Abstract:
This invention discloses a projection screen including a diffusion layer, a cap layer, an interference layer and an absorption layer. The projection screen of this invention is based on the concept of inverse reflection. The interference filter includes a plurality of high and low refractive index layers, both of which alternately stacked. The projection screen conveys a small spectral shift and a narrow bandwidth at different incident angles so that the better contract ratio of the image signal can be displayed.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a reflective screen for projectors, and more particularly to a reflective screen capable of increasing image contrast. 
   2. Description of the Related Art 
   A front projection display, referred to as a projector, typically uses a reflective screen to display images. Image signals projected on and reflected by the reflective screen are seen by viewers as images. Conventional reflective screens typically comprise flexible material, thus, they are easily rolled up for storage. Reflective screens, however, have same drawbacks, such as reduced image contrast due to reflected environmental light. 
   Recent improvements in reflective screens include increased image contrast in bright environmental light, increasing flexibility and durability, and reduced fabrication costs. 
   U.S. Pat. No. 5,335,022 discloses a screen with a high image contrast. The screen selects polarized optical signals from a projector but not environmental light to increase the image contrast. A polarizing filter layer is adhered to the screen, thus, passage of optical signals from the projector and the optical signals parallel to the polarizing filter layer is allowed. Polarized environmental light vertical to the polarizing filter layer is, however, filtered. Environmental light must be polarized prior to increasing the image contrast of the screen, thus, the screen is less workable. 
   U.S. Pat. No. 5,625,489 discloses a combination screen comprising one reflective layer and three polarization controlling layers. The polarization controlling layer has biconvex lens disposed between a photosensitive polarization film and a ¼ wave plate. Since optical signals from a projector are focused and reflected through the lens, the photosensitive polarization film can be activated to allow passage of light. When incident, half of the environmental light is blocked by the photosensitive polarization film. The other half of the light is reflected from the reflective layer after passing through the ¼ wave plate, and then passes again through the ¼ wave plate. Thus, the photosensitive polarization film blocks the other half of the light. The screen of this invention is relatively less flexible. 
   U.S. Pat. No. 6,987,610 discloses a screen for front projection displays sequentially comprising a substrate, an absorbing material and an uppermost luminescent material. Image signals from a projector excite the luminescent material simultaneously emitting image signals from the projector. The absorbing material absorbs all environmental light except for light of three primary colors (red, green and blue), thus the image contrast of the screen increases. 
   US patent publication No. 2004/0150883 discloses a screen for a projector having high image contrast and excellent flexibility. Fabrication of the screen is inexpensive and easily to be carried. As shown in  FIG. 1A , the screen includes a substrate  20  capable of absorbing light, an optical film  21 , a diffusion layer  22  and a protection layer  23 . The optical layer  21  comprises layers of high refractive index material H 1 -H 51  and low refractive index material L 1 -L 51 . The optical layer  21  does not absorb light, and intensely reflects the light of three-primary colors. Since image signals from the projector consist of three-primary colors, the screen is able to reflect the image signals.  FIG. 1B  shows a shift of the spectral reflectance of the optical film when the incident angles of light are 0° and 30°. 
   US patent publication No. 2004/0240053 discloses a reflective screen having an optical film. The screen is made of polymer material with solvent. As shown in  FIG. 2A , the screen comprises a light absorption layer  34 , a transparent substrate  30 , a stacked optical film  31 , an angle calibrating layer  33  and a diffusion layer  34 . The stacked optical film  31  comprises alternate layers of high refractive index material  1 H- 9 H and low refractive index material  1 L- 9 L.  FIG. 2B  shows a reflection spectrum of the optical film when the incident angles of light are 0° and 30°. 
   Although the above-mentioned conventional reflective screens may increase image and color contrast, they still suffer from some drawbacks in system and structure. A novel reflective screen capable of increasing image contrast in bright environmental light with lower fabrication cost is thus desirable. 
   BRIEF SUMMARY OF THE INVENTION 
   A reflective screen is provided. An embodiment of a reflective screen comprises an absorption layer. An interference filter is formed on the absorption layer to reflect light and generate a shift in a spectrum when an incident light is introduced. A cap layer is formed on the interference filter. A diffusion layer is formed on the cap layer. 
   A detailed description is given in the following embodiments with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
       FIG. 1A  shows cross section of a conventional reflective screen; 
       FIG. 1B  shows a shift of spectral reflectance of the optical film when the incident angles of light are 0° and 30° respectively according to  FIG. 1A ; 
       FIG. 2A  shows cross section of another conventional reflective screen; 
       FIG. 2B  shows a shift of spectral reflectance of the optical film when the incident angles of light are 0° and 30° respectively according to  FIG. 2A ; 
       FIG. 3  shows a schematic view showing a reflective screen according to an embodiment of the invention; 
       FIG. 4A  shows cross section of a reflective screen according to another embodiment of the invention; 
       FIG. 4B  shows a shift of spectral reflectance of the interference filter  102  according to  FIG. 4A ; 
       FIG. 5  shows a schematic view showing a reflective screen according to an embodiment of the invention; 
       FIG. 6  shows a schematic view showing a phase difference for light reflected at different interfaces; 
       FIG. 7  shows shifts of spectral reflectance of the various optical films when incident angles of light are 0° and 60° respectively according to an embodiment of the invention; and 
       FIGS. 8A to 8D  show shifts of spectral reflectance of the optical film with various refractive indexes according to an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
     FIG. 3  is a schematic view showing a reflective screen according to an embodiment of the invention. Referring to  FIG. 3 , a reflective screen according to an embodiment of the invention comprises a diffusion layer  103 , a cap layer  101 , an interference filter  102  and an absorption layer  104  which are sequentially disposed (on the cap layer  101 ) along the incident direction of light. Image signals projected on the reflective screen consist of the three primary colors, i.e. red (R), green (G) and blue (B). In addition to the image signals, white environmental light (W) is projected on the reflective screen. The interference filter  102  includes alternate layers of high refractive index layers and low refractive index layers. The interference filter  102  reflects light of specific wavelengths and therefore the reflective screen reflects light of the three primary colors while a portion of the environmental light not reflected by the interference filter  102  passes through the interference filter  102  and absorbs by the absorption layer  104 . 
   Assuming image quality is equal to the ration of “signal intensity/non-signal intensity” and includes chroma and contrast. For image signals from a projector of 1000 Lumen, the environmental light of 100 Lumen, and the three primary colors which constitute white light to be equal in illumination, image quality of a conventional reflective screen is:
 
(1000/3+100/3)/(100×2/3)=5.5
 
   In a preferred embodiment of the invention, half the light is, however, preferably reflected by the interference filter  102  while the other half of the light passes through the interference filter  102 . Since a ratio of a reflective illumination of the interference filter  102  is about 0.5, image quality of the reflective screen according to an embodiment of the invention is:
 
(1000/3+0.5×100/3)/(0.5×100×2/3)=10.6
 
   Therefore, image quality of the reflective screen according to this embodiment is substantially twice as that of a conventional reflective screen. 
   The interference filter  102  is provided with a narrower spectral bandwidth, that is, the interference filter  102  has high selectivity with respect to light of various wavelengths. When an LED or laser is used as a light source of a projector and a ratio of a reflective illumination of the interference filter  102  is about 0.1, image quality of the reflective screen is substantially tenfold in comparison with that of the conventional reflective screen. The interference filter  102  is disposed behind the cap layer  101  along the incident direction of light and therefore image signals from the projector enter the reflective screen through the diffusion layer  103 . 
     FIG. 4A  is cross section of a reflective screen according to an embodiment of the invention. Referring to  FIG. 4 , a reflective screen  100  comprises a diffusion layer  103 , a cap layer  101 , an interference filter  102  and an absorption layer  104  which are sequentially disposed along the incident direction of light (on the top of the cap layer  101 ). The interference filter  102  comprises alternate layers of a plurality of high refractive index layers H 1 -H 23  and a plurality of low refractive index layers L 1 -L 22 . In one embodiment of the invention, the cap layer  101  can be consisted of solid transparent material. The interference filter  102  is disposed on a rear surface of the cap layer  101 , and the diffusion layer  103  is disposed on a front surface of the cap layer  101  facing toward environmental light and image signals from a projector. The absorption layer  104 , comprising black or dark color pigments, is disposed behind (on the top of) the interference filter  102 . The absorption layer  104  is used to absorb light passing through the interference filter  102  to prevent the light from being reflected again. 
   The high or low refractive index described in the invention is not limited to a specific range of refractive indexes, while the high or low refractive index is relative with respect to each layer. 
   The cap layer  101  has a refractive index of greater than about 1.4 and is consisted of polyethylene terephthalate(PET), poly carbonate (PC), poly (methyl methacrylate)(PMMA), combinations thereof or other likes. 
   The interference filter  102  comprises alternate high refractive index layers and low refractive index layers stacked on the cap layer  101 . For flexibility, a total number of the high and low refractive index layers is less than 60. In one embodiment of the invention, the interference filter  102  is constituted by alternate stacked layers of 23 layers of ZrO 2  as the high refractive index layers and 22 layers of Al 2 O 3  as the low refractive index layers, wherein the ZrO 2  and Al 2 O 3  has a refractive index of about 2.05 and 1.62 respectively. 
     FIG. 4B  is showing a spectral reflectance of the interference filter  102  according to an embodiment of the invention. Referring to  FIG. 4B , when an incident angle of light is 0° and 30°, a shift in the spectral reflectance of the red, green and blue light is about 22 nm, 19 nm and 16 nm respectively. The average shift in spectral reflectance is less than about 0.66 nm/degree. 
   The cap layer  101  is preferably used as waveguide interface for projected or environmental light. Referring to  FIG. 5 , the refractive index n(z) of the cap layer  101 , having a thickness of “d”, is greater than the refractive index no of an incident medium which is typically air. Light from outside the reflective screen is scattered by the diffusion layer and travels at an angle of α in the waveguide interface and then reflected by the interference filter  102 . Light is then reflected back to the surface of the reflective screen at the same angle of α and then projected into incident medium of the reflective screen at an angle of α 0  greater than α in accordance with the Snell&#39;s law. A visual angle of the reflective screen is thus widened. Adjusting the thickness and refractive index of the cap layer  101  changes divergence of the visual angel of the reflective screen. 
   Referring again to  FIG. 4 , the diffusion layer  103  may be formed by patterning the surface of the cap layer  101 . The diffusion layer  103  may be an array-arranged pattern or a random pattern. In some embodiments, the diffusion layer  103  is formed by adhering a biconvex lens sheet or bead-shaped material on the cap layer  101 . The diffusion layer  103  is used to scatter light. 
   The absorption layer  104  preferably comprises black or dark color pigments. The absorption layer  104  may alternatively comprise a plurality of black absorption films adhered to the interference filter  102 . The absorption layer  104  may be a rigid or flexible material capable of absorbing light. The material of absorption layer  104  can be chosen as desired. 
   For the purpose of illustrating the concept of the invention, please refer to  FIG. 6 . A film  200  with a thickness of “d′” and a refractive index of n is disposed between an incident medium, typically air, with a refractive index of no and a substrate with a refractive index of ns. Light  201  is projected into the film  200  through the incident medium. A portion of the light  203  is reflected by the surface of the film  200 . The other portion of light  205  passes through the film  200  and is refracted by the film  200 . The light  205  is then reflected by the substrate and back into the incident medium. The phase difference between light  203  and  205  is:
 
φ=(2π/λ)2 nd  cos α
 
   Wherein α is an incident angle of the light, d is thickness, and n is a refractive index of the film  200 ; π is 180 degrees; λ is a wavelength of the light; and φ is the phase difference. To maintain a constant phase difference at a specific incident angle, the wavelength of light has to increase when an effective refractive index of a film increases. A spectral shift in spectral reflectance is thus reduced. 
     FIG. 7  shows shifts of spectral reflectance of various optical films when the incident angles of light are 0° and 60°. The films have the same optical thicknesses and characteristics of the air/film/glass structure. As the equation shows, a film, such as the interference filter, with a high refractive index causes a reduced spectral shift. 
     FIGS. 8A to 8D  show shifts of spectral reflectance of the optical film with various refractive indexes, wherein the incident angles of light are 0° and 60° respectively. [H, 3L] 6  represents stacking a high refractive index layer and a low refractive index layer 6 times to form an alternating stack layer including a total of 12 layers. The high refractive index layer has a thickness of 1 unit and the low refractive index layer has a thickness of 3 units. [3H, L] 6  represents stacking a high refractive index layer and a low refractive index layer 6 times to form an alternating stack layer including a total of 12 layers. The high refractive index layer has a thickness of 3 units and the low refractive index layer has a thickness of 1 unit. The 1 unit implies ¼ wave of the reference wavelength, say 550 nm in the present case. As  FIG. 8  shows, spectral shifts in the [3H, L] 6  are smaller than in [H, 3L] 6 . Thus, when the thickness of the high refractive index layer is greater than that of the low refractive index layer, the spectral shifts decrease. Preferably, a ratio of the thickness of the high refractive index layers to that of the low refractive index layers is preferably greater than 2. 
   Residual stress exists in conventional reflective screens due to the several optical films disposed therein and the property of material, such that the stability and durability thereof suffers. The reflective screen of the invention can prevent the adverse effects of residual stress. Since conventional reflective screens are typically made of SiO 2  and Nb 2 O 5  by sputtering process, which causes the residual stress problem. The thickness of the sputtered layer must be carefully controlled to prevent excess residual stress. Preferably, the interference filter  120  comprises alternate layers of ZrO 2  and Al 2 O 3  to reduce the effects of residual stress. In an interface between ZrO 2  and Al 2 O 3 , Al 2 O 3  has effect to stabilize ZrO 2 . ZrO 2  may also be doped with Y 2 O 3  to eliminate residual stress. The reflective screen of the invention may provide improved flexibility. Reflective screens are typically required to be flexible, so that the screens may be easily rolled up and stored. 
   Compared with conventional reflective screens, the invention has fewer shifts in spectral reflectance and accommodates a narrower bandwidth in a spectrum at different incident angles, thus, better image contrast may be obtained. Table 1 illustrates the comparison between the reflective screen of the invention and conventional reflective screens. 
   
     
       
             
             
             
             
           
         
             
                 
             
             
                 
               Nb 2 O 5 /SiO 2   
               Solvent 
               ZrO 2 /Al 2 O 3 (an 
             
             
                 
               (conventional 
               layer 
               embodiment 
             
             
               Material of 
               reflective 
               (conventional 
               of the 
             
             
               interference filter 
               screen) 
               reflective screen) 
               invention) 
             
             
                 
             
           
           
             
               Number of 
               51/50 
               9/9 
               27/26 
             
             
               layers (high/low 
             
             
               refractive index 
             
             
               layers) 
             
             
               Refractive 
                2.4/1.47 
               1.68/1.41 
               2.01/1.63 
             
             
               index (high/low 
             
             
               refractive index 
             
             
               layers) 
             
             
               Thickness (high/low 
               1.14/2.16 
               9.2/7.0 
               3.38/1.12 
             
             
               refractive index 
             
             
               layers) (nm) 
             
             
               Spectral 
             
             
               shift (0°-30°) 
             
             
               red (nm) 
               28 (634-606) 
               32 (640-608) 
               22 (630-608) 
             
             
               green (nm) 
               22 (539-517) 
               22 (532-519) 
               19 (539-520) 
             
             
               blue (nm) 
               18 (469-451) 
               20 (460-440) 
               16 (459-443) 
             
             
               Shift of bandwidth 
             
             
               (50%) 
             
             
               Red, green, blue 
               45, 31, 22 
               15, 10, 7 
               34, 28, 25 
             
             
               (nm) 
             
             
                 
             
           
        
       
     
   
   ADVANTAGES OF THE INVENTION 
   (1) In the reflective screen of the invention, the cap layer can be used as waveguide interface; and by adjusting its thickness and refractive index, it can provide a widened diffusion angle when the interference filter reflects light back to the surface of the reflective screen. 
   (2) The interference filter is more flexible and easier to roll up and store. 
   (3) Spectral shifts in reflectance are reduced and a visual angle of the reflective screen is widened while contrast and chroma of the reflective screen is constant. 
   (4) Image contrast can be improved by using fewer layers to form the interference filter. 
   (5) The interference filter has less residual stress, thus, the reflective screen is more durable. 
   While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.