US 3787206 A
A photoelectrophoretic color imaging system employing an imaging suspension comprising a plurality of colored electrically photosensitive pigment particles in a carrier liquid positioned between two electrodes wherein the imaging suspension is exposed to flood illumination of activating electromagnetic radiation in the presence of an electric field. Imaging is effected by means of imagewise patterns carried by at least one of the electrodes. The color and/or density of the reproduced images are controlled by the color and the intensity of the activating electromagnetic radiation.
Description (OCR text may contain errors)
IJiiite ties 1 [1 1 Goiie PHOTOELECTRICPHORETIC IMAGING METHOD INCLUDING AT LEAST ONE ELECTRODE CARRYING A PATTERN  Inventor: William L. Goife, Webster, NY.
 Assignee: Xerox Corporation, Stamford,
22 Filed: Oct. 21, 1971 21 Appl. No.: 191,300
 ILS. Cl 96/1.2, 96/1 PE, 96/l.3, 117/37 LE  Int. Cl G03g 13/00  Field of Search 204/181, 300, 18 PC; 117/37 LE; 96/1.2, 1.3
 References Cited UNITED STATES PATENTS 3,642,598 2/1972 Gundlach et a1 204/181 PE 3,145,156 8/1964 Oster 204/18 PC 3,384,566 5/1968 Clark 117/37 LE 3,594,161 7/1971 Kaufman 117/37 LE Ian. 22, 1974 3,409,528 ll/1968 Lennon 204/181 PE 3,672,884 6/1972 Mayer 3,689,399 9/1972 Ota 3,655,370 4/1972 Carreira et a1 1 17/37 LE Primary Examiner-John H. Mack Assistant Examiner W. l.- Solomon Attorney, Agent, or FirmJames J. Ralabate et al.
[5 7 ABSTRACT A photoelectrophoretic color imaging system employing an imaging suspension comprising a plurality of colored electrically photosensitive pigment particles in a carrier liquid positioned between two electrodes wherein the imaging suspension is exposed to flood illumination of activating electromagnetic radiation in the presence of an electric field. Imaging is effected by means of imagewise patterns carried by at least one of the electrodes. The color and/or density of the reproduced images are controlled by the color and the intensity of the activating electromagnetic radiation.
8 Claims, 5 Drawing Figures PHOTOELECTRICPIIORETIC IMAGING METHOD INCLUDING AT LEAST ONE ELECTRODE CARRYING A PATTERN BACKGROUND OF THE INVENTION This invention relates to a color imaging system and more particularly to a color printing method employing photoelectrophoretic techniques.
A number of duplicating techniques utilizing previously formedv printing masters are well known, one such method being the lithographic or offset printing process. Generally speaking, lithography is a method of printing from a flat plate which depends upon different properties of the image and non-image areas for printability. In conventional lithography, the non-image area is hydrophilic or water receptive while the image area is hydrophobic or water repellent. The printing master is first contacted with a fountain solution which wets all portions of the surface not covered by the hydrophobic image. An oilbase printing ink is then applied to achieve the desired selective inking in the image areas. Usually the ink image on the master is then transferred to an offset roller from where the actual printing takes place.
Another printing method is letterpress printing which is considered the oldest and most commonly used printing method. In letterpress printing ink is applied to a raised surface and transferred directly to a support substrate through pressure. The areas to be printed are raised above the non-printing areas and the ink rollers touch only the top surface of the raised areas. The surrounding non-printing areas are lower and do not receive ink.
In the reproduction of documents, there often arise situations where only a relatively small number of copies are required. When reproductions in black and white are to be made, the known printing methods are quite suitable since the change from a particular original to another requires only a change of printing masters, However, when colored reproductions are desired, printing only a relatively small number of copies of an original by the prior art methods is impractical. To prepare the apparatus for color printing every time a new original must be reproduced, requires not only a change in the printing master, but also a time consuming clean up to remove all of the printing ink from the previous printing. With respect to the situation where successive originals are to be reproduced in different colors and only a relatively small number of copies of each original are required, it becomes prohibitively expensive to clean the apparatus between each successive printing.
SUMMARY OF THE INVENTION Now there has been found a novel printing process by which reproduced colored images may be conveniently formed and which does not require extensive cleaning of the apparatus in order to be able to print successive images of diffferent colors or of varying shades of the same color. The present invention employs a photoelectrophoretic imaging technique utilizing an imaging suspension comprising a plurality of colored electrically photosensitive pigment particles in a carrier liquid.
Photoelectrophoretic imaging, generally speaking, comprises exposing an imaging suspension of colored light-absorbing particles in a carrier liquid postitioned between two electrodes to imagewise illumination in the presence of an electric field. As these steps are completed, selective particle migration takes place in image configuration, providing a visible image at one or both of the electrodes. Detailed discussions relating to the apparent mechanism by which photoelectrophoretic imaging operates are found in US. Pat. Nos.
3,383,933; 3,384,488; 3,384,565 and 3,384,566. Generally speaking, a thin film of the imaging suspension is placed on a transparent electrically conductive imaging plate or injecting electrode and exposure is made through the transparent plate while a second or blocking electrode is placed or rolled over the top of the imaging suspension. The particles present in the suspension migrate in response to activating electromagnetic radiation to form a visible image pattern at one or both of the electrodes, the images being opposite in sense to one another. In addition to employing imagewise illumination, the imaging particles can be made to deposit according to imagewise electrical and/or physical surface patterns formed on one of the electrodes.
Thus, in photoelectrophoretic imaging the lightabsorbing particles in the imaging suspension may be made to deposit on the electrodes according to an imagewise pattern of illumination; or according to an imagewise electrical pattern such as is present when an electrostatic latent image is formed on one of the electrodes; or according to physical modification of one of the electrodes. These modifications may comprise, for example: (a) a relief image on eitherelectrode which provides differences in the gap spacings of image and non-image areas with respect to the other electrode; (b) forming a conductivity pattern on the injecting electrode; (0) forming a blocking layer pattern on the blocking electrode; and (d) forming an electrostatic potential pattern on the insulating blocking electrode.
According to the present invention, which is particularly suited for the formation of monochromatic images but which may also be adapted to provide polychromatic images, the imaging suspension comprising a plurality of finely divided electrically photosensitive pigment particles in a carrier liquid is placed on one of the electrodes and exposed to uniform illumination. The photosensitive pigment particles are caused to deposit on the electrodes according to an imagewise pattern carried by at least one of the electrodes with the color and/or density of the images formed being controlled by the color and/or the intensity of the light illumination.
It should be understood that by the term imagewise pattern is meant imagewise electrical patterns as well as imagewise physical modifications such as have been previously mentioned but is not limited thereto, it being intended to encompass any imagewise pattern carried by an electrode which will effect imaging according to the method of the invention.
Since the novel imaging method of the invention is particularly suited for monochromatic (or single color) image reproduction, the electrically photosensitive pigment particles may be all of the same type in which case the density of the images will be controlled by the ent response to light exposure. In this instance the color and/or the density of the images formed may be controlled by the color and the intensity of the light illumination. Thus according to a particularly preferred embodiment of the invention wherein the imaging suspension comprises three differently colored electrically photosensitive pigments particles, namely cyan, magenta and yellow, simply by selecting a particular wavelength or wavelengths of light and regulating the intensity of the selected light a printed image of any desired color may be obtained. According to another embodiment of the invention polychromatic images may be obtained by sequentially varying the light illumination to which the imaging suspension is exposed as will be discussed in detail hereinafter.
It will be readily appreciated that the novel printing method of the invention offers a convenient and rapid means for changing from reproduced images of one color to those of another color or another shade of the same color. Such results can be obtained merely by changing the color and/or the intensity of the uniform light illumination. Since the imaging suspension preferably has at least three differently colored photosensitive pigment particles which are selected so that images of any color may be obtained therefrom, as will be explained in detail hereinafter, the same imaging suspension can be employed in all cases thereby obviating the necessity for changing the imaging suspension and cleaning the imaging apparatus between the formation of reproduced images of different colors.
Accordingly, it is an object of the present invention to provide a novel imaging system which overcomes the above described disadvantages and which provides the above-described desirable features.
It is another object of the invention to provide a printing process which utilizes photoelectrophoretic imaging techniques.
It is still another object of the invention to provide a photoelectrophoretic imaging system wherein the color and/or the density of the reproduced images can be changed without extensive preparation or cleaning of the apparatus.
It is a further object of the invention to provide a photoelectrophoretic imaging system whereby images of any color may be made from the same imaging suspension.
A still further object of the invention is to provide a photoelectrophoretic imaging system wherein uniform light illumination is employed.
Another object of the invention is to provide a photoelectrophoretic imaging system wherein imaging is effected by means of an imagewise pattern carried by at least one of the electrodes.
Still another object of the invention is to provide a photoelectrophoretic imaging system wherein imaging is effected by means of an imagewise electrical pattern carried by one of the electrodes.
Yet another object of the invention is to provide a photoelectrophoretic imaging system wherein imaging is effected by means of an imagewise physical modification of one of the electrodes.
The foregoing and other objects and advantages of the invention will become more readily apparent from the following detailed description of various preferred embodiments of the invention, particularly when read in conjunction with the accompanying drawings in which:
FIG. 1 represents a side sectional view ofa simple exemplary system for carrying out the imaging phase of the process of the invention;
FIG. 2 illustrates another embodiment for carrying out the imaging phase of the process of the invention;
FIG. 3 illustrates another embodiment for carrying out the imaging phase of the process of the invention;
FIG. 4 shows still another embodiment for carrying out the imaging phase of the process of the invention; and
FIG. 5 represents a side sectional view of an apparatus suitable to carry out another embodiment of the imaging phase of the process of the invention wherein images of more than one color are formed.
Referring now to FIG. 1, there is seen a transparent electrode, generally designated 10, which in this exemplary instance is made up of an optically transparent substrate 12 overcoated with a thin optically transparent conductive layer 14. A typical suitable electrode material is commercially available under the name NESA glass from Pittsburgh Plate Glass Company and comprises a thin optically transparent layer of tin oxide deposited on a transparent glass substrate. This electrode shall hereinafter be referred to as the injecting electrode. Coated on the surface of injecting electrode 10 is a thin layer of a suspension 16 comprising a plurality of differently colored, finely divided electrically photosensitive particles dispersed in a carrier liquid. The term electrically photosensitive as applied to the particles dispersed in the carrier liquid means any particle which, once attracted to the injecting electrode, will migrate away from it underthe influence of an applied electric field when it is exposed to activating electromagnetic radiation; however, for a detailed theoretical description of the apparent mechanism of operation of photoelectrophoretic imaging see the patents previously mentioned.
Above the liquid suspension 16 is arranged a second electrode generally designated 18, commonly referred to as the blocking electrode, which in this illustrative instance is shown as a roller but which may be of any suitable configuration, such as, for example, a flat plate. Blocking electrode 18 is connected to one side of potential source 30 through a switch 32. The opposite side of potential source 30 is connected to the conducting layer 14 of injecting electrode 10 so that when switch 32 is closed an electric field is applied across the liquid suspension 16 through electrodes 10 and 18. The imaging suspension 16 is exposed to uniform electromagnetic radiation, represented by arrows 34 through transparent injecting electrode 10. The activating electromagnetic radiation may be provided by any suitable light source.
Blocking electrode 18, in this illustrative instance, has a conductive central core 20 surrounded by an image layer 21 which comprises conductive image material portions 22 and insulating material portions 24 which together form a conductivity pattern, and optional outer layer 26 which may be any suitable blocking electrode material such as, for example, Tedlar or Baryta paper. It is preferred to employ outer blocking layer 26 to eliminate the possibility of electrical shorting occurring across the electrodes.
In accordance with the practice of the invention, electrode 18 is caused to roll across the top surface of injecting electrode with switch 32 closed during the period of uniform exposure. This light exposure causes exposed particles originally attracted to electrode 110 to migrate through the liquid and adhere to the surface of theblocking electrode in an imagewise pattern determined by the conductivity pattern of the image layer 21 surrounding the central core of electrode 18. It can be seen that the electric fields extending through blocking material layer 26 will correspond to the conductivity pattern of imaging layer 21. The portions of blocking material layer26 which are directly above the conductive material portions 22 of image layer 21 will be at a higher potential than those portions of blocking material layer 26 which are-directly above insulating material portions 24 of image layer 21 with the areas of higher potential having a field strength which is sufficient to cause the imaging particles to migrate to the surface of the blocking electrode whereas the fields in the areas of lower potential are insufficient to cause particles to migrate to the blocking electrode in those areas. Thus, the photosensitive pigment particles adhere to the outer surface of blocking electrode 18 in a pattern corresponding to the conductive material portions 22 of image layer 21.
The roller electrode 18 can then be removed from the surface of the injecting electrode it), brought into contact with a final copy member and the image transferred to the latter by any suitable transfer technique and fixed thereto. Roller electrode 18 then can be cleaned of any residual image particles and subsequently re-used.
It is to be understood that, although the invention has been described for purposes of illustration with respect to a particular electrode configuration, this novel and advantageous imaging system is not limited thereto. The invention may be practiced with either or both electrodes in the form of drums, rollers, flat plates or traveling webs.
Although the invention has been described with respect to a particular embodiment thereof, it will be recognized that it is not limited thereto but rather that various modifications of either electrode may be utilized and similar results obtained; Another embodiment of the invention is illustrated in FIG. 2. The overall apparatus configuration is similar to that shown in FIG. 1 and like elements are designated by the same numerals. However, according to this embodiment, the blocking electrode 18 has a conductive central core 20 surrounded by an outer layer 40 of insulating or photoconductive insulating material which may be any suitable material. Typical suitable materials include, any standard photoconductive materials such as, for example, selenium, sulfur, anthracene and tellurium; finely ground photconductive insulating materials dispersed in a high resistance electrical binder such as are disclosed in U.S. Pat. No. 3,121,006; inorganic photoconductive pigments dispersed in a photoconductive insulating material such as are disclosed in U.S. Pat. No. 3,121,007; an organic photocon'ductor such as phthalo cyanine in a binder; or generally any photoconductiveinsulating material which is suitable for use in xerographic reproduction. In this illustrative instance layer 40 comprises a photoconductive insulating material.
An electrostatic latent image corresponding to the original image to be reproduced is formed on the surface of the blocking electrode by uniformly electrostatically charging the outer surface thereof such as, for
image 4l8 and a lens 50. The roller electrode 18 is then rolled across the injecting electrode 10 with switch 32 closed. An image corresponding to the latent electrostatic image is formed on the blocking electrode.
Other methods of forming the electrostatic latent image may be used such as, for example, by selective deposition of electrostatic charge, as by impressing a charge through a stencil, imposing a potential on a shaped conductor'or electrode; or by transfer of electrostatic image techniques and others.
FIG. 3 illustrates another preferred embodiment of the invention. According to this embodiment imaging is effected by means of a relief image carried by one of the electrodes. Referring now to FIG. 3 there is seen a blocking electrode 60 carrying a relief image 62 on the outer surface thereof. The relief image provides a difference in the relative gap spacing of image and nonimage areas between the electrodes thus causing imaging to occur. The relief image 62 maybe formed of any suitable material having the required characteristics such as, for example, a photoresist image, a xerographic toner image, photoelectrophoretic imaging particles, etc.
FIG. 4 illustrates still another preferred embodiment of the invention wherein imaging is effected by means of an insulating material pattern deposited on the injecting electrode. Referring now to FIG. 4 there is seen a relatively thin imagewise pattern of insulating material on the conducting surface of injecting electrode 10. In a particularly preferred embodiment the insulating material will be optically transparent in order to permit activating electromagnetic radiation to pass through. Again, the imagewise pattern of insulating material may be formed of any suitable material such as, for example, a photoresist image, a transparent xerographic toner image, an image formed by photoelectrophoretic imaging particles, etc.
While not illustrated herein, it will be recognized by those skilled in the art that other imagewise patterns may be utilized to effect imaging according to the invention. For example it is possible to form an imagewise electrical conductivity pattern on the surface of the injecting electrode by removing the conductive surface thereof in imagewise fashion such as by etching, etc.
The present invention is particularly adapted for monochromatic image reproduction. Thus, at least two differently colored pigment particles, and preferably three, which have a different response to light exposure are incorporated in the imaging suspension. The photosensitive pigment particles are selected so that those of different colors respond to different wavelengths in the visible spectrum corresponding to their principal absorption and further so that their spectral response curves do not have substantial overlap thus making the system most suitable for subtractive color imaging. For full subtractive color imaging, several different particles are employed, namely a cyan colored particle sensitive mainly to red light, a magenta colored particle sensitive mainly to green light and a yellow colored particle sensitive mainly to blue light. While this is the simplest combination, additional particles having difof any color may be obtained.
The method of the invention may be further adapted to produce reproduced images of more than one color by selectively exposing various areas of the blocking electrode to different wavelengths of light. This can be accomplished by arranging appropriate filters in the path of the uniform light illumination in a manner such that different areas of the imaging suspension are contacted by different wavelengths of light exposure. Referring now to FIG. there is seen the same general apparatus configuration as shown in FIG. 1 with the exception that filters 72, 74, and 76 are positioned in the path of the impinging light. According to this embodiment of the invention, if the incident activating radiation is white light or composed of colors which are passed by the filters then the color of any particular segment of the reproduced image will correspond to that passed by one of the filters and the area of the image taken up by this particular segment will correspond to the area of the filter positioned in the path of the impinging light.
By practicing this embodiment of the invention with an imaging suspension having at least three differently colored photosensitive pigment particles it is possible to form reproduced polychromatic images of any color. For example, by forming an imaging suspension which has cyan, magenta and yellow colored photosensitive pigment particles and employing red, green and blue filters respectively, polychromatic images having red, green and blue colors can be formed on the imaging electrode. Of course by varying the filters other colors could be formed.
A wide range of voltages may be employed between the electrodes in this photoelectrophoretic printing system. For good image resolution, high image density and low background, it is preferred that the potential applied be such as to create an electric field of at least 300 volts per mil across the imaging suspension. The applied voltage necessary to obtain this field strength will, of course, vary depending upon the inter-electrode gap and/or the thickness and type of blocking material used on the blocking electrode surface. For the very highest image quality, the optimum field is at least about 2,000 volts per mil. The upper limit of field strength is limited only by the breakdown potential of the suspension and the material which forms the outer layer of the blocking electrode. Fields below about 300 volts per mil, while capable of producing images, generally produce images of low modulation.
Any suitable substantially insulating liquid may be used in the imaging suspension. Typical suitable substantially insulating liquids include the following saturated hydrocarbons: decane, dodecane, N-tetradecane, molten paraffins, molten beeswax or other molten thermoplastic materials, Sohio Odorless Solvent (a kerosene fraction available from Standard Oil Co. of Ohio) and lsopar G (a long chain saturated aliphatic hydrocarbon available from Humble Oil Co. of New York) and mixtures thereof; silicone oil, fluorinated hydrocarbons and mineral oil.
Any suitable electrically photosensitive particle or mixtures of such particles may be used according to the invention regardless of whether the particular particle is organic, inorganic and is made up of one or more components in solid solution or dispersed one in the other or whether the particles are made up of multiple layers of different materials. An extensive list of suitable particles is found in U.S. Pat. Nos. 3,384,488 and 3,383,933 both of which are hereby incorporated herein by reference.
As stated above any suitable particle structure may be employed. Typical particles include those which are made up of only the pure photosensitive material or a sensitized form thereof, solid solutions or dispersions of the photosensitive material in a matrix such as thermoplastic or thermosetting resins, copolymers of photosensitive pigments and organic monomers, multilayers of particles in which the photosensitive material is included in one of the layers and where other layers provide a light filtering action in an outer layer or a fusable or solvent softenable core of resin or a core of liquid such as dye or other marking material or a core of one photosensitive material coated with an overlayer of another photosensitive material to achieve broadened spectral response. Other photosensitive structures include solutions, dispersions or copolymers of one photosensitive material in another with or without other photosensitively inert materials.
While the above structural and compositional variations are useful it is preferred that each particle be primarily composed of an electrically photosensitive pigment, such as those referred to above, wherein the pigment is both the primary electrically photosensitive ingredient and the primary colorant for the particle. These particles have been found to give optimum photographic sensitivity and highest overall image quality in addition to being simple and economical to prepare. Of course it may often be desirable to include other ingredients such as spectral or electrical sensitizers or secondary colorants and secondary electrically photosensitive materials.
Having given a detailed description of various preferred embodiments, the invention will now be further described with respect to specific preferred embodiments by way of Examples to further aid those skilled in the art to practice the same, it being understood that these are intended to be illustrative only and are not to be construed as limiting the invention to the materials, conditions and steps recited therein. All parts and percentages are by weight unless otherwise specified.
EXAMPLES The following examples are carried out with an apparatus of the general type illustrated in FIG. 1 with the imaging suspension coated on a NESA glass electrode through which exposure is made. The NESA glass surface is connected in series with a switch, a potential source and the conductive parts of an image layer which comprises conductive material portions and insulating material portions defining an imagewise pattern and which has a coating of Baryta paper on its surface. The image layer is approximately 10 mils thick and surrounds a conductive roller about 2 k inches in diameter. The conducting roller, image layer and Baryta paper coating comprise the blocking electrode. The
blocking electrode is moved across the NESA glass surface at a speed of about 1.45 cm per second. The NESA glass electrode is about 3 inches square and is exposed with a light intensity of 1,800 foot candles.
Unless otherwise indicated, about 7 percent by weight of the indicated pigment in each example is suspended in Sohio Odorless Solvent 3440 and the magnitude of the applied potential is 2,500 volts with respect to the NESA glass plate. A spacing of about 1 mil isv EXAMPLE I A suspension including equal amounts of Watchung Red B, C.l. No. 15865, Monolite Fast Blue G.S., C.l. No. 74100 and Algol Yellow G.C., C.l. No. 67300 is prepared. The pigments are magenta, cyan and yellow respectively. A layer of this suspension is coated on the NESA glass electrode and exposed alternatively with one of the three color filters between the light source and the NESA plate so that a given color is projected onto the suspension as the roller blocking electrode is moved across the surface of the NESA electrode. When the roller electrode passes over the NESA plate, a subtractive color image is found on the NESA. For exposure with the red filter, a red image is obtained, a blue image with the blue filter and with the green filter, a green image. The colored image'areas correspond with the imagewise conductivity pattern of the image layer of the roller electrode.
EXAMPLES, u vn Six different pigment suspensions are formed each including equal amounts of the three different pigments listed.
EXAMPLE ll The pigments used are Duol Carmine, C.l. No. 15850, Algol Yellow and Monolite Fast Blue G.S.
EXAMPLE Ill The pigments used are Watchung Red B, Monolite Fast Blue G.S. and Velvaglow Fluorescent Pigment available from Radiant Color Co., Oakland, California.
EXAMPLE iv The pigments used are Monolite Fast Blue G.S., Lemon Cadmium Yellow, C.l. No. 77196 and Watchung Red B.
EXAMPLE V EXAMPLE V1 The pigments used are Cyan Blue Toner GTNF, C.l. No. 74160, Algol Yellow and Watchung Red B.
EXAMPLE VII The pigments used are lndofast Yellow Toner, Cyan Blue Toner GTNF and Watchung Red B.
While the invention has been described in detail with respect to various embodiments thereof, it is not intended to be limited thereto but rather it will be appreciated by those skilled in the art that modifications and variations are possible which are within the spirit of the invention and the scope of the claims. For example, the imaging suspension may be initially applied to the surface of the transparent electrode electrophoretically in accordance with the method described in copending application Ser. No. 764,718, filed Oct. 3, 1968 now US. Pat. No. 3,620,948.
What is claimed is: r
11. A photoelectrophoretic imaging method for fonning an image having a predetermined color comprising the steps of at least partially transparent and at least one of which carries an imagewise pattern capable of affecting an electrical field established between the electrodes when a potential is applied thereto;
b. arranging a layer of an imaging suspension between said electrodes, said suspension comprising a plurality of at least two differently colored finely divided particles in a carrier liquid, each of said particles comprising an electrically photosensitive pigment;
c. applying a potential to said electrodes, wherein there is established an imagewise electrical field across said suspension layer; and
d. substantially simultaneously exposing said suspension to uniform illumination to which less than all of said particles are substantially photoresponsive from a sourceof activating radiation whereby an image in the desired color is formed.
2. The method as defined in claim 1 wherein said imaging suspension comprises cyan, magenta and yellow particles in a carrier liquid.
3. The method as defined in claim 1 wherein the imagewise pattern carried by one of said electrodes comprises an electrostatic latent image.
4. The method as defined in claim 1 wherein the imagewise pattern carried by one of said electrodes comprises a relief image.
5. The method as defined in claim 1 wherein the imagewise pattern carried by one of said electrodes comprises an imagewise layer of insulating material.
6. The method as defined inclaim 1 wherein the imagewise pattern carried by one of said electrodes comprises an imagewise conductivity pattern formed by conductive material portions and insulating material portions.
7. The method as defined in claim 1 wherein at least a part of the electromagnetic radiation which contacts a part of the imaging suspension is passed through an tive different optical filters.
a. providing two electrodes, at least one of which is optical filter before passing through said transparent