US 3748035 A
An illumination system is provided for a polychromatic imaging process of the type wherein a polychromatic original is illuminated by light from a continuous spectrum light source and the resultant light image is projected onto a narrow imaging zone which progressively traverses the length of an imaging layer which comprises at least two differently-colored, electrically photosensitive materials which are sensitive to light of predetermined, different wavelengths. A filter member having at least one group of at least two differently colored filters is rotated between a continuous spectrum light source and the imaging layer. The colored filters are selected to reduce or eliminate certain radiation bands which interfere with the production of the best quality image. Transverse speed pattern lines are effectively eliminated by rotating the disc at filter disc rotation/imaging zone speed values of above about 40 cycles/in. At lower ratios the transverse speed pattern lines can be effectively eliminated by bypassing a predetermined amount of unfiltered light.
Claims available in
Description (OCR text may contain errors)
United States Patent [1 1 Mannik [451 July 24,1973
[ METHOD FOR SEQUENTIAL ILLUMINATION IN A POLYCHROME PROCESS [7 5] lnventor: Kallis H. Mannik, Webster, N.Y.
 Assignee: Xerox Corporation, Rochester, N.Y.
 Filed: Dec. 28, 1971  Appl. N0.: 213,0;17
 U.S; Cl. 355/4, 96/12  Int. Cl 603g 15/00  Field of Search 355/35, 38, 3, 4; 96/ 1.2
 References Cited UNITED STATES PATENTS 2,960,911 11/1960 Coote 355/35 3,282,190 11/1966 Neale 355/38 Primary Examiner-John M. Horan Attorney-James J. Ralabate [5 7] ABSTRACT An illumination system is provided for a polychromatic imaging process of the type wherein a polychromatic original is illuminated by light from a continuous spectrum light source and the resultant light image is projected onto a narrow imaging zone which progressively traverses the length of an imaging layer which comprises at least two differently-colored, electrically photosensitive materials which are sensitive to light of predetermined, different wavelengths. A filter member having at least one group of at least two differently colored filters is rotated between a continuous spectrum light source and the imaging layer. The colored filters are selected to reduce or eliminate certain radiation bands which interfere with the production of the best quality image. Transverse speed pattern lines are effectively eliminated by rotating the disc at filter disc .rotation/imaging zone speed values of above about 40 cycles/in. At lower ratios the transverse speed pattern lines can be effectively eliminated by bypassing a predetermined amount of unfiltered light.
8 Claims, 6 Drawing Figures Patented July 24, 1973 EL (/5 GREEN METHOD FOR SEQUENTIAL ILLUMINATION IN A POLYCHROME PROCESS This invention relates to an illumination system for polychromatic imaging, and more specifically to a system for producing high quality polychromatic images through the use of a simple continuous spectrum light source in systems of the type wherein an original polychromatic image is illuminated by the light source and the resultant light image is projected onto a progressively moving imaging zone of an imaging surface.
The illumination system of the present invention is useful for all polychrome systems which form an image upon a polychromatic imaging surface by projecting light from a polychromatic original onto a narrow imaging zone which progressively traverses the length of imaging surface. The progressively moving imaging zone is typically formed by projecting a narrow slit of light onto the imaging surface, with the slit moving progressively relative to the imaging surface; or by causing a progressive rolling contact between one of the two electrodes and the imaging suspension of a photoelectrophoretic or a migration imaging system while at least the area of contact isbeing exposed to projected light from the polychromatic original.
The illumination system of the present invention is especially advantageous when utilized in photoelectro: phoretic, migration, and manifold imaging systems.
In photoelectrophoretic imaging, a suspension of colored, photosensitive particles is placed between at least two electrodes subjected to a potential difference and exposed to a light image. Ordinarily, in carrying out the process, the imaging suspension in the form of a thin liquid film, is placed on a transparent, electrically conductive electrode and'the exposure is made through the transparent electrode while a second, biased electrode is passed across this suspension. The particles are believed to bear an initial charge once suspended in the liquid carrier. Upon application of an electric field between the electrodes, the charged particles are attracted'to the transparent electrode. Upon exposure to actinic electromagnetic radiation, the particles change polarityv by exchanging charge with the transparent electrode so that the exposed particles migrate to the second electrode thereby forming images on each of the electrodes, by'particle subtraction, each being complementary one to theother. The image formed on the transparent electrode is generally a photographically positive image; whereas, the image formed on the second electrode is a photographically negative image. Typically, the positive image is used directly or transferred to'a receiving web such as paper, whereas the negative image is generally erased by cleaning of the second electrode. An extensive and detailed discussion of photoelectrophoretic imaging techniques can be found in U.S. Pat. Nos. 3,383,933, 3,384,488, 3,384,565 and 3,384,566, which are hereby incorporated by reference. I
In a typical embodiment of polychromatic migration imaging systems, an imaging member comprising a conductive substrate or a substrate having a conductive layer with a layer of softenable or soluble material, conto'the original on one of the sheets. Although imaging ing it to a solvent or heat which dissolves or softens only the softenable layer. The colored photosensitive particles which have been exposed to activating radiation migrate through the soltenable layer as it is dissolved or softened, leaving an image on the conductive substrate conforming to a positive of the original by subtractive color imaging. Those portions of the photosensitive layer which do not migrate to the conductive substrate may be washed away by the solvent with the softenable layer or, depending upon whether a solvent or heat was employed, the softenable layer may at least partially remain behind on the substrate. Migration imaging is described and explained in much greater detail in U.S. Pat. No. 3,520,681, to W. L. Goffe; U.S. Ser. No. 837,591, filed June 30, 1969; U.S. Ser. No. 837,780, filed June 30, 1969; and U.S. Ser. No. 199,683, filed Nov. 17, 1971; all of which are incorporated herein by reference.
A further imaging system which can advantageously employ the illumination system of the present invention is a polychromatic manifold imaging system utilizing a manifold set comprising an electrically photosensitive, cohesively weak imaging layer sandwiched between a donor sheet and a receiver sheet. For polychromatic manifold imaging, the imaging layer is'coated onto the donor substrate as a plurality of small contiguousareas' of at least two different colors which respond to lights of different colors. An electric field is imposed across layers can be prepared which are themselves sufficiently cohesively weak to respond to the application of light and electric field, a larger range of materials may be used if an activating step is included in the process; The activating step serves to weaken the imaging layer so that it can be more easily fractured along a sharp line which defines the image to be reproduced; Conventionally, the imaging layer is activated by heating or by treating it with a swelling agent or partial solvent for the materialprior to placing the imaging layer between the donor and receiver sheets. The activating step can be omitted if, for example, thelayer retains sufficient residual solventafter having beencoated on a substrate from a solution, or paste'to render thelayer cohesively weak. I
The structure of the manifold imaging member can take many forms.- For example, the manifold member may include separate electrodes on opposite sides of the donor substrate and receiver sheet for the application of the field or they may be directly on the backsub faces of these members and integral therewith. Alternatively, one or both of the donor substrate and receiver sheet may be made of a conductive material. Conventionally, at least one of these is transparent so as to permit exposure of the imaging layer through this electrode. Where both separate electrodes and a receiving and/or donor sheet are used, the receiving sheet and receiving side electrode or the donor sheet and donor side electrode may be transparent to permit exposure of the imaging layer. The imaging layer may be exposed from either the receiver sheet side or the donor sheet side.
In one form of the polychromatic manifold imaging process, an imaging layer comprising a plurality of differently colored areas is coated onto a transparent, insulating donor sheet. The areas are of at least two different colors and are distributed uniformly as to their different colors. All of the areas of a particular color are photosensitive to light of a particular color, which ideally does not photosensitize the areas of the other colors. The donor is placed imaging layer side up on a transparent conductive electrode. The imaging layer is then activated by spraying or brushing a swelling agent or partial solvent for the imaging layer onto the surface of the imaging layer. An insulating receiver sheet is placed on the activated imaging layer. The electrode is then placed on the receiver sheet. An electric field is then applied between the electrodes and a polychromatic light image is scanned across the length of the manifold set. The light image passes through the donor side electrode and donor sheet to the imaging layer, The electrodes are then removed and, while maintaining an electric field across the imaging layer, the receiver sheet and donor sheet are separated providing a positive polychrome image, corresponding to the polychrome light image, on the donor sheet. The manifold imaging system in general is described in more detail in copending U.S. application Ser. No. 708,3 80, filed Feb. 26, 1969 by W. G. Van Dorn; and polychromatic manifold imaging is explained in more detail in U.S. Pat. No. 3,556,783 to Kyriakakis; both of which are incorporated herein by reference.
Light sources for such polychrome systems have, until the present time, been of either the continuous spectrum kind, typified by tungsten iodide lamps and conventional tungsten lamps, or various line spectrum light sources, typified by mercury lithium lamps. The line spectrum lamps with the lines or line groups for red, green and blue will generate images of good quality. However, for optimum working conditions line spectrum lamps require expensive power supplies,
elaborate controls, precision cooling systems, and other complex and costly devices in order to keep the light output constant. Continuous spectrum light sources are less complex and costly; however, the light which they emit is not ideal for the production of high quality polychromatic images. In an attempt to correct the deficiencies of the continuous light sources it has been found necessary to bias them by means of various color correction filters and to reduce illumination from certain radiation bands to zero, for example, by using didinium, infrared, or ultraviolet filters. However, even with these precautions it has not been possible before the present invention to equal the color quality of the line spectrum light sources in a polychromatic process.
Accordingly, it is an object of this invention to overcome these and other disadvantages.
it is a further object of this invention to provide a simple, inexpensive system for lighting in a polychrome process.
It is still a further object of this invtntion to provide a lighting system for a polychrome process which possesses the principle advantages of both the continuous spectrum light sources and the line spectrum light sources while avoiding the principle disadvantages of each.
These and further objects of the present invention are accomplished by the present invention which provides an improved system for forming a polychromatic image of a polychromatic original wherein the original image is illuminated by light from a continuous spectrum light source and the resultant light image is projected onto a narrow imaging zone which progressively traverses the length of an imaging layer which comprises at least two differently-colored, electrically photosensitive materials which are sensitive to light of predetermined, different wavelengths, the improvement which comprises: cyclically interposing in the path of said light a filter group having filters of at least two different colors to thereby remove from the light image color components which undesirably sensitize the photosensitive materials at a filter member rotation/imaging zone speed value of at least l/i, were i is the width of the slit in inches.
The present invention will become more apparent from the ensuing discussion and the drawings wherein:
FIG. 1 is a side view illustrating a simple exemplary system for carrying out the present invention;
FIGS. 2-6 are front views illustrating color wheels which can be used in carrying out the present invention.
The sizes and shapes of elements of the drawings should not be considered as actual sizes or even proportional to actual sizes because some elements have been purposely distorted in size or shape in order to more fully and clearly describe the invention.
While the illumination system of the present invention is advantageously employed in a number of polychrome systems, the invention will be specifically described for purposes of illustration as being employed in a polychromatic photoelectrophoretic imaging system. The adaptation of the illumination system of the present invention into other polychrome systems, such as migration and manifold systems, is, however, within the scope of this invention.
Referring now to H6. 1, there is seen a transparent electrode generally designated 2 which, in this exemplary instance, is made up of a layer of optically transparent glass 4 overcoated with a thin optically transparent layer of tim oxide 6. Tin oxide coated glass of this nature is commercially available under the trademark NESA glass. This electrode is referred-to as the injecting electrode. A thin layer of electrically photosen-- sitive particles dispersed in an insulating liquid carrier is coated on the surface of the injecting electrode 2. The term electrically photosensitive particle may be thought of, in the environment of photoelectrophoresis, as describing any particle which, once attracted to the injection electrode will migrate away from it under the influence of an applied electric field when it is exposed to actinic electromagnetic radiation. This term is explained in more detail in U.S. Pat. Nos. 3,384,488, 3,384,565 and 3,384,566. The liquid suspension 8 may also contain a sensitizer and/or k binder for the pigment particles which is at least partially soluble in the suspending or carrier liquid. Above the liquid suension 8 is a second electrode, alternatively referred to as the blocking or imaging electrode 9, which is shown in the form of a roller having a conductive central core 10 covered with a layer of a blocking electrode material 11 which may, for example, be baryta paper. The conductive core 10 of roller 9 is connected to one side of the potential source 12. The oposite side of potential source 12 is connected to the injecting electrode 2 through a switch 14, such that when switch 14 is closed, an electric field is applied across the suspension 8 between the electrodes 2 and 9. An image projection system, typically comprising a continuous spectrum light source 16',a transparency l8 and a lens 20, is provided to expose the suspension 8 to a light image of theoriginal transparency 18 to be reproduced. It should be noted, at'this point, that injecting electrode 2 need not necessarily be optically transparent but that instead, electrode 9 may be optically transparent and exposure may be made through it by the use of mirrors and other optical devices. Also, the image may be produced throughthe useof reflected light from a photograph or similar object instead of through the use of light transmitted through a transparency.
A continuous spectrum light source is utilized in the present invention, to provide images of equal quality to those produced through the use of line spectrum light sources. Typically, photoelectrophoretic and other polychrome images produced through the use of continuous psectrum light sources are less then ideal. This is due to the fact that the intensityof the light in certain radiation bands varies from that which would be achieved by ideal white light. Also, the pigments used are often non-ideal in that they are electrically photosensitive to light of wavelengths outside those of their compliment. To overcome these deficiencies and to produce high quality polychrome images using a continuous spectrum light source, a rotatable member such as disc 22 which is driven by any suitable means 24, such as a variable speed electric motor, is provided in accordance with the present invention. The disc may be of the type shown in FIGS. 2 and 4. The disc in FIG. 2 has one filter group divided into three angular segments, each of which comprises a filter capable of transmitting one of the three primary colors, red, blue or green. The intensity of the light of a particular color may be varied by varying the relative angular extent of that particular color filter, or by superimposing a neutral gray filter of the desired opacity over the particular color filter the intensity of which is to be adjusted. The band width or hue of the particular primary colors may be varied by usingfilters which pass light of only designated wavelengths. By interposing the disc 22 between the light source 16 and the injecting electrode 2 as shown in FIG. 1, the light from the continuous spectrum light source 16 can-be corrected to nearly ideal white light, and wavelengths which would adversely sensitize pigments beyond their ideal band width sensitivity can-be eliminated. The disc may be placed at any point between the light source 16 and the injecting electrode 2, and need not be positioned as shown in FIG. I between the lens 20 and the injecting electrode 2'. It may, in fact, be desirable to position the disc 22 between the transparency l8 and lens 20.
' It is to be understood that the invention is not limited to the use of a disc, but that other devices, such as an endless belt having one or more filter groups and supported about at least two rollers, may be used.
Any suitable insulating carrier liquid can be employed as the carrier for the photoelectrophoretic imaging suspension employed in the present invention. Typical carriersinclude .decane; dodecane; tetradecane; Sohio solvent 3454, a kerosene fraction available from StandardOil Company of Ohio;- dimethyl siloxane; olive oil; linseedoil, mineral oil; cottonseed oil;
' tures thereof.
For polychromatic photoelectrophoreticimaging, the particles employed in the imaging suspension are selected so that those ofdifferent colors respond to different wavelengths in the visible spectrum corresponding to their principle absorption and further so that spectral response curves do'not have substantial overlap; thus allowing for color separation insubtractive multicolor image formation.
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 different absorption maxima may be added to improve color synthesis. When mixed together in the carrier liquid, these particles produce a black-appearing suspension, and when one or more of the particles are caused to migrate from the transparent electrode toward the blocking electrode, they leave behind particles which produce a color equivalent to the color of the impinging light source. Thus, for example, red light exposure causes the cyan colored pigment to migrate toward the blocking electrode, thereby leaving behind the magenta and yellow pigments which combine to produce red in the final image. In the same manner, blue and green colored images are reproduced by removal of yellow and magenta respectively; and, of course, when white light impinges on the mix, all pigments migrate toward the blocking electrode leaving behind the color of the white or transparent substrate. No exposure in a particular area leaves behind all pigments which combine to produce a black image. For a more detailed discussion of pigments suitable for polychromatic photoelectrophoretic imaging suspension, reference is again made to US. Pat. Nos. 3,384,488 and 3,384,566.
In a typical operation, asuspension of suitably photosensitive yellow, cyan, and magenta pigments is uniformly coated on the injecting electrode 2. Switch vl4 is then closed, continuous spectrum light source 16 is energized, the disc 22 is rotated at high speed. between the light source 16 and the injecting electrode 2. Upon closing the switch 14 the conductivesurface 6 of the electrode 2 is thereby renderedpositive with respect to the back surface of the blocking electrode material 1 1. Any negatively charged particles within the system tend to move toward electrode 2, while any positively charged particles in the system would move toward blocking electrode 16. The existence of any positively I charged particles within the system and the movement thereof will be disregaded so as tofacilitate the expla nation. of the movement of negatively charged particles in the carrier liquid. The imaging roller or blocking electrode 9 is moved across the top of suspension coated electrode2 in the direction shown by the arrow. All particles which are on the surface 6 of electrode 2 which are exposed to electromagnetic radiation of a wavelength to which they'are sensitive move. away from surface 6 upto the surface of electrode 9 as it passes over the suspension-coated electrode 2. In areas other than the relatively narrow zone between the electrodes 2 and 9, there is essentailly no field in which the thus activated particles will migrate from the injecting electrode 2. Therefore, it is found that imaging takesplace mainly in the area under the imaging roller 9 in what is called the imaging zone.
The imaging zone has a width less than the length of the imaging surface and is progressively traversing the length of the imaging surface. The rotating disc is cyclically interposing filter elements of different colors into the light path. The combination of movement of the imaging zone and rotation of the disc causes the generation of finite lines of the different colors of the filters in the filter groups. These lines are referred to as transverse speed pattern lines.
It has been determined that there are certain relationships between the rate of rotation of the disc, the imaging speed and size of the imaging zone which can be used to effectively eliminate transverse speed pattern lines. The relationship can be simply expressed in terms of the filter member rotation/imaging zone speed value r which is determined in accordance with the equation:
r [(R'N)/(60 8)] cycles/inch,
where R denotes the speed of rotation of the filter member in revolutions per minute, N is the number of filter groups in the disc, and S defines the linear speed of the imaging zone in inches per second. In order to achieve full exposure of the imaging zone to all colors of the filters in each filter group, r must be at least 1/1, where i is the width of the imaging zone, in inches, in the direction of movement of the roller 9. Where the imaging roller has a diameter of 4 inches, the imaging zone will be about one-sixth inch. Thus, r must in this case be at least 6 cycles/inch. For a disc having three primary color filter segments, as those shown in FIGS. 2, 3 and 6, the value of N will be 1. Using such a disc and imaging at a speed of 10 inches per second, it would be necessary to rotate the disc at a rate of at least 3600 RPM to achieve an r value of at least 6 cycleslinch. However, such low apeed rotation of the disc will cause the generation of transverse speed pattern lines. In this case, the lines would have an approximate width of about one-eighteenth inch and would berelatively pure primary colors. At higher speeds the lines become smaller and less well defined as to the primary colors. At the relatively short viewing distances which are usual for images made according to this procedure, the eye can distinguish the individual transverse speed pattern lines and will not see an apparently uniformly colored image. It has been discovered that the transverse speed pattern lines can be effectively eliminated-by operating at r values of greater than about 40 cycles/inch, and preferably greater than about 50 cycles/inch. Thus, to achieve a high quality polychrome image in the above exemplary situation, the disc would have to be rotated at speeds of above about 24,000 RPM, and preferably above about 30,000 RPM. It is obvious that the necessary rotational speed of the disc can be reduced by a factor of 2 by merely using a disc having two, three primary color filter groups, i.e., a disc having six primary color filter elements in all, such as the disc shown in FIG. 4.
It has further been discovered that moderate r values may be employed to achieve high quality polychrome images by bypassing a predetermined amount of the continuous spectrum light source illumination. This may be accomplished by utilizing discs of the type shown in FIGS. 3, 5 and 6. The disc in FIG. 3 has one filter group comprising three primary color filter elements and one transparent segment of predetermined angular extent. The disc in FIG. 5 has two filter groups comprising three primary color filter elements and a transparent segment of predetermined angular extent, or a total of eight segments in all. The disc shown in FIG. 6 has one filter group comprising three primary color filter elements and two transparent, concentric rings of predetermined radial extent. By thus bypassing a portion of the light, high quality polychrome images may be achieved at r values of between about 6 to 40 cycles/inch. Preferably, such bypass should be used up to r values of about 50 cycles/inch. The advantages of this embodiment are obvious, especially when high imaging speeds are contemplated. For example, utilizing the above exemplary situation but imaging at 40 inches per minute, the disc would have to be rotated at about 100,000 RPM if no bypass were used. Even with the disc of FIG. 4 having 6 color segments, the disc would have to be rotated at speeds of about 50,000 RPM if no bypass were used. By using the bypass, high imaging speeds can be utilized without requiring less practical high disc rotation speeds. The amount of light bypass can be easily determined for each individual system at the time that the primary color filter segments are selected. The amount of bypass will be roughly proportional to the decrease in r value from the preferred range of 40-50 cycles/inch. At r values less than 6 cycles/inch, full bypass would be required and the advantages of the present invention would not be achieved.
With this illumination system it is possible to deliberately leave out any portion of spectral radiation which is not ideal for the imaging system being used. In this manner, and by using properly selected band width and filter characteristics for the primary colors used, the imaging light can be exactly matched to every photoelectrophoretic, migration, and manifold polychrome process imaging media. Although the above discussion describes the use of primary color filter elements for purposes of illustration, the above-described sequential illumination system is not limited to use with primary color filters only. For example, didinium filters can be replaced with a combination of two filters, neither of which has any pass band around 555 millimicrons, but which enable the passage of spectral radiation of other wavelengths. For example, a combination of Kodak Wratten No. 30 and No. 65A filters will give a good approximation for this purpose. Further, instead of combining three primary color filters it is entirely feasible that some combination of three other color filters, or four or more narrow band filters will improve the quality of the polychrome images.
A wide range of voltages may be applied between the transparent injecting electrode and the blocking electrode to affect imaging in the photoelectrophoretic polychrome system. It is preferred, in order to obtain good image resolution, that the potential be such as to create an electric field of at least about 60 volts per micron across the imaging layer. The applied potential necessary to obtain the desired field strength will, of course vary depending upon the inter-electrode gap and upon the thickness and type of material used on the respective imaging electrode surfaces. Potentials as high as 8,000 volts have been applied to produce images of high quality. The upper limit of the field strength is limited only by the breakdown potential of the suspension and blocking electrode material.
The system herein described produces continuous tone polychromatic color images. Any suitable material such as paper may be used as a receiving substrate for the image produced. Or, if one desires to produce a transparency, the use of transparent film such as Mylar and Tedlar films might be desirable. In fact, the transparent electrode can, itself, be a transparent electrically conductive or insulative sheet or web which, upon image formation, can be directly employed as a transparency. Simila ly, Such a web can be wrapped around a transparent drum or passed through the nip formed by the blocking electrode. and a transparent surface thereby obtaining a readily removable or disposable web which functions both as an electrode and as the ultimate transparency.
After being formed, the image can be fixed in place, for example, applying a protective coating over its top surface, such as by spraying with a thermoplastic composition or in any other suitable manner. When a fusible polymeric material such as a thermoplastic resin is used in conjunction with the pigment particles, the image can be fixed by utilizing known heat fixing or vapor fixing techniques. In addition, the application of heat in this manner further assists in the fixing process by accelerating the solvent removal from the image aras.
For photoelectrophoretic polychrome systems, it si preferred that the transparent electrode be capable of enhancing the process of charge injection with the photosensitive particles of the imaging suspension. Most preferably, the transparent electrode comprises'an optically transparent material, such as glass, overcoated with transparent or semi-transparent conductive materials suchas tin oxide, indium oxide, copper oxide, aluminum or the like; however, other suitable materials including many semi-conductive materials may be used. Exemplary of such is raw cellophane, which, al-
.though not ordinarily thought of as being a conductor,
.is. still capable'of accepting injected charge carriers of the. proper polarity under the influence of an applied electric field. The use of more conductive materials allows: for cleaner charge separation and prevents possible charge build-up on the electrode, the latter tending to diminish the electric field across the suspension in an undesirable manner.
. The blocking electrode of the photoelectrophoretic system electrode pair which together define the imaging zone is'preferablycapable of injecting electrons into, and receiving electrons from, said photosensitive particles ata negligible rate as compared to the transparent electrode. Generally, although not necessarily, theblocking electrode comprises a conductive central core and a layer of blocking material surrounding said core capable of preventing or greatly retarding the injection of electrons into the photosensitive particles as compared to the transparent electrode.
1 The core of the blocking electrode generally will consist of a material which is fairly high in electrical conductivity. Typical conductive materials include ocnductive rubber, steel, aluminum, copper and brass.
Preferably, the core of the electrode will have a high electrical ocnductivity in order to establish the required polarity differential in the system; however, if a material-having a low conductivity is used, a separate electrical connection may be made to the back of the blocking layerof the blocking electrode. For example, the blocking layer orsleeve may be a semi-conductive polyurethane material having a conductivity of from about 10' to 10 ohm-centimeters. If a-h'ard rubber, non-conductive core is used, then a metal foil may be used as a backing for the blocking-sleeve. Although a blocking layer or sleeve need not necessarily-be used in the system, the use of such a layer is preferred because of the markedly improved result which it is capable of producing. It is preferred that the blocking layer, when employed, be either an insulator or a semiconductor which will not allow for the passage of sufficient charge carriers, under the influence of the applied field, to discharge the particles finally bound to its surface, thereby preventing particle oscillation in the system. The result is enhanced image density and resolution. Even if the blocking layer does allow for the passage of some charge carriers to the photosensitive par ticles, it still will be considered to fall within the class of preferred materials if it does not allow for the passage of sufficient charge so as to recharge the particles to the opposite polarity. Illustrative blocking materials which can be employed are baryta paper; Tedlar polyvinyl fluoride; Mylar poly(ethylene terephthalate); and
polyurethane. Any other suitable material having a revention will become apparent to those skilled in the art upon reading the above disclosure of the present invention which has as a principle feature the provision of a method which allows the use of a continuous spectrum light source to preare high quality polychrome images in polychromatic imaging systems of the type wherein light from an illuminated polyehromatic original is projected onto a narrow imaging zone which progressively traverses the length of an imaging layer.
What is claimed is:
1. In a process of forming a polychromatic imageof a polychromatic original wherein the original image is illuminated by light from a continuous spectrum light source and the resultant light image is projected onto a narrow imaging zone which progressively traverses the length of an imaging layer which is positioned within an electric field in at least the area of the ima'ging zone and comprises at least two differently-colored, electrically photosensitive materials which are sensitive to light of predetermined, different wavelengths, the improvement which comprises: cyclically interposing in the path of said light a filter group having filters of at least two different colors,
to thereby remove from the light image color components which undesirably sensitize the electrically photosensitivematerials, at a filter member rotation/imaging zone speed value of at least I'li, where i is the width of the imaging zone in inches.
2. The process according to claim 1 wherein the color filter elements are angular segments of a disc which is rotatedat a predetermin3d speed.
3. the process according to claim 1 wherein a predetermined amount of illumination from the continuous than about 6 cycles per inch.
7. The process according to claim 1 wherein the filter group comprises filters of the three primary colors red, green and blue.
8. The process according to claim 3 wherein the color filter elements are angular segments of a disc and light bypass is effected by providing at least one transparent, concentric ring of predetermined radial extent.