WO2010019381A2

WO2010019381A2 - Enhanced uniformity backlight

Info

Publication number: WO2010019381A2
Application number: PCT/US2009/052037
Authority: WO
Inventors: Quinn D. Sanford; Kevin M. Hoffman; Kenneth A. Epstein
Original assignee: 3M Innovative Properties Company
Priority date: 2008-08-15
Filing date: 2009-07-29
Publication date: 2010-02-18
Also published as: WO2010019381A3; TW201013261A

Abstract

Enhanced uniformity backlights and methods of assembling a backlight are disclosed. The backlights maintain light uniformity by generating a plurality of images of a linear light source instead of by increasing the number of light sources. A microstructured film receives light from the linear light source and refracts the light to an image receptor. The microstructured film includes an array of microstructures that have a symmetry that provides multiple images of the light source. The positioning and brightness of the images on the image receptor is dependent on the microstructure and the orientation of the microstructure to the linear light source. A method is disclosed to identify the relative orientation of symmetric microstructures which can provide more uniform images of the linear light source.

Description

ENHANCED UNIFORMITY BACKLIGHT

Background

Uniform back-illumination is desired for many liquid crystal displays (LCDs) used in TVs and monitors, as well as in graphic displays and general lighting. Currently, it is difficult to satisfy the desire for a direct backlight that is thin, lightweight, consumes little energy, and is spatially uniform in brightness. Commercially available backlights often include an array of linear or serpentine cold cathode fluorescent lamps (CCFL). Often, the total CCFL tube length is minimized to decrease the energy consumption, thermal heating, and weight, which in turn increases the center-to-center spacing, S, of the lamps. The region of the backlight directly above the CCFL tube is much brighter than surrounding regions between adjacent CCFL tubes, so techniques have been devised to hide this bright region and spread the light more uniformly.

One technique of improving the uniformity is to use a thick diffusing plate to spread the light over a larger area. The CCFLs are positioned proximate a reflector, and a diffuser plate is set at a distance D above the reflector (distance P above the lamp array). The reflector is often a diffuse reflector, and additional films such as brightness enhancement films (BEF) can also be placed above the diffuser plate. Such backlights may achieve uniformity with S/D ratio of 1.6 or less or S/P of about 1.9 or less. The non- uniformity, called lamp Mura, is observed as a bright band above the lamps and darker zones between the lamps. Lamp Mura becomes more objectionable as the bulb spacing increases or as the cavity depth decreases, i.e., as the S/D ratio increases.

Mura reduction components, such as a specular back reflector and/or a linear prismatic optical surface proximate the bottom surface of the diffuser plate, can be used to improve uniformity in backlights with S/D ratios in the range 1.6 to 2.3, for example, as shown in U.S. Patent Publication No. 2007/0030415. A two dimensional structured sheet placed between an array of light sources and a diffuser plate can be used to improve uniformity in backlights, as described for example in U.S. Patent Publication No. 2007/0047254. The structures can include 4-sided objects with curved surfaces. For example, PCT Publications WO2006/121690 and WO2007/016076 describe curved surface pyramidal protrusions with rounded peaks. US6752505 describes varieties of protrusions including cones and pyramids, also useful for improving uniformity. Summary

Generally, the present disclosure is directed to a backlight, and method of assembling a backlight, that maintains light uniformity by generating a plurality of images of a linear light source instead of by increasing the number of light sources.

In one aspect, the backlight includes a microstructured film, a reflector, a linear light source and an image receptor. The microstructured film includes a first surface and a second surface opposite the first surface. The second surface includes an array of microstructures. The reflector is positioned facing the first surface, and defines a cavity between the reflector and the microstructured film. The linear light source is placed in the cavity so that the light source is parallel to the microstructured film. The image receptor faces the second surface of the microstructured film. The microstructures are positioned so that light from the linear light source refracts through the microstructures to form a first plurality of images of the linear light source on the image receptor. In another aspect, a method for assembling a backlight includes providing a microstructured film having a first surface and a second surface opposite the first surface, where the second surface includes an array of microstructures. The method further includes disposing a reflector facing the first surface, to form a cavity between the reflector and the microstructured film. The method further includes disposing a linear light source within the cavity, parallel to the microstructured film. The method further includes disposing an image receptor facing the second surface of the microstructured film. The method further includes rotating the microstructured film around a line normal to the reflector so that a plurality of images of the linear light source is refracted through the microstructures to the image receptor. In another aspect, a display includes a backlight and a liquid crystal display module. The backlight includes a microstructured film, a reflector, a linear light source and an image receptor. The microstructured film includes a first surface and a second surface opposite the first surface. The second surface includes an array of microstructures. The reflector is positioned facing the first surface, and defines a cavity between the reflector and the microstructured film. The linear light source is placed in the cavity so that the light source is parallel to the microstructured film. The image receptor faces the second surface of the microstructured film. The microstructures are positioned so that light from the linear light source refracts through the microstructures to form a first plurality of images of the linear light source on the image receptor. The liquid crystal display module is disposed facing the image receptor, opposite the microstructured film.

These and other aspects of the present application will be apparent from the detailed description below. In no event, however, should the above summaries be construed as limitations of the claimed subject matter, which subject matter is defined solely by the attached claims, as may be amended during prosecution.

Brief Description of the Drawings Throughout the specification reference is made to the appended drawings, where like reference numerals designate like elements, and wherein:

FIG. 1 is a schematic cross-sectional view of a backlight;

FIG. 2 is a schematic representation of the path of a light ray through a backlight; FIG. 3a is a schematic representation of images from a cube-corner; FIG. 3b is a schematic representation of images from a four-sided pyramid;

FIG. 4a is a schematic top view of a truncated cube-corner; FIG. 4b is a schematic side elevation view of a truncated cube corner; FIG. 5 is a schematic cross-section of a backlight; FIG. 5a is a schematic cross-section of a microstructure of FIG. 5; and FIG. 6 is a schematic top photographic view of a backlight.

The figures are not necessarily to scale. Like numbers used in the figures refer to like components. However, it will be understood that the use of a number to refer to a component in a given figure is not intended to limit the component in another figure labeled with the same number.

Detailed Description

Light uniformity is an important parameter in backlights for displays. Energy efficiency is another important parameter in backlit displays. As backlit displays decrease in thickness or increase in area, the number of light sources typically has been increased to maintain light uniformity. There is a need to maintain adequate light uniformity in backlit displays without increasing the number of energy consuming light sources. The uniformity and brightness of an LCD direct- lit backlight unit, such as used in a TV or a monitor, can be improved by generating multiple images of the light source used to illuminate the LCD panel. Each of these multiple images can have essentially the same shape and size, and can be uniformly spaced apart. Each of the images can be a real image, such as a focused image of the light source. Each of the images can instead be a virtual image, such as a non-focused image of the light source. The brightness of each of the images can be essentially the same, i.e., an equivalent fraction of the brightness of the light source that generates the images. The images are intercepted by the output surface of the backlight, which can be an image receptor. The image receptor can include diffuser. The image receptor can instead include a microstructure, such as a pair of brightness enhancement films (BEF) that are in a crossed orientation. The image reflector can be any other device that can spread the light corresponding to each of the images.

The refraction of light from a light source passes through a microstructured film to generate the images of the light source. In some embodiments, the microstructured film has a first surface which is smooth and planar, and a second surface which includes an array of microstructures. The array of microstructures can be a two-dimensional array of microstructures. The microstructured film has sufficient symmetry so that any point on the light source corresponds to a plurality of points which make up a portion of the plurality of images. In this way, a bright light source is broken up into a plurality of light source images, each of which contributes to the uniformity of the backlight.

In one aspect, the present disclosure describes uniform direct backlights with S/D ratios (i.e., the ratio of lamp spacing to depth of backlight) in the range 1.6 to 2.5 or greater, for example from 2 to 3, from 2 to 4, or from 2 to 5. In some embodiments the diffuser plate may be replaced with an inexpensive diffuser sheet, further decreasing the thickness and weight of the backlight. Large S/D ratios gain value as optical efficiency and bulb/inverter costs can lead backlight designers toward use of enhancement films and fewer light sources.

Relationships between a structured surface geometry, a lamp array, and a backlight cavity all can influence the uniformity of a backlight. Generally, the facets of the surface structure can establish images of the lamp when viewed through the structure. The intent is to spread the images of the lamp evenly across the output surface of the backlight, so that the images fill in the gaps between the actual lamps. The structured surface includes an array of repeated unit cells, which can be tiled together to cover the entire surface of the film.

Each of the unit cells may include one or more surface microstructures which can be the same or different. The surface microstructures generally are made up of surface facets which can be planar or curved. The surface facets can have smooth surfaces, or the surfaces can be either roughened or include smaller structures. Suitable roughened or structured surfaces can be those described in, for example, U.S. Patent Application Publication No. 2008/0166190 (Gardiner et al); also in U.S. Patent Application Serial Nos. 61/013782 (Attorney Docket No. 63574US002, filed December 14, 2007); 11/926902 (Attorney Docket No. 63707US002, filed October 29, 2007); and 11/952438 (Attorney Docket No. 63825US002, filed December 7, 2007).

Each of the unit cells can also include planar regions that can be parallel to the smooth surface of the microstructured sheet, so that the light source generating the images can also be observed directly. The planar regions, if included, can be located between the surface structures within the unit cell. The planar regions can also be a part of the surface structures (e.g., such as in a truncated pyramid). The base angle of the structured surface facet, the refractive index of the structure, and the orientation of the structure unit cell (with respect to the linear lamp direction) can be varied, in order to evenly space the images. In one aspect, a backlight includes a back reflector, an array of linear lamps, a microstructured film, a diffuser sheet and optional additional films. The microstructured film includes structures which can be useful as a uniformity enhancement component in thin direct backlights for LCD displays and signboards. A first side of the microstructured film facing the linear lamps is smooth. The second side has a tiled array of microstructures that divert light by refraction to improve uniformity at the output surface of the backlight.

Each face of the structured surface refracts light to form an image displaced from the source. There can be an orientation of the unit cell or surface structure with respect to the source, which spaces the images equally such that the light distribution on the output surface of the backlight is uniform. In one embodiment, each image has an essentially equal brightness, i.e., the brightness of each image varies less than 10%, less than 5%, less than 3%, less than 2%, or less than 1% of the average brightness of all of the images. In another embodiment, each image has an essentially equal spacing from an adjacent image, i.e., the spacing between adjacent images varies less than 10%, less than 5%, less than 3%, less than 2%, or less than 1% of the average spacing between all of the adjacent images. In one embodiment, the images from one light source are adjacent to the images from adjacent light sources. In another embodiment, the images from one light source are interleaved with at least a portion of the images from the adjacent light sources. The present disclosure includes a methodology to design and generalize such films for applications in direct- lit LCD backlights.

FIG. 1 shows a schematic cross section of backlight 100 according to one aspect of the present disclosure. Backlight 100 includes a microstructured film 110 that has a first surface 105 and a second surface 115. First surface 105 can be a planar smooth surface, and second surface 115 includes an array of microstructures 170. A reflector 120 faces the first surface 105 of microstructured film 110, and is spaced a distance D from first surface 105, defining a cavity 125. A first linear light source 130 and a second linear light source 140 (both viewed on end, in cross-section) are placed in the cavity 125 so they are parallel to the microstructured film 110. The first and second linear light sources, 130, 140, are disposed at a separation S apart from each other, and are below first surface 105 by a distance P.

The separation S between any two linear light sources in a display can be constant over the display, or the separation can be varied. Generally, the microstructured film has a uniform distribution of microstructures across the surface (i.e., each portion of the film is "translationally invariant"), and the separation S between any two light sources is the same across the display. However, in some embodiments it may be desirable to vary the distribution of microstructures in different portions of the microstructured film, and the separation S may vary also across the display.

First linear light source 130 is imaged through microstructured film 110 onto image receptor 150 as a plurality (i.e., 5 as shown in FIG 1) of images 130a-130e, and second linear light source 140 is imaged through microstructured film 110 onto image receptor 150 as a plurality (i.e., also 5 as shown in FIG 1) of images 140a-140e. The plurality of images 130a-130e, 140a-140e are shown in the figure to be equally sized and equally spaced from each other, although this is not necessarily a requirement. Backlight 100 further includes optional optical film 160 disposed between first surface 105 of microstructured film 110 and first and second linear light sources 130, 140. Optional optical film 160 can be a rigid support for microstructured film 110, and can also thermally insulate microstructured film 110 from heat generated by the light sources. In one embodiment, image receptor 150 can be any known diffuser. The diffuser can be a surface diffuser that can include particulate additions to the surface of a film or plate. The surface diffuser can have a textured surface, such as a matte finish, or a coating. In another embodiment, diffuser can be a bulk diffuser that can include particulate additions to the film or plate. The bulk diffuser can include phase separated components, microdomains, or microvoids that serve to diffuse light. The diffuser can also be a holographic diffuser, a diffuse adhesive, or a cellular diffuser such as microcellular polyethylene terephthalate (MCPET available from Furukawa America, Inc.). The surface diffuser or the bulk diffuser can be thin such as a film, or thick such as a plate. In another embodiment, image receptor 150 can be a microstructured surface that further serves to blend the boundaries of the images refracted onto the receptor. In one embodiment, such a microstructured surface can be a pair of crossed brightness enhancement films (BEF).

The reflector 120 can be a specular, semi-specular, or a diffuse reflector. One example of a specular reflector that may be used as the reflector 120 is Vikuiti™ Enhanced Specular Reflection (ESR) film available from 3M Company, St. Paul,

Minnesota. A semi-specular reflector can provide a balance of specular and diffusive properties, and are further described in, for example, PCT Patent Application No. US2008/064115 (Attorney Docket No. 63032WO003). Examples of suitable diffuse reflectors include polymers, such as polyethylene terephthalate (PET), polycarbonate (PC), polypropylene (PP), or polystyrene (PS). The polymers can be loaded with diffusely reflective particles, such as titanium dioxide, barium sulphate, calcium carbonate or the like. Other examples of diffuse reflectors include particulate loaded vinyl films such as Light Enhancement Film 3635-100, available from 3M Company. Still other examples of diffuse reflectors, including microporous materials and fibril-containing materials, are discussed in U.S. Patent Application Publication 2003/0118805.

The first and second linear light sources, 130, 140, can be linear light sources such as cold cathode fluorescent lamps (CCFL), hot cathode fluorescent lamps (HCFL), external electrode fluorescent lamps (EEFL), electroluminescent lamps (EL), organic electroluminescent lamps (OLED), light emitting diodes (LED), and the like. In some embodiments, the linear light sources are individual linear lamps aligned parallel to each other. In some embodiments, the light sources are a single lamp distributed in a serpentine manner within the cavity 125. In some embodiments, the linear light sources can be made up of discrete point light sources, such as individual LEDs aligned in a series of rows. In some embodiments, the individual LEDs include collimating optics to merge the discrete point lights into a linear light source.

The microstructured film 110 can have an array of microstructures. The array of microstructures can be a two-dimensional array of microstructures. The microstructures can be prisms, cube corners, rhombohedra corners, cones, pyramids, truncated pyramids and other geometric shapes that exhibit symmetry. As used herein, the word "pyramid" means any three dimensional microstructure having a polygonal base and triangular sides which intersect at a common peak. Pyramids can be based on polygons having three, four, five, six or more sides. In one embodiment, the microstructures can be tiled to cover the entire surface of the microstructured film with no remaining unstructured portions (e.g., no planar regions). Examples of microstructures that can be readily tiled to cover the entire surface include three, four, and six-sided pyramids.

In another embodiment, a unit cell can be tiled to cover the entire surface of the microstructured film. A unit cell can include any combination of microstructures so that there are no remaining unstructured portions, or a unit cell can include both unstructured portions and microstructures. A unit cell having a combination of microstructures that can be tiled, include a unit cell having both three- and four- sided pyramids joined along a common edge. An array of tiled microstructures can be formed from unit cells consisting of three-, four-, five-, six- or more sided pyramids or any combination thereof. A unit cell having a combination of both unstructured portions and microstructures include a unit cell having a five-sided pyramid microstructure.

The optional optical film 160 can be any optical film suitable to provide support for the microstructured film, or provide protection from undue heating by the light source, or a combination. In some embodiments, the optional optical film 160 is not a diffusing structure, since this could alter the images formed by the microstructured surface. A clear acrylic plate, for example, absorbs some of the thermal radiation emitted from the light source, and protects the lower coefficient of thermal expansion microstructured films. The acrylic plate does not contain bulk scattering (and/or absorbing) particles, so it does not warp appreciably upon exposure to the light sources. Also, without scattering and absorbing particles, more light is transmitted, resulting in a brighter backlight. FIG. 2 shows a schematic cross-section of a microstructured film 210 positioned a distance "P" above linear light source 230 according to one aspect of the disclosure. A light ray 234 traced backward through a facet 236 on a microstructure 270 emerges from the planar first surface 205 at an emergent ray angle Θi determined by the refractive index of the microstructured film 210, microstructure 270, and a base angle β between the facet 236 and the planar first surface 205. The distance P is the perpendicular distance between the linear light source 230 and the planar first surface 205. The emergent ray angle Θi establishes a ratio between P and an image displacement dM, such that dM/P = tan (Θi).

For uniform symmetric structures such as a cube-corner or a pyramid, a point source viewed through the structured film appears as a pattern of image points displaced an equal distance dM from the physical source. The displacement directions depend upon the symmetry of the facets. FIG. 3a shows a relationship of image points 330a-330f to point source 330 for a cube-corner microstructure, according to one aspect of the disclosure. A cube-corner 370 has 6-fold rotational symmetry (i.e. six rotational axes of symmetry), and the point source 330 appears as 6 image points 330a-33 Of located at the corners of a hexagon as shown in FIG. 3a. Point source 330 lies on linear light source

330L, positioned at alignment angle α to an axis of symmetry. Exemplary calculations for determining the position of each of the image points are discussed elsewhere in the disclosure.

FIG. 3b shows a relationship of image points 330g-330j to point source 330' for a four-sided pyramid microstructure, according to one aspect of the disclosure. A four-sided pyramid 370' casts 4 image points 330g-330j onto the corners of a square or diamond as shown in FIG. 3b. Point source 330 lies on linear light source 330L, positioned at alignment angle α to an axis of symmetry. Exemplary calculations for determining the position of each of the image points are discussed elsewhere in the disclosure. The two structures shown in FIGS 3a-3b, cube-corner and four-sided pyramid, are two of the simplest structures that can be tiled on a flat surface with 100% fill, i.e., the structures can be arranged so that there is no planar region between adjacent structures, across the entire surface.

Planar regions, or flats, can be used in the tiling design, in particular for a symmetric tile that does not (or cannot) fill the surface, but has interstitial flats. Typically, for both cube-corner and four-sided pyramids with 100% fill of the surface by tiling, interstitial flat areas are not considered. However, flats can be added to the cube-corners and four-sided pyramids by truncating the structure tips to create regions that are parallel to the planar first surface. Facets coplanar with the bottom planar surface of the microstructured film result in an image point located directly above the point source. FIGS 4a and 4b show a top view and side elevation view, respectively, of an exemplary truncated cube-corner microstructure 400. Truncated cube-corner microstructure 400 includes facets 490 and flats 480 parallel to first surface 405 of microstructured film 410. The truncated fraction "Tp" can be expressed as a decimal fraction of the peak height "h". The source is typically an extended shape, such as a linear CCFL, and each point on the lamp surface may be mapped to each image. The displacement direction and distance X is approximately the same for each point down the length of the source. Therefore, a linear source appears as a linear image oriented in the same direction as the source, but displaced a distance "X" from the source in a direction determined by the facet orientation. An analytical procedure can be used to determine the orientation directions and displacement distances from an extended source such as a line source. This procedure is described below, for the cube-corner structure shown in FIG 3a. The image points 330a- 330f in X₁, V₁ pairs for a single point source 330 located at x = 0, y = 0, are determined numerically. For the 6-fold cube-corner the image points lie at the vertices of a hexagon whose size depends upon the refractive index of the structured surface, the angle β, and the distance P as described elsewhere. The linear light source 330L can be represented as a line through the origin with equation y = x Cot (α), where α is the angle that the source line makes with the Y-axis.

The equation of a line through an image point that is perpendicular to the source line is y = - x Tan (α) + B. The parameter B is solved at the image point. One then solves for (x_s, y_s) at the intersection of the two lines. x_s = B /(cot(α) + tan(α)) , y_s = B x cot(α) /(cot(α) + tan(α)) The displacement distance from an image point to the linear source is di is given by:

The image displacement for each of the facets of the structured surface can be determined by following the above procedure. The angle α is then changed until the images are equally spaced from each other and the source.

The procedure can be generalized to structures with other rotational symmetries. In one embodiment, translational symmetric tile forms can have 2-fold, 4-fold, or 6-fold rotational symmetries, such as included in linear prisms, and 3, 4, or 6-sided pyramids. In another embodiment, an object of any rotational symmetry can be placed within a tile, but the translational symmetry will be that of the tile lattice. As a result, there will be flat or unfilled areas of the surface, which can be useful, e.g., to add an additional image of the source. In yet another embodiment, the surface can be filled by overlapping objects that have a different symmetry than that of the tile lattice. As a result, the image will have features of the tile lattice symmetry in addition to that of the object symmetry.

According to one embodiment, the orientation of the line source and the cube- corner tile is α =19.1° for 6 evenly spaced images. A seventh image can be generated directly above the light source, by truncating the cube-corners into flats. According to another embodiment, the orientation of the line source and a 4-sided pyramid tile is α =26.55° for 4 evenly spaced images. A fifth image can be generated directly above the light source, by truncating the four-sided pyramids into flats. Additional films and diffuser sheets may be added between the cube-corner film and the viewer to further homogenize the light emitted from the backlight. In the absence of the microstructured film, the viewer would see only the image represented by the light source and none of the images.

Examples

The performance of the backlight was modeled using an optical raytrace program. The optical raytrace program provides results comparable to public commercial raytrace software, such as TracePro® (available from Lambda Research Corp., Littleton MA), and LightTools® (available from Optical Research Associates, Pasadena CA). A model of a 16-CCFL backlight was developed to simulate the cube-corner film in a backlight. FIG. 5 shows the layout and dimensions of components in the backlight simulation.

In FIG. 5, backlight 500 includes microstructured film 510 having cube-corner microstructures 570 tiling the surface. FIG. 5a shows a schematic cross section of an individual cube-corner microstructure 570 which includes base angle β, and is optionally truncated with flats 580 parallel to first surface 505 of microstructured film 510. The truncated fraction "Tp" is a fraction of peak height "h" of microstructure 570. Optical sheet 560 supports microstructured film 510 and provides thermal resistance, as described elsewhere. A plurality of linear light sources 530 are positioned in cavity 525 having depth "D" defined between microstructured film 510 and diffuse reflector 520. The linear light sources 530 have diameter "d" (in the model, d = 3 mm), and are separated "S" apart, "H" from the back reflector, and "P" from the first surface 505 of microstructured film 510. The diffuse reflector 520 was set to reflectance R = 0% or R = 97% to compare the appearance with or without a reflector. The structural parameters of a cube-corner microstructured film are shown in

Table 1. The peak height (0.0408 mm) and the base facet length (0.1 mm) of the structure are linked to the base angle β (54.7°) and the corner face angle (90°). When the peak height changes, an alternative rhombohedron corner structure results, but the corner face angle is no longer 90° and the base angle is no longer 54.7°. Rhombohedron corners can be useful embodiments, as described elsewhere. The peak may be truncated parallel to the planar surface of the microstructured film, such that a flat occurs as shown in FIG. 5. The parameter Tp is the fractional height at which the peak is removed.

Table 1

The relationship between the maximum image displacement, CdM, from the CCFL and the center to center spacing S of the CCFLs can be considered in the backlight design.

C is a geometric factor close to 1 (e.g., C = 0.98 for the cube-corner; for the four-sided pyramid C = 0.89). The parameter C provides the relationship between the global maximum image displacement and the maximum displacement for the prism orientation with respect to the bulb axis. The global maximum image displacement is the image displacement when a facet is aligned parallel with the bulb axis. Therefore, when the optimum orientation angle does not place a facet parallel to the lamp axis, the maximum image displacement is less than the global maximum. In the case of the cube-corner and 7 evenly spaced images (the seventh image is the source imaged through the truncated cube- corner), CdM = (3/7) S. For the four- sided pyramid and 5 evenly spaced images (the fifth image is the source imaged through the truncated pyramid) CdM = (2/5) S. A generalized representation for the maximum image displacement for N evenly spaced images CdM = ((N-1)/(2N)) S.

Modeling simulations were run by tracing normal rays (such as ray 234 in FIG 2) backward into the backlight from a distance above the structured film. The optical raytrace program calculates the brightness observed at the launch point in the direction of launch; therefore, a point- wise linear scan simulates the cross-sectional brightness of the backlight observed by a viewer located above the microstructured film. The backlight brightness was scanned over one cycle from the midpoint between neighboring lamps up to the next midpoint. A lamp is positioned in the center at x = 0. The model parameters are tabulated in Table 2, and refer to the parameters described in FIG 5. The 4 scans represent orientation angles α, of the cube-corner structure with respect to the lamp direction, between 0° and 30°.

Table 2

The orientation angle α = 0° resulted in three evenly spaced lamp images, two with double the brightness of a primary lamp image from one of the facets. The center image had additional brightness due to the truncation of the cube-corner peaks, causing light to leak through to the center. For not-truncated peaks, the lamp images were equally bright. The orientation angle α = 15° resulted in overlap and superposition of the images of the lamp. Three wide images resulted: two overlapping doublets centered at around x = +8 mm and x = -8 mm and one overlapping triplet centered at x = 0 mm.

The orientation angle α = 19.1° resulted in multiple images split into 7 evenly spaced images. This orientation provides an orientation angle for uniform brightness and spacing of the images.

The orientation angle α = 30° resulted in five evenly spaced images. Two of the images were centered at + 5 mm and -5 mm and are double brightness, due to the overlap of 2 primary images.

The model was then run with the back reflector turned on (i.e., the value of R was set to 97% reflectivity). The average brightness increases, but the fine structure described above remained. There was a slight increase of the brightness toward the midpoints and away from lamp positioned at x = 0. The model was run again, with the orientation angle fixed at α = 19.1°. The truncation factor Tp was varied from 0.6 to 0.75, and a truncation factor of Tp = 0.65 was determined to be approximately optimum for uniform brightness.

The model was run to determine optimal designs for different cavity depths ranging from 13.8 mm up to 22.7 mm based upon the variation of the corner peak height from 0.03 mm to 0.05 mm. The reflectivity of the back surface was set to R = 0%. The resultant brightness scans are nearly identical for the parameter sets shown in Table 3.

Table 3

The ray trace examples above were based upon a lamp array spacing of S = 23.7 mm. However, results are general for a given ratio S/P. Small differences are expected if other dimensions such as H or the lamp diameter are not scaled accordingly. Additional components such as diffuser sheets, gain films, or a diffuser plate are expected to modify the results and can improve uniformity, especially off-axis (i.e., viewed from locations other than directly above the backlight). However, the optimal structured film design may change, since diffuse light or light refracted by additional elements can divert light in a more complicated manner. For example, a strong diffuser plate tends to concentrate light closer to the lamp, and may require a raytrace model including all of the critical optical components. In addition, variations in the cavity depth can squeeze or stretch the images, which can cause abrupt holes or peaks in the lamp array image. Diffusion may soften the overlap of neighboring bands.

FIG. 6 is a top view photograph of a backlight 600 according to one aspect of the description. A portion of backlight 600 includes a microstructured sheet to show the images formed by the light sources. In FIG. 6, the microstructured sheet included cube- corners (similar to the structure shown in FIG. 3a and FIG. 5a) tiled to cover the entire surface of the microstructured sheet. Each of the edges of the cube-corner base measured 227 microns, the height of the cube corner was 89 microns (distance "h" in FIG. 5a), the base angle "β" was 57.5 degrees, and the angle "α" was 15 degrees. The lamp diameter was approximately 3.4 mm (distance "d" in FIG. 5) and was approximately 15-18 mm from an acrylic sheet supporting the microstructured sheet (distance "P" in FIG. 5). The lamps were separated by approximately 35 mm (distance "S" in FIG. 5). Backlight 600 includes a first portion 605 which shows a first, second and third light source 620, 630 and 640. A second portion 610 of backlight 600 is covered by a microstructured sheet placed over the first, second and third light sources 620, 630 and 640. Multiple images of each of light source appear in second portion 610. For clarity, second light source 630 and first through sixth images 630a - 63Of associated with second light source 630 are labeled in FIG. 6.

The microstructured film can be made by any of the techniques known for production of microreplicated surfaces. Masters for microreplication can be manufactured by plunge cutting a flat copper surface with a v- shaped diamond tool in slotted rows. Cuts in two perpendicular directions form 4-sided pyramids. Cuts in three directions, rotated 60° to one another, form corner peaks. Patterned surfaces can replicated on plastic by thermo-compression, extrusion replication, cast and cure, and other techniques. Microreplicated films may be adhered to rigid plates, or the microstructures may be formed directly on a mechanically stable plate.

According to one embodiment, a curved prism face can be formed using curved facet tools. A curved prism face can be used to broaden the image bands. Optical design and ray-tracing can determine the curved surfaces that provide improved uniformity of the images. According to another embodiment, the base angles can be varied within a design. Diverse base angles can generate multiple image displacement distances. The diverse base angles can be formed with multiple tools having different apex angles and/or a single tool that is tilted at various angles during cutting of the master. According to another embodiment, "chaos cuts" can be used during fabrication of the master, to further spread the images. Chaos cuts can result from regular or randomized oscillation of the diamond tool as it cuts.

The embodiments described can be applied anywhere that thin, optically transmissive structures are used, including displays such as TV, notebook and monitors, and used for advertising, information display or lighting. The present disclosure is also applicable to electronic devices including laptop computers and handheld devices such as Personal Data Assistants (PDAs), personal gaming devices, cellphones, personal media players, handheld computers and the like, which incorporate optical displays. The backlights of the present disclosure have application in many other areas. For example, backlit LCD systems, luminaires, task lights, light sources, signs and point of purchase displays can be made using the described embodiments.

Unless otherwise indicated, all numbers expressing feature sizes, amounts, and physical properties used in the specification and claims are to be understood as being modified by the term "about." Accordingly, unless indicated to the contrary, the numerical parameters set forth in the foregoing specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings disclosed herein.

All references and publications cited herein are expressly incorporated herein by reference in their entirety into this disclosure, except to the extent they may directly contradict this disclosure. Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations can be substituted for the specific embodiments shown and described without departing from the scope of the present disclosure. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this disclosure be limited only by the claims and the equivalents thereof.

Claims

What is claimed is:

1. A backlight, comprising: a microstructured film, comprising: a first surface; a second surface opposite the first surface, the second surface comprising an array of microstructures; a reflector facing the first surface, defining a cavity between the reflector and the microstructured film; a first linear light source disposed in the cavity, parallel to the microstructured film; and an image receptor facing the second surface of the microstructured film, wherein the microstructures are positioned so that light from the first linear light source is refracted through the microstructures to form a first plurality of images on the image receptor.

2. The backlight of claim 1 , wherein the image receptor comprises a diffuser.

3. The backlight of claim 2, wherein the diffuser comprises a bulk diffuser or a surface diffuser.

4. The backlight of claim 1 , wherein the image receptor comprises a pair of crossed brightness enhancement films.

5. The backlight of claim 1, wherein the array of microstructures comprises at least two rotational axes of symmetry.

6. The backlight of claim 1 , further comprising an optically transparent film disposed between the linear light source and the first surface.

7. The backlight of claim 1, wherein the first plurality of images have a substantially equal brightness.

8. The backlight of claim 1, wherein the first plurality of images are substantially equally spaced apart.

9. The backlight of claim 1, wherein microstructures of the array of microstructures are selected from the group consisting of linear prisms, pyramids, truncated pyramids, and combinations thereof.

10. The backlight of claim 1, wherein each of the microstructures of the array of microstructures comprises a plurality of faces, and at least one of the plurality of faces is planar.

11. The backlight of claim 1 , wherein each of the microstructures of the array of microstructures comprises a plurality of faces, and at least one of the plurality of faces is curved.

12. The backlight of claim 1, wherein at least a portion of microstructures of the array of microstructures are separated by planar regions devoid of microstructure.

13. The backlight of claim 1 , wherein microstructures of the array of microstructures are tiled to cover the second surface of the microstructured film.

14. The backlight of claim 1, further comprising a second linear light source disposed parallel to the first linear light source, both light sources positioned at a height P from the microstructured film and spaced apart by a separation S, so that light from the second linear light source is refracted through the microstructures to form a second plurality of images on the image receptor.

15. The backlight of claim 14, wherein the first plurality of images are adjacent the second plurality of images.

16. The backlight of claim 14, wherein at least a portion of the first plurality of images and the second plurality of images are interleaved.

17. The backlight of claim 15 or claim 16, wherein each of the first plurality of images and the second plurality of images have a substantially equal brightness.

18. The backlight of claim 17, wherein the brightness of each image varies less than 5% from an average brightness of all images.

19. The backlight of claim 15 or claim 16, wherein each of the first plurality of images and the second plurality of images are substantially equally spaced apart.

20. The backlight of claim 14, wherein the cavity further has a depth D, and a ratio S/D ranges from 2 to 5.

21. A method of assembling a backlight, comprising: providing a microstructured film, comprising: a first surface; a second surface opposite the first surface, the second surface comprising an array of microstructures; disposing a reflector facing the first surface to define a cavity between the reflector and the microstructured film; disposing a linear light source within the cavity, parallel to the microstructured film; disposing an image receptor facing the second surface of the microstructured film; and rotating the microstructured film around a line normal to the reflector so that a plurality of images of the linear light source are refracted through the microstructures to the image receptor.

22. The method of claim 21 , wherein each of the plurality of images have a substantially equal brightness.

23. The method of claim 21 , wherein the plurality of images are substantially equally spaced apart.

24. The method of claim 21 , wherein the image receptor is a diffuser.

25. A display, comprising: the backlight of claim 1; a liquid crystal display module disposed facing the image receptor, opposite the microstructured film.