Abstract:
A display includes a backlight ( 40 ). In addition, the display includes for each of at least one wavelength band, a plurality of layers placed above the backlight ( 40 ). The plurality of layers includes an absorption layer ( 13,23,33 ), a luminescent layer ( 14,24,34 ) and a reflective layer ( 15,25,35 ). The absorption layer ( 12,22,32 ) is capable of absorbing light in a wavelength band while being substantially transparent to light in other wavelength bands. The luminescent layer ( 14,24,34 ) is capable, in response to light from the backlight ( 40 ), of emitting light within the wavelength band. The reflective layer ( 15,25,35 ) is adapted to reflect light in the wavelength band while being substantially transparent to light in other wavelength bands.

Description:
BACKGROUND 
       [0001]    Reflective displays typically use little power and are good for use in sunlight or bright indoor lighting. Because reflective displays are typically not fit, they perform less well when there is a lack of ambient light. Front lights can be used to light reflective displays, for example, by using a structured optical film to take light from the side of the display and direct it back to the display which reflects the light out to the user. Such optical film, however, is complex and can reduce the contrast of the display by scattering incoming light back to the user. 
       SUMMARY 
       [0002]    In accordance with an embodiment of the invention, a display includes a backlight. In addition, the display includes for each of at least one wavelength band, a plurality of layers placed above the backlight. The plurality of layers includes an absorption layer, a luminescent layer and a reflective layer. The absorption layer is capable of absorbing light in a wavelength band while being substantially transparent to light in other wavelength bands. The luminescent layer is capable, in response to light from the backlight, of emitting light within the wavelength band. The reflective layer is adapted to reflect light in the wavelength band.  FIG. 1  is a schematic sectional view of a reflective display device in accordance with an embodiment of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0003]      FIG. 1  is a schematic sectional view of a reflective display device in accordance with an embodiment of the present invention. 
           [0004]      FIG. 2  is a schematic sectional view of a reflective display device in accordance with another embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0005]      FIG. 1  is a schematic sectional view of a reflective display device  41  that includes a backlight  40 . For example, backlight  40  provides ultraviolet (UV) light with a wavelength in the range of 300 to 380 nanometers or deep blue light with a wavelength in the range of 380 to 420 nanometers. 
         [0006]    Reflective display device  40  includes absorption layers  12 ,  22  and  32 , which can be made to absorb different colors of light. For example, absorption layers  12 ,  22  and  32  are electro-optical layers composed of selective light absorbing material such as dichroic dyes in liquid crystal hosts, electrophoretically actuated pigment particles, or another of known selective light absorbing material that can be controlled to change from transparent to absorbing colors in specific wavelength bands. For example the thickness of each of absorption layers  12 ,  22  and  32  is 3 micrometers. 
         [0007]    Absorption layers  12 ,  22  and  32  are sandwiched between transparent conductive material. What is meant herein by transparent material is material that lets light let light through. Thus the term transparent material is meant to include translucent material where there is a significant amount of diffusion of light as well as clear transparent material where there is relatively little or even only a negligible amount of diffusion of light. 
         [0008]    For example, absorption layer  12  is between a transparent conductive layer  11  and a transparent conductive layer  13 . Absorption layer  22  is between a transparent conductive layer  21  and a transparent conductive layer  23 . Absorption layer  32  is between a transparent conductive layer  31  and a transparent conductive layer  33 . For example, each of transparent conductive layers  11 ,  13 ,  21 ,  23 ,  31  and  33  is 50 to 100 nanometers thick and is composed of, for example, indium tin oxide (ITO) or poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (Pedot-PSS) or some other transparent conductive material known to those skilled in the art of display manufacture. The transparent conductive material carries electric control signals that control at what locations in the display the selected light band is absorbed. 
         [0009]    As shown in  FIG. 1 , the transparent conductive layers are immediately adjacent to the corresponding absorption layer so as to minimize the voltages required for operation. In some cases, however, there may be intervening layers. For example, an alignment layer for the liquid crystals in a liquid crystal actuated dichroic dye guest-host system may separate an absorption layer from one or both of its corresponding transparent conductive layers. 
         [0010]    Below absorption layers  12 ,  22  and  32  light is reflected at wavelengths absorbed by the above absorption layers. For example, the reflection of light is implemented by a plurality of reflective layers. A reflective layer  15  operates as a wavelength selective mirror. Reflective layer  15  is located below absorption layer  12  and selectively reflects light that can be absorbed by absorption layer  12 . A reflective layer  25  operates as a wavelength selective mirror. Reflective layer  25  is located below absorption layer  22  and selectively reflects light that can be absorbed by absorption layer  22 . Reflective layer  25  can optionally also reflect light that can be absorbed by layer  12  while not reflecting light that can be absorbed by layer  32 . A reflective layer  35  operates as a wavelength selective mirror. Reflective layer  35  is located below absorption layer  32  and selectively reflects light that can be absorbed by absorption layer  32 . Reflective layer  35  can optionally also reflect light that can be absorbed by layers  12  and by layers  22 . 
         [0011]    For example, the reflective layers  15 ,  25  and  35  are made from reactive mesogen cholesteric films. For example, Merck material RMSO3-008 can be used to selectively reflect blue light. Merck material RMSO3-010 can be used to selectively reflect green light. Merck material RMSO3-009 can be used to selectively reflect red light. 
         [0012]    For more information on constructing reflective layers  15 ,  25  and  35 , see United States Patent Application Publication 2009/0140961 A1, published Jun. 4, 2009 for a Reflective Display. 
         [0013]    Immediately above each of reflective layers  15 ,  25  and  35  is placed a luminescent layer that when stimulated by light from backlight  40  emits light that is predominantly within the wavelength range that can be absorbed by the corresponding absorption layer. Thus, a luminescent layer  14  emits light within a wavelength band that is predominately within the wavelength range that can be absorbed by absorption layer  12 . A luminescent layer  24  emits light within a wavelength band that is predominately within the wavelength range that can be absorbed by absorption layer  22 . A luminescent layer  34  emits light within a wavelength band that is predominately within the wavelength range that can be absorbed by absorption layer  32 . For example, the luminescent layers are either fluorescent or phosphorescent. For example, each of luminescent layers  14 ,  24  and  34  absorbs approximately one third of the light emitted by luminescent backlight  40 . Alternatively, in accordance with design choices based on materials used and performance desired, luminescent layers  14 ,  24  and  34  can absorb disproportional amounts of the light emitted by luminescent back light  40 . Luminophores within the luminescent layers can optionally be chosen to absorb some visible wavelengths rather than just the wavelength of light from the backlight if their emission efficiency is high enough and the emission between the primary colors can be balanced. Allowing this visible absorption can broaden the choice of luminescent materials. Reflective layers  15 ,  25  and  35  are substantially transparent at the wavelengths of light emitted from backlight  40  that stimulate the luminescent layers. 
         [0014]    Composition of luminescent layers  14 ,  24  and  34  can be chosen based upon the composition of absorption layers  12 ,  22  and  32 . For example, if in absorption layers  12 ,  22  and  32  an electrophoretic cell is used to sweep pigment in or out of the pixel area to achieve the color absorption, no polarization is needed in luminescent layers  14 ,  24  and  34 . In this case luminescent layers  14 ,  24  and  34  can be composed of, for example, luminescent dye-doped transparent polymers or luminescent polymers or dendrimers having a thickness of 1-20 microns. 
         [0015]    If absorption layers  12 ,  22  and  32  achieve color absorption based on polarization, luminescent layers  14 ,  24  and  34  may need to be polarized as well. This can be achieved by composing luminescent layers  14 ,  24  and  34  using dichroic fluorescent dyes, for example, which can be aligned in a curable liquid crystal polymer layer. Such dyes are described in XUELONG ZHANG; GOROHMARU Hideki; KADOWAKI Masami; KOBAYASHI Takako; ISHI-I Tsutomu; THIEMANN Thies; MATAKA Shuntaro; Benzo-2,1,3-thiadiazole-based, highly dichroic fluorescent dyes for fluorescent host-guest liquid crystal displays; Journal of Material Chemistry; ISSN 0959-9428 2004, vol. 15, no12, pp. 1901-1904n. 
         [0016]      FIG. 1  shows the layers grouped in accordance with color spectrums. Layers  10  include absorption layer  12 , luminescent layer  14 , reflective layer  15  and substrate layers  11  and  13 , which implement display of a first color. Layers  20  include absorption layer  22 , luminescent layer  24 , reflective layer  25  and substrate layers  21  and  23 , which implement display of a second color. Layers  30  include absorption layer  32 , luminescent layer  34 , reflective layer  35  and substrate layers  31  and  33 , which implement display of a third color. For example, the first color is blue, the second color is green and the third color is red. Alternatively, the first color is red, the second color is green and the third color is blue. In general, the order of colors is a design choice based, for example, on the color absorption and reflectivity of chosen materials and desired optimization of performance of the display when the display operates using primarily ambient light and when the display operates using light generated from the luminescent layers. 
         [0017]    Alternatively, the colors can be arranged in a different order or different colors can be used. While it is standard practice to use three primary colors to implement a color display, the present invention can be also be used to implement a display with one, two, four, five or more primary colors. 
         [0018]    When ambient light is high, display  41  acts as a reflective display in which ambient light is selectively reflected to a user based on the wavelengths of light selectively absorbed by the absorption layers. Thus when there is sufficient ambient light, backlight  40  may not be needed to generate a display adequate for a user&#39;s needs. When backlight  40  is turned on, reflection of ambient light can be augmented by photoluminescent light generated within the luminescent layers under stimulation of the backlight. This photoluminescent light is selectively allowed to escape the display based on the wavelengths of light selectively absorbed by the absorption layers. 
         [0019]    While  FIG. 1  shows one stacking order, in alternative embodiments the layers are grouped in accordance with color spectrums and other schemes can be used to group layers. For example,  FIG. 2  shows a reflective display device  42  with a different order of layers. As shown in  FIG. 2 , transparent conductive layer  11 , absorption layer  12  and transparent conductive layer  13  are grouped at the top of reflective display device  42 . Then are located transparent conductive layer  21 , absorption layer  22  and transparent conductive layer  23 , transparent conductive layer  31 , absorption layer  32  and transparent conductive layer  33 , as shown. Arranged below these layers are luminescent layer  34 , reflective layer  35 , luminescent layer  24 , reflective layer  25 , luminescent layer  14  and reflective layer  15 . Alternatively, reflective layer  35 , selective mirror  25  and selective mirror  15  could be replaced by a single broadband mirror placed below luminescent layer  14 , luminescent layer  24  and luminescent layer  34 . 
         [0020]    In alternate embodiments, for example, luminescent layer  14 , luminescent layer  24  and luminescent layer  34  are all located above reflective layer  15 , reflective layer  25  and reflective layer  35  (or a single broadband mirror replacing reflective layers  15 ,  25  and  35 ) and all located below absorption layer  11 , absorption layer  21  and absorption layer  31 . For each color the luminescent layer is located below the corresponding absorption layer and is located above the corresponding reflective mirror.