the top surface of the thick layer which is also referred to as a substrate. In various embodiments of the backlight unit of the present teachings, there is a space above the lens and below the LCD panel. This space can preferably be air and have a thickness comparable to or greater than the LED pitch. In some embodiments of the BLU, an additional layer is positioned between the planar lens layer and the LCD panel.

[0009] In some embodiments, the planar lens collimates light more in one direction than in the other. The dimensions can be adjusted to change the collimation. In one embodiment, the emission area of the LED is larger in one direction than in the other. In one embodiment, each of the small lenses has perturbations superimposed on the surface to provide additional spreading in one or two directions. In one embodiment, the lenses can be slightly shifted such that they are not directly above each LED, resulting in an inclined output beam.

[0010] In one embodiment, there can be an additional film positioned at a distance above the plurality of lenses. This additional film can diffuse or spread the light to improve visual uniformity. The additional film can also spread more light in one direction than in the orthogonal direction to achieve a desired angular distribution. The additional film can also equalize the angular distribution of the light leaving the second film by incorporating a spatially varying angle bend function. The additional film can also incorporate light diffusion or spreading features which, in some embodiments, are different microlenses on opposite sides of the film.

[0011] In one embodiment, the surface of the material between the LEDs can absorb to minimize the crosstalk between dimming zones and / or enhance collimation, since the recycled reflected light will not be collimated. In one embodiment, there is a reflective polarizer below the lens assembly. In one embodiment, the collimation from each lens results in a substantially Gaussian angular distribution. In various embodiments, the refractive index of at least one of the plurality of planar lenses is between 1.5 and 1.57. Moreover, in various embodiments, the plurality of planar lenses can be configured to collimate in one or two dimensions with a FWHM of less than 40 degrees.

[0012] These and other aspects, features, and characteristics of the present invention, as well as the methods of operation and functions of the related structural elements and the combination of parts and manufacturing economy, will become more apparent when considering the following description and the appended claims in reference to the accompanying drawings, all of which form a part of this specification. However, it should be clearly understood that the drawings are for illustrative and descriptive purposes only and are not intended as a definition of the limitations of the present invention. Description of the Drawings

[0013] According to the preferred and exemplary embodiments, the present teachings and their further advantages will be described more specifically in the following detailed description 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 drawn to scale; instead, emphasis is generally placed on illustrating the principles of the teachings. The drawings are not intended to limit the scope of the applicant's teachings in any way.

[0014] Figure 1 is a schematic diagram of a typical backlight unit including an LED array for an LCD monitor.

[0015] Figure 2a is a three-dimensional graph showing the relationship between the distribution of light output from the LEDs and the angle, measured using a goniophotometer.

[0016] Figure 2b is the measured light distribution of Figure 2a represented in two dimensions.

[0017] Figure 3 is a schematic diagram of a two-dimensional array of LEDs.

[0018] Figure 4a is the measured output distribution leaving the crossed BEF.

[0019] Figure 4b is the measured output distribution leaving a single BEF.

[0020] Figure 5 is an array of LEDs with an array of collimating lenses placed close to the LEDs.

[0021] Figure 6 is an array of LEDs with an array of collimating lenses placed at a certain distance from the LEDs.

[0022] Figure 7 is an array of LEDs with an array of collimating lenses placed at a certain distance from the LEDs, with a relatively thick layer of high refractive index material between the LEDs and the lenses.

[0023] Figure 8 is an illustration of the small lenses of a conventional Fresnel lens.

[0024] Figure 9 is an illustration of the small lenses of the collimating lens of an embodiment of the present invention.

[0025] Figure 10It is a diagram of Example 1 of a backlight unit (BLU) according to the present teachings.

[0026] Figures 11a and 11b are diagrams of a microlens having a functional facet formed by a part of a spherical lens.

[0027] Figure 12 The angular beam profile of a lens section with a spherical radius equal to 3.5 mm for Example 1 of the BLU according to the present teachings is illustrated.

[0028] Figure 13a gives a table that shows the optimization of the lens shape for Example 1 of the BLU according to the present teachings to improve the collimation length. Figure 13b shows the energy uniformity and angular distribution for the optimal case presented in the table described in conjunction with Figure 13b for an SZ ratio equal to 2.4.

[0029] Figure 14a shows the energy distribution for this case (Example 1, R = 80%). Figure 14b shows the energy distribution for Example 2 of the BLU with an absorptive printed circuit board (PCB).

[0030] Figure 15a shows the angular distribution of light for Example 2 of the BLU according to the present teachings. Figure 15b shows the angular distribution of light for Example 3 of the BLU according to the present teachings.

[0031] Figures 16a - 16c illustrate the results for Example 4 of the BLU according to the present teachings, where Figure 16a shows a table of energy collimation, Figure 16b presents the intensity distribution measured at approximately 13 mm above the LED, and Figure 16c shows the light intensity emitted at an angle of less than plus or minus 10 degrees at approximately 13 above the LED.

[0032] Figures 17a, 17b, and 17c show tables of the angular energy distribution of an embodiment of a microlens array diffuser for Example 5 of the BLU according to the present teachings, where the circular FWHM diffusion angles are 20, 30, and 40 degrees, respectively, facing away from the LED. Figure 17d shows a table of the angular energy distribution of an embodiment of the microlens array diffuser, where the circular FWHM diffusion angle is 40 degrees, facing towards the LED.

[0033] Figures 18a, 18b, and 18c show the narrow-angle intensity distributions for the case of a microlens array diffuser, where the circular FWHM diffusion angles are 20, 30, and 40 degrees, facing the LED. Figure 18d shows the narrow-angle intensity distribution for the case of a microlens array diffuser, where the circular FWHM spread angles are 40 degrees, facing away from the LED. Figure 18e shows the energy uniformity for the case of a microlens array diffuser for Example 6 of the BLU according to the present teachings, where the circular FWHM diffusion angles are 20 degrees, facing the LED, and the diffuser is located approximately 15 mm above the LED. Figure 18f shows the narrow-angle intensity distribution for the case of a microlens array diffuser for Example 6 of the BLU according to the present teachings, where the circular FWHM diffusion angles are 20 degrees, facing the LED, and the diffuser is located approximately 15 mm above the LED.

[0034] Figures 19a and 19b show the uniformity of the intensity of the narrow-angle light (detector 86) emitted from the diffuser for Example 7 of the BLU according to the present teachings. Figure 19a is an embodiment without angular bending. Figure 19b is an embodiment in which the inner lens is replaced with an angular bending structure.

[0035] Figure 20 Shows the narrow-angle intensity (detector 86) for Example 7 of the BLU according to the present teachings.

[0036] Figure 21 Shows the lens array on the back side of the diffuser for Example 8 of the BLU according to the present teachings.

[0037] Figures 22a and 22b show the optical angle distribution leaving the top diffuser for Embodiment 8 of the BLU according to the present teachings. Figures 22c and 22d show the uniformity of the total energy and narrow-angle energy for Embodiment 8 of the BLU according to the present teachings, respectively.

[0038] Figures 23a and 23b show the resulting collimation for Example 9 of the BLU according to the present teachings, where the exit facets of each small lens are calculated using the Fresnel equation.

[0039] Figures 24a, 24b, 24c, 24d, and 24e illustrate the resulting collimation for Example 10 of the BLU according to the present teachings, where the scaling factors are 0.5, 0.65, 0.75, 1.0, and 1.5 for the vertical heights of the respective small lenses. That is, the height of the prismatic microstructures of the annular segments.

[0040] Figures 25a and 25b illustrate the resulting collimation of an embodiment with a scaling factor of 1 for the microlenses in Example 11 for a BLU according to the present teachings. Figures 25c and 25d illustrate the resulting collimation of an embodiment with a scaling factor of 0.65 for the microlenses in Example 11 for a BLU according to the present teachings.

[0041] Figures 26a and 26b illustrate the illuminance (energy) and narrowband luminance for Example 12 for a BLU according to the present teachings with a well-collimated scaling factor of 1.0. Figures 26c and 26d illustrate the illuminance (energy) and narrowband luminance for Example 12 for a BLU according to the present teachings with a well-collimated scaling factor of 0.65.

[0042] Figure 27a illustrates the angular dependence achieved by adding diffusion with a scaling factor of 1.0 and adding 30-degree (FWHM) diffusion. Figure 27b illustrates the angular dependence achieved by adding diffusion with a scaling factor of 0.65 and 15-degree diffusion.

[0043] Figure 28a illustrates the narrowband luminance of Embodiment 13 for a BLU according to the present teachings, where the LEDs are arranged in a square array. Figure 28b is the narrowband luminance of Embodiment 13 for a BLU according to the present teachings, where the LEDs are arranged in a hexagonal array.

[0044] Figure 29 Shows the output distribution of the square LED array in Example 13 for a BLU according to the present teachings if a 1×60 diffuser is added to the lower surface of the diffuser. The top side (away from the LEDs) of the diffuser has a 36-degree FWHM.

[0045] Figures 30a, 30b, and 30c respectively show the total illuminance, angular distribution, and surrounding energy directly above the collimating lens of Example 14 for a BLU according to the present teachings.

[0046] Figures 31a, 31b, 31c, and 31d respectively show the total illuminance directly above the collimating lens of Example 14 for a BLU according to the present teachings, with lens thicknesses of 3.5, 4.0, 5.0, and 6.0 mm respectively. When each individual collimating lens intersects with an adjacent lens, it is truncated at the midpoint.

[0047] Figures 32a, 32b, 32c, and 32d respectively show the surrounding energy of Example 14 for a BLU according to the present teachings, where the lens thicknesses are 3.5, 4.0, 5.0, and 6.0 mm respectively.

[0048] Figure 33a, 33b, 33c, and Figure 3d respectively show the total energy at a height of 12.5 mm for Example 14 of the BLU according to the present teachings, where the lens thicknesses are 3.5, 4.0, 5.0, and 6.0 mm respectively.

[0049] Figures 34a, 34b, and 34c illustrate the surrounding energy for Example 14 of the BLU according to the present teachings, where the lens radius is 50 mm and the thicknesses are 5.3 and 1 mm respectively, and the air gaps below the solid layer are 0, 1, and 2 mm respectively.

[0050] Figures 35a, 35b, and 35c illustrate the total energy uniformity at a height of 12.5 mm for Example 14, where the lens radius is 50 mm and the thicknesses are 5.3 and 1 mm respectively, and the air gaps below the solid layer are 0, 1, and 2 mm respectively.

[0051] Figures 36a and 36b respectively show the outgoing beam profile at the top surface and the narrow aperture (10 degrees) brightness at the top surface of Example 15 of the BLU according to the present teachings.

[0052] Figures 37a and 37b respectively show the outgoing beam profile at the top surface and the narrow aperture (10 degrees) brightness at the top surface of Example 15 of the BLU according to the present teachings, where a 12-degree angle bend of the prism and a 1 by 40-degree top hat diffuser structure are added to the bottom surface of the 1.5 mm diffuser at 12.5 mm.

[0053] Figure 38 Shows the structure on the bottom surface of the top layer of Example 16 of the BLU according to the present teachings.

[0054] Figures 39a, 39b, 39c, and 39d respectively illustrate the total illuminance directly above the collimator, i.e., the energy uniformity, when the air gaps are 0, 1.7 mm, 3.2 mm, and 4.7 mm for Example 17 of the BLU according to the present teachings.

[0055] Figures 40a, 40b, 40c, and 40d respectively illustrate the total illuminance directly above the top layer, i.e., the energy uniformity, when the air gaps are 0, 1.7 mm, 3.2 mm, and 4.7 mm for Example 17 of the BLU according to the present teachings.

[0056] Figures 41a, 41b, 41c, and 41d respectively illustrate the brightness directly above the top layer, i.e., the angular uniformity, when the air gaps are 0, 1.7 mm, 3.2 mm, and 4.7 mm for Example 17 of the BLU according to the present teachings.

[0057] Figures 42a, 42b, 42c, and 42d respectively illustrate the angular distribution of light directly above the top layer when the air gaps are 0, 1.7 mm, 3.2 mm, and 4.7 mm for Example 17 of the BLU according to the present teachings.

[0058] Figure 43 Illustrates the angular distribution of light directly above the top layer of Example 18 for BLU according to the present teachings. Detailed Description

[0059] The present teachings will now be described in more detail with reference to the exemplary embodiments shown in the accompanying drawings. Although the present teachings are described in conjunction with various embodiments and examples, the present teachings are not intended to be limited to such embodiments. On the contrary, as will be recognized by those skilled in the art, the present teachings encompass various alternatives, modifications, and equivalents. Those of ordinary skill in the art will recognize additional embodiments, modifications, and examples, as well as other fields of use, after exposure to the teachings herein, all of which are within the scope of the present disclosure as described herein.

[0060] References in the specification to "one embodiment" or "an embodiment" mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the teachings. The phrase "in one embodiment" appearing throughout the specification does not necessarily refer to the same embodiment. Moreover, as used in the specification and claims, unless the context clearly dictates otherwise, the singular forms "a," "an," and "the" include plural referents.

[0061] It should be recognized that the various steps of the methods of the present teachings can be performed in any order and / or simultaneously, so long as the present teachings remain viable. Additionally, it should be understood that the apparatus and methods of the present teachings can include any number or all of the described embodiments, so long as the present teachings remain viable.

[0062] Light-emitting diodes (LEDs) typically have emission areas ranging from a few hundred micrometers to over one millimeter. A large emission area is necessary to achieve high brightness with a relatively small number of LEDs. For example, Figure 5 An array of LEDs is shown, spaced 6 mm apart in each direction and having an emission surface diameter of 1 mm. Then, if an array of Fresnel collimating lenses on a thin glass sheet or polymer film (115) is placed 0.5 mm (114) above the LEDs, the collimation will be only +-45 degrees. And even in a configuration with a very small emission surface area, narrow collimation can be achieved, but the width of the collimated beam leaving the lens will be very small relative to the LED pitch, which will make it difficult to achieve uniform light intensity entering the LCD panel.

[0063] On the other hand, if the film with collimating lenses thereon is spaced 3 mm (115) above the LEDs, there is air between the lens array layer and the LEDs and it almost covers the entire surface, as Figure 6If the situation is as seen in [reference], the collimation effect will also be very poor because the high-angle light leaving the LED will not enter the lens located above the LED. Instead, the light will enter one of the adjacent lenses and exit at a high angle, resulting in high crosstalk between the dimming zones and only a small portion of the light being collimated.

[0064] In Figure 7 In the embodiment of the present invention shown in [figure], a thick layer of optically transparent material 117 with a refractive index of 1.5 is placed between the LED 112 and the lens film 115, with a small air gap between the LED emission area and the lower surface of 117. In another embodiment, the transparent material layer 117 can be part of the lens layer 115. It should be noted that the substrate layer of 115 or the lens layer 115 can have a different refractive index relative to the transparent material layer 117. The light emitted from the LED 112 has an angular distribution of all angles between plus and minus 90 degrees. However, once it enters the transparent material layer 117 with a refractive index of 1.5, the light is restricted to angles between plus and minus 45 degrees. If the refractive index of the transparent material of layer 117 is 1.57, then the light is restricted to angles between plus and minus 39.6 degrees. If the refractive index of the transparent material of layer 117 is 1.75, then the light is restricted to angles between plus and minus 34.8 degrees. If the refractive index of the transparent material of layer 117 is 2.0, then the light is restricted to angles between plus and minus 30 degrees.

[0065] Now, this allows the thickness of layer 117 to be adjusted such that the maximum lateral spread of the light from a given LED is half of the spacing between the LEDs. For an embodiment with a refractive index of 1.5, an LED spacing of 6 mm, and an emission area diameter of 1 mm, the thickness can be approximately 2.5 mm. This increased distance makes the LED more like a point source, allowing the light to be collimated to less than plus and minus 15 degrees. Another advantage of this configuration is that substantially all of the light leaving a given LED is captured by the lens directly above the LED. This configuration minimizes crosstalk and achieves the narrowest collimation. In addition, the collimated light leaving the lens layer 115 has a lateral dimension that almost spans the LED spacing, which greatly facilitates achieving a uniform intensity for entering the LCD panel.

[0066] Each annular micro-lens can be approximated and described by a prism that has a facet along the surface parallel to the LED surface and an exit facet that refracts light in the desired direction. The annular micro-lens can also be referred to as an annular segment. The shape of the segment can be described as having a micro-structure in the shape of a prism. Although the shape is nominally a prism structure and can be described by the angles and positions of the individual facets of the prism, it can be understood that in practice, the light exit facet usually has a certain curvature. For a conventional Fresnel lens, one facet 200( Figure 8)Coplanar with the surface having the LED 112, the second facet 201 is perpendicular to this surface, while the remaining surface 202 from which the refracted light exits becomes increasingly steep 210 from being almost coplanar with the first facet 200 at the center of the lens as it moves radially outward. The problem is that when we reach the outer edge of the lens, the exit facet of the small lens 220 needs to be almost perpendicular to the facet 200. This results in an excessive number of small lenses, which poses great manufacturing difficulties in creating very narrow facets and very steep small apex angles. A large number of small lenses will also result in more undesirable scattering.

[0067] In one embodiment of a planar lens including annular segments of a prism shape, Figure 9 Half of the lens is shown. The annular segments can also be referred to as annular sections. In the shown configuration, the bottom facet remains coplanar with the plane of the LED, and the exit facet 202 varies continuously from being almost horizontal at the center of the lens to being almost vertical at the outermost small lens. The exit surface 202 of the third prism 205 (which is vertical for a conventional Fresnel lens in this configuration) varies continuously.

[0068] Such lens structures can be fabricated using many techniques known in the art. For example, a suitable master mold can be used to cast the shape of the diffractive microstructure onto a substrate, and then a thermosetting polymer or an ultraviolet (UV) light-curing polymer can be used to form the lens structure. Alternatively, the shape can be imprinted into a thermoplastic substrate by compression molding or other molding, or can be created simultaneously with the substrate using extrusion embossing or injection molding. The microstructure can also be produced by replicating a master mold. For example, an optical film can be fabricated by replicating a master mold containing the desired shape, such as described in U.S. Patent No. 7,190,387 B2, titled "Systems and Methods for Fabricating Optical Microstructures Using a Cylindrical Platform and a Rastered Radiation Beam," issued to Rinehart et al.; U.S. Patent No. 7,867,695 B2, titled "Methods for Mastering Microstructures Through a Substrate Using Negative Photoresist," issued to Freese et al.; and / or U.S. Patent No. 7,192,692 B2, titled "Methods for Fabricating Microstructures by Imaging a Radiation Sensitive Layer Sandwiched Between Outer Layers," issued to Wood et al. (which patent has been assigned to the assignee of the present invention). The entire disclosure of all of these references is incorporated herein by reference. The master mold itself can be fabricated using the laser scanning techniques described in these references and can also be replicated using the replication techniques described in these references to provide the microstructure.

[0069] In one embodiment, the microstructure can be exposed to a photosensitive material using projection or contact lithography (such as used in semiconductors, displays, circuit boards, and other common techniques known in the art). In one embodiment, the microstructure including the markings can be created in the material using laser ablation (either using a mask or using a focused and modulated laser beam). In another embodiment, the desired microstructure can be created from a solid material using micromachining (also known as diamond machining). In yet another embodiment, the desired microstructure can be created in a solid material using additive manufacturing (also known as 3D printing) known in the art.

[0070] Accordingly, a backlight unit for a backlight display in accordance with the present teachings includes a backlight unit that includes a plurality of light emitting diodes positioned on a substrate and a plurality of planar lenses. In one embodiment, at least one of the plurality of light emitting diodes includes an emission area that is larger in one dimension. A respective one of the plurality of planar lenses is positioned a distance above a respective one of the plurality of light emitting diodes. In one embodiment, at least some of the plurality of planar lenses are configured to collimate light generated by adjacent ones of the plurality of light emitting diodes in a Gaussian angular distribution.

[0071] In one embodiment, a common center of each of the plurality of annular segments of the plurality of planar lenses is vertically aligned with a respective emission center of the plurality of light emitting diodes. In another embodiment, the common center of the plurality of annular segments of the plurality of planar lenses is spatially offset from the emission centers of at least some of the respective light emitting diodes of the plurality of light emitting diodes.

[0072] At least one of the plurality of planar lenses may be a Fresnel lens. In one embodiment, a thickness of at least one of the plurality of planar lenses is between 45 - 55% of a pitch distance of the plurality of light emitting diodes. In one embodiment, at least one of the plurality of planar lenses is configured to collimate asymmetrically. Moreover, in one embodiment, at least one of the plurality of planar lenses is configured to provide the same collimation in all directions. Moreover, in one embodiment, at least one of the plurality of planar lenses includes perturbations superimposed on a surface, the perturbations being configured to provide additional spreading in one or two dimensions. In some embodiments, a reflective polarizer film is positioned adjacent to the plurality of planar lenses.

[0073] Each of the plurality of planar lenses includes a plurality of annular segments having a common center. Each of the plurality of annular segments includes a prismatic microstructure having a first facet configured to refract light in a desired direction and a second facet oriented in a plane parallel to the plane of the emission surface of the corresponding light emitting diode of the plurality of light emitting diodes. In one embodiment, at least one of the plurality of annular segments includes a circular segment. Moreover, in one embodiment, the first facet of the innermost one of the plurality of annular segments is positioned parallel to the plane of the emission surface of the corresponding light emitting diode of the plurality of light emitting diodes. Moreover, in one embodiment, the first facet of the outermost one of the plurality of annular segments is positioned perpendicular to the plane of the emission surface of the corresponding light emitting diode of the plurality of light emitting diodes. In one embodiment, the first facet of at least one of the plurality of annular segments is determined to match a spherical lens having a radius. In one embodiment, the angle of the first facet of at least one of the plurality of annular segments is determined to match a spherical lens. The angle of the first facet of at least one of the plurality of annular segments can be defined by the Fresnel equation and subsequently scaled by a factor between 0.5 and 1.0.

[0074] Moreover, in one embodiment, the first facets of successive annular segments of the plurality of annular segments are positioned to have an angle that varies continuously from the first facet of the innermost one of the plurality of annular segments positioned parallel to the plane of the emission surface of the corresponding one of the plurality of light emitting diodes to the first facet of the outermost one of the plurality of annular segments positioned parallel to the plane of the emission surface of the corresponding one of the plurality of light emitting diodes.

[0075] In many embodiments, an optical film is positioned between the plurality of planar lenses and the plurality of light emitting diodes. The optical film can be configured to diffuse light transmitted through the optical film to improve visual uniformity. The optical film can be configured to spread light transmitted through the optical film in two dimensions to achieve a predetermined angular distribution. In one particular embodiment, the two dimensions are orthogonal dimensions. Moreover, the optical film can be configured to equalize the angular distribution of light transmitted through the optical film by incorporating spatially varying angular bends. In one particular embodiment, the optical film is formed with a plurality of microlenses formed on a first surface. A second plurality of microlenses can also be formed on a second opposite surface. In some configurations, at least some of the microlenses on the first surface are different from some of the microlenses on the second surface. In another embodiment, the optical film can include a light absorbing material that minimizes crosstalk between dimming zones and / or enhances collimation by recycling reflected light.

[0076] The following are specific examples of a backlight unit for a backlight display in accordance with the present teachings. In Example 1, a 3x3 array of LEDs spaced 7 mm apart is modeled using an optical tool, with a height of 0.5 mm, and the Lambertian emission area of each LED is approximately 1.5 mm x 1.5 mm. The vertical boundaries are assumed to be perfect specular reflectors in order to simulate the case of an infinite array of LEDs. The PCB is assumed to have a reflectivity of 80%.

[0077] Reference Figure 10 , shows the positions of different virtual detection surfaces represented by dashed lines. The LCD panel 180 is located approximately 16 mm above the PCB. In this case, the lens array layer 115 includes a thick substrate with a total thickness of 3.5 mm and a refractive index of 1.5. The shape of the light-emitting (functional) facets on the small lenses is assumed to be from a spherical lens with a radius of 3.5 mm, as shown in FIGS. 11a and 11b. The resulting angular output distribution is shown in Figure 12 , which indicates very poor collimation.

[0078] FIG. 13a shows in tabular form the results of optimization by adjusting the lens radius and stretching the lens by different amounts in the vertical direction. It can be seen that when the lens radius is 3.6 mm and the stretch ratio (SZ) is 2.4, very good collimation is achieved, with 71% of the energy being confined within an angle of +-20 degrees. FIG. 13b shows the energy uniformity and angular distribution for the optimal case presented in Table 13a for an SZ ratio equal to 2.4. FIGS. 14a and 14b show the energy uniformity at the detector 84 located 13.7 mm above the PCB. It can be seen that good energy uniformity is achieved.

[0079] In Example 2, the reflectivity of the PCB is changed from 80% to 0 (i.e., absorption). Any light reflected back to the PCB will not be well collimated. Therefore, collimation can be improved by removing the reflected light. FIGS. 14a and b compare the energy distributions of Example 1 (R = 80%) with Example 2 (with an absorbing PCB). It can be seen that changing the PCB from reflective to absorbing increases the energy within +-20 degrees from approximately 71% to approximately 75%.

[0080] Example 3 is the same as Example 2, except that the lens array is shifted 0.5 mm relative to the LEDs in the L = 0 direction. FIGS. 15a and b show that a 0.5 mm shift results in the output distribution being shifted by approximately 8 degrees and the efficiency being reduced by approximately 2%, which is significantly better than what can be achieved by using a prism direction turning film.

[0081] Example 4 is similar to Example 3, except that the emission area of the LED is smaller, being 1 mm × 1 mm, and the detectors 84 and 86 are located approximately 13 mm above the LED. Figure 16a shows that the smaller emission area results in better collimation and similar intensity uniformity, which is illustrated in Figure 16b. However, it is also important to examine the uniformity of the angular distribution of the light approximately 13 mm above the LED. Figure 16c illustrates the light intensity emitted at less than plus or minus 10 degrees approximately 13 mm above the LED. As can be seen from Figure 16c (detector 86), the intensity of the more collimated light is much higher directly above the LED than between the LEDs. From the perspective of visual uniformity, this is often undesirable.

[0082] In Example 5, for LEDs with a pitch of 7 mm and an emission area of 1.5 × 1.5 mm, a diffusive microlens array was added at the positions directly below the detectors 84 and 86. Figures 17a-d illustrate that adding a circular microlens diffuser reduces collimation, and the higher the diffusion, the greater the effect. Also, comparing Figure 17c with Figure 17d, it can be seen that if the microlens structure faces the LED, then better collimation is retained. Figures 18a-f illustrate how the intensity of the diffusion affects the uniformity of the intensity of the narrow-angle light of detector 86. The stronger the diffusion, the better the uniformity. Also, in Figure 18d, it is seen that if the microlens array texture faces away from the LED, then the uniformity is better than when it faces the LED.

[0083] In Example 6, the detectors 84 and 86 and the diffuser were moved up approximately 2 mm. As can be seen in Figures 18e and 18f, both the energy intensity and the narrow-angle intensity are significantly improved.

[0084] Example 7 is the same as Example 6, except that the inner part of each Fresnel lens was modified. At the center of each lens, the previous small lens within a radius of 1.2 mm was replaced by a prism angle bend with a base angle of 7.5 degrees. As can be seen in Figure 19a (Example 6 without angle bend) and Figure 19b (Example with central angle bend), modifying the central small lens significantly improves the narrow-angle intensity.

[0085] By aligning the array of radially symmetric prism angle bend small lenses with a base angle of 16 degrees and a radius of 1 mm with the space at the equal distance between 4 LEDs, the distribution of the light exiting the diffuser at the central region between the LEDs can also be locally modified. This was modeled in Example 7, and Example 7 is the same as Example 6 except for these periodic lenses on the back of the diffuser. Figure 20 The resulting narrow-angle intensity (detector 86) is shown.

[0086] In Example 6, as shown in FIGS. 18e and 18f, it is demonstrated that a uniform energy intensity can be achieved at the exit of the diffuser while maintaining approximately 50% of the contained energy within +-20 degrees. However, the angular distribution of the light varies periodically across the diffuser surface, with more on-axis light concentrated above the LED and more off-axis light concentrated between the LEDs. It is possible to use a spatially varying microlens array (such as a prism angle bend) to balance the angular distribution. For example, light approaching a position between the LEDs approaches at an off-axis angle of +-20 degrees. This light can be bent to enter the diffuser more perpendicular to the surface.

[0087] In Example 8, the same conditions as in Example 6 are used, except that there is a spatially varying prism angle bend on the back of the 20-degree FWHM diffuser, as Figure 21 shown. The lenses between the LED positions are 3 mm in radius, and the prism angle varies from 20 degrees at the center to zero degrees at the outer radius. The lens above the LED has a radius of 1.5 mm, and the prism angle varies between 10 degrees at the center and zero degrees at the outer radius. The resulting characteristics are shown in FIGS. 22a, 22b, 22c, and 22d. FIGS. 22a and 22b show the optical angular distribution leaving the top diffuser. FIGS. 22c and 22d show the uniformity of the total energy and the narrow-angle energy, respectively.

[0088] As opposed to approximating the angles of the exit facets of the annular rings in accordance with the shape of a spherical lens and then stretching the vertical dimension by different amounts in some cases, it is possible to use the well-known Fresnel equation n1sin(θ1) = n2sin(θ2) to calculate the angle of each exit facet to obtain optimal collimation. In this case, we want θ Figure 9 = 90 degrees, which means sin(θ 2 ) = 1.0 and m2 = 1.0 (for air). 2

[0089] In Example 9, a 1 mm x 1 mm LED has Lambertian emission, using a spatial 7 of a focusing lens, where Figure 9 the exit facets are calculated using the Fresnel equation. The reflectivity of the bottom surface of the PCB is assumed to be 80%. The thickness of the substrate layer is 3.5 mm, the refractive index is 1.5, and the lens diameter is 3.5 mm. No additional diffuser is included. FIGS. 23a and 23b show the resulting collimation.

[0090] Example 10 is the same as Example 9, except that the height of each small lens is scaled by a certain factor. We should be clear because this is an important parameter. FIGS. 24a, 24b, 24c, 24d, and 24e are the resulting collimation of Example 10, where the scaling factors for the vertical height of each small lens are 0.5, 0.65, 0.75, 1.0, and 1.5, respectively.

[0091] Example 11 is an example using the small lens of Example 10, which has an array of 3×3 LEDs spaced 7.0 mm apart, where each LED has a Lambertian emission area of one by one millimeter. The reflectivity of the PCB is 80% and a 30-degree diffuser is placed at a height of 15 mm, with the detector located directly above it. Figures 25a and 25b present the data for a scaling factor of one. It can be seen that the collimation effect is excellent. Figure 25a shows the intensity as a function of angle measured above the collimating lens. The presented data illustrate good uniform narrowband (+-10 degrees) brightness. Figure 25b shows the intensity as a function of angle measured above the diffuser at a height of 15 mm. Figures 25c and 25d present the data for an embodiment with a scaling factor of 0.65 for the small lens in Example 11 for the BLU according to the present teachings. These data indicate that for a scaling factor of 0.65, the more gradual decrease in intensity with angle (Figure 25c) results in a more uniform narrowband (+-10 degrees) brightness above the diffuser at a height of 15 mm (Figure 25d).

[0092] In Example 12, the 30-degree diffuser and the detector located directly above the 30-degree diffuser are moved to a vertical height of 20 mm. Figures 26a and 26b present the data for illuminance (energy) and narrowband brightness for a well-collimated scaling factor of 1.0. Figures 26c and 26d are the illuminance (energy) and narrowband brightness for a scaling factor of 0.65. When the intensity of the collimating lens decreases more gradually with angle, the uniformity is again much better.

[0093] By either adding a diffuser above the collimating lens or superimposing a diffractive microstructure on the exit facet of the collimating lens, the angular dependence of the light from the collimating lens can be changed. Figures 27a and 27b illustrate the angular dependence achieved by adding a diffuser. Figure 27a has a scaling factor of 1.0 and a 30-degree (FWHM) diffuser is added, while Figure 27b shows the data for a scaling factor of 0.65 with a 15-degree diffuser. The more gradual variation of intensity with angle significantly contributes to achieving good uniformity of brightness and illuminance. One way to characterize this is to consider the slope of the angular intensity of the light leaving the collimating lens as a function of angle. One metric is the difference between the angle at which the intensity is 10% of the maximum and the angle at which the intensity is 80% of the maximum, divided by the angle at which the intensity is 50% of the maximum. This ratio is called S(80,10). In many embodiments of the present teachings, S(80,10) is desirably greater than 0.7 and preferably greater than 1.0.

[0094] The configuration of the LEDs in the array can affect the achievement of good uniformity. Example 13 includes Lambertian LEDs of 1 mm by 1 mm, with the emitting LEDs spaced 7 mm apart, having a diffuser with approximately 36 degrees FWHM at a height of 12.5 mm, where the PCB has an 80% Lambertian reflectivity. The scaling factor of the collimating lens on the upper surface with a thickness of 3.5 mm is 0.65, and a 15-degree FWHM diffusion is added. The narrowband luminance is shown for the square array of LEDs in Fig. 28a and the hexagonal array in Fig. 28b. It can be seen that the hexagonal array results in better uniformity.

[0095] It is also possible to customize the angular distribution leaving the top diffuser. An elliptical output distribution is often desired. Figure 29 The output distribution of the square array of LEDs in Example 13 is shown if a 1 by 60 diffuser is added to the lower surface of the diffuser, which has 36 degrees FWHM on the top side (away from the LEDs).

[0096] Example 14 has a square array consisting of nine LEDs with a Lambertian emission spacing of 1 mm x 1 mm, and the 7PCB has an 80% Lambertian reflectivity. The scaling factor of the collimating lens on the upper surface with a thickness of 3.5 mm is 0.70, and a 15-degree FWHM diffusion is added. Fig. 30a shows the total illuminance, angular distribution, and surround energy directly above the collimating lens for Example 14 respectively. It can be seen that there is little energy in the space between the LEDs. A thicker lens layer will allow the light to spread more and have a smaller very low energy region, which will facilitate the achievement of uniform luminance at the upper diffuser.

[0097] Figs. 31a, 31b, 31c, and 31d show the total illuminance directly above the collimating lenses with lens thicknesses of 3.5, 4.0, 5.0, and 6.0 mm for Example 14 respectively. Each individual collimating lens is truncated at the midpoint when intersecting with the adjacent lens. It can be seen that as the lens thickness and radius increase, the overlap of energy becomes more significant. Figs. 32a, 32b, 32c, and 32d show the surround energy for lens thicknesses of 3.5, 4.0, 5.0, and 6.0 mm for Example 14 respectively. The best collimation is shown in Fig. 32b (4.0 mm), where the percentage of collimated energy within a 20-degree range is 62%, dropping to 59.6% for Fig. 32c (5.0 mm) and to 51.8% for Fig. 33c (6.0 mm).

[0098] Figures 33a, 33b, 33c, and 33d respectively show the total illuminance at a height of exactly 12.5 mm when the lens thicknesses for Example 14 are 3.5, 4.0, 5.0, and 6.0 mm. These data indicate that the structure with a lens size and thickness of 5.0 mm has the best uniformity. To further improve the uniformity, the collimating lens can be designed to spread the light slightly more along the diagonal.

[0099] It should be understood that the collimating lens layer can be solid or air. Figures 34a, 34b, and 34c illustrate the surrounding energy for Example 14, where the lens radius is 50 mm, the thicknesses are 5.3 and 1 mm respectively, and the air gaps below the solid layer are 0, 1, and 2 mm respectively. It can be seen that as air is added below the collimating lens, the collimation decreases significantly. The surrounding energy decreases from 59.6% without an air gap to 49.5% with a 2 - mm air gap. Figures 35a, 35b, and 35c show the total energy uniformity at a height of 12.5 mm for Example 14, where the lens radius is 50 mm, the thicknesses are 5.3 and 1.0 mm respectively, and the air gaps below the solid layer are 0, 1, and 2 mm respectively. It can be seen that as the air gap increases, the total energy uniformity also deteriorates.

[0100] Example 15 has a square array of nine LEDs, each LED being 1 mm × 1 mm, with Lambertian emitters spaced 7 mm apart, and the Lambertian reflectivity of the PCB being 80%. The scaling factor of the collimating lens on the upper surface with a thickness of 5.0 mm is 0.75 and a 15 - degree FWHM diffusion is added. It includes a second layer with a thickness of 1.5 mm, and the top texture gives a circular diffusion with a 30 - degree FWHM. The top of this layer is located at 12.5 mm. Figures 36a and 36b respectively show the outgoing beam profile of the top surface and the narrow - aperture (10 - degree) luminance at the top surface for Example 15.

[0101] Figures 37a and 37b respectively show the outgoing beam profile at the top surface and the narrow - aperture (10 - degree) luminance at the top surface for Example 15, where a 12 - degree angular bend of the prism and a top - hat - shaped structure of a 1 × 40 - degree diffuser structure are added to the bottom surface of the 1.5 - mm diffuser at 12.5 mm. This illustrates the ability to tilt the beam in one direction while spreading it in the opposite direction.

[0102] In Example 15, there are 4 layers: the top layer has a 30 - degree FHM diffuser on the upper surface and a 1 × 40 - degree diffuser on the lower surface. The next layer below it is a 12.5 - degree angular bend of the prism with the texture facing the LEDs, and immediately above the collimating lens is a layer with a 15 - degree FWHM diffuser, and the structure is on the side facing away from the LEDs. The bottom - most layer is the collimating lens, and the lens is also on the surface away from the LEDs. In some configurations, reducing the number of layers is beneficial.

[0103] In Example 16, there are two layers. The top layer has a 30-degree FWHM diffuser on the top surface and its structure is similar to the structure described in combination with Figure 38 which combines an elliptical spread in the horizontal direction and a 12.5-degree angular bend in the vertical direction. The bottom layer has a collimating lens on the top surface and a diffuser on its bottom surface facing the LED. Reducing the number of layers from four to two increases the efficiency by approximately 5% due to reduced back reflection. It also improves crosstalk and collimation due to the reduction of back-reflected light.

[0104] Example 17 has two layers with a square array of nine LEDs, 1 mm × 1 mm, Lambertian emission spaced 7 mm apart, and the Lambertian reflectivity of the PCB is 80%. The top surface of the upper layer is located 14 mm above the LEDs and has a diffuser with an FWHM of approximately 12 degrees. The bottom surface of this layer facing the collimator lens and the LEDs has a structure as shown in Figure 38 which spreads light in the horizontal direction with an FWHM of 60 degrees and tilts the light at 12.5 degrees in the vertical direction. The thickness of this layer is 0.5 mm. The bottom layer has a diffuser with an FWHM of approximately 10 degrees on the bottom surface. The collimator lens on the top surface is designed to have a radius of 90 mm (truncated when intersecting with adjacent lenses, scaling factor of 0.8). The total thickness of this layer and the air gap below it is 6.2 mm. The air gap varies from 0 to 1.7 mm to 3.2 mm to 4.7 mm, while the corresponding thickness varies from 6.2 mm to 4.5 mm to 3 mm to 1.5 mm.

[0105] Figures 39a, 39b, 39c, and 39d show that for larger air gaps, the total illuminance as the energy uniformity directly above the collimator is more uniform, especially when the air gap is approximately half or more of the LED pitch. Figures 40a-d and 41a-d show that as the air gap gets larger, the energy uniformity and angular uniformity above the top surface also improve. It can also be seen from Figures 42a-d that for larger air gaps, the angular distribution in the vertical direction also narrows significantly.

[0106] Example 18 is the same as Example 17, except that the top mixing diffuser is increased from approximately 12-degree FWHM to 18 degrees, and the air gap is set to 4.7 mm. As can be seen from Figure 43 the brightness of the top surface is significantly improved.

[0107] The parameters depend largely on the LED pitch. If the pitch is increased by 50%, then the thickness of the optically transparent material layer (117) needs to be scaled approximately proportionally. Also, the vertical height of the diffuser should be scaled approximately proportionally.

[0108] Equivalents

[0109] Although the applicant's teachings are described in connection with various embodiments, the applicant's teachings are not intended to be limited to such embodiments. On the contrary, as will be recognized by those skilled in the art, the applicant's teachings cover various alternatives, modifications, and equivalents that may be made therein without departing from the spirit and scope of the teachings.

Claims

1. A backlight unit, comprising: a) a plurality of light emitting diodes positioned on a substrate; and b) a plurality of planar lenses, wherein a respective one of the plurality of planar lenses is positioned at a certain distance above a respective one of the plurality of light emitting diodes, each of the plurality of planar lenses includes a plurality of annular segments having a common center, each of the plurality of annular segments includes a prism-shaped microstructure, and the prism-shaped microstructure has a first facet configured to refract light in a desired direction and a second facet oriented in a plane parallel to the plane of the emission surface of the respective light emitting diode among the plurality of light emitting diodes.

2. The backlight unit according to claim 1, wherein at least one of the plurality of planar lenses includes a Fresnel lens.

3. The backlight unit according to claim 1, wherein the thickness of at least one of the plurality of planar lenses is between 45% and 55% of the pitch distance of the plurality of light emitting diodes.

4. The backlight unit according to claim 1, wherein the refractive index of at least one of the plurality of planar lenses is between 1.5 and 1.

57.

5. The backlight unit according to claim 1, wherein at least one of the plurality of planar lenses is configured to collimate in one dimension, with an FWHM of less than 40 degrees.

6. The backlight unit according to claim 1, wherein at least one of the plurality of planar lenses is configured to collimate in two dimensions, with an FWHM of less than 40 degrees.

7. The backlight unit according to claim 1, wherein at least one of the plurality of planar lenses is configured to collimate asymmetrically.

8. The backlight unit according to claim 1, wherein at least one of the plurality of planar lenses is configured to provide the same collimation in all directions.

9. The backlight unit according to claim 1, wherein at least one of the plurality of planar lenses includes a perturbation superimposed on the surface and configured to provide additional scattering in one dimension.

10. The backlight unit according to claim 1, wherein at least one of the plurality of planar lenses includes a perturbation superimposed on the surface and configured to provide additional scattering in two dimensions.

11. The backlight unit according to claim 1, wherein at least one of the plurality of annular segments includes a circular segment.

12. The backlight unit according to claim 1, wherein the first facet of the innermost one of the plurality of annular segments is positioned parallel to the plane of the emission surface of the respective light emitting diode among the plurality of light emitting diodes.

13. The backlight unit according to claim 1, wherein the first facet of the outermost one of the plurality of annular segments is positioned perpendicular to the plane of the emission surface of the respective light emitting diode among the plurality of light emitting diodes.

14. The backlight unit according to claim 1, wherein the first facets of successive annular segments among the plurality of annular segments are positioned at an angle that varies continuously from the first facet of the innermost one of the plurality of annular segments positioned parallel to the plane of the emission surface of the corresponding one of the plurality of light-emitting diodes to the first facet of the outermost one of the plurality of annular segments positioned parallel to the plane of the emission surface of the corresponding one of the plurality of light-emitting diodes.

15. The backlight unit according to claim 1, wherein the angle of the first facet of at least one of the plurality of annular segments is determined to match a spherical lens.

16. The backlight unit according to claim 1, wherein the angle of the first facet of at least one of the plurality of annular segments is determined to match a spherical lens.

17. The backlight unit according to claim 1, wherein the angle of the first facet of at least one of the plurality of annular segments is determined using Fresnel's equations.

18. The backlight unit according to claim 1, wherein the angle of the first facet of at least one of the plurality of annular segments is determined using Fresnel's equations and is subsequently scaled by a factor between 0.5 and 1.

0.

19. The backlight unit according to claim 1, wherein the common center of each of the plurality of annular segments of the plurality of planar lenses is vertically aligned with the corresponding emission center among the emission centers of the plurality of light-emitting diodes.

20. The backlight unit according to claim 1, wherein at least some of the common centers of the plurality of annular segments of the plurality of planar lenses are spatially offset from the emission centers of at least some of the corresponding light-emitting diodes among the plurality of light-emitting diodes.

21. The backlight unit according to claim 1, wherein at least one of the plurality of light-emitting diodes includes an emission area that is larger in one dimension.

22. The backlight unit according to claim 1, further comprising an optical film positioned between the plurality of planar lenses and the plurality of light-emitting diodes, wherein the optical film is configured to diffuse the light transmitted through the optical film, thereby improving visual uniformity.

23. The backlight unit according to claim 1, further comprising an optical film positioned between the plurality of planar lenses and the plurality of light-emitting diodes, wherein the optical film is configured to spread the light transmitted through the optical film in two dimensions to achieve a predetermined angular distribution.

24. The backlight unit according to claim 23, wherein the two dimensions are orthogonal dimensions.

25. The backlight unit according to claim 1, further comprising an optical film positioned between the plurality of planar lenses and the plurality of light-emitting diodes, wherein the optical film is configured to equalize the angular distribution of the light transmitted through the optical film by combining spatially varying angular bends.

26. The backlight unit according to claim 1, further comprising an optical film positioned between the plurality of plano lenses and the plurality of light emitting diodes, wherein the optical film includes a plurality of microlenses formed on one surface.

27. The backlight unit according to claim 1, further comprising an optical film positioned between the plurality of plano lenses and the plurality of light emitting diodes, wherein the optical film includes a plurality of microlenses formed on both a first surface and a second surface.

28. The backlight unit according to claim 27, wherein at least some of the microlenses on the first surface are different from some of the microlenses on the second surface.

29. The backlight unit according to claim 1, further comprising an optical film positioned between the plurality of plano lenses and the plurality of light emitting diodes, wherein the optical film includes a light absorbing material that minimizes crosstalk between dimming zones.

30. The backlight unit according to claim 1, further comprising an optical film positioned between the plurality of plano lenses and the plurality of light emitting diodes, wherein the optical film includes a light absorbing material that enhances collimation by recycling reflected light.

31. The backlight unit according to claim 1, further comprising a reflective polarizer film positioned adjacent to the plurality of plano lenses.

32. The backlight unit according to claim 1, wherein at least some of the plurality of plano lenses are configured to collimate light generated by adjacent light emitting diodes among the plurality of light emitting diodes in a Gaussian angular distribution.

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