Patent Publication Number: US-8971680-B2

Title: Waveguide with Controlled Light Collimation

Description:
RELATED APPLICATIONS 
     This application is a Continuation-in-Part of an application entitled, ULTRA-THIN WAVEGUIDE WITH CONTROLLED LIGHT EXTRACTION, invented by Jiandong Huang et al., Ser. No. 13/484,346, filed May 31, 2012 now U.S. Pat. No. 8,630,518, 
     which is a Continuation-in-Part of an application entitled, METHOD FOR THE DESIGN OF UNIFORM WAVEGUIDE LIGHT EXTRACTION, invented by Jiandong Huang et al., Ser. No. 13/477,922, filed May 22, 2012. Both these applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention generally relates to light waveguide mediums and, more particularly, to a system and method for controlling the collimation of light extracted from a backlight device. 
     2. Description of the Related Art 
       FIG. 1  is a plan view of representing light extracted from a liquid crystal display (LCD) backlight (prior art). Mura is a Japanese term for unevenness, inconsistency in physical matter, or human spiritual condition. This word is used in LCD to describe undesired illumination non-uniformity due to design or fabrication defects. Mura can come from both front and back panels. As shown in the figure, more light is being extracted near the input light emitting devices (LEDs) on the left side of the panel, than on the right side of the panel. The significant amount of light extracted near the light source leaves an insufficient amount of light to be extracted from the right side of the panel. Backlight panels are conventionally designed using a significant degree of trial-and-error to find the correct balance of light extraction and illumination. 
       FIG. 2  is a partial cross-sectional view of a liquid crystal display (LCD) backlight system (prior art). Ideally, the system is intended to extract and collimate light (from the light source) up, through the waveguide top surface, to illuminate an LC panel (not shown). 
       FIG. 3  is a diagram comparing the intensity of light extracted from the waveguide top and bottom surfaces (prior art). Often, as shown in  FIG. 2 , a reflection pattern is added to the bottom of the waveguide to minimize the amount of light exiting the waveguide through the bottom surface. Alternatively or in addition, as shown in  FIG. 2 , a reflector can be added under the waveguide bottom surface. However, both these solutions undesirably increase the thickness and complexity of the backlight system. 
       FIG. 4  is a partial cross-sectional view of a waveguide with a light extraction feature (prior art). With an LED  400  light input of 0° (parallel to the waveguide top surface  404 ), the maximum angle of light propagation α′ inside the waveguide is:
 
Ω˜sin −1 (1/n W )
 
     where n W  is the index of waveguides. For a conventional backlight polymer with a refractive index ˜1.49, the angle Ω is roughly 42°. Light rays within the cone of ±42° (where horizontal is 0°) result is light rays exiting the waveguide top surface in a cone of ±48° (where vertical is 0°). However, in different situations, either greater or lesser amounts of light collimation are desired. 
     With the addition of air bubble light extraction features  406  and a design that avoids the critical angle for total internal reflection, the light rays within the horizontal cone may be directed toward the top surface for light extraction with a greater degree of dispersion. In the best case dispersion scenario, light inside this angular cone of ±42° (horizontal) can be extracted within ˜±32° (vertical). Increasing the dispersion of extracted light is useful for wide angle viewing. 
       FIG. 5  is a partial cross-sectional view of a waveguide light extraction feature with various angles of incident light (prior art). Depending on where light strikes the bubble structure, it may be reflected to the waveguide bottom surface (the rays marked “ 1 ”), extracted from the waveguide top surface due to total internal reflection (the rays marked “ 2 ”), or realigned at an angle where it is likely to strike another bubble structure at a favorable angle (the rays marked “ 3 ”). Conventionally, rays ( 1 ) reflected to the waveguide bottom surface have been an undesirable limitation associated with the use of light extraction features. 
     As noted in the application entitled, ULTRA-THIN WAVEGUIDE WITH CONTROLLED LIGHT EXTRACTION, invented by Jiandong Huang et al., Ser. No. 13/484,346, filed May 31, 2012, bubble structures can be used to enhance the degree of light collimation, as might be useful in narrow (private) angle viewing. 
     It would be advantageous if backlight panels and waveguide devices could be more efficiently designed to control the collimation of extracted light from a specified waveguide surface. 
     SUMMARY OF THE INVENTION 
     Disclosed herein are a system and method that take advantage of the shape and spatial arrangement of bubble structures in a waveguide, in combination with lenses, to control the angles of light collimation from the front side (top surface) of a backlight device. 
     Accordingly, a method is provided for controlling the collimation of light from a backlight top surface. A backlight device is used, which includes a first waveguide and a transparent top film overlying the first waveguide top surface. A plurality of bubble structures is formed in the top film bottom surface, having a refractive index less than a first waveguide medium. The bubble structures have a base and sides formed at an acute angle upwards with respect to the base, are separated by gap (W), and have a height (H). A plurality of lenses overlies the top film top surface, where each lens is aligned overlying a corresponding gap (W). A maximum angle (α) of light propagation is formed through the first waveguide medium relative to a horizontal direction parallel to the first waveguide top surface. In response to the values W and H, light, having the maximum angle (α) of light propagation, is reflected off the bubble structure sides into the top film. The method collimates in a vertical direction, orthogonal to the horizontal direction, light exiting the top film through the lenses. At least part of the angle of collimation is due lens radius of curvature and lens cross-sectional area. Light having an angle of propagation through the first waveguide medium of less than the maximum angle (α), and greater than a minimum angle (B) reflects off the bubble structure sides. Otherwise, light intercepting the top film bottom surface at less than a critical angle for total internal reflection (TIR), less than the minimum angle (B), is reflected back into the first waveguide. 
     In another aspect, the backlight device includes a second waveguide with a top surface underlying first waveguide bottom surface. For wider dispersion light extraction, the first waveguide accepts light from the second waveguide, having a worst case vertical angle orthogonal the horizontal direction, and distributes the second waveguide light from the lenses in a plurality of directions between the vertical direction and the horizontal direction, in response to passing through the lenses. 
     Additional details of the above-described method, and a backlight device with controlled light collimation, are provided below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of representing light extracted from a liquid crystal display (LCD) backlight (prior art). 
         FIG. 2  is a partial cross-sectional view of a liquid crystal display (LCD) backlight system (prior art). 
         FIG. 3  is a diagram comparing the intensity of light extracted from the waveguide top and bottom surfaces (prior art). 
         FIG. 4  is a partial cross-sectional view of a waveguide with a light extraction feature (prior art). 
         FIG. 5  is a partial cross-sectional view of a waveguide light extraction feature with various angles of incident light (prior art). 
         FIG. 6  is a partial cross-sectional view of a backlight device with controlled light collimation. 
         FIG. 7  is a more detailed depiction of the backlight device of  FIG. 6 . 
         FIG. 8  is a partial cross-sectional view depicting a variation of the backlight device of  FIG. 6 . 
         FIG. 9  is a graph of the percentage of light power exiting the waveguide top surface as a function of angle with respect to the normal (vertical) direction, contrasting the use of a lens with no lens. 
         FIGS. 10A  though  10 C depict a process for fabricating the backlight device of  FIGS. 6 and 7 . 
         FIG. 11  is a flowchart illustrating a method for controlling the collimation of light from a backlight top surface. 
         FIG. 12  is a partial cross-sectional view depicting the backlight device being used to create narrow and wide angles of collimation. 
         FIG. 13  is a graph comparing viewing angles (angles of collimation). 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 6  is a partial cross-sectional view of a backlight device with controlled light collimation. The backlight device  600  comprises a first waveguide  602  with a top surface  604  and a bottom surface  606 . A transparent top film  608  has a top surface  610 , and a bottom surface  612  overlying the first waveguide top surface  604 . A plurality of bubble structures  614  is formed in the top film bottom surface  612 , having a refractive index less than a first waveguide medium. For example, if the first waveguide is made from a polymer or glass, then it has an index of refraction (n) of about 1.5. Then, the bubble structures can be made from air, having a refractive index of 1. Other gases and materials may also be used to form the bubble structures. 
     The bubble structures  614  have a base  616  and sides  618  formed at an acute angle  620  upwards (i.e. towards the top surface  610 ) with respect to the base. The bubble structures  614  are separated by gap (W)  622 , and have a height (H)  624 . The bubble structures are frustum bubble structures with flat top surfaces  625  parallel to the waveguide top surface  604 . Some examples of frustum structures include a frustum-pyramid (as shown) and a frustum-cone. Other bubble structures need not have a flat top surface. A plurality of lenses  626  overlies the top film top surface  610 , where each lens is aligned overlying a corresponding gap (W)  622 . 
     W  622  and H  624  are defined with respect to a maximum angle (α) of light propagation  628  through the first waveguide medium relative to a horizontal direction  630  parallel to the first waveguide top surface  604 . The lenses  626  accept light reflected off the bubble structure sides  616  into the top film  608 , and collimate the light in a vertical direction  632 , orthogonal to the horizontal direction  630 . Although the values of W and H have some effect on the angles of collimation, they have a greater effect on the spread of light rays propagating though the first waveguide that can be collected and reflected up towards the lenses. As used herein, the word “collimate” is defined as tending to confine light into a narrow range of extraction angles. It should also be understood that while collimated light may ideally be extracted from the backlight device in only the vertical direction  632 , practically there exists a rage of collimation angles±the vertical direction. 
     In one aspect as shown, the lenses  626  are plano-convex lenses, where the planar lens surface  634  overlies the top film top surface  610 , and where light exits the lens convex surface  636  with an angle of collimation  638  responsive to the convex radius of curvature and lens cross-sectional area  640 . In the case of a circular lens, the cross-sectional area  640  is a diameter. 
       FIG. 7  is a more detailed depiction of the backlight device of  FIG. 6 . The bubble structure sides  618  reflect light into the top film  608 , accepted at an angle of light propagation through the first waveguide medium that is less than the maximum angle (α), and greater than a minimum angle (B). Rays with incidental angles outside this range are not fully collimated, resulting in a slightly broader distribution of angles than 0° (vertical). The distribution angles can be broadened by tuning the angle of the bubble structure sides  618  and the lens&#39; radius of curvature. As shown, rays associated with angle (α)  700  are marked  700  and rays associated with angle (B) and marked  702 . The maximum light intensity from the light source (not shown) into the first waveguide panel  602  is presented in the horizontal direction  630 . 
     Each lens  626  forms a set of reflected focal points  704  in the first waveguide medium. Each reflected focal point  704  is associated with reflections off a corresponding bubble structure side  618 , incident to the sides with angles greater than the critical angle for total internal reflection, so that stray light rays and losses are minimized. The reflected focal points  704  are located under that bubble structure  614  from which the light rays are reflected. The first waveguide  602  has a thickness  706 . The lens reflected focal points  704  are formed at a first distance  708  from the first waveguide top surface  604 , less than the first waveguide thickness  706 . Also shown is the lens focal position  710  for rays that would reach the lens  626  without reflecting off a bubble structure side  618 . The angles of these rays (about 90° with respect to horizontal  630 ) are greater than (α)  700  and, therefore, do not occur as a result of the first waveguide light source  712  through regular coupling. However, as explained in greater detail below, light rays with these angles may be sourced from an optional underlying second waveguide. 
     It can be seen from the figure that light rays not passing through the reflected focal points  704 , and therefore not extracted by reflection off the bubble structure sides  618 , would encounter the bubble structure bottom surfaces  616 . Thus, light extraction is a function of the aperture (W  622 ) between bubble structures  614 . The bubble structure bottom surfaces  616  reflect incident light since the light rays are at less than a critical angle for total internal reflection (TIR). That is, the critical angle for TIR is less than, or equal to the minimum angle (B)  702 . 
     The critical angle, in accordance with Snell&#39;s law, is the angle of incidence above which total internal reflection occurs. For a light ray passing from glass into air, the light emanating from the interface is bent. When the incident angle is increased sufficiently, the transmitted angle in air reaches 90 degrees, e.g., no light is transmitted into the bubble structure, but rather, is reflected. The critical angle can be found as follows:
 
n 1  sin θ i  =n 2  sin θ t .
 
     where n 1  is the refractive index of first waveguide  602  and n 2  is the refractive index of the bubble structure  614 . Rearranging, the angle of incidence is: 
     
       
         
           
             
               
                 
                   
                     
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       FIG. 8  is a partial cross-sectional view depicting a variation of the backlight device of  FIG. 6 . In this aspect the backlight device  600  further comprises a second waveguide  800  with a top surface  802  underlying first waveguide bottom surface  606 . Typically, the waveguides are separated by a material  803  having a lower index of refraction than either of the waveguides. In the worst case scenario, the first waveguide  602  accepts light  804  from the second waveguide, having a vertical angle orthogonal the horizontal direction  630 . The lenses  626  distribute light from the second waveguide in a plurality of directions between the vertical direction  632  and the horizontal direction  630 . It should be understood that any light sourced from the second waveguide that is not vertically aligned, it is also distributed in a plurality of directions and angles exiting the lenses. 
     W  622  and H  624  can be defined to randomize the distribution of second waveguide light meeting the bubble structure bottom surfaces  616  with incident angles greater than the critical angle for TIR. Then, the lenses  626  further randomize the distribution of second waveguide light passing through the bubble structures  614 . Further, the lenses  626  distribute second waveguide light in a plurality of directions responsive the radius of curvature of the lenses and the lenses&#39; cross-sectional area  640 . 
       FIG. 12  is a partial cross-sectional view depicting the backlight device being used to create narrow and wide angles of collimation. Thus, the first light source  712  adjacent to first waveguide side  806  can be energized to supply light collimated in a first range of angles  1200 , defined with respect to the vertical direction. A second light source  808  adjacent to the second waveguide side  810  can be energized to supply light distributed over a second range of angles, greater than the first range. 
       FIG. 13  is a graph comparing viewing angles (angles of collimation). 
     Functional Description 
       FIG. 9  is a graph of the percentage of light power exiting the waveguide top surface as a function of angle with respect to the normal (vertical) direction, contrasting the use of a lens with no lens. The flat surfaces of the bubble structures are used to bend the focal positions inside the waveguide to collect light rays with an angle of less than (α) and greater than (B). In order to gauge the improvements, half magnitude points (50% of total power) for the angles of collimation were contrasted as a means of measuring design progresses. The same bubble structure design was compared with, and without, the use of lenses (see  FIG. 6 ). At a 50% Full Width at Half Maximum (FWHM) light energy level, ˜30 degrees of collimation half angular width were required for the design with no lenses, while only ˜13 degrees of collimation half angular width were required for the same design with lenses. 
       FIGS. 10A  though  10 C depict a process for fabricating the backlight device of  FIGS. 6 and 7 . The process begins with the transparent top film  606 . As shown in  FIG. 10B , the lenses are adhered to one side (i.e. the top surface) of the top film  608  and forms  1000 , in the reverse shape of the bubble structures, are adhered to the top film bottom surface. As shown in  FIG. 10A  a compression process is shown to implant the forms into the top film, creating the bubble structures  614  between the forms. Then, the first waveguide is added. A detailed depiction is shown in  FIG. 10C . 
       FIG. 11  is a flowchart illustrating a method for controlling the collimation of light from a backlight top surface. Although the method is depicted as a sequence of numbered steps for clarity, the numbering does not necessarily dictate the order of the steps. It should be understood that some of these steps may be skipped, performed in parallel, or performed without the requirement of maintaining a strict order of sequence. Generally however, the method follows the numeric order of the depicted steps. The method starts at Step  1100 . 
     Step  1102  provides a backlight device comprising a first waveguide with a top surface and a bottom surface, and a transparent top film with a top surface, and a bottom surface overlying the first waveguide top surface. Step  1102  also provides a plurality of bubble structures formed in the top film bottom-surface, having a refractive index less than a first waveguide medium. If the first waveguide medium is polymer or gas, the bubble structures may be an air medium. The bubble structures have a base and sides formed at an acute angle upwards with respect to the base, are separated by gap (W), and have a height (H). Further, a plurality of lenses overlies the top film top surface, where each lens is aligned over a corresponding gap (W). Step  1104  forms a maximum angle (α) of light propagation through the first waveguide medium relative to a horizontal direction parallel to the first waveguide top surface. In response to W and H values, Step  1106  reflects light, having the maximum angle (α) of light propagation, off the bubble structure sides into the top film. Step  1108  collimates in a vertical direction, orthogonal to the horizontal direction, light exiting the top film through the lenses. In one aspect, Step  1102  provides plano-convex lenses, where the planar lens surface overlies the top film top surface. Then, collimating light in Step  1108  includes controlling the angle of collimation in response to the convex radius of curvature and lens cross-sectional area. 
     In one aspect, reflecting light off the bubble structure sides in Step  1106  includes reflecting light off the bubble structure sides, having an angle of light propagation through the first waveguide medium less than the maximum angle (α), and greater than a minimum angle (B). Further, Step  1106  forms a set of reflected focal points in the first waveguide medium for each lens, where each reflected focal point is associated with reflections off a corresponding bubble structure side, and located under the reflecting bubble structure. The reflected focal points are formed a first distance from the first waveguide top surface, less than the first waveguide thickness. 
     In another aspect, reflecting light off the bubble structure sides in Step  1106  additionally includes the reflecting light intercepting the top film bottom surface at less than a critical angle for TIR, less than the minimum angle (B). 
     In one aspect, Step  1102  additionally provides a second waveguide with a top surface underlying first waveguide bottom surface. Then, in Step  1110  the first waveguide accepts light from the second waveguide, having a worst case vertical angle orthogonal the horizontal direction. Step  1112  distributes the second waveguide light from the lenses in a plurality of directions between the vertical direction and the horizontal direction, in response to passing through the lenses. In another aspect, in response to W and H values, Step  1111  randomizes the distribution of second waveguide light meeting the bubble structure bottom surfaces with incident angles greater than the critical angle for TIR. In other words, H and W may be adjusted to effect the distribution (range) of second waveguide light ray angles occurring as a result of intercepting the bubble structures. Then, Step  1112  randomizes the distribution passing through the lenses. That is, if the lenses receive a greater distribution of light ray angles, the lenses further increase the range of light ray angles exiting the lenses. Further, the distribution of second waveguide light from the lenses is responsive a radius of curvature of the lenses and the lenses&#39; cross-sectional area. 
     In one aspect, Step  1103  energizes the first waveguide light source to supply light collimated in a first range of angles, defined with respect to the vertical direction, and Step  1109  energizes the second waveguide light source supplies light distributed over a second range of angles, greater than the first range. 
     A backlight device and method for controlling light collimation have been presented. Examples of particular bubble structures and lens shapes have been presented to illustrate the invention. However, the invention is not limited to merely these examples. Other variations and embodiments of the invention will occur to those skilled in the art.