US2190185A - Printing plate - Google Patents

Description

Feb. 13, 1940.l 1 A. c. HARDY 2,190,185

PRINTING I@LATE Original Filed Feb. 17, 1937 3 Sheets-Sheet 1 1 ATTORNEY A. C. HARDY Feb. 13, 1940.

1937 3 Sheets Sheefl 2 wam-OR MB 9M/Q27 ATTORNEY A. C. HARDY PRINTING PLATE Feb. 13, 1940.

3 Sheets-Sheet 3 Original Filed Feb-.l7, 1937 R l m wf, mC

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ATTRNEY Patented Feb. 13, 1940 .PATENT OFFICE PRINTING PLATE Arthur C. Hardy, Wellesley, Mass.,`assignor to Interchemical Corporation, New York, N. Y., a

corporation of Ohio Original application February 17, 1937, Serial No. 126,110. Divided and this application April 15,

1938, Serial N0. 202,177

(Cl. il-17) 4 Claims.

This invention relatesto printing plates, to contrast images for making printing plates, and to methods of making such contrast images.

'I'he present application is a division of my 5 application led February 17, 1937, Serial No. 126,110 on which patent No. 2,136,340 was issued on November 8, 1938.

The invention aims to provide a printing plate from which one may make prints containing printed and unprinted areas so proportioned and arranged as to give to the eye the effect of the variations in tone of an original subject or a continuous-tone photographic image thereof. Such a print I shall term a contrast image of the original to indicate that it contains only two contrasting tones, variations in the areas of which are utilized to simulate continuous variation in tone. Although such a print may be made with ink of any color on paper of any color, I shall, for convenience, term the two tones of the contrast image black and white," using black to refer to the inked areas of the print and white to refer to the uninked areas.

Printing plates are ordinarily made photo'-A chemically. This involves making a photographic contrast image of the original, which may be made either directly in a resist applied to a metal plate to control the etching of the plate or may be made as a positive or negative photographic plate or film and then transferred negative on a photographic plate, I shall terni the transparent areas black and the opaque areas white. In a resist on a. plate to be engraved for letter-press printing, I shall term the hardened areas black and the unhardened areas whitej and in a resist for engraving an intaglio plate, I shall term the unhardened areas black and the hardened areas White ,3o In order that a contrast image may give to i the eye the effect lof the tone variations of a continuous-tone image, the black and the white areas are so arranged and proportioned that the ratio of black area to white area corresponds 5,; to the tone of the original in each part of the contrast image. The area of the contrast image thus consists of a large number of small elemental areas, each of which includesa black area and a white area so proportioned that the ratio of the size of the black area to the size 6 of the whole elemental area corresponds (more or less closely accordingv to the. method used) f to the tonal value of the area of the continuoustone original corresponding to this elemental area of the contrast image. In order to save 10 repetition and to simplify the terminology used in this application, I shall refer to such an elemental area in which theV ratio of black area to .the total area corresponds. to the value of `the tone of the corresponding area of the continuous- 15 tone original as an element of the area of the contrast image.

Contrast images for making printing plates have'customarily been made by photographing a subject or a continuous-tone image of the sub- 201 ject through a half-tone screen. Methods involving photo-electric scanning have also been suggested. 'I'he effect of al1 the customary methods, including the most customary one of photographing through a half-tone screen, is to make 25' the elements of the area of the contrast image of uniform size. This method is satisfactory in representingr the middle tonesuof the original; but it has proved unsatisfactory in representing ,both the tones which are much darkertha the so middle tones andthe tones' which are. much lighter than the middle tones, because it requires the use of excessively small white dots and black spots, and uncontrollablervariations in the sizes of very small black dots or White spots necessarily 35 occur during etching and during printing.

By my invention, I have avoided the dimcultle and disadvantages caused by the presence of very smallblack dots and very small white spots in ordinary'contrast images, and in the printing 4o correspond accurately to the tone of the corresponding area of the original by means of black areas and white areas of not less than a predetermin'ed minimum size. Black areas of the size occur in tones lighter than the middle tone, while white areas oi minimum size occur in tones darker than the middle tone. 'I'he predetermined minimum size may be made great' enough to avoid the dillculties in etching and printing which are created by the use of very small black dots and white spots.

In describing my invention in detail, I shall refer to the accompanying drawings, in which Fig. 1 is a greatly enlarged view of part oi' a contrast image, or diagrammatic face view of part of a printing plate, embodying my invention;

Fig. 2 is a similar view oi the corresponding part of a contrast image or printing plate made by the ordinary half-tone process;

Fig. 3, 4 and 5 are enlarged fragmentary elevations of printing plates whose printing surfaces are diagrammatically represented by Fig. 1, Fig. 3 being a relief printing plate, Fig. 4 an intaglio printing plate and Fig. 5 a lithographie printing plate; l

Fig. 6 is a diagrammatic view of an electromechanical scanning apparatus which may be used in making contrast images embodying my invention;

Fig. 7 is an enlarged elevation of the recording light shown in Fig. 6; and

Fig. 8 is an end view of this light.

Figs. 1 and 2 show three parts A, A', B. B', and C, C', of two contrast images representing respectively a dark tone, a middle tone and a light tone of an original or a continuous-tone image of an original; The tonal value of the tone represented by the parts A and A' is 95, of the tone represented by the parts B and B', 50, and of the tone represented by the parts C and C', 5. The shading in these two views represents black areas in the sense hereinabove defined, while the unshaded areas represent white areas. I'he elements oi.' the two contrast images (that is, the elemental areas in which the ratio oi black to white corresponds to the tonal value of the corresponding area in the original) are outlined by dotted lines in Figs. 1 and 2 and identified by the symbois G, b and c in Fig.' 1 and a', b and c' in Fig. 2. In the middle-tone parts, B and B',-ofV each contrast image, each element b, b' consists of black and white areas of equal size. In the contrast image made by the ordinary half-tone process, shown in Fig. 2,'the elements a,*oi' the dark-tone area Av and the elements cf offthe light-tone area C'areof the same size as the elements b' of the middle-tone area' B'. Consequently, thekwhite areas inthe elements a' and the black areas in the elements c'l take the form of small spots or dots each 4having an area equal to only one-twentieth of onev size of the element. For still darker or lighter tones,"it is apparent that the spot or dot in each element must be made still smaller.

lThe contrast image which I have invented differs from an ordinary halt-tne image in that the elements a o f the darkf'tone area A are much larger than the elements b'of the'middle-tone area B; and the elements c o'f thelight-,tone areal C are also larger than the elementsb. As the result of the, increased size ofthe elements of the dark-tone area A, each element'a can include black and white areas having a ratio equal to 95% without the use oi' excessively small white areas; and in the same way the use of excessiveiy small black areas in the light-tone area C is avoided. ig

In the contrast image embodying my invention shown in Fig. 1,'it is to be understood that areas representing the tonesfbetween 95 and 50 consize White areas used in my contrast image.

In the same way,areas representing tones between and 5 consist of elements intermediate in size between the elements b and the elements c, containing black areas larger than those contained in the elements c and smaller than those contained in the elements b; but, in tones lighter than 5, the size of the elements is increased above the size of the elements c, while the size of the black areas which they contain is not decreased. In the particular form illustrated, the minimum size of the white areas and the black areas is the same, but this is not essential as each minimum may be made such as is necessary to secure invarying results Awith any particular method of etching and oi printing which is to be used.

From a comparison of the Figs. 1 and 2, it is apparent that while the contrast image of my invention resemblesan ordinary half-tone contrast image in the middle-tone parts of the image, it differs radically from the ordinary halitone contrast image in the parts representing tones which are either darker or lighter than the middle-tones. The light-tone parts of my image and of an ordinary half-tone image both consist of a white area containing isolated black areas, but in the half-tone image the isolated black areas in the light-tone parts are separated by the same distance (from center to center) as the black areas in the middle-tone part, while in my image, the isolated black areas are more widely separated in the light-tone parts than are the black areas in the middle-tone parts, and they are separated more and more widely as the tone becomes'lighter. A similar difference between jmy contrast image and the ordinary half-tone contrast image exists Ain the spacing of the isolated white areas which occur in the black area representing tones darker than the middle tones.

My invention includes a method and apparatus/,

for making a contrast image of a character such as that shown in Fig. l, but the method and apparatus Vare not claimed herein as they form the subject' oi' my parent 'application filed February 1'1,19av,ser1a1No. 126,110,011 which ,Pat-

ent No.2,136,34 0 was issuedon November 8, 1938.

v .Any embodiment. of the apparatus features of my invention is shown in Fig. 6. It includes an lordinary electro-mechanical scanning apparatus y Cil original mounted on the drum I8. The light from the scanning lamp I3, as modified by passing through the transparent original, is received by a photo-electric cell I5. The mechanical arrangement shown in Fig. 6 is merely illustrative and not essential as other known forms of scanning apparatus including such as are designed to scan an opaque original may be used in carrying out my invention.

A recording lamp I6 illuminating a shield I1- containing a long narrow slit or aperture I8 is provided with a lensv system I9 which focuses an image of the slit I8 on a recording fllm placed on the drum I I. In the form illustrated in Fig. 7, the shield I1 containing'the slit I8 is the anode of the lamp, but this is not essential as the shieldcontaining the slit may be separate from the lamp. The lamp is a gas-discharge lamp having no afterglow. The length of the image of the slit I8 is equal to the distance between the turns of the screw 28 of the driving mechanism, so that, as the mechanism operates, the image of the slit scans the entire area of the recording film without any overlap. The length of the image of the slit I8 on the recording lm detr mines the width of the elemental areas of the contrast image produced on the recording illm. In this connection, it should be noted that the elemental areas a, b, cshown in Fig. 1 are all of the same width. The lengths ofthe elemental areas are determined by the time length of the cycle of the recording lamp I8; and the proportion of black and white which each elemental area contains is determined by the time during which the recording lamp I6 is lighted and the time periods during which it is extinguished during one cycle. These time periods are controlled by the light passing through the original to the photo-electric cell I by means of interconnecting electric circuits between the photo-electric4 cell I5 and the recording lamp I6.

The interconnected electric circuits include a photo-cell circuit which charges a condenser 35, a constant current circuit which discharges the condenser 35, a trigger circuit actuated by the voltage acrossv the condenser 35, and a control circuit for the recording lamp I6 actuated by the trigger circuit. The circuits are so interconnected that the condenser 35 is alternately charged and discharged in a cycle controlled by the amount of light reaching the photo-cell I 5 and controlling the cycle of the recording lamp I6 and thus the length and character of the elemental areas of the contrast image.

The photo-celi-circuit contains a source of electro-motive force 21, a resistance I2 and the photo-electric cell I5, and is connected to the plate 35a of the condenser so that a positive potential is built up on this plate when the current flows in the photo-cell circuit. The current or rate of charge of this condenser in this circuit is proportional to the amount of light reaching the photo-cell.

The discharging circuit is connected to the plate 35a of the condenser and contains a source y of electro-motive force 28 opposed to the source of electro-motive force 21 in the photo-cell circuit. The discharging circuit also contains means 2 Ia maintained at a constant potential and holding the ow of current in the plate circuit at a constant value greater than the greatest current in the photo-cell circuit. lWhile greater than the current in the photo-cell circuit, the discharging current should be of the same order as that in the photo-cell circuit which requires that the tube 2| be operated at very low voltage. The strength of the discharging current may be regulated by a potentiometer 53 connected to the grid 2Ia and spanning the source of electro-motive force 28. The tube 2I also contains a control grid 2 I b which permits the plate current to iiow when it is neutral with respect to the cathode of the tube and cuts off the flow of current when it is at a certain negative bias with respect to the cathode of the tube. This grid is connected by a variable `tap 43 to the resistor 38 of the trigger circuit hereinafter described. When the current is owing in the discharging circuit, it reduces the positive charge on the plate 35a, because the current in this circuit is greaterthan the current in the photo-cell by which it is opposed. In addition to the charging and dischargin circuits connected to the plate 35a of the condenser, a means for varying the condenser charge is connected with its other plate 35h. This means is the resistor 38 to which the plate 35h is connected by a variable tap 50 so that the existence of a voltage drop in the resistor 38 applies a negative potential to the plate 351i equal to the voltage drop between the high end 38a 'of the resistor 38 and the variabletap 58, while the absence of a voltage drop in the resistor 38 eliminthe plate h of the circuit is connected to cause a flow of current through a resistor 31. The resistor 31 is connected through an oscillator and detector (enclosed in dotted lines in Fig. 1) to the resistor 38. The operation of this oscillator and detector (hereinafter described in detail) is such that it causes a iiow of uni-directional current through the resistor 38 only when the ilow of current through the resistor 31 is such as occurs in the plate circuit of the tube 22 when its grid is below the trigger value.

'Ihe control circuit for the recording lamp I6 is connected to the resistorl 38 of the trigger circuit, and serves to keep the recording lamp I6 lighted only when there is a drop in potential in the resistor 38 or only when there is no drop in potential across this resistor, according to the setting of a reversing switch 40. The control circuit includes a source of electro-motive force 34, the recording lamp I6, and the plate circuit of the tube 28. Current iiows in this circuit only when the grid of the tube 26 is neutral. The grid of the tube 26 is connected directly to the low end of the resistor 33 when the reversing switch is inthe position Amarked .Neg. on Fig. 6, so that current flows through the recording lamp I6 only when there is no potential drop in the resistor 38. This results in making a negative contrast image of a positive subject on the drum I8. When the switch 40 is thrown to the position marked "Pos," in Fig. 6, the grid circuit of the t-Jbe 26 is connected across a resistor 39 in the plate circuit of a tube 25 whose grid is then connected to the low end of the resistor 38. In this case, the current in the plate circuit ot the tube 26 and the recording lamp i8 flows only when there is no current in the plate circuit of the tube '25, that is, when there is a potential drop in the resistor 38. In this case, a positive contrast image is made.

The operation of the interconnected circuits which have been described is cyclic and may conveniently be described by beginning at the end of the discharge of the condenser t5. At this time, there is a potential drop in the resistor 38, as the condenser voltage is below the trigger value; if the reversing switch is in the positive position, the recording lamp is on and no current is flowing in the discharge circuit which includes the plate circuit of the tube 2i. The condenser, therefore, becomes charged by the current flowing through the photo-cell circuit, and the charge continues to increase until it reaches the trigger value. At this point, the trigger circuit operates to stop the flow of current in the resistor 38 and eliminating the potential drop in this resistor. The elimination of the potential drop in the resistor 38 causes three eiects: (l) It increases the charge of the condenser by a predetermined value depending upon the setting of the variable tap 50. (2) It eliminates the negative bias on the control grid 2th of the tube 2i and starts the flow of current in the discharge circuit. (3) It operates the control circuit to extinguishthe recording lamp iii. After these three eects of the elimination of the voltage drop in the resistor itt, which take place simultaneously, the condenser voltage decreases, since the current in the discharging circuitis greater than the current in the photoelectric circuit which is opposing it. The discharge of the condenser continues until its voltage reaches the trigger value. lThe trigger circuit then starts a flow of current through the resistor 3S, causing a potential drop in this resistor. The potential drop inthe resistor 3@ has three effects: (l) It decreases the charge in the condenser by the same amount as this charge was increased by the elimination oi the voltage drop in the resistor 38. (2) It produces a negative bias on the control grid @ib of the tube 2i sumcient to stop the iiow of current in the discharge circuit. (3) It lights the recording light it. This completes the cycle and the charging of the condenser by the current in the photocell circuit begins again. f

In the cyclic operation which has been described, the rate at which the condenser is charged is proportional to the amount of light reaching the photo-cell, as this determines the strength of the current in the photo-cell circuit. The rate at which the condenser is discharged is an inverse function of the amount of light reaching the photo-cell, since the photo-cell current opposes the constant current in the discharging) circuit. The amount of time required for the discharging .and charging of the condenser depends upon the discharging and charging rates and upon a fixed capacity of the condenser and the xed voltage by which the condenser voltage is raised above and dropped below the trigger value by the voltage drop in the resistor 38 applied to the plate 35h of ,the condenser through the variable tap 50. Since the time for charging and discharging of the conjdenser determines the length of the black and white portions of each elemental area of the contrast image, the device which has been described operates to produce a` positive contrast image such as that shown in Fig. 1 when the switch 4t is in Pos. position.

When a dark tone area of the original is between the scanning lamp and the photo-cell, the photo-cell current is weak, so that the charging of the condenser to the trigger value takes a long period of time making a long black area on the contrast image, and the discharge of the condenser is rapid making a short white area. Because of the slowness of the charging, the Whole cycle is long, and consequently the elemental area is long.

When a light tone area of the original lies between 'the scanning lamp and the photo-cell, the photo-cell current is large, charging the condenser rapidly and opposing the discharging current so that the discharge is slow. This also results in a long cycle producing a long elemental area which, in this case, contains a long white area and a short black area.

When a middle tone of the original is between the scanning lamp and the photo-cell, the current in the photo-cell circuit is approximately half the current in the discharge circuit, so that both the charging and discharging of the condenser are comparatively rapid, giving a short cycle producing a short elemental area which is half black and half white.

No black area of the contrast image can be less than a predetermined length, which is determined by the time required for the maximum photo-cell current to charge the condenser to the trigger value. At the same time, no white area can be less than a predetermined length, which is determined by the time required for the constant current in the discharge v circuit to discharge the condenser When'the photo-cell current is at its lowest value.

Adjustments may be made to determine the size of the elemental areas in the middle tones and the minimum sizes of the black and white areas as desired in the making of contrast images for etching printing plates. Best results are secured when the discharging current is made equal to the maximum photo-cell current plus the minimum photo-cell current. This or other desired adjustment of the discharging current may be made as follows: r

The proportion of black area in the contrast image is indicated by a milliammeter it which is connected in the recording lamp circuit and provided with a variable shunt iii which is adjusted so that the milllammeter reads lil@ when the recording light it is lighted without interruption, a condition which may be attained by closing the switch 5i to maintain a constant potential on the grid of tube 2t. The switch 5i is then opened and the indications oi the milliammeter it are then used as a guide in adjusting the photo-cell current and the discharging current. The adjustment is made after a positive continuous-tone image has been Wrapped around the cylinder i. The darkest tone of this image is moved between the scanning lamp i3 and the photo-cell i 5 with the reversing switch 40 in positive position. The strength" of the photo-cell current is then adjusted by adjusting the brightness of the scanning lamp I3, for example by a rheostat 41, until the reading of the milliammeter 46 equals the percentage of black area desired to represent the darkest tone of the origif the original, for example means of the potentiometer 53 until the reading of the milliammeter 46 is equal to the percentage of black desired to represent the' lightest tone of It is desirable to select the percentages so that 100 minus the percentage selected for the darkest tone equals the percentage selected for the lightest tone, as this results in making the discharging current equal to the sum of the maximum and minimum photo-cell currents. After the relation ofthe minimumsize white and black areas to the size of the elemental area has been adjusted in this manner, the size of the elemental areas is adjusted by varying the lposition of the variable tap 58 on the resistor 38 to control the frequency of the charging and discharging of the cycle of the condenser 35. The adjustment is most desirably such as makes the elemental areas inthe middle tones about equal in size to the elemental areas' of an ordinary half-tone contrast image used for the sort of plate for which the contrast image made on the apparatus is to be used.

A further feature of the apparatus illustrated consists iny a means for indicating the, total amount of black area in a contrast image produced by the apparatus. This is of value since it enables a printer using a plate made from such an image to know the area of the printed plate which must be covered with ink without the experimentation necessary to' determine this as in using ordinary printing plates. The means for indicating the total black area consists of an ampere hour meter 60 connected in the circuit of a recording lamp IG. The number of ampere hours indicated by this meter during the making ,is in' Pos position and'a negative contrast image when the reversing switch 40 is in Neg.

position. It should,.of course, be understood that,

ii desired, a negative continuous-tone image may .be placed upon the dr'um I 8 and that, in this case, a positive contrast image will be obtained by placing the reversing `switch 40 in` the Ne g.

position and a negative contrast limage by placingthe reversing switch 40 in the Pos. position The operation of the oscillator and detector enclosed in dotted line in Fig. 1 is as follows:

When sufilcient current flows through resistor 31, the grid of tube 23 is held' at a negative po- 'tential suiiicient to prevent a current in the plate l primary of transformer Il. This induces a volt-.`

'i age in the secondary'of the transformer which causes the grid of tube 23 to become more posicrease.

tive by virtue of the potential induced on one plate of condenser 36. This results in a further increase in `the plate currentand a correspondingly further increase in the grid voltage until saturation conditions are attained. At this point,

the current. through the primary of .transformer 4I becomes momentarily constantand the secondary voltage` drops to zero. This causes a de- When the current through resistor 31 decreases, there is a'corresponding increase in the potential of the grid of tube 23.

r'I'his causes the plate 4current of this tube to in- .ing upwardly from crease in the potential of the grid of tube 23, which produces a corresponding reduction in the current through'the primary of transformer Il. 'I'his induces in the secondary of the transformer a voltage of the opposite sign and tends to make the grid of tube 23 even more negative. This process continues until the grid of tube' 23 be'- comes so negative that the plate current ceases. Current thenflows through the resistor 44 in such a direction as to again cause the potential of the grid of tube 23 to increase. These oscillations continue so long as the voltage drop across resistor 31 biases the grid of tube 23 above the cut-off potential of this tube.

Tube 2l is operated as a detector, its grid being biased normally slightly below the cut-off potential. During one-half of the oscillation period of tube 23, the voltage induced in the secondary of transformer li causes the grid oi tube 24 to become positive allowing current to flow in the plate circuitthrough theresistor 38. Since the oscillations of tube 23 are at a radio frequency, the oscillations of this tube result in a pulsating unidirectional through resistor 38 which to all intent and'purpose 'may be regarded and treated asa direct current. It is obvious to one skilled in the art that this oscillator and Idetector might be replaced by a direct current amplifier con taining an odd number of stages. What I claim is: s 1. A contrast -image comprising black and white areas, the size of the black areas ranging upwardly from a predetermined minimum size occurring only in tones lighter than the middle tone of the image, the size of the white areas ranging upwardly from a predetermined mini- .mum size occurring only in tones darker than the middle tone of the image, and the black and white areas being in the middle tones of the image equal and greater than their minimum sizes.

2. A contrast image comprising black andk white areas. in which the size of the black areas ranges upwardly from a predetermined minimum size occurring only in tones lighter than the middle tone of theimage and the size of the white area-s ranges upwardly from a predetermined minimum size occurring only in tones darker' than the middle tone or the image.

3. A contrast image having parts representing dierent tones and consisting of a plurality of elements in each of which the ratio of white area to-black area corresponds to the value of the tone represented, the size of the black areas ranging `upwardly from a predetermined minimum size occurring only in tones lighter than the middletone and the size of the white areas ranging upwardly from la predetermined minimum size occurring only in tones darker than the middle tone, and the size of the elements ranging upwardly from a minimum size occurring in the middle tone.

4. A'contra'st'image having parts representing different tones and consisting of a plurality of elements in each of which the ratio` of white area to yblack area corresponds to the value of the tone represented, the size of the black areas rangsize occurring. only in tones lighter than the middle tone andthe size of the white areas ranging upwardly from a predetermined minimum size occurring onlyjin tones darker than the middletone. l C.' HARDY.

a predetermined minimum

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