Abstract:
A homogenizer for collimated light limits the angular distribution of the light by passing the light through a mild diffuser followed by a slab light guide which has top and bottom surfaces covered with optical constraining layers and optical absorbing layers where the optical absorbing layer has a higher refractive index than the optical constraining layer and the optical constraining layer has a lower refractive index than the slab light guide.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is based on U.S. Provisional Application Serial No. 60/439886, entitled “Homogenizer for Collimated Light With Controlled High Angle Scatter”, filed on Jan. 14, 2003, the teachings of which are incorporated herein by reference. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates to efficiently homogenizing collimated light entering a light guide and more specifically to backlighting a liquid crystal display (LCD).  
         BACKGROUND OF THE INVENTION  
         [0003]    It is known that the use of a collimated backlight and a front diffusing screen can greatly improve the quality of an LCD. One such approach is described in Saccomanno (U.S. Pat. No. 6,428,198), which is incorporated herein by reference. Saccomanno describes the use of an arc lamp, whose light is collected, homogenized, and coupled into an array of optical conduits. Each conduit then illuminates a non-imaging optic, which collimates the light and subsequently illuminates the edge of a light-extraction guide.  
           [0004]    Even though my prior patent teaches an effective collimated light and diffuser screen arrangement, for certain applications such as medical imaging there is a need to improve black-level contrast and image sharpness even at the expense of a slightly larger and less light efficient device.  
         SUMMARY OF THE INVENTION  
         [0005]    In accordance with my present invention, a mild diffuser, having controlled scattering angles, is placed at the input aperture of a slab light guide. This mild diffuser is inserted between the collimation source (e.g. non-imaging optics) and the light extraction guide. Unlike the diffusers that have been previously used in diffuse backlights, the diffuser in accordance with my invention has a controlled scattering angle of less than about eight degrees and most advantageously of less than ±5 degree full-width half-maximum (FWHM) scatter and is referred to herein as a ‘mild diffuser’ to contrast it from the prior art diffuser arrangements. The slab light guide further serves to homogenize the collimated beam. The slab light guide may be a separate element from the light extraction guide or the light extraction guide may have a “lead-in” portion that comprises a homogenizing slab section.  
           [0006]    This homogenizer technique is especially useful in overcoming irregularities due to periodic structures that supply the source of collimated light. Since any diffuser will naturally increase the overall beam divergence, an optical constraining layer, having a refractive index slightly less than the refractive index of the slab light guide, is positioned on one or more outer surfaces of the slab light guide. A light absorbing black layer is then positioned on the optical constraining layer or layers, the light absorbing layer having a higher refractive index than the slab light guide and the optical constraining layer. The result of this combination is that the slab light guide now can strip out high angle light.  
           [0007]    Such high angle light will cause increasing fuzziness between adjacent pixels and also cause a net lowering of the black-level contrast; this effect is described in Yamaguchi (U.S. Pat. No. 6,421,103). The light exiting the slab light guide is thus homogenized and stripped of high-angle light and can be fed into the light extraction guide, providing a uniform output. 
       
    
    
     DESCRIPTION OF THE DRAWING  
       [0008]    [0008]FIG. 1 illustrates a homogenizer in accordance with one illustrative embodiment of the present invention.  
         [0009]    [0009]FIG. 2 illustrates a homogenizer in accordance with my invention in combination with a wedge shaped light extraction guide.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0010]    Referring first to FIG. 1, an acrylic (although other optical quality materials may be used) slab light guide  12 , having a first refractive index, is covered on its top surface  43  with an optical constraining layer  15 , such as an acrylic pressure sensitive adhesive (PSA). A type of PSA that is suitable for my invention is Rexam OCAV3. The optical constraining layer  15  has a second refractive index, which is slightly less than the refractive index of the slab light guide  12 . In one embodiment of my invention, the acrylic slab light guide  12  has a refractive index of 1.4893 while the optical constraining layer  15  has a refractive index of 1.4800.  
         [0011]    Because of the slight difference in refractive index, the optical constraining layer  15  acts to trap light within the light guide under certain conditions. Accordingly, collimated light that enters the acrylic slab light guide  12  at surface  41  through a mild diffuser  11  with an angular spread below a certain threshold value is contained within the slab light guide  12  by total internal reflection (TIR). Light with an angular spread above the threshold value exits the slab light guide  12  and enters the optical constraining layer  15 . In embodiments of my invention using PSA as the optical constraining layer  15 , it also mechanically functions to adhesively fasten an optical absorbing layer  16 , such as for example, Dupont Kapton CB black polyimide, to the slab light guide  12 , forming a sandwich structure therewith.  
         [0012]    In other embodiments of my invention the optical absorbing layer is disposed on the optical constraining layer, for example, in certain embodiments, the optical constraining layer  15  is a thin film coating on the acrylic slab  12  and the optical absorbing layer is a black paint overcoat, such as for example Krylon Ultra-Flat or Tetenal Kameralack. Note that the optical constraining layer must be thick enough, for example three wavelengths of light, so that the total internally reflected light is not inadvertently absorbed due to the evanescent aspect of light reaching the black layer.  
         [0013]    The optical absorbing layer  16  has a refractive index that is greater than the refractive index of optical constraining layer  15 . This difference in refractive indices causes the light within the optical constraining layer  15 , that is, the light that has not been contained by TIR within the light guide, to exit into the optical absorbing layer  16  where it is absorbed.  
         [0014]    Advantageously, the mild diffuser allows for the mixing of discrete collimated light sources, such as non-imaging collimators  22  that are optically driven from optical fibers  21 . Suitable mild diffusers are available from Reflexite (Avon, Conn.), part numbers BP336, BP302 and BP321 having symmetric half angles of ±3.9 degrees, ±3.8 degrees, and ±2.8 degrees, respectively. From lab testing, it has been determined that BP321 is preferred when used in combination with a “SolarTec CL Light” fiber optic illuminator from Wavien, Inc. (Santa Clarita, Calif.), ESKA SK60 fibers from Mitsubishi Rayon Co. (Tokyo, Japan), and Poly II acrylic from Polycast (Stamford, Conn.). In other embodiments of my invention, the mild diffuser  11  is embossed on the entrance aperture of the slab light guide  12 .  
         [0015]    Light that is angularly limited below the threshold limit passes through the slab light guide  12  and exits at surface  42 . Advantageously, this angularly limited collimated light is especially suitable for a wedge light extraction guide  23  as may be found behind a liquid crystal display (LCD).  
         [0016]    In certain embodiments of my invention, the lower surface  44  of the slab light guide  12  has a second optical constraining layer  17  and a second optical absorbing layer  18  disposed thereon. These optical layers function in the same manner as previously described optical constraining layer  15  and optical absorbing layer  16 .  
         [0017]    Referring now to FIG. 2, there is depicted another illustrative embodiment of the present invention. In this embodiment, the mild diffuser, slab light guide, and wedge light extraction guide are fashioned from the same monolithic substrate  50 , preferably acrylic. The monolithic substrate  50  comprises two distinct regions, a constant cross-section slab light guide region  61  and a wedge-shaped light extraction guide region  62 . The light enters the slab light guide region  61  through an embossed entrance diffuser  51 . Similar to the previous embodiment, the slab light guide region  61  includes an upper surface  53  and a lower surface  54 .  
         [0018]    The upper surface  53  and the lower surface  54  are covered with optical constraining layers  15  and  17 , respectively as in the prior embodiment. The optical constraining layers  15  and  17  each have a second refractive index, which is slightly less than the refractive index of the monolithic substrate  50 . Because of the slight difference in refractive index, the optical constraining layers  15  and  17  act to trap light within the slab light guide region  61  under certain conditions. Accordingly, collimated light that enters the monolithic substrate  50  through embossed entrance diffuser  51  with an angular spread below a certain threshold value is contained within the monolithic substrate  50  by total internal reflection (TIR). Light with an angular spread above the threshold value exits the monolithic substrate  50  and enters the optical constraining layers  15  and  17 .  
         [0019]    Disposed on the optical constraining layers  15  and  17  are optical absorbing layers  16  and  18 , respectively. The optical absorbing layers  16  and  18  each have a refractive index that is greater than the refractive index of optical constraining layers  15  and  17 . This difference in refractive indices causes the light within the optical constraining layers  15  and  17 , that is, the light that has not been contained by TIR within the monolithic substrate  50 , to exit into the optical absorbing layers  16  and  18 , where it is absorbed.  
         [0020]    Table 1 below details results of the Snell&#39;s law calculations for a certain illustrative embodiment of my invention comprising a 6-millimeter thick acrylic slab with a refractive index of 1.4893, and an optical constraining layer formed from a PSA with a refractive index of 1.4800. These calculations detail input light angles from 5 to 23 degrees in air. The calculations show that light with a divergence angle of greater than 10 degrees is absorbed. Also shown in Table 1 is the minimum slab length required for the input light to have at least one reflection into the optical constraining layer. For example, for light having angles 10 degrees and greater to get absorbed the slab length needs to be at least two inches long.  
                                                         TABLE 1                                   Input   Light Angl   Minimum   Light Angle           Light Angle   within slab ( )   Slab Length   into PSA           (degrees)   (degrees)   (mm)   (degrees)                                        5.0000   3.3549   102.36   TIR           6.0000   4.0247   85.28   TIR           7.0000   4.6938   73.08   TIR           8.0000   5.3620   63.93   TIR           9.0000   6.0294   56.81   TIR           10.000   6.6958   51.11   88.049           11.000   7.3610   46.45   86.367           12.000   8.0249   42.56   85.157           13.000   8.6875   39.27   84.120           14.000   9.3486   36.45   83.177           15.000   10.008   34.00   82.295           16.000   10.666   31.86   81.455           17.000   11.322   29.97   80.646           18.000   11.976   28.29   79.861           19.000   12.627   26.79   79.096           20.000   13.277   25.43   78.347           21.000   13.924   24.21   77.612           22.000   14.568   23.09   76.889           23.000   15.210   22.07   76.176                      
 
       ALTERNATE EMBODIMENTS  
       [0021]    Alternate embodiments may be devised without departing from the spirit or the scope of the invention. For example, an array of collimated light emitting diodes (LED) or low numerical aperture fibers can be ass input sources in lieu of the non-imaging collimated light sources comprising collimators  22  and optical fibers  23 . Also, the light guides need not be solid, but can be hollow by use of TIR films, such as that described in Whitehead (U.S. Pat. No. 4,260,220).