Patent Publication Number: US-7221847-B2

Title: Optical elements having programmed optical structures

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional of U.S. Ser. No. 09/415,471, filed Oct. 8, 1999, now U.S. Pat. No. 6,845,212, the disclosure of which is herein incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates generally to optical elements and more particularly to lightguides, optical films and other optical elements suitable for use in display devices and having programmed optical structures. 
   2. Description of the Related Technology 
   Backlit display devices, such as liquid crystal display (LCD) devices, commonly use a wedge-shaped lightguide. The wedge-shaped lightguide couples light from a substantially linear source, such as a cold cathode fluorescent lamp (CCFL), to a substantially planar output. The planar output is then used to illuminate the LCD. 
   One measure of the performance of the backlit display is its uniformity. A user can easily perceive relatively small differences in brightness of a display from one area of the display to the next. Even relatively small non-uniformities can be very annoying to a user of the display. 
   Surface diffusers or bulk diffusers, which scatter the light exiting the lightguide, are sometimes used to mask or soften non-uniformities. However, this diffusion also results in light being directed away from a preferred viewing axis. A net result can be a reduction in overall brightness of the display along the preferred viewing axis, which is another performance measure of a display device. 
   Unlike non-uniformities, from a subjective standpoint relatively small increases or decreases in overall brightness are not easily perceived by the user of the display device. However, the display device designer is discouraged by even the smallest decreases in overall brightness including decreases so small they might only be perceived by objective measurement. This is because display brightness and power requirements of the display are closely related. If overall brightness can be increased without increasing the required power, the designer can actually allocate less power to the display device, yet still achieve an acceptable level of brightness. For battery powered portable devices, this translates to longer running times. 
   SUMMARY OF THE INVENTION 
   In accordance with the invention, an optical element, such as a lightguide or an optical film, is formed with a predetermined, programmed pattern of optical structures. The optical structures may be arranged to selectively correct for non-uniformities in the output of a lightguide, or may be arranged to otherwise effect the performance of the display in a predetermined, and designed manner. 
   In a first aspect of the invention, an optically transmissive film having a first surface and a second surface and a first edge and a second edge is formed with a plurality of optical structures formed in the first side. The plurality of optical structures are arranged on the first side in a predetermined pattern, and each optical structure has at least one characteristic selected from the group consisting of an amplitude, a period and an aspect ratio. Each characteristic has a first value for a first predetermined location on the film between the first edge and the second edge and the characteristic has a second value, different from the first value, for a second predetermined location on the film, different than the first predetermined location on the film, between the first edge and the second edge. 
   In another aspect of the invention, the structure in accordance with the invention is part of a thick optical element, such as for example, a lightguide wedge. The structure is achieved on the thick element through injection molding, compression molding, or by bonding a film with the structure to the additional optical element. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The many advantages and features of the present invention will become apparent to one of ordinary skill in the art from the following detailed description of several preferred embodiments of the invention with reference to the attached drawings wherein like reference numerals refer to like elements throughout and in which: 
       FIG. 1  is a perspective view of an illumination device adapted in accordance with an embodiment of the invention; 
       FIG. 2  is a perspective view of an optical film incorporating a programmed pattern of optical structures in accordance with one embodiment of the invention; 
       FIG. 3  is a perspective view of an optical film incorporating a programmed pattern of optical structures in accordance with another embodiment of the invention; 
       FIG. 4  is a perspective view of a lightguide wedge incorporating an in-phase programmed pattern of optical structures in accordance with another embodiment of the invention; 
       FIG. 5  is a cross-section view taken along line  5 — 5  of in  FIG. 4 ; 
       FIG. 6  is a perspective view of a lightguide wedge incorporating an out-of-phase programmed pattern of optical structures in accordance with another embodiment of the invention; 
       FIG. 7  is perspective view of a linear lens structure incorporating a programmed pattern of optical structures in accordance with another embodiment of the invention; 
       FIG. 8  is a perspective view of an optical film incorporating a programmed pattern of optical structures in accordances with an alternate preferred embodiment of the invention; 
       FIG. 9  is a perspective view of an optical film incorporating a programmed pattern of optical structures in accordances with an alternate preferred embodiment of the invention; 
       FIG. 10  is a perspective view of an optical film incorporating a programmed pattern of optical structures in accordances with an alternate preferred embodiment of the invention; 
       FIG. 11  is a side view of a lightguide incorporating first programmed pattern of optical structures in a top surface and a second programmed pattern of optical structures in a bottom surface in accordance with a preferred embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention is described in terms of several preferred embodiments, and particularly, in terms of an optical film or a lightguide suitable for use in a backlighting system typically used in flat panel display devices, such as a laptop computer display or a desktop flat panel display. The invention, however, is not so limited in application and one of ordinary skill in the art will appreciate that it has application to virtually any optical system, for example, to projection screen devices and flat panel televisions. Therefore, the embodiments described herein should not be taken as limiting of the broad scope of the invention. 
   Referring to  FIG. 1 , an illumination system  10  includes a light source  12 ; a light source reflector  14 ; a lightguide  16  with an output surface  18 , a back surface  20 , an input surface  21  and an end surface  22 ; a reflector  24  adjacent the back surface  20 ; a first light redirecting element  26 ; a second light redirecting element  28 ; and a reflective polarizer  30 . The lightguide  16  may be a wedge, a modification thereof or a slab. As is well known, the purpose of the lightguide is to provide for the distribution of light from the light source  12  over an area much larger than the light source  12 , and more particulary, substantially over an entire area formed by output surface  18 . The lightguide  16  further preferably accomplishes these tasks in a compact, thin package. 
   The light source  12  may be a CCFL that inputs light to the edge surface  21  of the lightguide  16 , and the lamp reflector  14  may be a reflective film that wraps around the light source  12  forming a lamp cavity. The reflector  24  backs the lightguide  16  and may be an efficient back reflector, e.g., a lambertian film or a specular film or a combination. 
   In the embodiment shown, the edge-coupled light propagates from the input surface  21  toward the end surface  22 , confined by total internal reflection (TIR). The light is extracted from the lightguide  16  by frustration of the TIR. A ray confined within the lightguide  16  increases its angle of incidence relative to the plane of the top and bottom walls, due to the wedge angle, with each TIR bounce. Thus, the light eventually refracts out of the output surface  18  and at a glancing angle thereto, because it is no longer contained by TIR. Some of the light rays are extracted out of the back surface  20 . These light rays are reflected back into and through the lightguide  16  by the back reflector  24 . First light redirecting element  26  is arranged as a turning film to redirect these light rays exiting the output surface  18  along a direction substantially parallel to a preferred viewing direction. 
   As shown in  FIG. 2 , the first light redirecting element  26  may be a light transmissive optical film with an output surface  32  and an input surface  34  formed with prisms (not shown), which refract and reflect the light exiting the lightguide  16  along the preferred viewing direction. The prisms may have a substantially uniform configuration, or may have a non-uniform configuration as described in commonly assigned U.S. patent application Ser. No. 90/415,873 “OPTICAL FILM WITH VARIABLE ANGLE PRISMS” filed of even date herewith, the disclosure of which is hereby expressly incorporated herein by reference. 
   Referring back to  FIG. 1 , the second light redirecting element  28  may not be required in every configuration of the illumination system  10 . When included in the system  10 , the second light redirecting element may be a diffuser, a lenticular spreader or a prism film, for example a brightness enhancing film such as the 3M Brightness Enhancement Film product (sold as BEFIII) available from Minnesota Mining and Manufacturing Company, St. Paul, Minn. The reflective polarizer  30  may be an inorganic, polymeric or cholesteric liquid crystal polarizer film. A suitable film is the 3M Diffuse Reflective Polarizer Film product (sold as DRPF) or the Specular Reflective Polarizer film product (sold as DBEF), both of which are available from Minnesota Mining and Manufacturing Company. Furthermore, at least the second light redirecting element  28  and the reflective polarizer  30 , and potentially the first light redirecting element  26 , may be combined into a single optical element. The commonly assigned U.S. patent application Ser. No. 09/415,471 entitled “DISPLAY ILLUMINATION DEVICE AND METHOD OF ENHANCING BRIGHTNESS IN A DISPLAY ILLUMINATION DEVICE” filed of even date herewith, the disclosure of which is hereby expressly incorporated herein by reference, describes several such combined optical structures. 
   With lightguides used for backlighting, such as lightguide  16 , it is common for there to be non-uniformities in the light output from the lightguide. These non-uniformities can frequently be concentrated near the input surface  21 . To mask these defects in applications of the lightguide, a diffuser that covers the output surface of the lightguide is typically used. However, a diffuser tends to reduce the overall brightness of the display and may not adequately mask all of the defects. 
   Referring now to  FIG. 2 , shown graphically is a film containing an in-phase varying amplitude pattern. The pattern described may be formed on a top or bottom surface of a wedge, on a plano film, or as described below, on a turning film. In that regard, in addition to the prisms formed on the input surface  34  of the first light redirecting element  26 , the output surface  32  may be formed with optical structures. More particularly, the first light redirecting element  26  has a first edge  36  and a second edge  38 . Extending from the first edge  36  toward the second edge  38  are a plurality of optical structures  40  arranged in a pattern  42 . Each optical structure  40  may have a number of characteristics, such as amplitude, period and aspect ratio of the peaks  44  and valleys  46 . The pattern  42  may also have characteristics, such as for example, a pitch, p, between optical structures  40 . The structures  40  in  FIG. 2  are shown having amplitude variation. In application of the first light redirecting structure  26 , the grooves may be arranged such that variation in amplitude is perpendicular to the lightsource  12 . 
   With continued reference to  FIG. 2 , it is observed that within the pattern  42 , the optical structures  40  are formed with larger amplitude at the first edge  36  and decrease in amplitude toward the second edge  38 . The larger amplitude produces more optical power along the groove axis because of the higher surface slopes. The optical power of this pattern then decreases as a function of the distance from the first edge  36 . This arrangement of the optical structures  40  and the pattern  42  is purposeful. As noted, non-uniformities in the output of lightguide  16  may be concentrated near the input surface  21  while there may be less non-uniformity farther from the input surface  21 . Thus, the optical structures  40  and the pattern  42  are arranged to provide more diffusion near first edge  36 . In application, first edge  36  will be disposed substantially adjacent the input surface  21  of the lightguide  16 . Pattern  42  may have a uniform pitch, p, as shown, and the depth of the optical structures  40  may decrease to naught toward the second edge  38 . This pattern, as will be discussed in more detail below, may be produced with any tool type. 
   It should be appreciated that using ray tracing and other analysis techniques, it is possible to determine particular arrangements for the optical structures  40  and the pattern  42  that best correct particular observed non-uniformities in the output of the lightguide  16 . That is, one or more of the characteristics of the optical structures  40  and the pattern  42  may be tailored to correct a particular non-uniformity. As described above, in connection with first light redirecting element  26 , the optical structures  40  and the pattern  42  provided optical power to the output of the lightguide  16  near the input surface  21  in order to mask non-uniformities that may occur near the input surface  21 . Less or no optical power is provided away from the input surface  21  as fewer or less intense non-uniformities are typically observed from the lightguide  16  farther from the input surface  21 . In this manner, optical power is provided where most needed to mask or soften non-uniformities, while less optical power is provided where there may be fewer non-uniformities to mask. Moreover, optical power may be added virtually anywhere to the output of the lightguide by adding optical structures and/or varying the characteristics of the optical structures. Furthermore, the addition of optical power need not be uniform. Instead, optical power may be added, as necessary, to discrete regions of the lightguide output if necessary to help mask a defect or create a particular optical effect. 
   Planar light guides, and some wedge light guides that operate using frustrated TIR, may include an extractor pattern on a back surface of the lightguide. Typically, the extractor pattern is a pattern of white dots disposed on the back surface of the lightguide. Light incident to one of the dots is diffusely reflected by the white dot, and a portion of this reflected light is caused to exit the light guide. In spite of the diffuse nature of this method of extracting light from the lightguide, the pattern of dots may itself be visible in the lightguide output. Thus, to hide the dot pattern, additional diffusion is typically provided. 
   With reference to  FIG. 3 , an extractor film  50  is shown. Formed in a surface  52  of the extractor film are a plurality of optical structures  54  disposed in a pattern  56 . The optical structures  54  are arranged essentially to replace the white dot pattern for providing extraction of light from the lightguide. While shown in  FIG. 3  as circles or dots, the optical structures  54  are not collectively limited to any particular shape nor are they limited to any one particular shape within the pattern  56 . Therefore, the optical structures  54  may be prisms, lines, dots, squares, ellipses or generally any shape. Moreover, the optical structures  54  may be spaced very closely together within the pattern  56 , much more so than the dots within a dot pattern may be spaced and, for example, within about 50–100 μm of each other. This very close spacing of the optical structures  54  eliminates or reduces the need for diffusion in the output of the lightguide that is ordinarily necessary to hide the pattern of white dots. The invention also permits the changing of the slope of the lightguide at a micro-level. That is, the slope of the lightguide may be locally increased or decreased at the micro-level. When a light ray hits a higher positive slope, it will be extracted from the lightguide faster than if it hit the nominal wedge angle. 
   While so far discussed in terms of optical films, the invention has application to the lightguide wedge itself. Referring to  FIGS. 4 and 5 , a lightguide  60  has in an input surface  62 , and an output surface  64  and a back surface  66 . The input surface  62  is arranged to be disposed adjacent a light source (not depicted) to provide a source of light incident to the input surface  62 . The light incident to the input surface  62  is extracted out of the output surface  64  as a result of frustrated TIR within the lightguide  60 . As discussed above, it is common for there to be non-uniformities in the light output from the lightguide  60 , particularly near the input surface  62 . 
   With continued reference to  FIGS. 4 and 5 , diffusion is added to the back surface  66  of the lightguide  60  and is further adjusted in intensity extending away from the input surface  62 . That is, the back surface  66  is formed with in-phase optical structures  68  arranged to provide diffusive extraction near the input surface  62  and to taper to naught away from the input surface  62 . The pattern can also be non-tapering, i.e., constant, over the entire surface, increasing from naught, randomly varying, or distributed in discrete regions. It is also possible for the optical structures to be out-of-phase, such as optical structures  68 ′ formed in a back surface  66 ′ of the lightguide  60 ′ shown in  FIG. 6 . It will be appreciated that patterns of optical structures may also be formed in the output surface  64  either separately or in conjunction with a pattern formed in the back surface  66 . The overall purpose of providing the optical structures is to achieve an effect that minimizes non-uniformities of the lightguide output wherever they may occur, and for the lightguide  60  shown in  FIGS. 4 and 5 , the non-uniformities appear primarily adjacent the input surface  62 . 
   With reference to  FIG. 5 , the optical structures  68  may be formed on a surface  72  of an optical film  70 . The optical film  70  may then be coupled to the wedge structure of the lightguide  60  using ultraviolet (UV) curing, pressure sensitive or any other suitable adhesive. Alternatively, the wedge may be molded in bulk to include the optical structures  68  in the back surface  66 . 
   As will be more generally appreciated from the foregoing discussion, virtually any configuration of optical structures may be formed into an optical film, and the optical film coupled, for example by bonding, to a lightguide or other bulk optical element. For example, glare reduction, anti-wetout, Fresnels, and virtually any other structure that may be formed in a surface of an optical film may be easily replicated into the film and then the film coupled to another optical element. 
   Films incorporating programmed optical structures may be manufactured using a microreplication process. In such a manufacturing process, a master is made, for example by cutting the pattern into a metal roll, and the master is used to produce films by extrusion, cast-and-cure, embossing and other suitable processes. Alternatively, the films may be compression or injection molded or roll formed. A preferred apparatus and method for microreplication is described in the commonly assigned U.S. patent application entitled “Optical Film With Defect-Reducing Surface and Method of Making Same,” Ser. No. 09/246,970, the disclosure of which is hereby expressly incorporated herein by reference. 
   As an example of the above-described feature of the invention, and with reference to  FIG. 7 , a linear Fresnel lens or prism  80  has a substantially planar input surface  82  and an output surface  84 . The output surface  84  is formed with lens structures  86  and superimposed on the lens structures  86  are additional optical structures  88 . The optical structures  88  have characteristics, such as amplitude, period, and aspect ratio, that vary from a first edge  90  of the lens  80  to a second edge  92  of the lens  80 . The lens  80  may be formed in bulk, or as shown in  FIG. 7 , the lens structures  86  including the optical structures  88  may be formed on a film  94  that is then bonded to a bulk optical substrate  96 . 
   Referring now to  FIG. 8 , shown graphically is a film  100  containing a varying amplitude pattern  102  that was formed using a “V” shaped tool. The pattern  102  may be formed on a top and/or bottom surface of the film  100 . Likewise, the pattern may be formed in a wedge or slab. The film  100  has a first edge  104  and a second edge  106 . Extending from the first edge  104  toward the second edge  106  are a plurality of optical structures  108  arranged in the pattern  102 . Each optical structure  108  may have a number of characteristics, such as amplitude, period and aspect ratio. The pattern  102  may also have characteristics, such as for example, a pitch, p, defining a spacing between optical structures  108 . The optical structures  108  in  FIG. 8  are shown having amplitude variation. In application of the film  100 , the grooves may be arranged such that variation in amplitude is perpendicular to a lightsource of the lightguide incorporating the film  100 . 
   With continued reference to  FIG. 8 , it is observed that within the pattern  102 , the optical structures  108  are formed with larger amplitude at the first edge  104  and decrease in amplitude toward the second edge  106 . The larger amplitude produces more optical power along the groove axis because of the higher surface slopes. The optical power of this pattern then decreases as a function of the distance from the first edge  104 . This arrangement of the optical structures  108  and the pattern  102  is purposeful. 
   With reference to  FIGS. 9 and 10 , films  110  and  112 , are shown respectively. Each film  110  and  112  has the same characteristics as film  100 , and like reference numerals are used to describe like elements therebetween. As opposed to the pattern created by using a “V” shaped tool, the film  110 ,  FIG. 9 , has a pattern  114  of optical structure  116  that is formed using a curved nose tool. The film  112 ,  FIG. 10 , has a pattern  118  of optical structures  120  that is formed using a square nose tool. The patterns  114  and  118  are arranged as described to provide optical power in the surface or surfaces of the films  110  and  112 . It will be appreciated that virtually any tool configuration may be used with the particular tool being selected to achieve a desired amount and form of optical power in the surface or surfaces of the film. 
   In the lightguide  121  illustrated in  FIG. 11 , a first pattern  122  of optical structures  124  is formed in a bottom surface  126  and a second pattern  128  of optical structures  130  is formed in a top surface  132  of the wedge  134 . The first pattern  122  may be arranged to facilitate the extraction of light from the wedge  134 , while the second pattern  128  may be arranged to mask non-uniformities in the light output from the wedge. It will be appreciated, however, that the patterns implemented in the wedge  134  will depend on the desired light output to be achieved from the wedge  134 . Moreover, as described above, the patterns  122  and  128  may be formed first in an optical film that is later coupled to the wedge, for example, by bonding. In another form, surfaces  122  and  128  are injection molded with the wedge. 
   Still other modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of the foregoing description. This description is to be construed as illustrative only, and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details of the structure and method may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which come within the scope of the appended claims is reserved.