Abstract:
A color projection system includes a light source, a rotatable drum having a plurality of differently Colored bands, and a light modulation panel on which light beams reflected by the colored bands can be directed by way of scrolling. The apparatus directs the light beam from the light source onto a first group of juxtaposed bands. The light beam on each band is split and reflected into light beams having complementary colors and different directions of polarization. Light beams having a first direction of polarization are directed onto the light modulation panel, and light beams having a second direction of polarization are reflected onto a second rope of bands of the drum. The second group of bands is shifted by at least one band with respect to the first group of bands.

Description:
FIELD OF TECHNOLOGY 
     The invention relates to a color projection system comprising a light source, a rotatable drum having a plurality of differently colored bands, and a light modulation panel on which light beams reflected by the colored bands can be directed by way of scrolling. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     In such a color projection system as described in non-prepublished European patent application EP 99202414.1, the light beam coming from the light source is directed onto the drum and split by the differently colored bands into light beams having complementary colors, while a light beam colored in conformity with a color of the band is directed onto the light modulation panel. The complementary color is absorbed by the band. This means that when using split colors of red, green and blue, one color of a light beam projected on a band is directed onto the light modulation panel, whereas the other two colors are absorbed. This results in a two-thirds loss of light. 
     It is an object of the invention to provide a color projection system in which there is less loss of light. 
     In the color projection system according to the invention, this object is achieved in that the color projection system further comprises means for directing the light beam from the light source onto a first plurality of juxtaposed bands, means for splitting and reflecting the light beam on each band into light beams having complementary colors and different directions of polarization, means for directing light beams having a first direction of polarization onto the light modulation panel, and means for reflecting light beams having a second direction of polarization onto a second plurality of bands of the drum which is shifted by at least one band with respect to the first plurality of bands. 
     Instead of absorbing the color(s) complementary to the colored band, these colors are reflected and subsequently reflected again into the direction of the drum, while the colors reach another band. This band has the same color as one of the colors reflected again so that this band as yet directs the relevant color onto the light modulation panel. In this way, a larger part of the light beam coming from the light source is directed onto the light modulation panel. 
     These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, 
     FIG. 1 is a diagrammatic elevational view of a device according to the invention, 
     FIG. 2 is a cross-section of a drum of the device shown in FIG. 1, 
     FIGS. 3A-3C show color bands directed onto the drum and the light modulation panel in a first embodiment of the drum, 
     FIGS. 4A-4C show color bands directed onto the drum and the light modulation panel in a second embodiment of the drum, 
     FIGS. 5A-5D show color bands directed onto the drum and the light modulation panel in a third embodiment of the drum. 
     Corresponding components in the Figures have the same reference numerals. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a color projection system  1  according to the invention, comprising a light source  2 , first and second integrator plates  3 ,  4  preceding the light source  2 , a lens  5  preceding the integrator plates  3 ,  4 , a polarized beam splitter (PBS)  6  preceding the lens  5 , a lens  7  located opposite one side of the PBS  6 , a drum  9  rotatable about a shaft  8  and located at a side of the lens  7  remote from the PBS  6 , a lens  10  located opposite another side of the PBS  6 , a second polarized beam splitter (PBS)  11  preceding the lens  10 , a light modulation panel  12  located opposite one side of the PBS  11 , and a projection lens  13  located opposite another side of the PBS  11 . 
     The second integrator plate  4  is provided with a plurality ofjuxtaposed lenses and interpositioned reflecting surfaces facing the drum  9 . Such an integrator plate is known from EP-A-0 902 907 in the name of the applicant. 
     FIG. 2 is a cross-section of the drum  9 . Drum  9  has a reflecting coating  14  provided with a ¼λ foil  15  and a dichroic layer  16 . The dichroic layer  16  is divided into a plurality of axially extending bands of different colors. A light beam  17  directed onto the dichroic layer  16  is split by the dichroic layer  16  and the ¼λ foil  15  and reflected in a light beam  18  having a color which corresponds to the color of the dichroic layer  16  and has a first direction of polarization S, and a light beam  19  having a color which is complementary to the color of the dichroic layer  16  and has a second direction of polarization P. 
     The operation of the device  1  will now be elucidated with reference to FIGS. 3-5. 
     FIG. 3A shows a part of the drum provided with a plurality of bands consecutively having the colors blue B, red R and green G. 
     An unpolarized light beam  20  coming from the light source  2  is projected via the integrator plates  3 ,  4 , the lens  5 , the PBS  6  and the lens  7  in an area  21  on the drum  9  in which four colored bands  22  are located. In the embodiment shown in FIG. 3A, the bands  22  located in the area  21  consecutively have the colors blue B, red R, green G and blue B. The light beam  17  directed onto the bands  22  is split by each band into the light beams  18 ,  19 , as has been elucidated with reference to FIG.  2 . 
     This means that the blue band  22  reflects a blue light beam  18  having a direction of polarization S and that the complementary colors red and green are reflected as light beam  19  having a direction of polarization P. The light beams  18 ,  19  are subsequently directed via lens  7  onto the PBS  6  where the light beam  18  having the direction of polarization S is bent into the direction of lens  10  and subsequently directed via PBS  11  onto a light modulation panel  12 . The light modulation panel  12  is synchronously band-controlled by information, associated with the relevant color band, about the image to be formed. The light beam  18  is reflected by means of the light modulation panel  12  and its direction of polarization is reversed and is guided as light beam  23  through the projection lens  13  and imaged on a screen (not shown). 
     The light beam  19  reflected by the drum  9  and having the direction of polarization P is projected by the PBS  6  on the reflecting surfaces of the integrator plate  4  via lens  5 . The light beam  19  is again reflected by the reflecting surfaces into the direction of the drum  9 , where the light beam  19  is directed onto an area  24  of the drum  9 . With respect to the area  21 , the area  24  is shifted by one band. This means that the light beam  19  reflected by the blue band B is imaged with the complementary colors red R and green G on a subjacent band  22 , as is indicated by arrow P 1  (see FIGS.  3 A and  3 B). In the embodiment shown in FIG. 3A, this band has the color red R. Consequently, the red part of the light beam  19  with the colors red R and green G will be reflected as a light beam  18  and imaged on the screen by the projection lens  13  via the light modulation panel  12  in a way as described above. The green part G of the light beam  19  will again be directed towards the second integrator plate  4 . However, upon this second reflection, this green light beam reaches a lens portion located next to a reflecting surface so that it will not be reflected again towards the drum  9 . 
     FIG. 3C shows the bands  25  ultimately imaged on the light modulation panel  12 , each band being composed of a first part which is imaged directly on the light modulation panel  12  via the drum, and a second part which is reflected via the second integrator plate  4  and subsequently presented to the light modulation panel  12 . 
     As is clearly visible from FIGS. 3A-3C, the light beam  17  is directed onto four bands  22 , while in the end only three bands  25  on the light modulation panel  12  are illuminated. If the area  21  were to have the same number of bands as the light modulation panel  12 , the upper band of the light modulation panel in the embodiment shown would not be provided with a light beam reflected via the second integrator plate  4  and the light modulation panel  12  would be unevenly illuminated. 
     In the color projection system as described in the opening paragraph of European patent application EP 99202414.1, the light beam is directed onto, for example, three bands with the colors red R, green G and blue B in an area  21  and subsequently imaged in three bands  25  on the light modulation panel  12 . Each band reflects only one of the three colors, while the other two colors are absorbed. This means that only 33% of the light beam reaches the light modulation panel  12 . 
     In the embodiment shown in FIGS. 3A-3C, four bands are illuminated, while three of them are imaged on the light modulation panel  12  and three times light beams are as yet directed onto the light modulation panel  12  via the second integrator plate  4 . In this embodiment, 50% of the light beam  17  thus reaches the light modulation panel  12 . 
     In both cases, possible losses in the different optical components have not been taken into account. 
     It will be evident that the bands in the area  21  are shifted by rotating the drum  9  about the centerline  8 , so that the bands imaged on the light modulation panel  12  are also shifted or are scrolled. 
     FIGS. 4A-4C show a second embodiment of the drum  9 , in which the light beam  20  is imaged on an area  26  of the drum  9  in which five bands  27  are located. In this embodiment, the bands consecutively have the colors blue B, red R, white W, green G. The lower four bands of these five bands are imaged as bands  28  on the light modulation panel  12 . The light beams  17  incident on the bands  27  are reflected as light beams  18 ,  19 , similarly as is the case with the bands  22 . The light beam  19  reflected by the blue band B and having the complementary colors red R and green G is presented again to the subjacent red band R, whereafter the red part is directed onto the light modulation panel  12 . The white band generates only a light beam  18  so that the green band located below the white band is not illuminated once more by a light beam  19 . 
     FIG. 4C shows the illumination of the light modulation panel  12  as a result of an illumination with light beam  17  of an area  26  as shown in FIG.  4 A. In the light beams imaged on the light modulation panel  12 , blue B is over-represented. Consequently, a division of bands as shown in FIG. 4A is suitable if blue B should be present to a stronger degree in the ultimate illumination of the light modulation panel than the other colors, or if blue B is represented to a lesser degree in the original light beam  17  so that a better approximation of white light can be obtained in this way. 
     If the integrator plate  4  is provided with one lens and two associated reflecting surfaces, the light beams  19  reflected for the second time can also be presented once more to the drum  9 . The area  29  (see FIGS. 5A-5C) on which light beam  17  is directed then preferably comprises two bands more than the number of bands which is imaged on the light modulation panel  12 . Such an embodiment is shown in FIGS. 5A-5D, in which an area  29  comprising five bands  30  on the drum  9  is illuminated, while only three bands  31  on the light modulation panel  12  are illuminated. In this embodiment, 60% of the light beam  17  reaches the light modulation panel  12 . 
     It is alternatively possible to image, for example, six bands on the light modulation panel  12  simultaneously, while the area which is illuminated on the drum comprises, for example, seven bands if the light beam  19  is reflected only once by the integrator plate  4 . In such a case,  12 / 21   st  part of the light beam  17  is directed onto the light modulation panel  12 . It is alternatively possible to further increase the number of bands so that an even larger part of the light beam  17  will ultimately reach the light modulation panel  12 . However, the control of the light modulation panel  12  simultaneously becomes more complicated because more bands are to be scrolled on the light modulation panel  12 . 
     It is alternatively possible to pass the light from the light source  2  through a polarization conversion means so that all the light from the light source  2  is directed by the PBS  6  onto the drum  9 .