US3517596A - Screening unit for half-tone color reproduction - Google Patents

Description

' June" 30, 1970 s. w-lJoHN o ET AL 3,517,596

SCREENING UNIT FOR HALF-TONE COLOR REPRODUCTICN Filed Dec. 27, 1967 4 Sheets-Sheet 1L .Mmv/A/ 4. 1.55.00!

s. w. JOHNSON ET AL June 30, 1970 SCREENING UNIT FOR HALF-TONE COLOR REPRODUCTION Filed Dec. 27, 1967 4 sheets sheet 2 Mam/w 4, 1 2mm 5 W1 mm m w \w M i w g O June 30, 1970 s. W.VJOHNSON ET AL 3,517,596

SCREENING UNIT FOR HALF-TONE COLOR REPRODUCTION Filed Dec. 27 967 4 Sheets-Sheet s v I IV VEN TOR: 5/60/90 JZ/M/mA/ i MflEV/A/ f]. 1 500 .ATTOIUEY June 30, 1970 I s. w. JOHNSON E AL 3,517,595

SCREENING UNIT FOR HALF-TONE COLOR REPRODUCTION Filed Dec. 27, 1967 4 Sheets-Sheet v mvnv Toni Swazi/0. Jam/$0M f Mew/A. Alina M/JW ATTORNEY 3,517,596 SCREENING UNIT FOR HALF-TONE COLOR REPRODUCTION Sigurd W. Johnson, Trenton, N.J., and Marvin A. Leedom,

Warminster, Pa., assignors to RCA Corporation, a corporation of Delaware Filed Dec. 27, 1967, Ser. No. 693,777 Int. Cl. G03b 33/00 US. Cl. 9512.2 12 Claims ABSTRACT OF THE DISCLOSURE A screening unit for use in a half-tone reproduction process comprises a carriage member and a rotary member supported for rotation about its axis within a through opening in the carriage member. Roller means support the carriage member over a recording element so that a half-tone screen, adjacent one end of the rotary member, is a predetermined distance from the recording element at an exposure station. Pawl and ratchet wheel means rotate the rotary member intermittently when the screening unit is moved along a predetermined path. The screening unit is adapted to use in combination with a reciprocating processing head to which the screening unit can be coupled selectively by magnetic means.

BACKGROUND OF INVENTION This invention relates generally to screening units used in the image reproduction arts, and more particularly to an improved screening unit for use in a half-tone process for reproducing a colored image on a recording element. The improved screening unit is particularly useful in combination with apparatus for making a multi-colored electrophotographic print from a colored, continuous tone transparency, such as a 35 mm. Kodachrome colored transparency or the like.

It has been proposed to use a photographic half-tone screen, that is, a meshwork of lines at right angles, ruled on glass, in a half-tone printing process to translate the varying (continuous) tones of an original image into dots of uniform tone but varying size on a reproduction of the original image. The half-tone process is used preferably to reproduce images on recording elements that have relatively poor continuous tone characteristics. In a multi-color, half-tone, printing process, either electrostatic or photographic, a plurality of half-tone colored images, each of a different color, are printed separately, in a predetermined order and in superimposition, to build up a composite multi-colored image corresponding to an United States Patent O original multi-colored subject. Since each of the separate SUMMARY OF INVENTION The improved screening unit is adapted for use with a recording element of the type that is exposed to a light image in a reproduction process. One embodiment of the improved screening unit comprises a carriage member and a rotary member supported by the carriage member and disposed for rotation about its axis. A half-tone screen is disposed adjacent one of the ends of the rotary member, and means are provided to support the carriage member over the recording element with the half-tone screen spaced a predetermined distance from the recording element.

In another embodiment of the invention the improved screening unit is combined with a processing head to which it can be coupled selectively for movement therewith over the recording element and for rotating the screen automatically.

The improved screening unit is particularly, although not exclusively, adapted for use with reproduction processing apparatus wherein the screening unit can be moved into, and out of, an exposure station without touching the'recording element, and wherein the half-tone screen may be rotated automatically through predetermined angles in preparation for particular exposures.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of an improved screening unit in combination with electrophotographic processing apparatus, the latter being illustrated with some parts broken away to expose internal parts;

FIG. 2 is an enlarged, side elevational view of the improved screening unit of FIG. -1 in combination with a portion of the apparatus thereof;

FIG. 3 is an enlarged perspective view of the processing head shown in FIGS. 1 and 2;

FIG. 4 is a perspective view, viewed from the bottom, of the processing head shown in FIG. 3;

FIG. 5 is an enlarged perspective view of the improved screening unit and the processing head shown in FIGS. 3 and 4;

FIG. 6 is a fragmentary cross-sectional view of the screening unit and the base on which it is mounted, taken along the line 6-6 in FIG. 5;

FIG. 7 is an enlarged, fragmentary view of the ruled portion of the half-tone screen of the improved screening unit, viewed in the direction of the arrow A in FIG. 6; and

FIG. 8 is a schematic diagram of a portion of an electrical control circuit including means for moving the processing head and for energizing the coupling means to couple the improved screening unit tothe processing head.

DESCRIPTION OF THE PREFERRED EMBODIMENTS General An improved screening unit is herein described in combination with electrophotographic apparatus of the type with which it is preferably adapted to cooperate, that is, with apparatus that has a reciprocating processing head.

Referring now to the drawings, particularly to FIGS. 1 and 2, there is shown apparatus 9 for making a halftone, multi-colored, electrophotographic print from a colored photographic transparency with the aid of an improved screening unit 10. The apparatus 9 comprises a platen or base 12 for supporting an electrophotographic recording element 14 of the type used in the electrophotographic arts. A fundamental explanation of an electrophotographic method and a recording element of the type used by the apparatus 9 is described in an article, Electrofax, Direct Electrophotographic Printing on Paper, by C. J. Young and H. C'. Greig, in RCAReview, December 1954, Vol. XV, No. 4.

The recording element 14 includes a photoconductive surface 18 and is disposed in a fixed relationship with a light image projector 16, such as a photographic enlarger, so that a light image can be projected onto the recording elements photoconductive surface 18. The projector includes a filter wheel 19 for disposing a desired one of a plurality of different colored filters in the path of the projected light image. The base 12 also supports the screening unit 10 and a processing head 20. The processing head 20, shown in detail in FIGS. 3 and 4, is adapted to be reciprocated over, and slightly spaced from, the recording element 14 to process it, and, in addition, to move and to operate the screening unit 10 by selective coupling thereto, in a manner to be described hereinafter in detail.

The screening unit Referring now particularly to FIGS. 5 and 6, the improved screening unit is shown on the base 12, decoupled from the processing head 20, and in the park (out of use) position. The screening unit 10 comprises a carriage member 22 and a rotary member 24, preferably a tubular member, disposed for rotation about a central vertical axis within a through opening 26 in the carriage member 22. The carriage member 22 comprises a relatively thick plate 28 formed with an inwardly extending flange 30 adjacent the lower surface 32 of the plate 28, narrowing and defining the lower portion of the through opening 26 and providing an annular shoulder on which the rotary member 24 is disposed.

Means are provided to maintain the carriage member 22 a predetermined distance over the base 12 and the recording element 14 and to allow the carriage member 22 to roll along a predetermined straight path on the base 12. To this end, four roller bearings are fixed to the plate 28 within four recessed corners 34-37 thereof. Thus, wheels or roller bearings 38 and 40 are fixed to vertical side walls in the recessed corners 34 and 35, respectively, as illustrated in FIG. 5. Similar roller bearings of which only roller bearing 41 is shown in FIG. 6, are fixed to the vertical side walls in the recessed corners 36 and 37.

Guide means are provided to maintain the carriage member 22 along a predetermined path over the base 12. To this end, strips 42 and 44 of a material having a relatively low coeflicient of friction, such as Teflon, for example, are fixed to opposite vertical sides 46 and 48, respectively, of the plate 28 and extend therebelow. The Teflon strips 42 and 44 substantially touch vertical, parallel walls 50 and 52, respectively, of the base 12 and serve to guide the carriage member 22 in its movement along the base 12, that is, along a path parallel to the parallel walls 50 and 52 of the base 12.

A rod 54 is slidably mounted within a fixed housing member 56 (FIG. 5) on the plate 28 of the carriage member 22, for slidable, reciprocal motion substantially parallel to the direction of travel of the carriage member 22. Therod 54 is spring-biased by a pin 58, extending through the rod 54 near one end 60 thereof, and a pair of symmetrically disposed springs, of which only one spring 62 is shown in FIG. 5. One end of the spring 62 is anchored to the housing member 5 6 and the other end is anchored to one end of the pin 58. Thus, the rod 54 is spring-biased in a direction away from the processing head 20, that is, to the left, looking at FIG. 5.

A gravity-actuated pawl 64 is loosely coupled to the end 60 of the rod 54 by a pivot pin 65 extending through the pawl 64 and the rod 54. The pawl 64 is adapted to engage a plurality of annularly arrayed ratchet teeth 66 formed in the upper end 68 of the rotary member 24. A screw 70 is threaded in the opposite end 72 of the rod 54 for adjusting purposes, as will be hereinafter explained. The head of the screw 70 is adapted to engage a fixed member, such as a bracket member 74, fixed to the apparatus 9, for exerting a force on the rod 54, against the spring tension of the spring 62, when the screening unit 10 is moved to the park position, that is, to the extreme left position, as shown in FIG. 5.

A pair of spaced- apart brackets 76 and 78 of magnetic material are disposed'on the upper surface 80 of the plate28 of the carriage member 22 so that vertical planar surfaces 82 and 84 of the brackets 76 and 78, respectively, are in substantially the same plane. The brackets 76 and 78 provide means to couple the screening unit 10 to the processing head 20, in a manner to be hereinafter explained.

The rotary member 24 is formed with an outwardlyextending flange 86 (FIG. 6) adjacent the upper end 68 thereof, and the ratchet teeth 66 are formed on the upper surface of the flange 86. Friction reducing means, such as small rounded studs 88 of Teflon, are disposed between the flanges 86 and 30 of the rotary member 24 and the carriage member 22, respectively, and also between the outer cylindrical wall 89 of the rotary member 24 and the flange 30 of the carriage member 22. This arrangement reduces friction between the rotary member 24 and the carriage member 22 and also disposes the rotary member 24 accurately within the carriage member 22 so that the rotary member 24 may rotate easily about its vertical axis. This arrangement also eliminates the need for any lubrication.

A half-tone screen 90 of any desired type, such as a Levi screen or an Instant Photo screen, for example, is fixed to the lower end 92 of the rotary member 24, as by glue or any other suitable means, so that the (ruled side) lower surface 94 of the screen 90, in its operative position, will always be a predetermined distance from the recording element 14 on the base -12. In the electrophotographic processv described herein, this distance is about 0.075 inch. The space between the screen 90 and the photoconductive surface 18 will be a function of the focal length and magnification of the projection lens. As illustrated in FIG. 7, the screen 90 comprises a meshwork of lines 96 at right angles to each other, usually rolled on glass. The screen 90 may comprise between about 33 to 300 lines per inch, depending upon its application. Developed photographic or electrophotographic images that have been exposed through a screen comprise a plurality of dots of uniform tone but varying size.

The rotary member 24 is adapted to be rotated about its axis when a force is applied to the end 72 of the rod 50, and the rod 54 is moved forward so that the pawl 64 engages the ratchet teeth 66 of the rotary member 24. This force is applied to the rod 54 when the carriage member 22 is pushed along the base 12 until the head of the screw 70 engages the fixed member 74 and the rod 54 is caused to move, relative to the carriage member 22, against the spring tension of the spring 62. The screw 70 can be extended adjustably to a predetermined length so that the rod 54 is forced to move a predetermined distance, against the tension of the spring 62, and the rotary member 24 is caused to rotate through a predetermined angle, usually 15 or 30 degrees. The angle of rotation of the rotary member 24 is also dependent upon the number of ratchet teeth 66. Releasing the force on the rod 54, as by releasing the push on the carriage member 22 causes the carriage member 22 to move slightly and the rod 54 to return to its normal springbiased position, as shown in FIG. 5.

The screening unit 10 may be used manually, that is, not in combination with a processing head 20, to dispose the half-tone screen 90 a predetermined distance from the recording element 14 at an exposure station 100, as shown in FIG. 1. Thus, a previously electrostatically charged photoconductive surface 18 of the recording element 14 can be exposed by a light image through the half-tone screen 90 to provide a half-tone latent electrostatic image on the photoconductive surface 18. The half-tone latent electrostatic image can be developed by any suitable manual means, or by the processing head 20 in a manner be hereinafter described.

The processing head 20 is adapted to cooperate with the screening unit 10 to move the latter selectively along a path on the base 12 between its park position and its exposure position and to rotate the rotary member 24 through a predetermined angle between exposures in a process requiring a plurality of exposures. Referring now particularly to FIGS. 3, 4, and 5, the processing head 20 is shown as comprising two separable parts,

fluid applicator means 102 and fluid removal means 104. The fluid removal means 104 is a member formed with an elongated slot 106 extending therethrough from a top surface 108 to a bottom flat surface 110 of the means 104. The fluid applicator means 102 comprises a relatively flat rectangular member of insulating material, such as Lucite, disposed within the slot 106, well spaced from the side walls of the fluid removal means 104 that define the slot 106. A pair of outwardly extending tabs 112 and 114 are fixed adjacent the upper edge of the fluid applicator means 102 and are disposed to rest on the top surface 108 of the fluid removal means 104, maintaining the fluid applicator means 102 in position to apply a selected fluid, such as a liquid electroscopic toner, to the recording element 14.

A plurality of flexible tubes 116120 communicate with separate openings in the fluid applicator means 102, which openings, in turn, communicate with a metal manifold 122 (FIG. 4) disposed along the lower edge of the fluid applicator means 102. Each of the tubes 116120 is connected to a separate one of tanks or reservoirs (not shown) through a separate one of solenoid operated valves 124-128, respectively, as shown in FIG. 1. The manifold 122 is formed with an elongated slit 130 through which a selected fluid can flow onto the photoconductive surface 18 of the recording element 14 when a selected solenoid valve is actuated. Thus, many fluids may be applied sequentially by the processing head 20 in a process of developing a latent image on the photoconductive surface 18.

The bottom surface 110 of the fluid removal means 104 is formed with a recess 132 spaced from the walls that define the slot 106. A conduit 134 in the fluid removal means 104 communicates with the recess 132 and with a flexible hose 136 which is adapted to be connected to a vacuum pump (not shown) for reducing the gas pressure within the recess 132. This reduced pressure removes any of the fluids that have been applied to the surface 18 of the recording element 14 during the developing process.

Electrostatic charging means is provided in the processing head 20 for applying an electrostatic charge of a desired polarity onto the photoconductive surface 18 of the recording element 14. To this end, the bottom surface 110 of the fluid removal means 104 is formed with a recess 138 (FIG. 4) for disposing a thin corona discharge wire 140 therein. The Wire 140 is electrically insulated from the fluid removal means 104 by a pair of insulators 142 and 144 (FIG. 3), and the wire 140 is electrically connected to a unidirectional, high voltage, power supply 146 (FIG. 1) (about 9 kv.) through a conductor 148. At appropriate times, a unidirectional voltage of suflicient amplitude is applied between the wire 140 and the metal base 12 to produce a corona discharge of a desired polarity around the wire 140 to charge the photoconductive surface 18 of the recording element 14 uniformly with a suitable electrostatic charge.

The processing head 20 is adapted to be reciprocated over, and slightly spaced from, the photoconductive surface 18 of the recording element 14 so that, in one direction of motion, an electrostatic latent image can be toned with an appropriate toner, and, in an opposite direction of motion, the toned image can be rinsed and the photoconductive surface 18 can be electrically charged in preparation for the formation of a subsequent electrostatic latent image. To this end, the processing head 20 has two plates 150 and 151 adjustably fixed to the opposite ends of the fluid removal means 104. The plate 150 has an ear 152 extending perpendicularly therefrom. The ear 152 is formed with a hole 154 for receiving therein a pin 156 on one side of a carriage 158. Similarly, the plate 151 has an ear 160 extending perpendicularly therefrom, and the ear 160 is formed with a hole 162 for receiving therein a pin (not shown) on the other side of the carriage 158.

The carriage 158 is adapted to move the processing head 20 in a reciprocal motion and comprises a plate 164 disposed below the base 12, as shown in FIG. 2. An internally threaded nut 166 is fixed beneath the plate 164 so as to engage a lead screw 167 threadably therein. One end of the lead screw 167 is journaled in a vertical bracket 168 that supports one end of the base 12. The other end of the lead screw 167 is coupled through a suitable gear train 169 to a reversible motor 170. The motor 170 may be controlled by a reversing switch 172, as shown schematically in FIG. 8, to rotate it in either of two opposite directions, as desired.

The screening unit 10 may be coupled selectively as by magnetic coupling means, to the processing head 20 for movement therewith. Thus, a pair of electromagnets 174 and 176 (FIG. 5) are disposed on the upper surface of the processing head 20 and fixed thereto by any suitable means. The electromagnets 174 and 176 are aligned with the vertical- surfaces 82 and 84 of magnetic material of the brackets 76 and 78 on the carriage member 22 of the screening unit 10'. Thus, the brackets 76 and 78 are attracted to, and held by, the electromagnets 174 and 176 when the processing head 20 is brought into substantial contact with the screening unit 10- and the electromagnets 174 and 176 are energized. Hence, the screening unit 10 can be pushed by the processing head 20 into the park position, that is away from the exposure station, but the processing head 20 cannot pull the screening unit '10 unless the electromagnets 174 and 176 are energized.

Alternatively, the electromagnets 174 and 176 can be disposed on the screening unit 10 and the magnetic brackets 76 and 78 can be disposed on the processing head 20 to accomplish the aforementioned coupling. Also, while two electromagnets 174 and 176 are shown and described herein, it has been found that only one electromagnet may be used with satisfactory results.

Each of the operations in the electrostatic printing process performed by the apparatus 9 can be sequenced manually in the .process of making a half-tone colored composite print from a colored transparency. Thus, for example, the light projector 16 may be turned on manually by closing a switch to a suitable circuit (not shown) known in the art. Also, the solenoid operated valves 124- 128 may be manually energized selectively to apply suitable toners sequentially to the processing head 20, and the corona wire may be energized manually from the power supply 146, when desired, by closing switches to appropriate circuits, well known in the art.

Referring now to FIG. 8, there is shown a schematic diagram of a portion of a control circuit including means for reciprocating the processing head 2-0 and for energizing the electromagnets 174 and 176 on the processing head 20 selectively. The reversible motor for reciprocating the carriage 158 which, in turn, reciprocates the processing head 20, is connected to a pair of input terminals and 182 of a source of alternating current through a main double pole-single throw switch 184, a rectifier 186, and the double pole-double throw reversing switch 172. Thus, the processing head 20 can be moved in one direction when the main switch 184 is closed and the reversing switch 172 is closed in one direction. By closing the reversing switch 172 in an opposite direction, the direction of travel of the processing head 20 is reversed.

The electromagnets 174 and 176 are also connected to the input terminals 180 and 182 through the main switch 184 and through a switch 188. Thus, by energizing the electromagnets 174 and 176 selectively through the switch 188, the screening unit 10 can be magnetically coupled to the processing head 20 for movement therewith.

A rinse solution solenoid switch and a liquid toner solenoid switch 192, such as for a black liquid toner, for example, is also connected to the input terminals 180 and 182 through switches 194 and 196, respectively, and the main switch 184. Other circuits, such as circuits for the corona discharge wire 140, the projector 16 light circuit, and the solenoid operated valves for the liquid toners and rinse, for example, may also be provided through switches 198, 200, etc. in a manner known in the art. While all of the circuits for the apparatus 9 may be actuated manually, they may be sequenced automatically by a master control circuit (not shown), such as a stepping drum programmer which may comprise a drum with adjustable plastic cams thereon adapted to actuate a plurality of micro-switches. Such a programmer is Programmer Model No. A31EZ-30, provided by the Tenor Company, New Berlin, Wis.

The screening unit 10 cooperates with the reciprocating processing head 20 to make a half-tone colored image on the photoconductive surface 18 of the recording element 14 in the following manner: Let it -be assumed that the screening unit 10 is originally in the park position, as shown in FIG. 5. The processing head 20- is moved over the recording element 14 at the exposure station 100 to charge the photoconductive surface 18 electrostatically, as by energizing the corona discharge wire 140 while the processing head 20 is moved to the extreme right position, shown in FIG. 1. During this movement (to the right), the electromagnets 174 and 176 are energized and the screening unit 10 is pulled into the exposure position, that is, at the exposure station 100. The photoconductive surface 18 is exposed to a light image through an appropriate filter on the filter wheel 19 and the screen 90 of the screening unit 10, thus providing a latent electrostatic image on the previously charged photoconductive surface 18. The processing head 20 is now moved to the left and an appropriate one of the solenoid operated valves 124-128 is actuated to tone the latent electrostatic image with an appropriate toner.

When the screening unit 10 reaches the end of its path of travel to the left, the head of the screw 70 on the rod 54 is pushed against the stationary bracket member 74, and the rod 54 is moved against the spring bias of the spring 62, causing the rotary member 24 to rotate through a predetermined angle. The length of the screw 70 protruding from the rod 54 can be adjusted for the rotation of the rotary member 24 to be about 30, or any other desired angle. The screening unit 10 is now ready to be pulled by the electromagnets 174 and 176 when the processing head 20 is moved again to the right. During this motion, the (first) toned image can be rinsed and the photoconductive surface 18 can be charged again in preparation for the next (second) exposure.

The second exposure of the photoconductive surface 18 can be through an appropriate filter on the filter wheel 19 of the light image projector 16, and through the rotated screen half-tone 90, producing a second latent electrostatic image on the photoconductive surface 18. The second latent electrostatic image is toned by moving the processing head 20 to the left again and actuating an appropriate one of the solenoid valves 124-428 to allow a desired color toner to develop the latent image. During this motion, the screening unit 10 is once more pushed to its extreme left position, thereby causing a force to be applied to the head of the screw 70 by the bracket 74 and, causing the screen 90 to turn through an angle of 30 again.

If it is desired that the next (third) exposure, through another filter on the filter wheel of the light image projector 16 be made without using the half-tone screen 90, the electromagnets 174 and 176 can be deenergized, as by opening the switch 188, and the screening unit 10 is left in the park position when the processing head 20 is moved to the right. If, however, it is desired that the third exposure be made through the screening unit 10, the electromagncts 174 and 176 are energized and the screening unit 10 is pulled along with the processing head 20 as the latter is moved to the extreme right. As many exposures as desired may thus be made on the photoconductive surface 18 at the exposure station 100, either through the half-tone screen of the screening unit 10 or without the half-tone screen 90, demanded by any particular developing process.

We claim:

1. A screening unit for use with a recording element,

said screening unit comprising:

a rotary member adapted to rotate about an axis,

a screen,

means supporting said screen on said rotary member substantially perpendicularly to said axis,

a carriage member adapted to be positioned over said recording element, and

means on said members for supporting said rotary member on said carriage member for rotation about said axis with said screen at a predetermined distance from said recording element when said carriage member is positioned thereover,

said carriage member comprising wheel means, whereby it can be moved along a path over said recording element,

said carriage means having a through opening, and

said means on said members for supporting said rotary member on said carriage member comprising a first flange adjacent the periphery of said rotary member, and a second flange defining a portion of said through opening, whereby said rotary member is disposed for rotation about its axis within said through opening with said first and second flanges in a substantial abutting relationship.

2. A screening unit for use with a recording element,

said screening unit comprising:

a rotary member adapted to rotate about an axis,

a screen,

means supporting said screen on said rotary member substantially perpendicularly to said axis,

a carriage member adapted to be positioned over said recording element, and

means on said members for supporting said rotary member on said carriage member for rotation about said axis with said screen at a predetermined distance from said recording element when said carriage member is positioned thereover,

said rotary member comprising rotating means to rotate it about its axis, and

spring-biased reciprocating means being mounted on said carriage member and being cooperatively associated with said rotating means on said rotary member to rotate said rotary member when a force is applied to said reciprocating means against the spring bias thereof.

3. A screening unit as described in claim 2, wherein said rotating means comprise ratchet teeth formed in a circular pattern, and

said spring-biased reciprocating means comprises a rod and a pawl mounted adjacent an end of said rod and adapted to engage said ratchet teeth, whereby reciprocating said rod in one direction causes said rotary member to rotate about said axis.

4. A screening unit for use with a recording element,

said screening unit comprising:

a carriage member,

means cooperating with said carriage member to support it over said recording element,

a through opening in said carriage member extending in a direction transversely to said recording element when said carriage member is over said recording element,

a rotary member having an axis of rotation and a pair of opposite ends,

a screen,

means to support said screen adjacent one of said ends substantially perpendicularly to said axis of rotation,

means to maintain said rotary member for rotation about said axis and within said through opening in said carriage member with said screen at a predetermined distance from said recording element when said carriage member is supported thereover, and

means on said rotary member for rotating it about said axis.

5. A screening unit as described in claim 4 wherein said recording element is supported on a base, and

said means on said carriage member to support it over said recording element comprises a plurality of roller means fixed to said carriage member and adapted to engage said base, whereby said carriage member may be rolled over said recording element.

6. A screening unit as described in claim 5 wherein guide means are fixed to said carriage member for cooperating with said base to guide said carriage member along a predetermined path when said carriage member is moved over said recording element.

7. A screening unit as described in claim 4 wherein said carriage member comprises a plate having upper and lower major surfaces,

said through opening extends between said major surfaces, and

said means to maintain said rotary member for rotation about, its axis and within said through opening comprise an inwardly extending first flange on said carriage member which defines and narrows a portion of said through opening adjacent said lower major surface, an outwardly extending second flange on said rotary? member adjacent the other end thereof, and friction reducing means disposed between said first and second flanges whereby said rotary member may be rotated easily about said axis.

8. A screening unit as described in claim 4 wherein said means on said rotary member for rotating it about its axis comprises a plurality of ratchet teeth formed adjacent the other end of said rotary member.

9. A screening unit as described in claim 8 wherein a rod is mounted on said carriage member for reciprocal movement with respect thereto,

a pawl is coupled to one end of said rod and adapted to engage said ratchet teeth for rotating said rotary member when said rod is moved in one direction of its reciprocal movement,

spring means biases said rod in an opposite direction to said one direction, and

the other end of said rod comprises means to engage a force for moving said rod against said spring means,

whereby said pawl can engage said ratchet teeth and rotate said rotary member. 10. In apparatus of the type for processing a recording element to reproduce an image thereon, wherein means are provided (1) to expose said recording element with a light image of said image to be reproduced to form a latent image, and (2) to reciprocate a processing head across said recording element to develop said latent image, the improvement comprising:

a screening unit comprising a screen adapted to be disposed over a predetermined location and spaced a predetermined distance from said recording element when said recording element is exposed by said light image, whereby said recording element is exposed through said screen, and

means including said processing head to move said screening unit selectively into, and away from, said predetermined location during the processing of said recording element by said apparatus.

11. In apparatus of the type described in claim 10',

wherein said screening unit comprises a carriage member and a rotary member adapted to be carried by said carriage member,

said screen is attached to said rotary member substantially perpendicularly to the axis of rotation of said rotary member, and

said means to move said screening unit selectively includes magnetic means cooperatively associated with said processing head and said screening unit for coupling said processing head to said screening unit.

12. In apparatus of the type described in claim 10,

wherein said apparatus comprises, in addition,

a base for supporting said recording element,

said screening unit comprises, in addition,

a carriage member, wheel means on said carriage member adapted to engage said base for rolling thereover, and guide means for guiding said carriage member over said base along a predetermined path including said predetermined location,

controllable electromagnetic means fixed to said processing head, and

magnetic means fixed to said carriage member and adapted to be attracted to said electromagnetic means when said processing head is adjacent said carriage member, whereby said carriage member and said processing head can be moved together simultaneously when said electromagnetic means areenergized.

References Cited UNITED STATES PATENTS 1,226,838 5/1917 Wolber -36 JOHN M. HORAN, Primary Examiner US. Cl. X.R.

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