A LIGHT VALVE

Abstract: A light valve (2) for framing a transparency on an overhead projector when the transparency is arranged in both portrait and landscape orientations comprises a liquid crystal display panel (3). Electrodes (30 and 31) are formed on a front side (27) and electrodes (32 and 33) are formed on a rear side (28) of the panel (3). The electrodes (30 to 33) define three rows (35a, 35b, 35c) of pixels (34) and a plurality of columns (36) of pixels (34). The light valve (2) may be operated with all the pixels (34) in the centre row (35b) operating in light transmitting mode and the pixels (34) in the side rows (35a, 35c) in light scattering mode for forming two side frames for partially framing the transparency (4) in portrait orientation. For framing the transparency in landscape orientation, a plurality of columns (36a) of pixels (34) on either side of the X-axis (25) are operated in light transmitting mode, and pixels (34) in columns (36b) adjacent the top and bottom of the light valve (2) are operated in light scattering mode for forming a top and bottom frame for the transparency in landscape mode.

Description: "A l i ght val ve" The present invention relates to a light valve, and in particular, though not limited a light valve for use in combination with an overhead projector for framing or partially framing a transparency, and in particular, for selectively framing or partially framing a transparency in portrait or landscape orientation. The light valve may also be used, for example, in connection with a transparency feeding device, X-ray inspection apparatus, photocopying apparatus, and with other apparatus where it is desired to frame or partially frame an object sheet in landscape or portrait orientation. The invention also relates to an overhead projector comprising the light valve. Light valves for framing or partially framing a transparency in landscape or portrait orientation in an overhead projector are known. In general, such light valves comprise a liquid crystal display panel which is foπned by top and bottom substrates which define respective front and rear sides of the panel. A suitable liquid crystal medium is provided between the substrates, and electrodes formed on the inner or outer surfaces of the top and bottom substrates define and form pixels in the panel. A typical liquid crystal medium is Polymer Dispersed Liquid Crystal also known as Nematic Curvilinear Aligned Phase. The medium consists of nematic liquid crystal droplets which are dispersed in a transparent polymer matrix. Such liquid crystal medium has a high light transmittance typically, 70%, symmetrical viewing characteristics, and a relatively low manufacturing cost relative to, for example, conventional Twisted Nematic Liquid Crystal Displays. On a voltage difference being applied across a pair of electrodes forming a pixel, the liquid crystal droplets in the liquid crystal medium are selected to either operate the pixel in light transmitting mode, or alternatively, in light scattering or light absorbing mode. When the voltage difference across the electrodes is removed, the liquid crystal droplets of the liquid crystal medium are selected to operate in the alternative mode. In general, when a voltage difference is applied across the electrodes forming a pixel, the pixel is operated in light transmitting mode, and on removal of the voltage difference, the pixel operates in a light scattering or absorbing mode. Such light valves are referred to as positive type light valves. However, in general, for such known light valves to operate effectively for framing or partially framing a transparency in portrait or landscape orientation, it is necessary to dynamically address the electrodes forming the pixels of the panel by known prior art methods which are referred to as passive matrix addressing. Such methods are unsuitable for addressing light valves formed from polymer dispersed liquid crystal panels. This is because a plot of the electro-optical characteristic of such polymer dispersed liquid crystal panels shows that the electro- optical characteristic is not sufficiently sharp to support even the simplest passive matrix addressing, which is two-way multiplex addressing. Thus, in general, such polymer dispersed liquid crystal panels are suitable only for static addressing, which is also referred to as direct addressing. Because of this, known light valves constructed from polymer dispersed liquid crystal panels have been unsuitable for selectively framing or partially framing a transparency in portrait and landscape orientation. To statically address such light valves constructed from polymer dispersed liquid crystal panels, because of the structure of the electrode patterns required on the respective substrates, electrical connections to the electrodes for facilitating addressing of the electrodes must be made by electrically conductive tracks which also are formed on the respective substrates. Such tracks, in general, are located in a working area of the light valve, in other words, the area of the light valve which is to be operated in a light transmitting mode. The provision of such tracks requires relatively wide spacing between the electrodes, which in turn leads to areas of the panel which remain dark. In general, such dark areas are in the form of long bands which may extend parallel to the X and/or the Y-axis of the light valve. Such dark bands are in turn projected onto the screen along with the image of the transparency, and this, needless to say, is undesirable. There is therefore a need for a light valve for use with an overhead projector or other similar type apparatus which overcomes the problems of known light valves. The present invention is directed towards providing such a light valve, and the invention is also directed towards providing an overhead projector with the light valve. According to the invention, there is provided a light valve comprising a liquid crystal display panel defining a working area and an X-axis and a Y-axis in the plane of the panel, and having a front side and a rear side, a plurality of electrodes being formed on the front and rear sides of the panel for forming a plurality of pixels in the working area, which are capable of being selectively operated in a scattering and/or absorbing mode for scattering and/or absorbing incident light and a transmitting mode for transmitting incident light therethrough, wherein the pixels are arranged to form at least three rows parallel to the Y-axis, and at least three columns parallel to the X-axis, and are formed by the electrodes for selectively and alternately forming at least two light transmitting areas in the working area for permitting the transmission of incident light therethrough, the electrodes being arranged and being statically addressable for selectively operating at least one of the pixels in each column in the transmitting mode and two of the pixels in that column simultaneously in the scattering and/or absorbing mode for forming at least a part of one of the light transmitting areas, and for selectively operating the said two pixels of that column in the transmitting mode while the at least one of the pixels is in the transmitting mode for forming at least a part of another one of the light transmitting areas, and each of the electrodes terminate adjacent a peripheral edge of the working area for facilitating connection adjacent the peripheral edge of the working area of the electrodes to a signal source. In one aspect of the invention at least two of the light transmitting areas are of width in a direction parallel to the X- axis different to each other. In another aspect of the invention the length parallel to the Y- axis of the respective light transmitting areas is variable. In a further aspect of the invention the position of the respective light transmitting areas is variable. In one embodiment of the invention some of the electrodes on one of the front and rear sides of the panel cooperate with two of the electrodes on the other of the front and rear sides for forming two pixels, for minimising the number of electrodes required, and for ensuring that all the electrodes terminate adjacent a peripheral edge of the working area. In another embodiment of the invention at least some of the electrodes on one of the front and rear sides of the panel cooperate with four of the electrodes on the other of the front and rear sides for forming four pixels, for minimising the number of electrodes and for ensuring that the electrodes terminate adjacent a peripheral edge of the working area, and also for facilitating manufacture of the liquid crystal display panel. Ideally, each of the electrodes on one of the front and rear sides of the panel cooperate with N of the electrodes on the other of the front and rear sides of the panel for forming N pixels, where N is a whole number greater than or equal to one. In another embodiment of the invention the electrodes are arranged so that in at least one of the columns of the pixels one of the electrodes on each of the front and rear sides of the panel, forms with two of the electrodes on the other of the front and rear sides of the panel, two pixels in the column of pixels. In another embodiment of the invention the electrodes are arranged so that each of the electrodes on each of the front and rear sides of the panel forms with two of the electrodes on the other of the front and rear sides of the panel two pixels in the corresponding row of pixels, for facilitating ease of manufacture of the liquid crystal display panel. In one embodiment of the invention one of the electrodes on the front sides of the panel is an elongated electrode and extends parallel to the Y-axis for forming all the pixels of one of the rows of pixels, for minimising the number of electrodes on the panel. The row of pixels which is formed by the one electrode on the front side of the panel may be formed to one of the peripheral edges of the working area of the panel relative to the Y-axis. In which case, the other electrodes on the front side of the panel are elongated electrodes and extend parallel to the X-axis, each electrode extending parallel to the X-axis forming the remaining pixels in the corresponding column of pixels. The electrodes on the rear side of the panel are provided by a plurality of elongated electrodes extending parallel to the X-axis the electrodes being of two different lengths, the longer electrodes each forming two pixels, one of which pixels is formed with the electrode on the front side of the panel which extends parallel to the Y-axis, and the shorter electrodes on the rear side of the panel forming one pixel with a corresponding one of the electrodes on the front side of the panel which extend parallel to the X- axis. Alternatively, the row of pixels which is formed by the one electrode on the front side of the panel is located intermediate a pair of rows of pixels which are located on opposite sides of the intermediate row, each side row of pixels being formed by a plurality of electrodes extending parallel to the X-axis on the front side of the panel and located on opposite sides of the intermediate electrode. The electrodes on the rear side of the panel are provided by a plurality of elongated electrodes extending parallel to the X-axis for forming with the corresponding electrodes on the front side of the panel three pixels in the corresponding column of pixels. In a further embodiment of the invention one of the electrodes on the rear side of the panel is an elongated electrode, and extends parallel to the Y-axis for forming all the pixels in another row of pixels other than the row formed by the elongated electrode on the front side of the panel which extends parallel to the Y-axis. In another embodiment of the invention a plurality of elongated electrodes extending parallel to the X-axis are provided on the respective front and rear sides of the panel on respective opposite sides of each of the electrodes which extend parallel to the Y-axis, the electrodes which extend parallel to the X-axis on the respective opposite sides of the respective electrodes which extend parallel to the Y-axis being of different length on each of the front and rear sides, the longer electrodes which extend parallel to the X-axis on the respective sides being to one side of the corresponding electrode which extends parallel to the Y- axis, and the shorter electrodes which extend parallel to the X- axis being located on the other side of the corresponding electrode which extends parallel to the Y-axis, the longer electrodes which extend parallel to the X-axis on each of the front and rear sides cooperating with the electrode which extends parallel to the Y-axis and the shorter electrodes which extend parallel to the X-axis on the other of the front and rear sides for forming two pixels in the corresponding column of pixels. In an alternative embodiment of the invention the electrodes on each of the front and rear sides of the panel are provided by elongated electrodes extending parallel to the X-axis of two different lengths, one being longer than the other, the longer electrodes on each of the front and rear sides of the panel cooperating with a corresponding one of the longer and one of the shorter electrodes on the other of the front and rear sides of the panel for forming two pixels in a corresponding column of pixels. Preferably, the shorter electrodes are arranged to form two of the rows of pixels, the said two rows of pixels being located on respective opposite sides of the Y-axis towards the peripheral edge of the working area. In one aspect of the invention three rows of pixels parallel to the Y-axis are formed by the electrodes. In one embodiment of the invention the electrodes are selectively addressable by a first signal and a second signal for defining the respective at least two light transmitting areas, the first and second signals being of waveforms which are similar with the exception that one of the first and second signals is out of phase with the other of the first and second signals so that when the waveform of one of the signals is low or going low, the waveform of the other signal is high or going high. The fact that two signals are sufficient for operating the light valve for selectively and alternately forming the at least two light transmitting areas is largely achieved by virtue of the arrangement of the electrodes. Preferably, the waveforms of each of the first and second signals are square waveforms. In another embodiment of the invention the low value of each of the first and second signals and the high value of each of the first and second signals are such that when one of the signals which are being applied to the respective electrodes on the front and rear side of the panel, which form one of the pixels is changed from one of the first and second signals to the other of the first and second signals the operating mode of the pixel changes. Preferably, in each column of pixels at least two of the adjacent pixels in the column are formed by one electrode on at least one of the front and rear sides of the panel, and the electrodes on the other of the front and rear sides of the panel forming the at least two pixels in the column are addressable so that when the signal being applied to the electrode forming the at least two pixels is changed for changing the operating mode of one of the at least two pixels, the signal being applied to the electrode on the other of the front and rear sides forming the other of the at least two pixels may be changed for maintaining the said other of the at least two pixels operating in the mode in which it had been operating prior to the change of signal on the electrode forming the at least two pixels. In an alternative embodiment of the invention the electrodes are selectively addressable by a third signal, the waveform of which is similar to the first and second signals, with the exception that the frequency of the third signal is different to that of the first and second signals. Preferably, the frequency of the third signal is 2N times the frequency of the first signal, or vice versa, where N is a whole number equal to one or greater than one. Advantageously, every second edge of the third signal coincides with a rising or a falling edge of the first and second signals, or vice versa. In another embodiment of the invention when at least one pixel, but not all of the pixels in at least one of the columns is operated in the transmitting mode, and all the pixels in at least another one of the columns are operated in the scattering and/or absorbing mode, the electrodes on one of the front and rear sides of the panel, which form a pair of adjacent columns of pixels which are located between the column in which the at least one pixel is operating in the transmitting mode and the column or columns of pixels operating in the scattering and/or absorbing mode, are addressed by the third signal for operating all the pixels formed in the adjacent pair or pairs of columns of pixels in the transmitting mode. Preferably, the edges of adjacent electrodes on each of the front and rear sides extend parallel to each other. Advantageously, the adjacent edges of adjacent electrodes on each of the front and rear sides are spaced apart from each other a distance which is just sufficient to break electrical continuity between the adjacent electrodes. Ideally, the width of each electrode over its length is constant, in the working area. Advantageously, the length of each electrode over its width is constant, in the working area. In one embodiment of the invention the electrodes are arranged so that each column of pixels is formed by four electrodes, and at least one electrode is formed on one of the front and rear sides of the panel, and the other electrodes are formed on the other of the front and rear sides. Advantageously, each light transmitting area is centred about the Y-axis. In one aspect of the invention one of the light transmitting areas is a first light transmitting area, and is of width in the X-axis direction which corresponds to the width in the X-axis direction of an object sheet arranged in portrait orientation, and another of the light transmitting areas is a second light transmitting area, and is of width in the X-axis direction which corresponds to the width in the X-axis direction of an object sheet in landscape orientation. In one aspect of the invention each of the light transmitting areas are of rectangular shape. In another embodiment of the invention the light valve is suitable for mounting on an object sheet receiving stage of an overhead projector for partially framing an object sheet. In a further embodiment of the invention the light valve is suitable for mounting just beneath the object sheet receiving stage of the overhead projector. Preferably, a signal generating means is provided for generating at least the first and second signals. Advantageously, a selecting means is provided for each electrode for selecting and applying the signal of the at least first and second signals to be applied to the corresponding electrode. Ideally, a control means is provided for controlling the signal generating means and the selecting means. In one embodiment of the invention the liquid crystal display panel is a positive liquid crystal display panel. In another embodiment of the invention the liquid crystal display panel is a polymer dispersed liquid crystal panel. The advantage of using such a polymer dispersed liquid crystal display panel is that it provides a panel with a relatively high light transmittance, typically, approximately 70%, symmetrical viewing characteristics, and relatively low manufacturing costs. Additionally, the invention provides a projector comprising the light valve according to the invention. In one aspect of the invention the light valve may be located adjacent an object sheet receiving stage of the projector. Alteratively, the light valve may be placed on the object sheet receiving stage of the projector, and the light valve acts as an object sheet receiving stage, or the light valve may be located just beneath the object sheet receiving stage. In one embodiment of the invention the light valve forms the object sheet receiving stage of the projector. Typically, the projector is an overhead projector for projecting from an object sheet onto a screen. The advantages of the invention are many. Firstly, by virtue of the fact that the electrodes on the respective front and rear sides of the liquid crystal display panel are arranged so that each electrode terminates adjacent the periphery of the working area of the panel, electrical connections to the respective electrodes may be made adjacent the peripheral edge of the working area, thereby eliminating the need to run electrically conductive tracks on the front and rear sides of the panel within the working area, which thus eliminates the formation of dark bands in the light valve. A second and important advantage of the invention is that because of the arrangement of the electrodes on the front and rear sides of the panel, the panel may be operated to frame or partially frame a transparency, both in portrait and landscape orientation by statically addressing the electrodes with the two signals, which are identical to each other, with the exception that one signal is phased shifted relative to the other. This, thus, leads to a relatively simple and inexpensive construction of light valve, and the control circuitry for providing the two signals and for selectively addressing the appropriate electrodes with the appropriate signal of the two signals is likewise relatively simple, and thus inexpensive. A third advantage of the invention is provided in the case where all the electrodes on each of the front and rear sides of the panel overlap two corresponding adjacent electrodes on the other side in the Y-axis direction, since this construction of panel significantly reduces the need for accurately aligning the electrodes on the respective front and rear sides in the Y-axis direction. The invention will be more clearly understood from the following description of some preferred embodiments thereof, which are given by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a sectional side elevational view of an overhead projector according to the invention, Fig. 2 is a perspective view of the overhead projector of Fig. 1, in use, Fig. 3 is a view similar to Fig. 1 of the overhead projector of Fig. 1 illustrated in use, Fig. 4 is a view similar to Fig. 2 showing the overhead projector of Fig. 1 also in use, Fig. 5 is a plan view of a working area of one side of a light valve also according to the invention for use in the overhead projector of Fig. 1 showing an electrode pattern on a front side of the light valve, Fig. 6 is a plan view similar to Fig. 5 of the rear side of the light valve of Fig. 5, Fig. 7 is a plan view of the working area of the light valve of Fig. 5 from the front side showing the electrode pattern on the rear side of the light valve in broken lines, Fig. 8 is a plan view of the working area of the light valve of Fig. 5 showing a pixel array of the light valve, Figs. 9(a) to (f) are plan views of the working area of the light valve of Fig. 5 illustrating the light valve in use in different configurations, Fig. 10 is a plan view of a front side of the light valve of Fig. 5, Fig. 11 is a plan view of the rear side of the light valve of Fig. 5, Fig. 12 is an enlarged transverse cross-sectional view of the light valve of Fig. 5, Figs. 13(a) and (b) illustrate waveforms of signals which are applied to the light valve of Fig. 5, Fig. 14 is a plan view similar to Fig. 9 of a light valve according to another embodiment of the invention, Figs. 15(a) and (b) are front and rear plan views similar to Figs. 5 and 6, respectively, of the light valve of Fig. 14, Figs. 16(a) to (c) illustrate waveforms of signals which are applied to the light valve of Fig. 14, Fig. 17 is a front plan view similar to Fig. 5 of a light valve according to another embodiment of the invention, Fig. 18 is a rear plan view similar to Fig. 6 of the light valve of Fig. 17, Fig. 19 is a front plan view similar to Fig. 5 of a light valve according to another embodiment of the invention, Fig. 20 is a rear plan view similar to Fig. 6 of the light valve of Fig. 19, and Fig. 21(a) and (b) are views similar to Figs. 5 and 6, respectively, of a light valve according to another embodiment of the invention. Referring to the drawings, and initially to Figs. 1 to 13, there is illustrated an overhead projector according to the invention indicated generally by the reference numeral 1 which also comprises a light valve also according to the invention indicated generally by the reference numeral 2. The light valve 2 comprises a liquid crystal display panel 3, which in this case is a polymer dispersed liquid crystal display panel, which is a positive type panel. The light valve 2, as will be described below, is provided for selectively framing or partially framing an object sheet, in this case an A4 transparency 4 when oriented in portrait and landscape orientation on the projector 1. Before describing the light valve 2, the overhead projector 1 will first be described. The overhead projector 1 comprises a light box 7 having a base 8, side and end walls 9 and 10, respectively, and a top wall 11, which together define a hollow interior region 12. The top wall 11 supports and frames a glass stage 15 for supporting the transparency 4, which may be placed on the stage 15 in portrait or landscape orientation. A light source, namely, a lamp 16, in the light box 7 projects an image of the transparency 4 onto a screen 17. A spherical mirror 18 reflects light which is backwardly directed from the lamp 16 and focuses the backwardly directed light at the centre of the lamp 16. A condensing lens 19 in the light box 7 directs incident light from the lamp 16 and the spherical mirror 18 through a Fresnel lens 20. The Fresnel lens 20 is arranged to have the lamp 16 at its object position, and forms an image at a projection lens 21 which is mounted in a carrier 22 which is in turn slidably mounted on a post 23, which extends upwardly from the light box 7. A plane mirror 24 also located on the carrier 22 directs light from the projection lens 21 onto the screen 17. The projection lens 21 is arranged to form an image of the transparency 4, stage 15 and Fresnel lens 20 on the screen 17. The overhead projector 1 up to here is substantially similar to a conventional overhead projector. The light valve 2 is located between the Fresnel lens 20 and the stage 15, and light having passed through the Fresnel lens 20 passes through the light valve 2 before passing through the transparency 4. Turning now to the light valve 2, and referring in particular to Figs. 5 to 13, the light valve 2 comprises the liquid crystal display panel 3 which defines a centrally located X-axis 25 and a centrally located Y-axis 26 in the plane of the panel 3, and has a front side 27 and a rear side 28. The panel 3 defines a working area 29 which in this embodiment of the invention is smaller than the area of the panel 3. In Figs. 5 to 9 only the working area 29 of the panel 3 is illustrated. Referring now in particular to Figs. 5 to 9, an electrode pattern comprising electrodes 30 and 31 is formed on the working area 29 of the front side 27, and an electrode pattern comprising electrodes 32 and 33 is formed on the working area 29 of the rear side 28. The electrodes 30 and 31 on the front side 27 are arranged to cooperate with the electrodes 32 and 33 on the rear side 28 for forming a plurality of pixels 34 arranged in three rows 35a, 35ώ and 35c, which are parallel to the Y-axis 26 and a plurality of columns 36, which are parallel to the X-axis 25, see Fig. 8. The electrodes 30, 31, 32 and 33 are selectively and statically addressable by two signals, namely, a first signal and a second signal, which will be described below for operating the pixels 34 in a light transmitting mode, and in a light scattering or absorbing mode. In the light transmitting mode each pixel 34 transmits incident light from the Fresnel lens 20 through the transparency 4 for projecting the portion of the transparency 4, which corresponds to the pixel 34 which is in the transmitting mode, onto the screen 17. In the light scattering or absorbing mode, each pixel 34 scatters or absorbs incident light from the Fresnel lens 20 for effectively acting as a mask for preventing effective passage of light through the pixel 34. The pixels 34 which are operated in the scattering or absorbing mode act as a mask for masking a portion of the transparency 4 or for partially framing the transparency 4. The electrodes 30, 31, 32 and 33 are arranged on the front and rear sides 27 and 28, respectively of the panel 3 and are selectively and statically addressable by the first and second signals so that the pixels 34 may be selectively operated to alternately form a first light transmitting area 40 for framing the transparency 4 along its opposite sides when placed in portrait orientation on the stage 15, and a second light transmitting area 41 for framing the transparency 4 along its top and bottom sides when placed in landscape orientation on the stage 15, see Figs. 9(a) and (b) . Typically, the area corresponding to a margin along a transparency side will be operated to frame the transparency. The first and second light transmitting areas 40 and 41, respectively, are centred around the X-axis 25 and the Y- axis 26. When the panel 3 is operated to form the first light transmitting area 40, all the pixels 34 in the centre row 35b are operated in the transmitting mode, while all the pixels 34 in the side rows 35a and 35c are operated in the scattering mode for forming two masking areas 42 for framing the transparency 4 in portrait orientation at opposite side edges thereof. The masking areas 42 are illustrated in Fig. 9(a) cross-hatched. The width in the X- axis direction and the length in the Y-axis direction of the first light transmitting area 40 are similar to the width and length, respectively, of the A4 transparency 4. In other words, the width of the pixels 34 in the centre row 35b is similar to the width of an A4 transparency in portrait orientation, and the length of the row 35b of pixels 34 is similar to the length of an A4 transparency in portrait orientation. When the electrodes 30, 31, 32 and 33 are addressed by the first and second signals for operating the pixels 34 to form the second light transmitting area 41, all the pixels 34 in a plurality of the columns 36 disposed on either side of the X-axis 25 are operated in the transmitting mode, while all pixels in a plurality of columns 36 adjacent the top and bottom of the panel 3 are operated in the scattering mode for forming masking areas 44 for in turn framing the top and bottom of the transparency 4 in landscape orientation. The masking areas 44 are illustrated cross-hatched in Fig. 9(b). The width in the X-axis direction and the length in the Y-axis direction of the second light transmitting area 41 is similar to the width and length, respectively of the A4 transparency in landscape orientation. The width of the first light transmitting area 40 in a direction parallel to the X-axis 25 is narrower than the corresponding width of the second light transmitting area 41. Additionally, when the light valve 2 is operated in the first and second light transmitting areas 40 and 41, respectively, as illustrated in Figs. 9(a) and (b) , the length of the light transmitting area 40 in a direction parallel to the Y-axis is longer than the corresponding length of the second light transmitting area 41. Figs. 2 and 3 illustrate the respective lengths A and B, and the respective widths C and D of an image of the transparency 4 projected onto the screen 17 when the light valve 2 is operated to form the first and second light transmitting areas 40 and 41, respectively, as illustrated in Figs. 9(a) and (b) . Additionally, the electrodes 30, 31, 32 and 33 are selectively and statically addressable by the two signals for operating the pixels 34 to form first and second light transmitting areas 40a and 41a, respectively, which are relatively short in the direction parallel to the Y-axis 26 so that as well as partially framing the transparency 4 in either portrait or landscape orientation, the light valve 2 also masks areas of the transparency 4 which may be subsequently either sequentially or progressively revealed. The widths of the first and second light transmitting areas 40a and 41a parallel to the X-axis 25 are similar to the corresponding widths of the respective first and second light transmitting areas 40 and 41, respectively. In Fig. 9(c) the first light transmitting area 40a is relatively short and is located adjacent the top of the light valve 2 for revealing, for example, a title and/or a first bullet point adjacent the top of the transparency 4 when placed on the stage 15 in portrait orientation. In Fig. 9(d) the first light transmitting area is indicated by the reference numeral 40b, and is illustrated having been moved down the light valve 2 for revealing, for example, another bullet point on the transparency 4 when placed on the stage 15 in portrait orientation. In Fig. 9(e) the light valve 2 is operated with the second light transmitting area 41a being located adjacent the top of the transparency 4 when placed on the stage 15 in landscape orientation. The second light transmitting area 41_? illustrated in Fig. 9(e), would typically be provided for revealing a title, and/or first bullet point on the transparency 4 in landscape orientation. In Fig. 9(f) the second light transmitting area 41ύ is illustrated as having been moved down the light valve 2 for revealing, for example, a lower bullet point on the transparency 4 when placed on the stage 15 in landscape orientation. To form the first light transmitting areas 40a and 40b illustrated in Figs. 9(c) and 9(d), only the pixels 34 in the first light transmitting areas 40a and 40b are operated in the transmitting mode, all the pixels 34 in the rows 35a and 35b and the remaining pixels 34 in the row 35c are operated in the scattering or absorbing mode. The pixels 34 which are operated in the light scattering or absorbing mode are cross-hatched in Figs. 9(c) and (d) . Similarly, to operate the light valve 2 with the second light transmitting modes 41Z? and 41c formed, only the pixels 34 in the areas 41b and 41c are operated in the transmitting mode, while the remaining pixels 34 in the cross-hatched area of the light valve 2 are operated in the scattering or absorbing mode. By appropriately addressing the electrodes 30, 31, 32 and 33 with the appropriate first and second signals, the first and second light transmitting areas 40a, 40b and 41a and 4lb may be scrolled upwardly and/or downwardly in the light valve 2 in a direction parallel to the Y-axis, and/or alternatively, the length of the first and second light transmitting areas 40a, 40b and 41a and 4lb may if desired be progressively lengthened or shortened in the direction parallel to the Y-axis for revealing more of or concealing more of the transparency 4. In Fig. 4 a typical length of the image of the transparency 4 in portrait orientation which is projected onto the screen 17 when the light valve 2 is operating as illustrated in Fig. 9(c) with the first light transmitting area 40a formed is indicated by the arrow E. Additionally, more than one first light transmitting area 40a and 406 or 41a and 41t> may be formed simultaneously in the light valve 2 by appropriately operating the relevant pixels 34 in the transmitting mode. For example, it is envisaged that a light transmitting area 40a or 41a may be formed for passing light through the top of a transparency 4, whether in portrait or landscape orientation, for example, for projecting information on the top of the transparency 4 onto the screen 17, typically, this information may be a title or the like. The other or other light transmitting area or areas 406 or 416 would then pass light through the transparency 4 for projecting, for example, a bullet point or bullet points on the transparency 4 onto the screen 17. It is also envisaged that one of the light transmitting areas 40a, 406, 41a and 416 may be maintained in a fixed position, and the other or others of the light transmitting areas may be sequentially scrolled upwardly or downwardly as desired to reveal points of information. For example, the light transmitting areas 40a or 41a adjacent the top of the transparency may be fixed to reveal a title. Fig. 1 illustrates the projector 1 with the light valve 2 operating with all the pixels 34 in the scattering mode. In this mode, the entire transparency 4 is blanked, and indeed, the entire stage 15 is blanked from the screen 17. Thus, no light is projected onto the screen 17. The light valve 2 is normally operated in this mode during changeover of transparencies. Turning now to the pattern of electrodes 30 and 31, and 32 and 33 on the front and rear sides 27 and 28, respectively, each of the electrodes 30, 31, 32 and 33 are of constant width along their length and of constant length along their width. The electrodes 31, 32 and 33 are of similar width, and the electrodes 31 and 32 are of similar length. In Figs. 7 and 9, the electrodes 32 and 33 on the rear side 28 are illustrated in broken lines and are illustrated as being wider than the electrodes 31 on the front side 27. This is only for the purpose of illustration so that the electrodes 32 and 33 can be seen in the views of Figs. 7 and 9. Additionally, adjacent electrodes 30 and 31 on the front side 27 and the electrodes 32 and 33 rear side 28 are arranged to be just sufficiently spaced apart from each other to break electrical continuity between adjacent electrodes 30 and 31, and 32 and 33. This thus minimises the gaps between the pixels 34 and minimises any danger of dark bands between the pixels 34 which are operating in a light transmitting mode being visible when projected onto the screen 17. The electrodes 30 to 33 are arranged on the front and rear sides 27 and 28 so that each column 36 of pixels 34 is formed by only four electrodes, namely, the electrodes 30 and 31 on the front side 27 and the electrodes 32 and 33 on the rear side 28. The electrode 30 on the front side 27 is parallel to the Y-axis and forms all the pixels 34 in the side row 35c. The electrodes 31 on the front side 27 are parallel to the X-axis 25 and each form two pixels 34 in the columns 36, the third pixel 34 in each column 36 being formed by the electrode 30. Each electrode 32 on the rear side 28 is parallel to the X-axis 25 and cooperates with and overlaps the electrode 30 and a corresponding one of the electrodes 31 on the front side 27 for forming two pixels in each column 36, in other words, the pixels 34 in the rows 356 and 35c. Each electrode 33 on the rear side 28 is parallel to the X-axis 25 and cooperates with a corresponding one of the electrodes 31 on the front side 27 for forming the third pixel 34 in each column 36, in other words, the pixel in the side row 35a. By virtue of the fact that only four electrodes 30 to 33 form the pixels 34 in each column 36, each electrode 30 to 33 terminates at a peripheral edge 45 of the working area 29 of the panel 3, and thus an electrical connection discussed below is made to each electrode 30 to 33 adjacent the peripheral edge 45 for applying the first and second signals to the electrodes 30 to 33. Referring now to Figs. 10 to 13 the detailed construction of the light valve 2 and the generation of the first and second signals and the addressing of the electrodes 30 to 33 with the first and second signals will now be described. Figs. 10 to 12 are not to scale, and are provided mainly for the purpose of illustrating the principle of the invention. The panel 3 is formed by a pair of substrates 46 and 47 which form the front and rear sides 27 and 28, respectively, and a liquid crystal medium 48 sandwiched between the substrates 46 and 47. The liquid crystal medium 48 is a polymer dispersed liquid crystal medium, which comprises nematic liquid crystal droplets dispersed in a transparent polymer matrix. The electrodes 30 and 31 are formed on the inner surface of the substrate 46, while the electrodes 32 and 33 are formed on the inner surface of the substrate 47. Selecting means, namely, drivers 50 for selectively applying the first and second signals to the electrodes 30 to 33 are secured to the substrates 46 and 47. Two drivers 50 are secured to each substrate 46 and 47. The drivers 50 are provided with sufficient outputs so that there is at least one output for each of the smaller electrodes, and more than one output for each of the larger electrodes. The outputs from the drivers 50 are connected to the electrodes by electrically conductive tracks 51 which are formed on the respective substrates 46 and 47. The drivers 50 and tracks 51 are arranged so that one driver 50 drives all the electrodes above the X-axis 25 on its corresponding substrate 46 or 47, and the other driver 50 on the substrate drives all the electrodes on the substrate below the X-axis 25. The tracks 51 are connected to the corresponding electrode 30 to 33 adjacent the peripheral edge 45 of the working area 29. A signal generating means is formed by a clock generator 52 in combination with the drivers 50 for generating and selectively applying the first and second signals to the electrodes 30 to 33 through the outputs of the drivers 50. A control means, namely, a microprocessor 54 controls the clock generator 52 and the drivers 50 for respectively generating the first and second signals and applying the first and second signals to the electrodes 30 to 33. Referring now to Fig. 13 the first and second signals which are generated by the clock generator 52 and the drivers 50 are illustrated. The first signal is illustrated by the waveform (a) and the second signal is illustrated by the waveform (b) . The first and second signals (a) and (b) are identical to each other, with the exception that the signal (b) is out of phase with the signal (a). The phase shift is such that when the first and second signals (a) and (b) are applied to respective electrodes which form one of the pixels 34 a voltage difference is maintained across the electrodes for causing the pixel 34 formed by the electrodes to operate in the transmitting mode. When the same signals either the first signal (a) or the second signal (b) are applied to the electrodes of one of the pixels 34 the voltage difference between the electrodes is maintained at zero, and thus the pixel 34 operates in the scattering or absorbing mode. In this embodiment of the invention since the panel 3 is a polymer dispersed liquid crystal display panel, the pixels 34 when the voltage across their respective electrodes is zero or close to zero operate in the light scattering mode. Additionally, in this embodiment of the invention the voltage difference which is applied across the electrodes when the respective first and second signals (a) and (b) are applied to the electrodes of one of the pixels 34 is dependent on the thickness of the polymer dispersed liquid crystal medium layer among other characteristics of the panel. The voltage difference required to cause the pixels to operate in the light transmitting mode may lie in the range between 15 volts and 130 volts. Accordingly, the electrodes 30 to 33 are addressed to operate the pixels 34 in the transmitting mode and the scattering mode as follows. To operate one of the pixels 34 in the transmitting mode, the drivers 50 of the two electrodes forming the pixel 34 are operated to apply the first signal (a) to one of the electrodes, and the second signal (b) to the other electrode for providing a voltage difference across the electrodes of the pixel 34. To operate one of the pixels 34 in the scattering mode, the drivers 50 of the electrodes forming the pixel 34 are operated to apply either the first signal to both of the electrodes of the pixel 34, or the second signal to both of the electrodes of the pixel 34, thereby providing a zero voltage difference across the electrodes of the pixel 34. To form the first light transmitting area 40, the pixels 34 in the side rows 35a and 35c are operated in the scattering mode, while the pixels 34 in the centre row 356 are operated in the transmitting mode. The pixels 34 in the side row 35c are formed by the electrode 30 on the top substrate 46 and the electrodes 32 on the bottom substrate 47. To operate the pixels 34 in the side row 35c in the scattering mode, the first signal (a) is applied to the electrode 30, and the first signal (a) is also applied to the electrodes 32 thereby maintaining the voltage across the electrodes 30 and 32 at zero volts and the pixels 34 in the side row 35c operate in the scattering mode. The pixels 34 in the centre row 356 are formed by the electrodes 31 on the top substrate 46 and the electrodes 32 on the bottom substrate 47. In order to maintain a voltage difference across the electrodes 32 and 31 the signal applied to the electrodes 31 and 32 must be out of phase with each other and thus the second signal (b) is applied to the electrodes 31, and the pixels 34 in the centre row 356 are operated in the transmitting mode. The pixels 34 in the side row 35a are formed by the electrodes 31 on the top substrate 46 and the electrodes 33 on the bottom substrate 47. In order to maintain the voltage across the electrodes 31 and 33 at zero volts, the second signal (b) is applied to the electrodes 33. Thus the signals applied to the electrodes 31 and 33 are in phase with each other, and the pixels 34 in the side row 35a operate in the scattering mode. To form the second light transmitting area 41 in the panel 3, the pixels 34 in the columns 36a are operated in the transmitting mode, while the pixels 34 in the columns 366 are operated in the scattering mode, see Fig. 9(b). To achieve this, the first signal (a) is applied to the electrodes 30 and 31 on the top substrate 46. Since the pixels 34 in the light transmitting area 41 are to operate in the light transmitting mode, the second voltage (b) is applied to the electrodes 32 and 33 on the bottom substrate 47 which are to form the pixels 34 in the second light transmitting area 41. Thus, a voltage difference is applied between the electrodes 30 and 32 on the one hand, and the electrodes 31 and 33, on the other hand which are to form the pixels 34 in the columns 36a of the second light transmitting mode 41, and thus, those pixels 34 operate in the transmitting mode. The pixels 34 in the columns 366 are to operate in the scattering mode, and accordingly, the first voltage (a) is applied to the electrodes 32 and 33 which are to form the pixels 34 in the columns 366. The addressing of the electrodes 30 to 33 with the relevant first and second signals (a) and (b) for operating the light valve 2 with the shorter first and second light transmitting areas 40a and 406, and 41a and 416 is substantially similar to that already described, and will be readily apparent to those skilled in the art. For scrolling or progressively lengthening or shortening the light transmitting areas 40a and 406, and 41a and 416 in the light valve 2, the appropriate drivers 50 are operated by the microprocessor 54 for applying the first and second signals (a) and (b) , respectively, to the appropriate electrodes 30 to 33. This will likewise be readily apparent to those skilled in the art. Manually operated controls (not shown) for commanding the microprocessor 54 for controlling the drivers 50 for in turn operating the light valve 2 are provided. The controls may be conveniently located on the light box 7, or may be provided on a hand held remote console connected to the microprocessor 54. The control of such a microprocessor for operating the clock generator 52 and the drivers 50 will be readily apparent to those skilled in the art. In use, when it is desired to project images of a series of transparencies 4 sequentially onto the screen 17 using the overhead projector 1, initially, the light valve 2 is operated with all the pixels 34 in the scattering mode, thereby blanking out the screen 17. The first of the transparencies 4 is placed on the stage 15, and the light valve 2 is operated in the desired operating configuration. Where the transparency 4 is placed on the stage 15 in portrait orientation, the light valve is operated to form the first light transmitting area 40, or one or more of the light transmitting areas 40a or 406 for portrait orientation, as desired. Where the transparency 4 is placed on the stage 15 in landscape orientation, the light valve 2 is operated to form the second light transmitting area 41, or one or more light transmitting areas 41a or 416 in landscape orientation. Between changeovers from one transparency 4 to the next, the light valve 2 is operated with all the pixels 34 in the scattering mode for blanking the screen 17. Referring now to Figs. 14 to 16 there is illustrated a light valve 60 according to another embodiment of the invention for use in an overhead projector, similar to the overhead projector 1. The light valve 60 comprises a liquid crystal display panel 61, which is substantially similar to the panel 3 and similar components are identified by the same reference numerals. The main difference between the light valve 60 and the light valve 2 is that the electrode patterns which comprise electrodes 62, 63, 64 and 65 on the substrates 46 and 47 which form the front and rear sides 27 and 28, respectively, of the panel 61 are different to the electrode patterns on the substrates 46 and 47 of the panel 3 of the light valve 2. In this embodiment of the invention the electrode pattern on the rear side 28 is substantially similar to the electrode pattern on the rear side 28 of the panel 3 with the exception that the electrodes 64 and 65, which are similar in length to the electrodes 32 and 33, respectively, are wider in width in a direction parallel to the Y-axis 26. The electrodes 62 and 63 on the front side 27 of the panel 61 are similar to the electrodes 64 and 65, respectively, but are located in reverse order so that each column 36 comprises three pixels 34, namely, two side pixels 34 and a centre pixel 34. The pixels 34 in the side rows 35a are formed by the electrodes 63 cooperating with the corresponding electrodes 65, while the pixels 34 in the side row 35c are formed by the electrodes 62 cooperating with corresponding electrodes 64. The pixels 34 in the centre row 356 are formed by the electrodes 63 and 64 cooperating with each other. Additionally, in this embodiment of the invention the electrodes 62 and 63 on the front side 27 are not aligned in a direction parallel to the Y-axis with the corresponding electrodes 64 and 65 on the rear side 28, rather, the electrodes 62 and 63 are staggered relative to the electrodes 64 and 65. In other words, each electrode 62 and 63 overlaps two corresponding electrodes 64 and 65 on the rear side 28. The overlapping is arranged so that a centre line extending through the electrode 62 and 63 parallel to the X-axis coincides with the spacing between the corresponding pairs of electrodes 64 and 65 on the rear side 28 and vice versa. Accordingly, the width of each pixel 34 in the direction parallel to the Y-axis is approximately half the width of the electrodes 62 to 65 in a direction parallel to the Y-axis, and the number of pixels 34 formed in a direction parallel to the Y-axis is equal to twice the number of electrodes 62 or 63 on the front side 27 of the panel 61. The advantage of the light valve 60 is that the panel 61 may be constructed with significantly less precision than the panel 3 of the light valve 2, and accordingly, the cost of manufacture of the panel 61 of the light valve 60 is lower than the cost of manufacture of the panel 3 of the light valve 2. In the case of the light valve 2 which is described with reference to Figs. 1 to 13, in order to minimise the spacings, in other words, the gaps in the Y-axis direction between the pixels 34 it is important that the edges of the electrodes extending parallel to the X-axis on one side of the panel 3 should be accurately aligned with the corresponding edges of the corresponding electrodes on the other side of the panel 3, otherwise, undesirable longitudinally extending gaps are formed between the pixels which extend parallel to the X-axis, and which lead to dark lines being projected onto the screen 17. This problem is overcome by overlapping the electrodes 62 and 63 with the electrodes 64 and 65, respectively, in the Y-axis direction, thereby eliminating or minimising the gaps. Additionally, gaps which extend parallel to the Y-axis 26 are also eliminated or minimised by virtue of the fact that each pixel 34 is formed by at least one electrode which also forms another adjacent pixel 34 in each column 36 of pixels. A further advantage of the light valve 60 is that the number of connections required to the electrodes 62 to 65 for forming a given number of pixels 34 is significantly reduced. In the case of the light valve 60, to address 3N pixels 2N connections only are required. However, a disadvantage of the construction of the light valve 60 is that when the light valve 60 is operated for framing a transparency 4 in portrait form, and a short first light transmitting area or areas 40a and/or 406 are formed a problem arises at the boundary 66 between the first light transmitting areas 40a and/or 406 and a masking area 67 in the centre row 356 of pixels 34, when the electrodes 62 to 65 are addressed by only the first and second signals. In such cases, one of the pixels 34 in one or other of the side rows 35a or 35c at the boundary 66 operates in the transmitting mode. The side row 35a or 35c in which the pixel operates in the transmitting mode depends on the location of the boundary 66, in other words, whether the boundary 66 coincides with an edge of the electrodes 62 and 63 or a centre line of the electrodes 62 or 63. To overcome this problem, the clock generator 52 and the drivers 50 cooperate for providing a third signal (c), see Fig. 16, which is similar to the first and second signals (a) and (b) , with the exception that the frequency of the third signal (c) is twice the frequency of the first and second signals (a) and (b) . Every second rising or falling edge of the third signal (c) coincides with a rising or falling edge of the first and second signals (a) and (b) . The third signal (c) is applied either to the electrodes 62 and 63, or to the electrodes 64 and 65, whichever the side edge of which coincides with the boundary 66. This, as will be described below causes all the pixels formed by the electrodes to which the third signal is applied to operate in the transmitting mode. Accordingly, all the pixels 34 in the two columns 36* and 36 which are closest to the boundary 66, and which contain pixels which are in the first light transmitting areas 40a or 406, as the case may be all operate in the transmitting mode. Thus, the pixels 34 in the columns 36 and 36 which also lie in the side rows 35a and 35c adjacent the boundary 66 also operate in the transmitting mode. While the pixels 34 operating in light transmitting mode in the side rows 35a and 35c are not required to operate in the transmitting mode, their operation in the transmitting mode is acceptable, since it provides a balance to the operation of the light valve 60. Furthermore, since the pixels 34 are of relatively narrow width in the Y-axis direction, the fact that two pixels 34 in the respective side rows 35a and 35c are operating in the transmitting mode is acceptable. Where the first light transmitting area 40a or 406 is bound at the top and the bottom by masking areas 67 the third signal is applied either to the electrodes 62 and 63, or to the electrodes 64 and 65 whichever, the side edges of which coincide with the top and bottom boundaries 66. In Fig. 14 the third signal is applied to the electrodes 64 and 65, the edges of which form the boundary 66. By virtue of the fact that the third signal (c) is at twice the frequency of the first and second signals, the voltage difference across the electrodes forming the pixels 34 in the columns 36 and 36/ is J5 times, namely, 0.7 times the voltage difference across the electrodes of the other pixels 34 operating in the transmitting mode. This is sufficient for operating the pixels 34 in the columns 36 and 36y with a transmittance, which is substantially similar to the light transmittance through the pixels 34 which are operated in the transmitting mode by applying the first and second signals to their respective electrodes. To operate the light valve 60 to form the first light transmitting area 40, in other words, to operate the light valve 60 with all the pixels 34 in the centre row 356 in the transmitting mode, and all the pixels 34 in the side rows 35a and 35c in the scattering mode, only the first and second signals (a) and (b) are required for addressing the electrodes 62 to 65. The addressing of electrodes 62 to 65 with the first and second signals for forming the first light transmitting area 40 will be readily apparent to those skilled in the art from the description of the addressing of the electrodes 30 to 33 of the light valve 2. The third signal is only required where the light valve 60 is to be operated to form a first light transmitting area 40a or 406, in other words, where some of the pixels 34 in the centre row 356 are to operate in the transmitting mode and others of the pixels 34 in the centre row 356 are to operate in the scattering mode. When the light valve 60 is being operated for framing a transparency 4 in landscape orientation, all the pixels 34 of the columns 36 which form the second light transmitting area or areas 41, 41a and/or 416 are operated in the transmitting mode, and thus the problem at the boundary 66 does not arise. Accordingly, the light valve 60 when operated to form the second light transmitting areas 41, 41a or 416 to frame or to frame and part mask a transparency 4 in landscape orientation is addressed by the first and second signals only. Referring now to Figs. 17 and 18 there is illustrated a light valve according to another embodiment of the invention indicated generally by the reference numeral 70 for mounting in the overhead projector 1 between the Fresnel lens 20 and the stage 15 in similar fashion as the light valve 2. In this embodiment of the invention, the light valve 70 comprises a liquid crystal display panel 71 which is substantially similar to the liquid crystal display panel 3, and similar components are identified by the same reference numerals. The main difference between the liquid crystal display panel 71 and the panel 3 is in the electrode patterns on the front and rear sides 27 and 28. In this embodiment of the invention, each column 36 of pixels 34 are formed by four electrodes, however, three of the electrodes, namely, the electrodes 73, 74 and 75 are formed on the front side 27, while only the electrode 76 is formed on the rear side 28. The electrode 74 cooperates with the electrodes 76 to form all the pixels in the centre row 356. The electrodes 73 and 75 cooperate with the electrodes 76 to form the pixels 34 in the side rows 35a and 35c, respectively. It will be evident to those skilled in the art how first light transmitting areas 40, 40a and 406 and second light transmitting areas 41, 41a and 416 can be formed by selectively and statically addressing the relevant electrodes 73 to 76 with the appropriate first or second signal from the description already given of the liquid crystal display panel 3 with reference to Figs. 1 to 13. Referring now to Figs. 19 and 20, there is illustrated a light valve 80 according to another embodiment of the invention which comprises a liquid crystal display panel 81. The liquid crystal display panel 81 is substantially similar to the panel 3 and similar components are identified by the same reference numerals. The main difference between the panel 81 and the panel 3 is that additional electrodes 82, 83 and 84 are provided on the front side 27, and corresponding electrodes 85, 86 and 87 are provided on the rear side 28. The electrode 82 cooperates with the electrode 85 to form an upper frame around the transparency 4 when in landscape orientation, or a part of the first light transmitting area 40 or 40a when the transparency is in portrait orientation. The electrodes 84 and 83 on the front side 27 cooperate with the electrodes 86 and 87, respectively, on the rear side 28 to form a frame area at the bottom of the transparency in landscape orientation. The electrodes 86 and 87 also cooperate to form part of the first light transmitting areas 40 and 40b when the transparency 4 is in portrait orientation. An electrode 90 on the front side 27 cooperates with electrodes 91 and 92 on the rear side 28 for forming two relatively wide pixels 34 in a column 36 in a direction parallel to the Y-axis. The electrode 91 on the rear side 28 cooperates with the electrode 90 on the front side 27 for forming a third relatively wide pixel 34 in the column 36 in the direction parallel to the Y-axis. The pixel 34 in the centre row 356 formed by the electrodes 83 and 87 is operated in the scattering mode for framing a US standard transparency when in portrait form. Otherwise, when the light valve 80 according to this embodiment of the invention is used with an A4 transparency in portrait form, the pixel 34 in the centre row 356 formed by the electrodes 83 and 87 is operated in light transmitting mode, unless it is required to mask a portion of the A4 transparency. Corners 94 of the liquid crystal display panel 81 are not provided with electrodes, and these portions of the panel 81 are permanently blank, since they fall outside the area of a transparency, whether in portrait or landscape orientation. Referring to Fig. 21 there is illustrated a light valve 100 according to another embodiment of the invention. The light valve 100 comprises a liquid crystal display panel 101 which is substantially similar to the liquid crystal display panel 3 of the light valve 2, and similar components are identified by the same reference numerals. The main difference between the light valve 100 and the light valve 2 is in the arrangement of the electrodes. In this embodiment of the invention electrodes 102, 103 and 104 are provided on the front side 27 of the panel 101, and electrodes 105, 106 and 107 are provided on the rear side 28 of the panel 101. This arrangement of electrodes, provides for the formation of four rows 35 of pixels parallel to the Y-axis 26, in other words, rows 35a, 356, 35c and 35d. The electrodes 103 and 106, on the front and rear sides 27 and 28, respectively, each form one row 35 of pixels 34 each, namely, the rows 356 and 35c. The electrodes 102 on the front side 27 form two pixels 34 in each column 36 of pixels 34 with the electrodes 105 and 106 on the rear side 28. The electrodes 107 on the rear side 28 of the panel 101 form the other two pixels 34 in each column of pixels with the electrodes 103 and 104 on the front side 27. In this embodiment of the invention the electrodes 102 and 104 on the front side 27 and 105 and 107 on the rear side 28 are aligned with each other in the Y-axis direction, in other words, they do not overlap in the Y-axis direction, as is the case in the light valve 60. By addressing the electrodes 102 to 107 with the first and second signals (a) and (b) , first and second light transmitting areas 40, 40a and 406, and 41, 41a and 416 may be formed as in the case of the light valve 2. The first light transmitting areas are formed by operating all or some of the pixels in the rows 356 and 35c in the transmitting mode, and all the pixels in the rows 35a and 35d in the scattering mode. The second light transmitting areas are formed by operating all the pixels in the relevant columns 36 in the transmitting mode, and all the pixels in the remaining columns 36 in the scattering mode. In addition, third and fourth light transmitting areas may also be formed by addressing the electrodes 102 to 107 with the first and second signals (a) and (b) . The third light transmitting area is formed by operating all the pixels in the row 356 in the transmitting mode, and all the pixels in the rows 35a, 35c and 35d in the scattering mode. The fourth light transmitting area is formed by operating all the pixels in the row 35c in transmitting mode, and the pixels in the rows 35a, 356 and 35d in the scattering mode. Additionally, as well as forming shorter first and second light transmitting areas 40a and 406, and 41a and 416, shorter third and fourth light transmitting areas may also be formed. The addressing of the electrodes for forming the first, second, third and fourth light transmitting areas, as well as the shorter first, second, third and fourth light transmitting areas in the light valve 100 will be readily apparent to those skilled in the art from the description of the addressing of the electrodes 30 to 33 of the light valve 2. While the light valves according to the invention have been described for use in an overhead projector also according to the invention, the light valves according to the invention may be used in conjunction with a transparency feeding apparatus, a photocopier, an X-ray viewing apparatus, or indeed, any other apparatus where it is desired to partially mask and/or frame or partially frame a transparency or an object sheet. It will of course be appreciated that while the light valves have been described for framing a transparency of A4 size, they may be used for framing a transparency of any other size, and this would merely require providing a panel with electrodes of appropriate dimensions to accommodate the dimensions of the transparency. While the pixels in the liquid crystal display of the light valves have been described as operating in a transmitting mode and a scattering mode, it will also be appreciated that instead of the scattering mode, the pixels may operate in an absorbing mode, in which case, a dye would be introduced to the polymer dispersed liquid crystal medium which would absorb the light when the liquid crystal droplets of the pixels are randomly oriented as they are when the voltage across the electrodes forming the pixels to operate in the scattering mode is zero volts. It is envisaged that the frequency of the third signal may be selected at frequencies other than twice the frequency of the first and second signals. For example, the frequency of the third signal may be any whole number of times the frequency of the first and second signals, or alternatively, the first and second signals could be any whole number of times the frequency of the third signal. Indeed, it will be appreciated that once the frequency of the third signal on the one hand, and the frequency of the first and second signals on the other hand are different, that is sufficient, although not preferable. The rising or falling edges of the third signal do not have to coincide with the rising or falling edges of the first and second signals.

Claims: CLAIMS 1. A light valve comprising a liquid crystal display panel (3,61,71,81,101) defining a working area (29) and an X-axis (25) and a Y-axis (26) in the plane of the panel (3,61,71,81,101), and having a front side (27) and a rear side (28), a plurality of electrodes (30,31,32,33,62,63,64,65,73,74,75,76,82,83,84,85,86, 87,90,91,92,102,103,104,105,106,107) being formed on the front and rear sides (27,28) of the panel (3,61,71,81,101) for forming a plurality of pixels (34) in the working area (29), which are capable of being selectively operated in a scattering and/or absorbing mode for scattering and/or absorbing incident light and a transmitting mode for transmitting incident light therethrough, characterised in that the pixels (34) are arranged to form at least three rows (35) parallel to the Y-axis (26), and at least three columns (36) parallel to the X-axis (25), and are formed by the electrodes (30 to 33, 62 to 65, 73 to 76, 82 to 87, 90 to 92, 102, 107) for selectively and alternately forming at least two light transmitting areas (40,41) in the working area (29) for permitting the transmission of incident light therethrough, the electrodes (30 to 33, 62 to 65, 73 to 76, 82 to 87, 90 to 92, 102, 107) being arranged and being statically addressable for selectively operating at least one of the pixels (34) in each column (36) in the transmitting mode and two of the pixels (34) in that column (34) simultaneously in the scattering and/or absorbing mode for forming at least a part of one (40) of the light transmitting areas (40,41), and for selectively operating the said two pixels (34) of that column (34) in the transmitting mode while the at least one of the pixels (34) is in the transmitting mode for forming at least a part of another one (41) of the light transmitting areas (40,41), and each of the electrodes (30 to 33, 62 to 65, 73 to 76, 82 to 87, 90 to 92, 102, 107) terminate adjacent a peripheral edge (45) of the working area (29) for facilitating connection adjacent the peripheral edge (45) of the working area (29) of the electrodes (30 to 33, 62 to 65, 73 to 76, 82 to 87, 90 to 92, 102, 107) to a signal source (50,52). 2. A light valve as claimed in Claim 1 characterised in that at least two of the light transmitting areas (40,41) are of width in a direction parallel to the X-axis different to each other. 3. A light valve as claimed in Claim 1 or 2 characterised in that the length parallel to the Y-axis of the respective light transmitting areas (40,41) is variable. 4. A light valve as claimed in any preceding claim characterised in that the position of the respective light transmitting areas (40,41) is variable. 5. A light valve as claimed in any preceding claim characterised in that at least some of the electrodes on one of the front and rear sides (27,28) of the panel (3,61,71,81,101) cooperate with two of the electrodes on the other of the front and rear sides (27,28) for forming two pixels (34). 6. A light valve as claimed in any preceding claim characterised in that at least some of the electrodes on one of the front and rear sides (27,28) of the panel (3,61,71,81,101) cooperate with four of the electrodes on the other of the front and rear sides (27,28) for forming four pixels (34). 7. A light valve as claimed in any preceding claim characterised in that each of the electrodes on one of the front and rear sides (27,28) of the panel (3,61,71,81,101) cooperate with N of the electrodes on the other of the front and rear sides (27,28) of the panel (3) for forming N pixels (34), where N is a whole number greater than or equal to one. 8. A light valve as claimed in any preceding claim characterised in that the electrodes are arranged so that in at least one of the columns (36) of the pixels (34) one of the electrodes on each of the front and rear sides (27,28) of the panel (3,61,71,81,101), forms with two of the electrodes on the other of the front and rear sides (27,28) of the panel (3,61,71, 81,101), two pixels (34) in the column (36) of pixels (34). 9. A light valve as claimed in any preceding claim characterised in that the electrodes (62,63,64,65) are arranged so that each of the electrodes (62,63,64,65) on each of the front and rear sides (27,28) of the panel (61) forms with two of the electrodes (62,63,64,65) on the other of the front and rear sides (27,28) of the panel (61) two pixels (34) in the corresponding row (35) of pixels (34). 10. A light valve as claimed in any preceding claim characterised in that one of the electrodes (30,74,103) on the front sides (27) of the panel (3,71,101) is an elongated electrode (30) and extends parallel to the Y-axis (26) for forming all the pixels (34) of one of the rows (35) of pixels (34). 11. A light valve as claimed in Claim 10 characterised in that the row (35) of pixels (34) which is formed by the one electrode (30) on the front side (27) of the panel (3) is formed to one of the peripheral edges of the working area (29) of the panel (3) relative to the Y-axis (26). 12. A light valve as claimed in Claim 11 characterised in that the other electrodes (31) on the front side (27) of the panel (3) are elongated electrodes and extend parallel to the X-axis (25), each electrode (31) extending parallel to the X-axis (25) forming the remaining pixels (34) in the corresponding column (36) of pixels (34). 13. A light valve as claimed in any of Claims 10 to 12 characterised in that the electrodes (32,33) on the rear side (28) of the panel (3) are provided by a plurality of elongated electrodes (32,33) extending parallel to the X-axis (25) the electrodes (32,33) being of two different lengths, the longer electrodes (32) each forming two pixels (34), one of which pixels (34) is formed with the electrode (30) on the front side (27) of the panel (3) which extends parallel to the Y-axis (26), and the shorter electrodes (33) on the rear side (28) of the panel (3) forming one pixel (34) with a corresponding one of the electrodes (31) on the front side (27) of the panel (3) which extend parallel to the X-axis (25). 14. A light valve as claimed in Claim 10 characterised in that the row (35) of pixels (34) which is formed by the one electrode (74,103) on the front side (27) of the panel (3,71,101) is located intermediate a pair of rows (35) of pixels (34) which are located on opposite sides of the intermediate row (35), each side row (35) of pixels (34) being formed by a plurality of electrodes (73,75,105,107) extending parallel to the X-axis (25) on the front side (27) of the panel (3,71,101) and located on opposite sides of the intermediate electrode (74,103). 15. A light valve as claimed in Claim 14 characterised in that the electrodes (76) on the rear side (28) of the panel (3,71) are provided by a plurality of elongated electrodes (76) extending parallel to the X-axis (25) for forming with the corresponding electrodes (73,74,75) on the front side (27) of the panel (3,71) three pixels (34) in the corresponding column (36) of pixels (34). 16. A light valve as claimed in any of Claims 10 to 14 characterised in that one of the electrodes (106) on the rear side (28) of the panel (3,101) is an elongated electrode (106), and extends parallel to the Y-axis (26) for forming all the pixels (34) in another row (35) of pixels (34) other than the row (35) formed by the elongated electrode (103) on the front side (27) of the panel (3,101) which extends parallel to the Y-axis (26). 17. A light valve as claimed in Claim 16 characterised in that a plurality of elongated electrodes (102,104,105,107) extending parallel to the X-axis are provided on the respective front and rear sides (27,28) of the panel (101) on respective opposite sides of each of the electrodes (103,106) which extend parallel to the Y-axis (26), the electrodes (102,104,105,107) which extend parallel to the X-axis (25) on the respective opposite sides of the respective electrodes (103,106) which extend parallel to the Y-axis (26) being of different length on each of the front and rear sides (27,28), the longer electrodes (107,102) which extend parallel to the X-axis (25) on the respective sides (27,28) being to one side of the corresponding electrode (103,106) which extends parallel to the Y-axis, and the shorter electrodes (104,105) which extend parallel to the X-axis (25) being located on the other side of the corresponding electrode (103,106) which extends parallel to the Y-axis (26), the longer electrodes (102,107) which extend parallel to the X-axis on each of the front and rear sides (27,28) cooperating with the electrode (103,106) which extends parallel to the Y-axis and the shorter electrodes (104,105) which extend parallel to the X-axis on the other of the front and rear sides (27,28) for forming two pixels (34) in the corresponding column (36) of pixels (34). 18. A light valve as claimed in any of Claims 1 to 9 characterised in that the electrodes (62,63,64,65) on each of the front and rear sides (27,28) of the panel (61) are provided by elongated electrodes extending parallel to the X-axis (25) of two different lengths, one being longer than the other, the longer electrodes on each of the front and rear sides (27,28) of the panel (61) cooperating with a corresponding one of the longer and one of the shorter electrodes on the other of the front and rear sides (27,28) of the panel (61) for forming two pixels (34) in a corresponding column (36) of pixels (34). 19. A light valve as claimed in Claim 18 characterised in that the shorter electrodes are arranged to form two of the rows (35) of pixels (34), the said two rows (35) of pixels (34) being located on respective opposite sides of the Y-axis (26) towards the peripheral edge of the working area (29). 20. A light valve as claimed in any preceding claim characterised in that three rows (35) of pixels (34) parallel to the Y-axis (26) are formed by the electrodes. 21. A light valve as claimed in any preceding claim characterised in that the electrodes are selectively addressable by a first signal and a second signal for defining the respective at least two light transmitting areas (40,41), the first and second signals being of waveforms which are similar with the exception that one of the first and second signals is out of phase with the other of the first and second signals so that when the waveform of one of the signals is low or going low, the waveform of the other signal is high or going high. 22. A light valve as claimed in Claim 21 characterised in that the waveforms of each of the first and second signals are square waveforms. 23. A light valve as claimed in Claim 21 or 22 characterised in that the low value of each of the first and second signals and the high value of each of the first and second signals are such that when one of the signals which are being applied to the respective electrodes on the front and rear side of the panel, which form one of the pixels (34) is changed from one of the first and second signals to the other of the first and second signals the operating mode of the pixel (34) changes. 24. A light valve as claimed in any of Claims 21 to 23 characterised in that in each column of pixels (34) at least two of the adjacent pixels (34) in the column are formed by one electrode on at least one of the front and rear sides (27,28) of the panel (3,61,71,81,101), and the electrodes on the other of the front and rear sides (27,28) of the panel forming the at least two pixels (34) in the column are addressable so that when the signal being applied to the electrode forming the at least two pixels (34) is changed for changing the operating mode of one of the at least two pixels (34), the signal being applied to the electrode on the other of the front and rear sides (27,28) forming the other of the at least two pixels (34) may be changed for maintaining the said other of the at least two pixels (34) operating in the mode in which it had been operating prior to the change of signal on the electrode forming the at least two pixels (34). 25. A light valve as claimed in any of Claims 21 to 24 characterised in that the electrodes are selectively addressable by a third signal, the waveform of which is similar to the first and second signals, with the exception that the frequency of the third signal is different to that of the first and second signals. 26. A light valve as claimed in Claim 25 characterised in that the frequency of the third signal is 2N times the frequency of the first signal, or vice versa, where N is a whole number equal to one or greater than one. 27. A light valve as claimed in Claim 25 or 26 characterised in that every second edge of the third signal coincides with a rising or a falling edge of the first and second signals, or vice versa. 28. A light valve as claimed in Claim 25 or 27 characterised in that when at least one pixel (34), but not all of the pixels (34) in at least one of the columns (36) is operated in the transmitting mode, and all the pixels (34) in at least another one of the columns (36) are operated in the scattering and/or absorbing mode, the electrodes on one of the front and rear sides (27,28) of the panel (61), which form a pair of adjacent columns (36) of pixels (34) which are located between the column (36) characterised in that the at least one pixel is operating in the transmitting mode and the column or columns of pixels (34) operating in the scattering and/or absorbing mode, are addressed by the third signal for operating all the pixels formed in the adjacent pair or pairs of columns of pixels in the transmitting mode. 29. A light valve as claimed in any preceding claim characterised in that edges of adjacent electrodes on each of the front and rear sides (27,28) extend parallel to each other. 30. A light valve as claimed in any preceding claim characterised in that the adjacent edges of adjacent electrodes on each of the front and rear sides (27,28) are spaced apart from each other a distance which is just sufficient to break electrical continuity between the adjacent electrodes. 31. A light valve as claimed in any preceding claim characterised in that the width of each electrode over its length is constant, in the working area. 32. A light valve as claimed in any preceding claim characterised in that the length of each electrode over its width is constant, in the working area. 33. A light valve as claimed in any preceding claim characterised in that the electrodes (30 to 33, 62 to 65, 73 to 76, 82 to 87, 90 to 92) are arranged so that each column (36) of pixels (34) is formed by four electrodes, and at least one electrode is formed on one of the front and rear sides (27,28) of the panel (3,61,71,81), and the other electrodes are formed on the other of the front and rear sides (27,28). 34. A light valve as claimed in any preceding claim characterised in that each light transmitting area (40,41) is centred about the Y-axis (26). 35. A light valve as claimed in any preceding claim characterised in that one of the light transmitting areas (40,41) is a first light transmitting area (40), and is of width in the X- axis (25) direction which corresponds to the width in the X-axis (25) direction of an object sheet (4) arranged in portrait orientation. 36. A light valve as claimed in any preceding claim characterised in that another of the light transmitting areas (40,41) is a second light transmitting area (41), and is of width in the X-axis (25) direction which corresponds to the width in the X-axis (25) direction of an object sheet (4) in landscape orientation. 37. A light valve as claimed in any preceding claim characterised in that each of the light transmitting areas (40,41) are of rectangular shape. 38. A light valve as claimed in any preceding claim characterised in that the light valve (2,60,70,80) is suitable for mounting on an object sheet receiving stage (15) of an overhead projector (1) for partially framing an object sheet (4). 39. A light valve as claimed in Claim 38 characterised in that the light valve (2,60,70,80,100) is suitable for mounting just beneath the object sheet receiving stage (15) of the overhead projector (1). 40. A light valve as claimed in any preceding claim characterised in that a signal generating means is provided for generating at least the first and second signals. 41. A light valve as claimed in Claim 40 characterised in that a selecting means is provided for each electrode for selecting and applying the signal of the at least first and second signals to be applied to the corresponding electrode. 42. A light valve as claimed in Claim 40 or 41 characterised in that a control means is provided for controlling the signal generating means and the selecting means. 43. A light valve as claimed in any preceding claim characterised in that the liquid crystal display panel is a positive liquid crystal display panel. 44. A light valve as claimed in any preceding claim characterised in that the liquid crystal display panel is a polymer dispersed liquid crystal panel. 45. A projector comprising the light valve (2) as claimed in any preceding claim. 46. A projector as claimed in Claim 45 characterised in that the light valve (2,60,70,80,100) is located adjacent an object sheet receiving stage (15) of the projector (1). 47. A projector as claimed in Claim 46 characterised in that the light valve (2,60,70,80,100) is placed on the object sheet receiving stage (15) of the projector (1), and the light valve (2,60,70,80) acts as an object sheet receiving stage. 48. A projector as claimed in Claim 46 characterised in that the light valve (2,60,70,80,100) is located just beneath the object sheet receiving stage (15). 49. A projector as claimed in Claim 45 characterised in that the light valve (2,60,70,80,100) forms the object sheet receiving stage (15) of the projector (1). 50. A projector as claimed in any of Claims 45 to 49 characterised in that the projector (1) is an overhead projector for projecting from an object sheet (4) onto a screen (17).

Inventors: O'KEEFFE DONAL MARTIN, O'KEEFFE, DONAL, MARTIN
Applicants: KEEFFE DONAL MARTIN O, PRIVAN HOLDINGS LIMITED, O'KEEFFE, DONAL, MARTIN
Application year: 1996
Main classification: G02F1
IPC codes: G02F1, G03B21