US3275445A - Photographic half-tone contact screens - Google Patents

Description

Sept. 27, 1966 H. MIDDLEMISS 3,275,445

. PHOTOGRAPHIC HALF-TONE CONTACT SCREENS Filed July 15, 1963 5 Sheets-Sheet 1 B4 -IOO- INVENTOQ 7W 2m Wig ATTORNEYJ Sept. 27, 1966 H. MIDDLEMISS 3,275,445

PHOTOGRAPHIC HALF'TONE CONTACT SCREENS Filed July 15, 1963 5 Sheets-Sheet 2 I33 LINES PER INCH 93/ LINES PER INCH Q W'MW 5W A: %ZIVL A-r-roRNEYSi p 27, 1966 H. MIDDLEMISS 3,275,445

PHOTOGRAPHIC HALF-TONE CONTACT SCREENS Filed July 15, 1963 5 Sheets-Sheet 5 \NVENTOR W {WW ATTORNEYS United States Patent 3,275,445 PHOTOGRAPHIC HALF-TONE CONTACT SCREENS Herbert Middlemiss, Nottingham, England, assrgnor of one-half to W. H. Howson Limited Filed July 15, 1963, Ser. No. 295,101 Claims priority, application Great Britain, July 17, 1962,

27,340/ 62 14 Claims. (Cl. 96116) This invention relates to photographic halftone contact screens of the cross-lined type for use in converting an image having a variety of continuous tones into differently sized dots, for the preparation of half tone printing plates.

Heretofore such screens have varied in fineness by having from about 65 to up to 300 lines to the inch with corresponding variations in sensitivity of adjustment to the original image and resolving power which determines the quality of reproduction of the original image. It will be appreciated that there is a limit to the number of basic lines that the screen can have before it becomes impracticable so that there has previously been a limit to the quality of reproduction that could be obtained.

The lines on such contact screens are usually made-up from a series of dots, the dots varying in density from their centers to their edges and the lines being crisscrossed so that a chequer board pattern is produced.

One method of making such screens is by projecting light through a camera screen, that is a glass screen provided with a criss-cross pattern of lines, onto a photosensitive film, the light passing through any square formed by the lines being vignetted to produce the subsequent dot which forms one of the row of dots which produce the lines on the contact screen, and such contact screens are usually used in direct contact with the photosensitive material on which the final image is to be produced.

According to the present invention a contact screen has a pattern of lines of spaced dots in which the centre of each dot is in the form of a central core having a midpoint of maximum density which is surrounded by a pattern of minor dots of smaller area, these having lessening density with displacement from the central core, the minor dots being spaced apart in a regular pattern to provide areas of minimum density between them.

Thus in the present invention each of the dots of normal type the edges of which are of diminishing density is replaced by a dot of the kind set forth above in which the area of diminishing density is pierced in a regular pattern by areas of very low density through which light can pass.

It has been found that in practice the areas of low density allow lines on the final image to grow in the required direction so that favorable definition of fine detail is achieved. The general dots which form the image remain but they tend to be shaped in accordance with the minor dots, although little of the pattern of the minor dots remain visible in the finished reproduction.

Preferably the minor dots cease or are of such density as to almost cease after a predetermined distance to provide an area of minimum density between adjacent dots which is of the same area as the central cores, and the minor dots may be arranged in rows which are parallel with the rows of dots forming the screen, each minor dot being substantially square and adjacent minor dots having their corners adjacent.

Preferably each central core is in the shape of a number of minor dots joined together along their sides, the areas of minimum density being in the shape of the spaces left between a number of minor dots.

In a method of making a contact screen as set forth above the pattern of dots is achieved by coordinating two basic patterns of dots one of which has more lines of dots to the inch, and in which the dots are of smaller area than the other in which the dots correspond to the cores of the final pattern. Thus one of the basic patterns of dots corresponds to the high density cores and the other corresponds to the rows of minor dots of lessening density with displacement from the central core.

In a convenient arrangement one basic pattern of dots has 133 lines of dots per inch and the other has 931 lines per inch.

Preferably the dots on both basic patterns are substant-ially square.

A convenient method of making a contact screen according to the invention comprises exposing a photosensitive material to a first pattern of light projected through a cross-lined screen, said pattern of light corresponding to an area of the pattern of dots and minor dots which forms the eventual screen and then projecting a second pattern of light in place of the first in the form of a low density image of a dot or dots corresponding in area and position to the area of a larger dot or dots in the basic larger pattern projected initially, and subsequently developing the photosensitive material.

Preferably the first pattern of light corresponds to the pattern on the screen at the area comprising a single large dot, and provides a central area of light intensity which is surrounded bya number of areas of equal in- "ice - tensity but smaller area which are spaced apart from one another in both linear directions in a regular pattern. Thus the first pattern of light projects an image which corresponds to the eventual pattern on the contact screen but without the minor dots of smaller area being of lessening density with displacement from the central core.

In a preferred method of projection the first pattern of light is achieved by shining the light from a light source through one of a pair of interchangeable opaque stops which are provided with apertures so shaped as to produce the patterns of light required.

The other stop will be used to control the second pattern of light and this stop may be provided with apertures in the form of concentric bands which are 'of diminishing width from the centre to the edge of the stop. Thus this stop provides a light pattern which simulates a vignetted dot obtained through a normal screen, and the diminishing intensity towards the edges of the spot produce the required diminishing intensity of the minor spots around the central core.

The invention may be performed in various ways buta screen according to the-invention will now be described together with one method of producing it, with reference to the accompanying drawings in which,

'FIGURE 1 shows a portion of the pattern on a screen according to the invention,

FIGURE 2 is a diagrammatic illustration of part of they screen,

, FIGURE 3 is a graph showing the density of each dot on the screen in relation to the width of the dot,

FIGURE 4 is a diagrammatic illustration of the basic. principle which is used in the production of a screen,

FIGURE 5 is a diagrammatic illustration of the method used for producing a number of dots on the screen and,

FIGURE 6 is another diagrammatic illustration of the method of adjusting the intensity of the dots.

\FIGURE 1 shows a portion of ascreen according to the invention, from which it will be seen that each dot comprises a central core E which is surrounded by a number of smaller square shaped dots F. The density of the central core and of the minor dots 1F lessens with displacement from the midpoint of the central core although this is not visible in the black and white line drawing,

however the minor dot at each corner is of such low density'that it does not re-produce 'so that areas of density whichlare of the same shape as the central cores are produced between adjacent dots. V v

A section of density through each dot will be asshown in FIGURE -3. In this figure'which corresponds to a cross section 111 -111 through the dot E on FIGURE ,2 each" minor dot is indicated by the reference letter B. The central core comprises the three dots B B B which aresubstantially opaque on the screen the dot B being the midpointof maximum density. The dots B and B are the minordots on each side of the central core and as will be seen their density is considerably less than that of a central core'itself, and the dots B B have a density which is lower. again. As will be seen the density between each of the dots is reduced.

FIGUREZ illustrates the method by which the pattern arranged as 1 3-3 lines to the inch, two of these lines being indicated at A and A and these lines also corresponding to two of the smaller spaced lines which are formed by small square or diamond shaped dots as indicated at B, B

The lines indicated at A,.A are crossed by similar linesC, C at right angles thereto, these similar lines also co-inciding with the small square or diamond-shaped dots which are indicated at D, D and the lines C, C A, A intersecting at points such as E, E, E and E Thus thecentres of the dots of the larger spaced lines are at E, E E 1i". and as a result of the process to :be dfir; scribed central .cores of maxiumintensity are formed at these positions as shown. 7

Combined with this basic screen arrangement indicated by the lines C, C A, A there is a secondary screen arrangement the lines of which are provided by'small dots similar to those indicated at B and which provide 931 lines to the inch. Six of these lines are indicated at F to F and are made up of square or diamond shape dots,

such as G, G which, as mentioned above are similar to I the dots B. Similarly secondary lines such as H to H are disposed at right angles to the lines F to F and are also made up of square or diamond shape dots such as In the screen pattern illustrated some of the dots H or F are of such low density as to substantially cease at the points I to J to provide areas of minimum densty, the reason for this being asset forth above when discussing the cross-section of the major dots which form the screen as shown in FIGURE 3. I

it will 'be appreciated that the minor dots B, 'D, G, I, act to provide the shading of density which would normally surround a vignetted dot on a 13'? line per inch' screen. so that although a gradual shading is provided from one dot to the next as they are displaced from the midpoint of the central core .there is an 'area of lesser density between each minor dot.

. The screen as shown in FIGURES '1, 2 and 3 can be the minor dots ends at a predetermined distance from made by the application of the arrangement shown in FIGURE 4. Inthis arrangement alight. source, indicated atK, can shine throughan opaque, stop L whichis provided with a pattern of apertures which correspond to the surrounding minordots-and core of a single major dot on the contact screen. In this figure the various points B to B of greaterintensity of light. intensity on FIGURE 3 are indicated in a corresponding row of apertures across the stop. Part of a crossline screen, such as If a number of apertures are provided in the photographic screen M, as shown in FIGURE15 then a corre-.

sponding number of patterns similar to N will be-projected, and for simplicity only the central aperture corre-. Thus'it will be sponding to B is shown in this figure. seen that if sufficient apertures are provided in the screen M a contact screen of the desired size can 'be produced in this manner. v

The latent image produced in the photosensitive material by the foregoing method will show little if any of the required 1 grading of the minor dots from thecenter to the edges of the major dots. initially made of low density and subsequently strengthw ened in the required pattern by the following method;

which grades the density of the dots and fills in' between them as shown in FIGURE 3.

In orderto grade .the density of the :dots and in; s between them as shown .in FIGURE 3, the film is-left in position afteran initial exposure to the light source and the stop L in FIGURE-4 isreplaced Iby a-stop O as shown in FIGURE 6. This stop 0 is provided with a series of apertures in the form of concentric bands P which are of diminishing width from the centers to the between the points B3, B and B in FIGURE 3 8.1'61S1lbfl stantially filled into provide a center core having ,a

midpoint of maximum density and sides. which are of diminishing intensity; 7

In FIGURE 6 thenormal vignetting type dots of V 133 line screen are indicated at Q.

It will be appreciated {that there are other ways of I making a screen according to the invention, andithat it would be possible to arrange stop Lito include morethan one basic dot. made of any convenient colour which is required.

What I claim is: 1

1. A photographic half-tone contact screen comprising a member having a pattern of linearly arranged spaced first dots each of a predetermined area, each first dot being in the form of a centralcore having a midpoint of maximum density which is surrounded bya pattern of minor dots each of a smaller area than said predetermined area, said minor dots. lessening in density with displacement from ,the central core, and the minor dots. being spaced apart in a regular pattern'to provide areas of minimum density between themselves.

' 2. The photographic half-tone contact screen as defined:

in claim 1 whereinfthe intensity of at least some of the central cores to define areas of minimum density between adjacent minor dots, and said last-mentioned areas are substantially identical rareawise to the areas of said first dots.

3. The photographic half-tone contact screen as defined.

in claim 1 wherein each central .core is definedby a number of plural sided minorqdots, the last-mentioned minor dotsbeing positioned in side-by-side relationship,: and the areas of minimum density being in the shape of;

' a square left between a number ofsa-id last-mentioned minor dots.

being in corner-tocorner adjacent relationship.

It is, therefore,

Moreover the contact screen may be I 5. The photographic half-tone contact screen as defined in claim 2 wherein said minor dots are arranged in rows which are parallel to rows of said first dots, each minor dot being substantially square, and adjacent minor dots being :in corner-to-corner adjacent relationship.

6. The photographic half-tone contact screen as defined in claim 3 wherein said minor dots are arranged in rows which are parallel to rows of said first dots, each minor dot being substantially square, and adjacent minor dots being in corner-to-corner adjacent relationship.

7. A method of making a contact screen comprising the steps of providing an element having a coating of light-sensitive material, forming first and second light patterns of lines of dots in which there are more light dots per inch and of a smaller area in the first pattern than in the second pattern and the light dots of the second pattern correspond in configuration to the final pattern of the contact screen, exposing the light-sensitive coating by exposing the same to the first and second light patterns, and thereafter developing the light-sensitive element.

8. The method of making a contact screen as defined in claim 7 in which the first light pattern has 133 lines of dots per inch and the other pattern has 931 lines of dots per inch.

9. The method of making a contact screen as defined in claim 7 wherein the dots of both patterns are substantially square resulting in the exposure of like square areas of the light-sensitive element.

.10. The method of making a contact screen as defined in claim 7 in which the light-sensitive coating of the element is first exposed to the first light pattern and is thereafter exposed to the second light pattern.

11. The method of making a contact screen as defined in claim 7 in which the first pattern of light corresponds in configuration to a :portion of the contact screen which forms a single large dot and provides a central area of light intensity which is surrounded by a number of areas of lesser intensity but smaller size which are spaced apart from one another in two linear directions and in a regular pattern.

12. The method of making a contact screen as defined in claim 7 wherein one of the lines of dots of the first light pattern is in intersecting relationship to one of the lines of dots of the second light pattern.

13. The method of making a contact screen as defined in claim 7 wherein the first light pattern is iormed by passing light from a light source through one of a pair of interchangeable opaque stops which is provided with apertures shaped to provide the pattern of light required.

14. The method of making a contact screen as defined in claim 7 in which the second pattern of light is formed by passing light from a light source through one of a pair of interchangeable opaque stops which is provided with apertures in the form of concentric bands which are of diminishing width from the centre of the stop to the edges thereof.

References Cited by the Examiner FOREIGN PATENTS 128,772 8/1948 Australia.

NORMAN G. TORCHIN, Primary Examiner.

ALEXANDER D. RICCI, Examiner.

R. H. SMITH, Assistant Examiner.

Claims (1)

1. A PHOTOGRAPHIC HALF-TONE CONTACT SCREEN COMPRISING A MEMBER HAVING A PATTERN OF LINEARLY ARRANGED SPACED FIRST DOTS EACH OF A PREDTEREMINED AREA, EACH FIRST DOT BEING IN THE FORM OF A CENTRAL CORE HAVING A MIDPOINT OF MAXIMUM DENSITY WHICH IS SURROUNDED BY A PATTERN OF MINOR DOTS EACH OF A SMALLER AREA THAN SAID PREDETERMINED AREA, SAID MINOR DOTS LESSENING IN DENSITY WITH DISPLACEMENT FROM THE CENTRAL CORE, AND THE MINOR DOTS BEING SPACED APART IN A REGULAR PATTERN TO PROVIDE AREAS OF MINIMUM DENSITY BETWEEN THEMSELVES.

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