US3401629A - Printing method and printing means for ambient light filters - Google Patents

Description

Sept. 17, 1968 v. E. HAMlLTON PRINTING METHOD AND PRINTING MEANS FOR AMBIENT LIGHT FILTERS 3 $heets-Sheet 1 Filed Feb. 10, 1964 Sept. 17, 1968 v. E. HAMILTON 3,401,629

PRINTING METHOD AND PRINTING MEANS FOR AMBIENT LIGHT FILTERS 5 Sheets-Sheet 2 Filed Feb. 10, 1964 NVENTOR. fiaz/vz. %4/1//A 0/1 BY 2 2 xffaz/vz/ Sept. 17, 1968 v. E. HAMILTON PRINTING METHOD AND PRINTING MEANS FOR AMBIENT LIGHT FILTERS 3 Sheets-Sheet 5 Filed Feb. 10, 1964 INVENTOR v 11g. -%ewA 4M/470/v United States Patent Oflice 3,401,629 PRINTING METHOD AND PRINTING MEANS FOR AMBIENT LIGHT FILTERS Vern E. Hamilton, Palos Verdes Estates, Caiif., assignor to Douglas McDonnell Corporation, a corporation of Maryland Filed Feb. 10, 1964, Ser. No. 343,763 25 Claims. (Cl. 101-426) ABSTRACT OF THE DISCLOSURE Accurate printing of multiplicity of extremely fine lines very closely spaced is accomplished with printing member having planar margin and planar grid-printing face recessed only a few thousandths of an inch. Sheet of material is contacted by margin and then moved by pressure differential to contact inked printing face. The uniform pressure causes every part of sheet to touch face and eliminates skips. Parallel grid lines at an acute angle to horizontal and sloping in opposite directions are printed on opposite sides of transparent sheet to produce diamond grid pattern without ink filleting in acute angled corners of each diamond.

This invention relates to space lattice type ambient light trapping filters and more particularly to novel components of such filters and combinations of the components, and apparatus and methods for producing such components and laminating them into completed filters on a production basis.

Cathode ray tubes are typical examples of diffuse radiant screens and particularly susceptible to the elfects of ambient light rays. When such rays strike the surface of a cathode ray tube at various angles they produce specular reflection off the glass or diffuse reflection off the phosphors or both. The more serious problem is the reflection off the phosphors because such reflection often illuminates the low lights to such extent as to confuse them with the highlights, thus degrading the contrast.

Various schemes have been proposed for overcoming this difficulty, such as hoods over the tubes and honeycomb cores located in front of the screens, but such schemes have met with only minor success. My copending application for patent on Ambient Light Filter, Ser. No. 230,644, filed Oct. 15, 1962, discloses a novel construction which solves the problem.

Briefly, that novel construction comprises a filter body of transparent material in which are embedded a plurality of filter elements having a grid pattern. Each element constitutes a tier of alternating transparent and high- 1y light absorbing material in closely spaced relation to constitute a multiplicity of light transmitting apertures bordered by such light absorbing material. The grid pattern may be formed of generally parallel straight or wavy lines, or lines crossing each other to produce cells of varying shapes including but not limited to squares, rectangles and diamonds. The tiers are in generally parallel relation to each other and are spaced depthwise of the filter body with th light transmitting apertures in registry to constitute depthwise directed viewing cells. The axes of the cells may be normal to the plan of the filter body or at some other preselected angle.

Ambient light rays striking the surface of the filter body at acute angles other than normal are refracted into the cells at an angle, striking one or another of the depthwise spaced lines of light absorbing material and are absorbed thereby. The success of such filter results from the fact that the cells are very small in at least one lateral dimension so as not to be resolvable by the eye, and the lines forming the cell boundaries are extremely thin in the depthwise direction so that their edges present no appreciable grazing surface to reflect light rays onwardly. In fact, in a typical example the width of the apertures may be from .006 to .030 inch and the line width from .002 to .010 inch. The line thickness may vary from .0001 to .0002 inch, or even thinner, and the depthwise spacing between lines may be of the order of .010 inch or less. Such a filter with six filter elements is only one sixteenth inch thick.

It will be readily appreciated that the manufacture of a filter of this type presents various difficult problems. The production of a diamond grid pattern is particularly vexing. Such pattern is made up of a multiplicity of straight parallel lines extending in one direction and another multiplicity of straight parallel lines extending in another direction at an acute angle to the first direction. In order to achieve as wide a viewing angle as possible when the filter is used on television receivers the lines usually extend at angles which are as close as possible to horizontal without producing a moire effect with the raster lines of the screen. As a result, they cross each other at very acute angles, and the printing medium tends to flow out into the corners of the acute intersections, filling up a part of each diamond shaped aperture. This undesirably reduces the total transparent area and decreases the percentage of image signal transmission. It also produces an optical effect of enlarged spots at the acute intersections which appear to line up vertically and produce annoying vertical lines. Because of the extremely small size of the apertures it is practically impossible to produce a printing grid by any of the usual etching methods without a slight rounding of the acute corners. This further compounds the difficulty.

This problem has been solved in the present invention by printing, on a thin sheet or web of transparent material, one set of straight parallel lines on one face of the material and a corresponding set of lines on the second face of the material at an appropriate angle to the first set. With this arrangement the printing grids are simplified to straight lines with no corners to fill in, and there is no printing of intersecting lines on the same surface to cause further filling. When the printed sheet is viewed in the depthwise direction, the sets of lines on opposite faces produce the desired diamond grid.

The complete filter is made by laying up the desired number of sheets in registry and cementing them together. Each of the sheets, having been printed in the same manner, has a front face with lines in a first direction and a rear face with lines in a second direction. When the sheets are stacked, a front face and a rear face of adjacent sheets will be in direct contact except for the almost infinitesimal layer of cement, and the lines extending in opposite directions will be in the same plane for all practical purposes. Hence, at each interface there will be a one-plane diamond grid just as though it had been printed in a single operation, but there will be no filling or fillet-ing of the corners because the ink will be dry at the time of cementing. Also, even though the printed lines are very thin, they do have a finite thickness which is enough to form cement entrapment cells in the closed diamond shape if printed on one side only. This arrangement of parallel lines only, printed on both sides of the sheet, permits egress of the cement without entrapped bubbles.

It should be noted at this point that it is not essential to have a complete diamond grid pattern at each surface or interface. The parallel lines extending in one direction intercept rays coming from one general direction and the other parallel lines intercept rays coming from another general direction. Therefore sheets with only single sets of parallel lines may be used, with equal numbers of each kind, and all of one kind may be stacked together and the two sets united. The viewing cells will be diamond shaped when viewed in a depthwise direction but this is a visual result rather than a goal to be obtained. While the technical result is the same, it requires twice as many sheets, increasing the cost and lowering the light transmission.

The registry problem is just as important and just as difficult as in the case where a complete diamond grid pattern is printed on one face of a web or sheet, for the apertures must be in registry so that the depthwise viewing cells will be collimated. Because of the narrowness of the lines and of the spaces between them, a misregistry of lines in succeeding tiers of more than two or three ten-thousandths of an inch cannot be tolerated, and registration within one ten-thousandth of an inch is the desirable tolerance limit. Another problem is that although the plastic sheets are very smooth and quite uniform, there is still a small percentage variation in thickness throughout the area. If a printing plate and a backing plate, or two printing plates, are both very hard and flat there will be areas of the sheet which will fail to be printed. Resilient printing grids are not a satisfactory solution because the printed lines will be distorted.

The problem of uniform printing is overcome by positioning a length of the thin flexible material to be printed very close to the face of the printing grid, with just a few thousandths of an inch clearance. Differential air pressure, either vacuum or pressure, or both vacuum and pressure, is then applied to the length, or sheet, of material in a direction to urge it into contact with the printing grid. The pressure is substantially uniform over the whole area to be printed and thus will press the sheet uniformly against the printing grid regardless of high spots or low spots. Egress of air on the vacuum side is made possible because of the parallel line configuration in which the end of each space between the lines is connected at each end to the surrounding vacuum conduit or gutter. In practice it has been found impossible to achieve perfect contact in the closed diamond shape because the ink forms a seal leaving large voids in the printed area.

To accomplish the above, the printing member, in one preferred form, is provided with a planar margin, which is generally rectangular for printing filters for television receivers but may be any other desired planform to produce filters for other purposes. The area within the margin comprises a shallow recess with the printing grid therein, and the surface of the grid is preferably from .002 to .005 inc-h below the surface of the margin. A conduit or gutter communicates with the recess and with a valved vacuum source. When the sheet is positioned in sealing contact with the margin, the vacuum source is connected to the recess and the air pressure differential across the thickness of the sheet presses it against all portions of the printing grid.

In order to obtain automatic registration of the lines and spaces in the process of laminating, each sheet or length of the web is provided with index formations in identical registry relation with the lines and spaces. These may be punched holes, or dimples, or elongate beads, or any other suitable formation. Punched holes are being used at the present time with complete success but may be replaced with one of the other systems when conditions warrant.

In one form of the apparatus, in which only one side of the sheet is printed at one station, a second member which may be termed a backing member is used in connection with the printing member described above. The backing member is substantially identical with the printing member except that its shallow recess does not bear a printing grid. It may, however, bear a grooved pattern to facilitate drawing the sheet into contact with it by means of a vacuum. One of the members is provided with a pinrality of punch pins spaced along its margin, and the margin of the other member bears corresponding punch holes.

The members are relatively movable toward and away from each other along a predetermined path, preferably about a fixed pivot. If desired, one or the other of the members may be stationary, while the companion member is movable. As the members approach each other with a sheet or length of material between them, the punch means produce index formations in a peripheral area of the sheet and then the margins sealingly grip the sheet between them and hold it immobile during the printing operation. It will be obvious that with this arrangement the printed grid pattern will always be in the same exact registry relation with the index formations, and their patterns will always be identical because they are produced by the same apparatus.

The present practice is to position the sheet in contact with the margin of the backing member and apply the vacuum to be sure to prevent the sheet from inadvertently touching the printing grid before it is firmly gripped. When the two members have come together the vacuum is reversed and the sheet is printed. The vacuum is then reversed again, drawing the sheet away from the printing plate, then the vacuum is released, the members separated, and the sheet removed. If desired, both pressure and vacuum sources may be available, and either or both may be used to produce the desired air pressure differential.

The backing plate may also hear a printing grid so that both sides of the sheet may be printed at one station; but with this form of the invention it is presently preferred to use two substantially identical stations and print only one side of the sheet at each station.

In a modification, two sets of two members each are provided, each set being relatively movable toward and away from each other along a predetermined path, and the paths bearing a definite, fixed relation to each other. The first set comprises open frame members having planar margins to grip a peripheral area of a sheet or length of material. Themeans for producing the index formations are carrled by these 'mangins. When the first members have gripped the sheet and are holding it immobile, the second members are brought through the openings in the first members into juxtaposition with the sheet. These are printing members and are substantially identical to those described above. Preferably one is brought into contact with the sheet first, vacuum is applied, and the one side printed. Then the second member is brought into contact, is reversed, and the second side printed. Since the paths of relative movement of the two sets of members are fixed, the printed grid patterns will always be in exact registry relation with the index formations.

Various other advantages and features of novelty will become apparent as the description proceeds in conjunction with the accompanying drawings, in which:

FIGURE 1 is a perspective view of a pair of processing members engaging a length of a web of material to print a grid pattern on one side thereof;

FIGURE 2 is a similar view showing a two station apparatus for printing the grid pattern on both sides of the material;

FIGURE 3 is a perspective view of the printing plate of FIGURES l and 2;

FIGURE 4 is a sectional view in elevation of the apparatus of FIGURE 1, taken on a line corresponding to line L4 of FIGURE 3;

FIGURE 5 is a sectional view taken on line 55 of FIGURE 3;

FIGURE 6 is a perspective view of a modified form of backing plate used in the apparatus of FIGURE 2;

FIGURE 7 is a sectional view of one form of preloaded bearing usable in the pivotal mountings of the appanatus;

FIGURE 8 is a perspective view of a laminated filter body with one layer peeled back; and

FIGURE 9 is a sectional view in elevation of a modified form of apparatus.

In the form of the invention disclosed in FIGURES 1 and 2, the apparatus generally comprises a pair of processing member and 12 arranged for relative movement toward and away from each other along a predetermined path to insure exactly the same registry in each closing operation. For the purposes of the invention both members may be movable, or one member may be movable and the other one fixed. They may be mounted on slides or tracks or guide pins but preferably they have a pivotal mountings, which may be merely the connection between them or it may be the support for the movable parts of the apparatus. As shown, member 10 has a pair of supporting legs 14 extending from the lower edge of the member as viewed in FEGURE 1, each leg having a pivot bearing boss 26. Member 12 has a pair of depending arms 18 pivotally supported by the bearing bosses through pins or axles 20. Simple friction bearings are shown for simplicity but in practice pre-loaded anti-friction bearings, such as shown in FIGURE 7, are used which allow substantially Zero radial and axial movement to insure registry.

As will be brought out in more detail below, in FIG- URE 1 member 19 is a backing member or plate and member 12 is a printing member or plate. A continuous web 22 of thin, flexible, transparent, plastic material is fed from a supply roll 24 by any suitable known means which may provide slack for intermittent movement of the web into the apparatus while the supply roll unreels at a uniform rate. A bracket 23 and grooved roller 25 may be provided to support and guide the web into the apparatus. By means about to be described, the length of material within the apparatus is imprinted on one side with a grid pattern and also provided with index formations and a pair of holes to receive conveyor suspension hooks. It may then be moved on to a severing station and a drying rack. Alternatively, it may be passed over a series of rollers for drying, and then severed.

The printing member may be of any suitable shape but, for simplicity and lightness, it is preferably in the form of a fiat plate. As seen in FIGURE 3, which is a view of the interior or printing side, the plate is generally rectangular and is provided with a planar margin 26. The margin bears a plurality of punch pins 28 to produce index formations in the sheet or length of material, and also bears a pair of punch pins 38 to produce conveyor hook holes in the sheet. Corresponding punch holes 32 and 34 respectively are formed in the corresponding margin 36 of plate 10 to coperate with the punch pins.

Within the margin 26 is a shallow, substantially planar recess 38, just a few thousandths of an inch deep, hearing a printing grid 40 in the form of narrow, straight,

parallel lines extending diagonally up and to the left as seen in FIGURE 3. The printing surfaces of the grid are preferably .002 to .005 inch below the level of margin 26, as best seen in FIGURE 5. Adjacent the border of the recess is a small shallow depression or gutter 42 extending all of the way around the printing grid. At each of its corners the gutter is connected to a conduit or passage 44 which extends through the thickness of plate 12. At the outer face of the plate these passages are connected by fittings 46 and conduits 48 -to a conduit 50 which leads to a unit 52 for providing pressure or vacuum as desired. Three Way valve 54 supplies pressure or vacuum through conduit 50 or connects it to atmosphere, as determined by the operator. These operations may be fully automated if desired.

Member 10 is substantially identical with member 12 except that in the form shown it is not provided with a printing grid in the recess 56. In fact, the recess may be nothing more than fine tool marks leading to the gutter so that vacuum or pressure may be applied to the whole area within its margin 36. Member 10 is supplied with a pressure-vacuum system like that of plate 12. In commercial practice it is more desirable to place both plates in communication with a single pressure-vacuum unit with appropriate valving.

As seen in FIGURE 4, plate 12 may be readily moved between inking and printing position by means of a simple conventional servomotor comprising a cylinder 58 pivotally mounted on bracket 69 and piston rod 62 pivotally connected to bracket 64 carried by plate 12. Pressure fluid is supplied to cylinder 58 through conduits 66, 68 from a hydraulic pressure unit 70, the flow being controlled by valve 72. When plate 12 is in the inking or idle position indicated by dotted lines it may be inked by roller 74 which may be operated manually or mechanieally.

The manner of operation of the apparatus is very simple and straightforward. With plate 12 in the dotted line position of FIGURE 4, the web, or the sheet if it is desired to sever before printing, is positioned in face-wise contact with margin 36 of the plate 10. Valve 54 is then operated to apply a vacuum to the recess in plate 10. Since the sheet is in sealing engagement with the margin, the vacuum will pull sheet 22 into the recess and hold it against inadvertent movement. By manipulating valve 72, plate 12 is brought up to the solid line position. As it approaches final position, the pins 28 and 30 punch index holes and conveyor hook holes in the sheet. Pivot 20 is preferably in the plane of margin 36 of plate 10 so that the pins are traveling almost exactly normal thereto as they pierce the sheet and enter the punch holes.

The margin 26 of plate 12 is now in gripping and sealing contact with a peripheral area of sheet 22. The vacuum in plate 10 is released, and a vacuum is now applied to plate 12, urging the sheet into contact with the printing grid to produce the desired grid pattern. Since the printing surfaces are raised a few thousandths above the bottom of the recess, the entire area communicates freely with the gutters and conduits, and the air pressure differential is applied uniformly to the sheet so that all parts come into good printing contact. The differential can be assisted by air pressure from the plate 10 side if desired. In fact the whole operation can be carried out by pressure instead of vacuum or by a combination of both.

It will be seen that the sheet is provided with index formations and gripped against any lateral movement in its plane during the printing operation. Since the printing grid is always in the same registry relation with the punch pins, the printed grid pattern will always be in the same relation with the index holes so that the filter elements when assembled on jig pins for laminating will be automatically collimated, and no visual or optical alignment will be necessary.

When the sheet or web emerges from the apparatus it is provided with a printed grid 76 and with index holes 78 which, as shown, extend around the perimeter of the printed area. It is also provided with holes 80 for the reception of conveyor hooks 82 depending from bars 84 of the conveyor 86. Any suitable severing means 88 is used to sever the web into individual sheets if this has not been done before printing. The severing means may be located at any position downstream of the printing apparatus depending on how the additional processing is to be carried out.

Insteaad of punched holes, the index formations may be dimples, beads, or other suitable means which will provide automatic mechanical alignment when the elements are stacked. Even greater accuracy may be obtained by first bringing the plates into gripping engagement with the sheets and then punching the holes or indenting the sheet. This can be done with individual movable servo-punches or by using a typical double acting press technique.

It will be apparent that a printing plate like plate 12 without the punch pins is eminently suitable for printing any sheets having a variable thickness which makes it diflicult to obtain total printing contact and it is not necessary to achieve perfect registry of a plurality of printed sheets. The method of applying an air pressure differential to a sheet in contact with a printing grid produces uniform pressure over the entire area so that a very light pressure will produce uniform printing and very accurate lines. If the matter to be printed includes closed figures, the plate bearing the printing grid is made porous to prevent entrapment of air which would otherwise hold some areas of the sheet out of printing contact.

The apparatus of FIGURE 1 prints only one side of the sheet, as illustrated. If the sheet material is stiff enough to prevent inadvertent contact with the printing grid before it is desired, plate 10 can be provided with a printing grid also. Such grid would be identical with the one shown in FIGURE 3 and, because it would be reversed in position, it would print a complementary pattern n the opposite side of the transparent sheet to produce a diamond grid effect when the sheet is viewed depthwise. In this case the servo-punches or double acting press mechanism would be utilized so that the sheet would be gripped and held immobile while being punched and during the entire printing phase. However, because of the extreme accuracy required in the manufacture of the present filters, it is preferred to avoid any possibility of inadvertent printing contact.

The preferred mode is illustrated in FIGURE 2, in which a second pair of processing members is located onward from the pair of FIGURE 1. The distance between sets is determined by the drying time of the ink. Since very quick drying ink is used, the distance is not excessive. Members 90 and 92 are substantially identical with members it) and 12 but are reversed in position. Member 90 is the backing plate and member 92 is the printing plate, which is located to print the complementary grid pattern on the second side of the sheet, as indicated by the dotted lines in FIGURE 2. Plate 90, as illustrated in FIGURE 6, is provided with pins 94 to receive the index holes 78, and with pins 95 to receive the conveyor hook holes 80. All of these pins are tapered so that the sheet can be fitted thereon readily, but the base diameters of at least pins 94 are exactly the same size as the punch pins 28 so that in the second printing the grid pattern will match the index holes as accurately as does the first grid pattern.

The predetermined path of travel mentioned above must be maintained with great accuracy in order to produce the desired .accurate end result. Pivotal connections between the relatively moving parts make this accuracy possible with no complication. Plain friction bearings can be used but present a wear problem because they must be pre-loaded to eliminate end play and radial play. A simple anti-friction bearing arrangement is illustrated in FIG- URE 7, although other suitable connections can be used. In this figure, arm 18 is pivotally connected to leg 14 through the medium of bolt 93, which is a push fit in bore 100. A shim washer 192 is fitted on the bolt to maintain an exact distance between members 14 and 18. The inner race 104 of a tapered roller bearing is a push fit on the bolt and the outer race 106 is a push fit in bore 108 in arm 18, with the usual tapered rollers 110 between the races. The assembly is completed by thrust washer 112 and nut 114 threaded on the bolt. As the nut is tightened, race 104 is pushed into snug contact with washer 102, and race 106 is pushed inwardly in bore 108 until all slack is removed from the bearing assembly. A similar bearing may be used in the other pivoted joint slightly modified to avoid over-restraint in the axial direction.

A laminate constituting a complete filter body 116 is illustrated in FIGURE 8. A corner of the top lamina 118 is turned back to show the direction of the lines. It will be observed that the lines on opposite sides of one lamina extend in opposite directions, forming between them a diamond grid pattern when viewed through the depth of the lamina. Since the lines of the printing grids did not cross each other there was no filleting of the corners in the manufacture of the grid itself during etching, and since the printed lines on the sheet are on opposite sides they do not touch each other at any point and hence no filleting results here.

It will be further noted that the lines on the two confronting faces of adjacent sheets extend in opposite directions and are depthwise separated only by the infinitesimal thickness of the cement so that for practical purposes they form a co-planar diamond grid. Since the ink is dried before laminating and is not soluble in the cement, there is no filleting of the corner. Thus, maximum light transmission is assured for any selected diamond grid pattern.

As mentioned earlier, it is not essential that each lamina or sheet 118 carry oppositely extending lines on its opposite faces. One group of sheets may carry lines extending up and to the left, trapping rays from the upper right quadrant, and another group may carry lines extending up and to the right, trapping rays from the upper left quadrant. The two groups can be then made up into a single laminate similar to 116. The diamond pattern at each interface as illustrated, however, has the advantage of reducing the total number of sheets required for a given light trapping function.

In a modification of the apparatus, illustrated in FIG- URE 9, the functions of gripping the sheet and holding it immobile and producing the index formations are performed by a first set of processing members, and the printing function is preformed by a second set of processing members. The first set comprises generally rectangular open frame members 120 and 122, pivoted at 124 on bracket or base 126 for relative movement toward and away from each other along a predetermined path. When swung together as shown they grip between them a length of the web 22 of thin, flexible, transparent material. If desired, either frame member may be maintained stationary and the other one moved. The means for operating the frames are conventional servomotors 128 and 130 substantially identical to that shown in FIGURE 4, include similar conduits and power source.

After the frames have gripped a peripheral area of the sheet or web, index formations are produced in this peripheral area. Frame 122 carries axially movable punch pins 132, arranged around the periphery of the margin 134 of the frame and individually actuated by small pneumatic servo cylinders 136. Frame 120 carries a series of punch apertures 138 in its margin 140 aligned with the punch pins to complete the punching operation. The punch pins may be blunt to produce dimples, or elongated to produce beads. Their actuation may be by the conventional double acting press technique instead of the individual servo cylinders.

Printing members 142 and 144 are substantially identical with printing member 12 of FIGURES 1 and 4 including the means for producing air pressure differential and the hydraulic system for controlling their movement, with the exception that ther margins 146 and 148 do not carry any means for producing index formations, since this is accomplished by the frame members. Stops 152 are provided as .a positive limit on their approach to center. Conventional inking rollers 150* are provided to ink the printing plates when they are in their idle position.

The printing operation may be performed in several different sequences. In one mode, plate 144 remains in idle 142 is moved in through the opening margin 146 contacts a marginal area of sheet 22. The air pressure differential is applied to urge the sheet into printing contact with the printing grid of plate 142. The differential is maintained to hold the sheet stationary while plate 144 is brought up until its margin 148 contacts the sheet. The differential is now reversed and the sheet contacts the printing differential is then released, plates 142 and 144 return to their idle position, frame members 120 and 122 move relatively apart, and sheet 22 is removed.

In another mode, plates 142 and 144 are moved up at the same time, with plate 142 ahead of 144 just enough to complete its printing operation as plate 144 arrives. This mode and the previous one require relatively slow grid of plate 144. The

movement of the plates because the unbalanced air pressure produced by the first moving plate against the unsupported center of the sheet might stress it enough to produce a momentary bulge which would cause defective printing.

The operation can be speeded up to some extent by a third mode in which both plates swing up rapidly at the same time, producing equal air pressure on both faces of the sheet. The plates are stopped when quite close to the sheet. Then plate 142 is advanced relatively slowly the short remaining distance into contact, and the printing is preformed on one side of the sheet. Then plate 144 is brought into contact, and the printing operation is completed.

All of the modes described may be controlled manually, or an automatic programming control for the hydraulic valves may be provided. It will be seen that the invention described above provides a novel and effective method and apparatus for producing unique filter elements for assembly into complete space lattice type ambient light trapping filters.

Various changes and modifications may be made in construction and operation described herein without departing from the spirit of the invention, and it is intended that all such changes and modifications shall be embraced .within the scope of the following claims.

I claim:

1. Apparatus for printing a grid pattern on thin flexible material, comprising: a pair of open frame members relatively movable toward and away from each other along a predetermined path; said frame members having confronting planar margins surrounding passages therethrough; said margins, when the frame members are moved toward each other, being adapted to grip a peripheral area of a length of thin flexible material therebetween; and a pair of printing members relatively movable toward and away from each other through said passages along a predetermined path; said printing members having confronting, substantially planar faces provided with printing grids having complementary patterns adapted to contact the opposite faces of said material in the area between said passages and print complementary portions of a complete grid pattern thereon.

2. Apparatus as claimed in claim 1; and, in addition thereto, selectively operable means to produce an air pressure differential across the thickness of said material to initially urge the portion to be printed toward a first one of said printing members and subsequently urge said portion toward the second one of said printing members.

3. Apparatus as claimed in claim 1; the predetermined paths of movement of said frame members and said printing members having a fixed relation to each other; and the margins of said frame members bearing co operative means for producing index formations in said peripheral area and being adapted to restrain said material against lateral movement in its plane during the printing process to insure that the printed grid pattern will always be in exact predetermined registry relation with said index formations.

4. Apparatus for printing a grid pattern on thin flexible material, comprising: a pair of open frame members relatively movable toward and away from each other along a predetermined path; said frame members having confronting planar margins surrounding passages therethrough; said margins, when the frame members are moved toward each other, being adapted to grip a peripheral area of a length of thin flexible material therebetween; and a pair of printing members relatively movable toward and away from each other through said passages along a predetermined path; said printing members having confronting, substantially planar faces including a planar margin and a shallow recess within said margin, and a printing grid in the recess; said printing grids having complementary patterns adapted to contact the opposite faces of said material and print complementary portions of a complete grid pattern thereon; passage means extending through each of said printing members and communicating with its respective recess; vacuum producing means; conduit means communicating with said vacuum producing means and said passage means to induce .a vacuum in said recesses; said members being movable toward each other into close parallel relation with said length of material therebetween; the margins of said printing members being adapted to grip a marginal area of said material in sealing engagement; and selectively operable means connected with said conduit means to initially induce a vacuum in a first one of said recesses to urge said material toward its associated printing member, and to subsequently release said vacuum and induce a vacuum in the second one of said recesses to urge said material toward its associated printing member.

5. Apparatus as claimed in claim 4; and, in addition thereto, pressure producing means also connected to said conduit means and controlled by said selectively operable means to substitute pressure for vacuum on occasion as determined by an operator.

6. Apparatus as claimed in claim 4; the predetermined paths of movement of said frame members and said printing members having a fixed relation to each other; and the margins of said frame members bearing cooperative means for producing index formations in said peripheral area of said material and being adapted to restrain said material against lateral movement in its plane during the printing process to insure that the printed grid pattern will always be in exact predetermined registry relation with said index formations.

'7. Apparatus for printing a grid pattern on thin flexible material, comprising: a pair of open frame members relatively movable toward and away from each other along a predetermined path; said frame members having confronting planar margins surrounding passages therethrough; said margins, when the frame members are moved toward each other, being adapted to grip a peripheral area of a length of thin flexible material therebetween; and a pair of printing members relatively movable toward and away from each other through said passages along a predetermined path; said printing members having confronting, substantially planar faces including a planar margin and a shallow recess within said margin, and a printing grid in the recess; said printing grids having complementary patterns adapted to contact the opposite faces of said material and print complementary portions of a complete grid pattern thereon; a first one of said printing members being movable into printing position with its planar margin coplanar with the planar margin of its associated frame member and in sealing contact with said material; selectively operable means to produce an air pressure differential across the thickness of said material to initially urge it into printing contact with the printing grid of said first printing member and to temporarily retain it in such contact; the second one of said printing members being subsequently movable into printing position with its planar margin coplanar with the planar margin of its associated frame member and in sealing contact with said material; and said selectively operable means being operable to reverse the direction of said air pressure differential to urge said material into printing contact with the printing grid of said second printing member.

8. Apparatus as claimed in claim 7; the margins of said frame members bearing cooperative means for producing index formations in said peripheral area of said material and being adapted to restrain said material against lateral movement in its plane during the printing process to insure that the printed grid pattern will always be in exact predetermined registry relation with said index formations.

9. Apparatus for printing a grid pattern on thin flexible material, comprising: a printing member having a generally planar face including a planar margin and a printing grid having a multiplicity of image-printing embossments lying in a common plane recessed slightly below said margin; said margin being adapted to sealingly contact a peripheral area of a sheet of thin flexible material and initially hold the portion of the sheet within said peripheral area spaced from said printing grid; and means to produce an air pressure differential across the thickness of said material acting on the portion of the sheet within said peripheral area to urge it into uniform printing contact with said printing grid.

10. Apparatus as claimed in claim 9; the printing surfaces of said printing grid being recessed about .003 to .005 inch below the plane of said margin.

11. Apparatus for printing a grid pattern on thin flexible material comprising: a printing member having a generally planar face including a planar margin and a printing grid having a multiplicity of image-printing embossments lying in a common plane recessed slightly below said margin; said margin being adapted to sealingly contact a peripheral area of a length of thin flexible material; passage means extending through said printing member and communicating with said recessed printing grid; vacuum producing means; and conduit means communieating with said vacuum producing means and said passage to induce a vacuum between said material and said printing grid to draw the material into uniform printing contact with said grid.

12. Apparatus as claimed in claim 11; and, in addition thereto, pressure producing means also communicating with said conduit; and valve means to selectively introduce pressure therefrom between said material and said printing grid to initially prevent inadvertent contact between said material and said printing grid.

13. Apparatus for printing a grid pattern on thin flexible material, comprising: a pair of members relatively movable toward and away from each other along a predetermined path; said members having confronting generally planar faces including a planar margin and a shallow recess within said margin; at least one of said members having a printing grid in its recess; said members being movable toward each other into close parallel relation with a length of thin flexible material therebetween; said margins being adapted to grip a peripheral area of said material in sealing engagement; and selectively operable means to produce an air pressure differential across the thickness of said material to initially urge the portion within said peripheral area toward a first one of said members and subsequently urge said portion toward the second one of said members.

14. Apparatus as claimed in claim 13; each of said members having a printing grid in its recess to produce complementary grid patterns on opposite faces of said material.

15. Apparatus as-claimed in claim 13; said margins bearing cooperative means for producing index formations in said peripheral area and being adapted to re strain said material against lateral movement in its plane during the printing process to insure that the printed grid pattern will always be in exact predetermined registry relation with said index formations.

16. Apparatus for printing a grid pattern on thin flexible material, comprising. a pair of members relatively movable toward and away from each other along a predetermined path; said members having confronting generally planar faces including a planar margin and a shallow recess within said margin; passage means extending through each of said members and communicaL ing with its respective recess; vacuum producing means; conduit means communicating with said vacuum producing means and said passage means to induce a vacuum in said recesses; at least one of said members having a printing grid in its recess; said members being movable toward each other into close parallel relation with a length of thin flexible material therebetween; said margins being adapted to grip a peripheral area of said Cit material in sealing engagement; and selectively operable means connected with said conduit means to initially induce a vacuum in a first one of said recesses to urge said material toward its associated member, and to subsequently release said vacuum and induce a vacuum in the second one of said recesses to urge said material towards its associated member.

17. Apparatus as claimed in claim 16; and, in addition thereto, pressure producing means also connected to said conduit means and controlled by said selectively operable means to substitute pressure for vacuum on occasion as determined by an operator.

18. Apparatus for printing a diamond grid pattern on a length of thin transparent material to be used as a component of a space lattice type ambient light trapping filter, comprising: a pair of plate-like printing members relatively movable toward and away from each other along a predetermined path; said members being generally rectangular in planform and having confronting generally planar faces; each of said faces bearing a printing grid on the major portion of its area; said grids being substantially identical and comprising a multiplicity of closely spaced, narrow, parallel, straight lines; said lines extending in a direction at an acute angle to a horizontal side of the generally rectangular face when viewed in elevation so that the lines on the confronting faces extend upwardly and laterally in opposite directions when the faces are in printing position; said printing grids being adapted to contact opposite sides of a length of thin transparent material arranged between them and print complementary grid patterns thereon; said grid patterns cooperating to produce a diamond grid pattern when viewed through the thickness of the material.

19. A method of printing accurately and uniformly a grid pattern on a length of thin flexible sheet material, comprising: positioning a length of thin flexible sheet material in very closely spaced, parallel, confronting relation with a substantially planar grid printing face of a printing member; sure differential across the thickness of said material over the major part of its area; and using said differential to urge said material into contact with said grid printing face with substantially uniform pressure over the entire printing area of said face.

20. A method as claimed in claim 19; including the further steps of producing index formations in said material and restraining it against lateral movement in its plane during printing to attain an exact predetermined registry relation between said grid pattern and said index formations.

21. A method of printing accurately and uniformly a grid pattern on a length of thin flexible sheet material, comprising: positioning a length of thin flexible sheet materia said material; and reversing the air pressure differential to urge said material into uniform printing contact with said grid printing face.

22. A method as claimed in claim 21; including the 23. A method of printing accurately and uniformly complementary portions of a composite grid pattern on opposite sides of a length of thin, flexible, transparent sheet material, comprising: positioning said length of material facewise juxtaposition with a first substantially planar grid printing face borne by a first printing member; applying an air pressure differential across the thickproducing a low, uniform air pres-" ness of said material to urge it into printing contact with said first face; temporarily maintaining said differential; positioning a second substantially planar grid printing face borne by a second printing member in facewise juxtaposition with said material; and reversing the air pressure differential to urge said material into printing contact with said second grid printing face.

24. A method as claimed in claim 23; including the further steps of producing index formations in said material and restraining it against lateral movement in its plane during printing to attain an exact predetermined registry relation between said composite grid pattern and said index formations.

25. A method of printing a diamond grid pattern on a sheet of thin transparent material with the lines intersecting at very acute angles without producing corner fillets at the acute intersections, comprising: printing spaced, parallel, straight lines on one face of said material in a predetermined direction; and printing spaced, parallel, straight lines on the other face of said material in a direction which is at a very acute angle to the direction of the lines on the first face.

References Cited UNITED STATES PATENTS Bement 101-296 X Beatty 101-296 Goodstein 101-296 X MacArthur 101-297 Kruczek 101-35 Jafiin et a1. 101-28 Hakogi 101-35 Haberstump et a1. 101-28 Stahmann 101-35 Childress 101-129 Weiss 101-129 Vasilantone 101-126 Early et a1. 101-3 WILLIAM B. PENN, Primary Examiner.

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