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 lit, 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. Figure 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] Figure 1 is a schematic sectional view of a reflective display device in accordance with an embodiment of the present invention. [0004] Figure 2 is a schematic sectional view of a reflective display device in accordance with another embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

[0005] Figure 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 1 1 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 1 1 , 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-ethylened ioxythiophene) poly(styrenesu Ifonate) (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 Figure 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 June 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- 1 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] Figure 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 1 1 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'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 Figure 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, Figure 2 shows a reflective display device 42 with a different order of layers. As shown in Figure 2, transparent conductive layer 1 1 , 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 1 1 , 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.