Abstract:
A field sequential dichroic light valve using ferro-electric liquid crystal components and multi-layer thin film broadband polarizing beam-splitting devices as an optical switch. The field sequential dichroic light valve has a fast time response and a memory function, as well as a switch frequency larger than 180 Hz and an optical transmittance larger than 90%. Accordingly, the field sequential dichroic light valve can effectively switch R, G, B-colored light beams in time sequence, and can be applied to a full-color projection display system.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a liquid crystal projection display, and particularly to a field-sequential dichroic light valve. 
     2. Prior Art of the Invention 
     A conventional electronic optical switch can be turned ON/OFF by the rotation of light polarization by a twisted neumatic (TN) liquid crystal component and the effect of an external electric field to the direction of LC principal axis. Referring to FIGS. 1 a  and  1   b , the unpolarized incident light I o  becomes a polarized light having an a-direction polarization after passing through the polarization film  10 . The a-direction polarized light changes the polarization direction to b-direction with the effect of the alignment films  12   a ,  12   b  and the liquid crystal molecules  14  in the cavity. Therefore, the polarized light can be emitted from the polarized film  16 . In FIG. 1 b , the liquid crystal molecules  14  will not change the polarization direction of incident light due to the effect of the external electric field of voltage source  18 . Therefore, the a-direction-polarized light cannot be emitted through the polarized film  16 . The conventional optical switch is not operated in a mechanical manner and has a high contrast. The switch speed of the optical switch can achieve the level of 20˜50 msec. However, such a switching speed is not fast enough for a field sequential liquid crystal display, especially for a tri-color projection display system (PDS). 
     Other prior arts, such as U.S. Pat. No. 5,868,482 of Edlinger et al., U.S. Pat. No. 5,448,314 of Heimbuch et al., and U.S. Pat. No. 5,371,543 of Anderson, utilize the color wheel technology used in conventional TV and PDS products. Operated in a mechanical manner, these prior arts have a low S/N ratio, and it is hard to adjust the time sequence. Furthermore, the different color sectors on the wheel waste optical energy. 
     Recently, ferro-electric liquid crystal cell (FLC-CELL) products, such as the FLC driving scheme of Displaytech Co., have raised the switch speed to 20 μsec, and have memory functions that TN LC cells do not have. However, like a TN LCD panel, a FLCD panel needs a dyed PVA polarizer, which has a high absorption for visible light. The light transmittance of a FLCD panel is only about 28˜30%. Energy loss is a serious problem for TN LCD and FLCD panels. 
     To improve the light transmittance of a polarizer, a structure of all dielectric multi-layer film is of interest. However, it is very difficult to design the film system. In the optical communications field, combining a multi-layer film polarized beam splitter with an electric switch to form a single unit has been reported. Many single units are serially arranged to control the direction of the optical path for a predetermined wavelength. However, this device does not have light-combining or light-splitting function. The structure of this device is as shown in FIG. 2., wherein the reference numbers  21 ˜ 26  indicate optical fibers, reference numbers  27 ˜ 32  indicate cylinder lenses, reference numbers  33 ˜ 37  indicate polarized beam splitter (PBS) prisms, reference numbers  38 ˜ 41  indicate electro-optical components, and reference numbers  43 ˜ 47  indicate PBSs. 
     To provide an improved LCD system having a fast time sequential switch mechanism, an electrically controlled switch having a high speed and a high transmittance is required. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of this invention is to provide a field sequential dichroic light valve, which has a fast time response and a memory function. 
     Another object of this invention is to provide a field sequential dichroic light valve, which has a switch frequency larger than 180 Hz and an optical transmittance larger than 90%. 
     With an appropriately designed optical thin-film filter, this invention can provide a field sequential light-splitting/combining module in an optical system having a white light source. 
     To achieve the above objects, this invention provides a time sequential dichroic light valve, which comprises: a first polarizing beam-splitting device for receiving a broadband incident light and emitting a second polarized light of a first spectral range when an energy field is applied thereto; a second polarizing beam-splitting device for receiving a first polarized light and a second polarized light of other spectral ranges except the first spectral range, and emitting the second polarized light of a second spectral range when an energy field is applied thereto; and a third polarizing beam-splitting device for receiving first polarized light and a second polarized light of other spectral ranges except the first spectral range and the second spectral range, and emitting the second polarized light of a third spectral range when an energy field is applied thereto, whereby the energy field is sequentially applied to the first polarizing beam-splitting device, the second polarizing beam-splitting device and the third polarizing beam-splitting device to switch the transmission of the second polarized lights of the first spectral range, the second spectral range and the third spectral range along a selective direction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following detailed description, given by way of example and not intended to limit the invention solely to the embodiments described herein, will best be understood in conjunction with the accompanying drawings in which: 
     FIGS. 1 a  and  1   b  are diagrams illustrating a conventional electric optical switch; 
     FIG. 2 is a diagram illustrating a conventional multilayer broadband PBS; 
     FIGS. 3 a  to  3   c  are diagrams illustrating the structure of a ferro-electric liquid crystal cell; and 
     FIGS. 4 a  and  4   b  are diagrams illustrating the field sequential dichroic light valve of this invention under an external electric field; 
     FIG. 5 a  is the reflective spectrum of a red vertical PBS; 
     FIG. 5 b  is the reflective spectrum of a green vertical PBS; 
     FIG. 5 c  is the reflective spectrum of a blue vertical PBS; 
     FIG. 6 is a diagram illustrating the tri-color field sequential switch prism module that is made of PBS of FIGS. 5 a ,  5   b  and  5   c;    
     FIG. 7 is a diagram illustrating the structure of a field sequential dichroic light valve according to one embodiment of this invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIG. 3 a  and  3   b , a ferro-electric liquid crystal cell comprises: a polarizer  50   a , an analyzer  50   b , ITO glasses  52   a  and  52   b , alignment layers  54   a  and  54   b , ferro-electric liquid crystal molecules  56  and DC electric field  58 . According to the drawings, switching the polarity of the DC electric field can make the ferro-electric liquid crystal molecules  56  switch between the bi-stable states of ±θ angle. Thus, the cell can serve as an electric optical path switch to control the optical path of incident light between the polarizer and the analyzer. Referring to FIG. 3 a , the incident light I o  becomes polarized light after passing through the polarizer. The polarization of the polarized light rotates to 90° when passing through the ITO glasses  52   a  and  52   b , the alignment layers  54   a  and  54   b  and the ferro-electric liquid crystal molecules  56 , which are controlled by the electric field +E. Thus, the polarized light can pass the analyzer  50   b . The ferro-electric liquid crystal cell is transparent. Referring to FIG. 3 b , when an electric field −E having an opposite polarity is applied, the position of the ferro-electric liquid crystal molecule is switched from +θ to −θ. At this time, the optical axis of the incident light passing through the polarizer has the same direction as the ferro-electric liquid crystal molecule. Therefore, the polarization of the polarized light will not change after passing through the ITO glasses  52   a  and  52   b , the alignment layers  54   a  and  54   b  and the ferro-electric liquid crystal molecules  56 . The polarized light cannot pass through the analyzer  50   b.    
     Referring to FIG. 3 c , the ferro-electric liquid crystal molecule has a spontaneous polarization P at the direction perpendicular to the long axis of the molecule. The spontaneous polarization fast switches between the bi-stable states of 2θ by the effect of electric field ±E. Furthermore, under this bi-stable state, the ferro-electric liquid crystal molecule has a memory function so that the liquid crystal molecule can remain in the same position even there is no applied electric field. In the application of a display, this implies that the original image remains even if the power supply is cut-off. 
     Referring to FIGS. 4 a  and  4   b , the fabrication of this invention is begun by preparing two right-angle prisms by coating a multi-layer thin-film system having a high polarization on the slope of one of the prisms, and then adhering the two prisms to form a cubic polarized beam-splitter. The upper and lower ITO glasses and alignment layers  67 ,  68  are placed between two cubic polarized beam-splitters. The ferro-electric liquid crystal molecules are then filled in the space between the alignment layers. 
     FIG. 4 a  shows the structure of this invention when the applied electric field is +E. The incident s-polarized light is reflected by the broadband polarized beam-splitter  62  on the slope between the prisms  60   a  and  60   b  to pass the upper ITO glass and the upper alignment layer  67 . Thereafter, the polarization of the polarized light rotates to 90° due to the birefringence of the ferro-electric liquid crystal molecules  69 . That is, the s-polarized light is converted to a p-polarized light. After passing through the lower ITO glass and the lower alignment layer  68 , the p-polarized light can pass through the broadband polarized beam-splitter  66  on the slope between the prisms  64   a ,  64   b.    
     On the other hand, referring to FIG. 4 b , when the applied electric field is −E, since the principal axis of the ferro-electric liquid crystal molecules is parallel to the electric field, the polarization of the incident s-polarized light is not changed while passing the ferro-electric liquid crystal molecules. Thus, the s-polarized light is reflected by the broadband polarized beam-splitter  66 . 
     In other words, if the incident light is a s-polarized light, FIG. 4 a  shows the “OFF” state of the ferro-electric liquid crystal light valve, and FIG. 4 b  shows the “ON” state. 
     In the above embodiment, various light valves can be obtained by using various thin-film systems as the multilayer thin film of the polarized beam-splitter  62 ,  66 . Refer to FIG. 5 a  to FIG. 5 c , the multi-layer thin film is a s-polarized red light reflective coating when the relation of reflectivity and the wavelength of incident light for s-polarized light and p-polarized light is as shown in FIG. 5 a . If the relation of reflectivity and the wavelength of incident light for s-polarized light and p-polarized light is as shown in FIG. 5 b , the multi-layer thin film is an s-polarized green light reflective coating. Thus, the device of FIG. 4 becomes a green light valve. The multi-layer thin film of FIG. 5 c  is a s-polarized blue light reflective coating. 
     Referring FIG. 6, the time sequential dichroic light valve can be obtained by serially arranging three light valves respectively having the features of FIG. 5 a , FIG. 5 b , and FIG. 5 c . The field sequential dichroic light valve comprises: a first polarized beam-splitting device comprising a first polarizing beam-splitting component  80 , a second polarizing beam-splitting component  81  and a first light valve  70  which can be controlled by an applied energy field, wherein when a broadband incident light is projected to the first polarizing beam-splitting component  80 , a first polarized light of first spectral range is reflected, a second polarized light of the first spectral range and the first and second polarized lights of other spectral ranges pass through the first polarizing beam-splitting component  80 , and when the energy field applied to the first light valve  70  is a negative electric field as shown in FIG. 4 b , the first polarized light passing through the first light valve  70  is reflected by the second polarizing beam-splitting component  81 , however, when the energy field applied to the first light valve  70  is an energy field having a reverse polarization to the negative electric field, the first polarized light of first spectral range is converted to a second polarized light by the first light valve, thus the second polarized light can pass through the second polarizing beam-splitting component  81 ; a second polarized beam-splitting device comprising a third polarizing beam-splitting component  82 , a fourth polarizing beam-splitting component  83  and a second light valve  72  which can be controlled by an applied energy field, wherein when the second polarized light of first spectral range and the first and second polarized lights of other spectral ranges passing through the first polarizing beam-splitting component  80  are projected to the third polarizing beam-splitting component  82 , the first polarized light of second spectral range is reflected, the second polarized lights of the first spectral range and the second spectral range and the first and second polarized lights of other spectral ranges except the first spectral range and the second spectral range pass through the third polarizing beam-splitting component  82 , and when the energy field applied to the second light valve  72  is a negative as shown in FIG. 4 b , the first polarized light of the second spectral range is reflected by the fourth polarizing beam-splitting component  83  to incorporate the first polarized light of the first spectral range reflected by the second polarizing beam-splitting component  81  to emit from the prism  93 , however, when the energy field applied to the second light valve  72  is an energy field having a reverse polarization to the negative electric field, the first polarized light of the second spectral range is converted to a second polarized light by the second light valve  72 , thus the second polarized light will pass through the fourth polarizing beam-splitting component  83  and emit from the prism  93   a ; and a third polarizing beam-splitting device comprising a fifth polarizing beam-splitting component  84 , a sixth polarizing beam-splitting component  85  and a third light valve  74  which can be controlled by an applied energy field, wherein when the first polarized light and the second polarized light of other spectral ranges except the first spectral range and the second spectral range passing through the third polarizing beam-splitting component  82  are projected to the fifth polarizing beam-splitting component  84 , the first polarized light of a third spectral range is reflected and the second polarized light of all spectral ranges passes through the fifth polarizing beam-splitting component  84 , and when the energy field applied to the third light valve  74  is a negative electric field as shown in FIG. 4 b , the first polarized light passes through the third light valve  74  and is reflected by the sixth polarizing beam-splitting component  85  to incorporate the first polarized lights of the first spectral range and the second spectral range emitted from the prism  93  to emit from the prism  95 , however, when the energy field applied to the third light valve  74  is an energy field having a reverse polarization to the negative electric field, the first polarized light of the third spectral range is converted to a second polarized light by the third light valve  74 , thus the second polarized light passes through the sixth polarizing beam-splitting component  85  to emit from the prism  95   a , so that the transmission of the lights of the three spectral ranges can be sequentially switched along a selected direction by applying an energy field to the first light valve  70 , the second light valve  72  and the third light valve  74  in turn. 
     The operation of the field sequential dichroic light valve is described in detail below. Referring to FIG. 6, when an unpolarized white light W is incident to the red light reflecting filter  80 , the s-polarized red light Sr is reflected, and the s-polarized blue light Sb, s-polarized green light Sg and the p-polarized white light are transmitted. When the electric field (−E) is applied to the ferro-electric liquid crystal component  70 , s-polarized red light Sr passes through the ferro-electric liquid crystal component  70  and is then reflected by the red light reflecting filter  81  to pass through the green light reflecting filter  83  and the blue light reflecting filter  85 , thus emitting from the prism  95 . When the electric field (+E) is applied to the ferro-electric liquid crystal component  70 , the polarization of the s-polarized red light Sr rotates to 90° after passing through the ferro-electric liquid crystal component  70  so that the s-polarized red light Sr is converted to a p-polarized red light Pr and is emitted from the prism  91   a.    
     The s-polarized blue light Sb, the s-polarized green light Sg and the p-polarized white light passing through the prism  90  are transmitted to the prism  92 . The s-polarized green light Sg is reflected by the green light reflecting filter  82 . When the electric field (−E) is applied to the ferro-electric liquid crystal component  72 , the s-polarized green light Sg passes through the ferro-electric liquid crystal component  72  and is then reflected by the green light reflecting filter  83  to pass through the blue light reflecting filter  85  and emit from the prism  95 . When the electric field (+E) is applied to the ferro-electric liquid crystal component  72 , the polarization of the s-polarized green light Sg rotates to 90° after passing through the ferro-electric liquid crystal component  72 , so that the s-polarized green light Sg is converted to a p-polarized green light Pg that is then emitted from the prism  93   a.    
     The s-polarized blue light Sb and the p-polarized white light passing through the prism  92  are transmitted to the prism  94 . The s-polarized blue light Sb is reflected by the blue light reflecting filter  84 . When the electric field (−E) is applied to the ferro-electric liquid crystal component  74 , the s-polarized blue light Sb passes through the ferro-electric liquid crystal component  74  directly and is reflected by the blue light reflecting filter  85 . When the electric field (+E) is applied to the ferro-electric liquid crystal component  74 , the polarization of the s-polarized blue light Sb rotates to 90° after the s-polarized blue light Sb passes through the ferro-electric liquid crystal component  74 , so that the s-polarized blue light Sb is converted to a p-polarized blue light Pb and emits from the prism  95   a.    
     Therefore, when the electric field (−E) is respectively applied to the ferro-electric liquid crystal components  70 ,  72  and  74  at a time sequence of t=t 1 , t 2 , t 3 , the s-polarized red light Sr, green light Sg and blue light Sb are sequentially emitted from the prism module. However, when the electric field applied to the ferro-electric liquid crystal components  70 ,  72  and  74  is +E, the p-polarized red light Pr, green light Pg and blue light Pb are respectively emitted from the prisms  91   a ,  93   a  and  95   a  along the direction perpendicular to the incident light W. The p-polarized white light Prgb passes through the prism  90 ,  92  and  94  to emit out along the path of the incident light W. Referring to FIG. 7, if the p-polarized lights Prgb, Pr, Pg, Pb are useless, the light-absorbing devices  100   a ˜ 100   d  can be used to absorb the p-polarized lights. The light-absorbing device, which is composed of at least one layer of thin film or a slab of selected absorbing materials, can be directly coated on the emerging surfaces of the p-polarized lights. FIG. 7 also illustrates a time sequential circuit used in the field sequential dichroic light valve of this invention. The time sequential circuit uses a time sequential signal generating device  110  to produce the time sequential signals R, G, B to respectively control the ferro-electric liquid crystal components  70 ,  72 ,  74 . 
     According to the above description, it is understood that the field sequential dichroic light valve of the present invention has the following advantages: First, the light valve has a fast time response in which the switching time is less than 20 μsec due to the use of ferro-electric liquid crystal. Second, the light valve has a memory feature, which a TN mode liquid crystal component doesn&#39;t have, when there is no power supply due to the bi-stability of ferro-electric liquid crystal component. Third, the light valve has a reduced noise and can increased lifespan since there is no mechanical manner in the operating process. Fourth, the light valve has a polarizing beam-splitting function. Fifth, as shown in FIG.  4  and FIG. 5, the light valve can be used to select the optical path or color field. 
     While the present invention has been particularly shown and described with reference to a preferred embodiment, it will be readily appreciated by those of ordinary skill in the art that various changes and modifications may be made without departing from the spirit and scope of the invention. It is intended that the claims be interpreted to cover the disclosed embodiment, those alternatives, which have been discussed above, and all equivalents thereto.