embodiments of a light shutter device according to the present invention are described with reference to the accompanying drawings . the embodiments below are light shutter devices which operate ( turn on and turn off the light ) based on the well - known principle illustrated in fig1 , and repetitious descriptions are avoided . [ 0040 ] fig1 shows a principal part of a light shutter device 1 which is a first embodiment of the present invention . this light shutter device 1 has a plurality of light transmitting portions ( light shutter elements ) 11 a , 11 b , . . . , which are shadowed in fig1 on a planar plzt chip 10 . the light shutter elements 11 a , 11 b , . . . are arranged in two lines x 1 and x 2 alternately , in other words , light shutter elements to form adjacent pixels in one line of an image are arranged in mutually different lines . the light shutter elements 11 a , 11 b , . . . in the two lines are driven to be turned on and off in synchronization with motion of a light receiving surface in a sub scanning direction so as to form one line on the light receiving surface . in order to drive the light shutter elements 11 a , 11 b , . . . , a common electrode 12 is provided in the center of the chip 10 , and individual electrodes 13 a , 13 a , . . . are provided at both sides of the common electrode 12 with the light shutter elements 11 a , 11 b , . . . in - between . the gap between the common electrode 12 and each of the individual electrodes 13 a , 13 a , . . . is supposed to be l , and the dimension of the individual electrodes 13 a , 13 a , . . . in the direction perpendicular to the direction of electric field is supposed to be w . according to an experiment conducted by the inventors , when l and w were so designed that w / l would be over 2 , the mutual influences of the electric fields which occur on the respective light shutter elements 11 a , 11 b , . . . could be inhibited , and crosstalk could be suppressed . in the first embodiment shown by fig1 w / l is 4 . 7 . the relationship between driving voltage and quantity of transmitted light in this embodiment is shown by fig4 b . in fig4 b , the curve a shows a light transmitting characteristic of one of the light shutter elements when all the light shutter elements are turned on , and the curve b shows a light transmitting characteristic of the light shutter element when only the light shutter element is turned on . compared with the case of fig4 a , in the first embodiment in which w / l is 4 . 7 , although there is a slight difference between the characteristic curves a and b in shape , there are almost no differences between the curves a and b in half - wave voltage and maximum quantity of light . [ 0044 ] fig3 shows a principal part of a light shutter device 2 which is a second embodiment of the present invention . this light shutter device 2 has a plurality of light transmitting portions ( light shutter elements ) 11 a , 11 b , . . . , which are shadowed in fig3 on a planar plzt chip 10 . the light shutter elements 11 a , 11 b , . . . are arranged in four lines x 1 , x 2 , x 3 and x 4 in such a way that light shutter elements to form adjacent pixels in one line of an image are arranged in mutually different lines . the light shutter elements 11 a , 11 b , . . . in the four lines are driven to be turned on and off in synchronization with motion of a light receiving surface in a sub scanning direction so as to form one line on the light receiving surface . in the second embodiment , the common electrode 12 has extensions 12 ′ which are drawn from both sides of the central portion of the common electrode 12 . the individual electrodes 13 a , 13 a , . . . face the central portion or the extensions 12 ′ of the common electrode 12 . according to an experiment using the second embodiment , by setting w / l over 2 , the mutual influences of the electric fields which occur on the respective light shutter elements 11 a , 11 b , . . . could be inhibited , and crosstalk could be suppressed . in the second embodiment shown by fig3 w / l is 7 . the relationship between driving voltage and quantity of transmitted light in this embodiment is shown by fig4 c . the curve a in fig4 c shows a characteristic of one of the light shutter elements 11 a , 11 b , . . . when all the light shutter elements 11 a , 11 b , . . . are turned on , and the curve b shows a characteristic of the light shutter element when only the light shutter element is turned on . compared with the case of fig4 a , in the second embodiment in which w / l is 7 , although there is a slight difference between the characteristic curves a and b in shape , there are almost no differences between the curves a and b in half - wave voltage and maximum quantity of light . when the light shutter devices 1 and 2 are installed in printers , as fig2 shows ( fig2 shows the first embodiment ), shield masks 20 which cover the respective light shutter devices other than the light shutter elements 11 a , 11 b , . . . are provided . the shield mask 20 can be formed by forming a film on the chip 10 and by making holes in the film on the light shutter elements 11 a , 11 b , . . . by photolithography . an insulating and light shielding material shall be selected for the film . for example , resin on which a black pigment is dispersed is usable . also , a mask plate on which a shield film is patterned may be placed on the chip 10 as the shield mask 20 , and other various shielding ways can be adopted . the width of the light shutter elements 11 a , 11 b , . . . , that is , the dimension of the light shutter elements in the main scanning direction is determined depending on the image resolution . for example , if the image resolution is 400 dpi , the width is 63 . 5 μm , and if the image resolution is 600 dpi , the width is 42 μm . each of the light shutter elements is preferably positioned in the center of the dimension w of the corresponding individual electrode . in the above - described first embodiment and the second embodiment , the image resolution is 400 dpi . [ 0050 ] fig5 and 6 show the results of experiments . fig5 shows the relationship between w / l and half - wave voltage shift δvd between the time of all lighting and the time of single lighting . fig6 shows the relationship between w / l and the rate of the quantity of transmitted light at the time of single lighting to that at the time of all lighting . as is apparent from fig5 and 6 , the larger w / l is , the smaller crosstalk is . there are no practical problems when w / l & gt ; 2 . the value w / l can be set smaller by setting the value l smaller . that l is small means that the area of the portion functioning as a light shutter element is small , and if l is too small , the illumination system will have too much burden to achieve a sufficient quantity of transmitted light . practically , l is preferably not less than 5 μm and shall be determined in consideration for other factors of the print system and the desired performance as a light shutter . if the individual electrodes 13 a , 13 a , . . . in the first and second embodiments have a dimension d in the direction of electric field , the value d / l influences the half - wave voltage . fig7 shows the results of an experiment about the relationship between d / l and half - wave voltage vd . table 1 shows the values d , d / l and vd at the points “ a ” through “ h ” in the graph of fig7 . as is apparent from fig7 when d / l becomes less than approximately 0 . 5 , the half - wave voltage vd suddenly rises . accordingly , by designing the individual electrodes 13 a , 13 a , . . . so that d / l & gt ; 0 . 5 , the driving voltage can be kept low . there are no particular limitations on the maximum of d / l ; however , the value d / l shall be determined in consideration for the size of the chip 10 . the electrodes may be of the shape shown by fig8 in which large electrodes 12 and small electrodes 13 face each other . in this case , even if the large electrodes 12 satisfy the conditions w 2 / l & gt ; 2 and d 2 / l & gt ; 0 . 5 , if the small electrodes 13 are on the conditions w 1 / l & lt ; 2 and d 1 / l & lt ; 0 . 5 , it is not possible to suppress crosstalk . it is necessary that the small electrodes 13 satisfy the conditions w 1 / l & gt ; 2 and d 1 / l & gt ; 0 . 5 . [ 0056 ] fig9 shows a principal part of a light shutter device 3 which is a fourth embodiment of the present invention . in the light shutter device 3 , a plurality of light shutter elements 11 a , 11 b , . . . are arranged in four lines x 1 , x 2 , x 3 and x 4 as in the second embodiment ( see fig3 ). the characteristic of the fourth embodiment is described referring to fig1 . the electrodes 12 and 13 a to drive the light shutter element 11 a have a gap l in - between . the individual electrode 13 c to drive the light shutter element 11 c is positioned at a distance not less than 5l from the sides 11 a 1 and 11 a 2 of the light shutter element 11 a . in other words , within an area a at a distance not less than 5l from the sides of a light shutter element , no other individual electrodes are arranged . the reason why no other individual electrodes are arranged within the area a is described . the inventors fabricated a model shown by fig1 , and the driving voltage and the quantity of transmitted light were measured . in order to drive a light shutter element 11 , an electric field was applied between a common electrode 12 and an individual electrode 13 . another electrode 13 ′ was provided at a distance m from the light shutter element 11 . while fixing the distance l between the electrodes 12 and 13 at 20 μm and at 40 μm and changing the distance m between 50 μm and 250 μm , the driving voltage and the quantity of transmitted light were measured to figure out the half - wave voltage . these measurements were carried out on the first condition that the electrode 13 ′ has the same potential as the individual electrode 13 ( corresponding to all lighting ) and on the second condition that the electrode 13 ′ has the same potential as the common electrode 12 ( corresponding to single lighting ). a difference in half - wave voltage vd between a case on the first condition and a case on the second condition is referred to as a shift voltage δ vd . the results of the experiment is partly shown by fig1 a and 12 b . fig1 a shows a case in which l = 20 μm and m = 70 μm on the first condition , and in this case , the half - wave voltage was 100v . fig1 b shows a case in which l and m are the same as in the case of fig1 a on the second condition , and in this case , the half - wave voltage was 130v . the shift voltage δvd was 30v . table 2 shows the relationship between distance m and shift voltage δ vd when l = 20 μm and when l = 40 μm . it is ideal that the shift voltage δ vd is zero ( which means that there is no crosstalk ); however , if the shift voltage δ vd is not more than 10v , the quantity of transmitted light decreases only by at most 5 %, which is practically no problem . therefore , the distance m shall be not less than five times the distance l . in the fourth embodiment , the shift voltage δ vd was 8v , and the quantity of transmitted light decreased by 3 %. [ 0063 ] fig1 shows a princiapal part of a light shutter device 4 which is a fifth embodiment of the present invention . this light shutter device 4 is basically of the same structure as the fourth embodiment shown by fig9 . however , the width of the extensions 12 ′ of the common electrode 12 and the width of the individual electrodes 13 a , 13 a , . . . are smaller than those of the fourth embodiment , and the width of the central portion of the common electrode 12 is larger than that of the fourth embodiment . the individual electrodes 13 a , 13 a , ... are arranged out of the areas a , that is , each of the individual electrodes is positioned at a distance not less than 5l from the light shutter elements driven by the other individual electrodes . in the fifth embodiment , the shift voltage a vd was 5v , and the quantity of transmitted light decreased by 2 %. [ 0064 ] fig1 shows a principal part of a light shutter device 5 which is a sixth embodiment of the present invention . the light shutter device 5 is basically of the same structure as the first embodiment shown by fig1 . the difference is that a groove 12 a is formed in the center of the common electrode 12 . by separating the common electrode 12 by the groove 12 a , crosstalk between the light shutter elements which are arranged on both sides of the common electrode 12 can be suppressed . in the sixth embodiment , the common electrode 12 has a width of 60 μm , and a groove 12 a with a width of 25 μm and a depth of 50 μm is formed . the groove 12 a may be formed by cutting , etching ( plasma etching , reactive ion etching , wet etching , etc .) or sand blasting . in the sixth embodiment , the shift voltage δ vd was 0v , and the reduction in quantity of transmitted light was 0 %. the width of the groove 12 a is preferably not more than the width of the common electrode 12 minus 10 μm , and the depth of the groove 12 a is preferably not less than 5 μm . if the depth is less than 5 μm , the effect of suppressing crosstalk is weak . [ 0067 ] fig1 shows a principal part of a light shutter device 6 which is a seventh embodiment of the present invention . this light shutter device 6 is basically of the same structure as the light shutter device 4 of the fifth embodiment . in the seventh embodiment , further , a groove 12 a is formed in the common electrode 12 . the groove 12 a is the same as the groove in the sixth embodiment . in the seventh embodiment , the shift voltage δvd was 0 . 2v , and the reduction in quantity of transmitted light was 0 %. as the material with an electro - optical effect , not only plzt but also linbo 3 and others are usable . the number of lines of the light transmitting portions ( light shutter elements ) and the details of the electrode pattern are arbitrary . although the present invention has been described in connection with the preferred embodiments above , it is to be noted that various changes and modifications are possible to those who are skilled in the art . such changes and modifications are to be understood as being within the scope of the present invention .