TECHNICAL FIELD

The present invention relates to a method of producing with the aid of exposure a screened image of a picture on the light-sensitive layer of a film, a plate, etc.

Exposing a picture for a film, a printing plate, a stencil, etc., via customary screens gives among other things inferior results the bigger the enlargement being created. This state of affairs is founded on the circum¬ stance that in customary screens use is made of uniformly distributed screen dots, which become larger and sparser the larger the chosen enlargement is.

BACKGROUND ART

Stochastic screens, i.e. screens with randomly distributed dots, are pre¬ viously known and are currently used primarily for eliminating moire patterns.

OBJECT OF THE INVENTION

The object of the present invention is to establish a method of pro¬ ducing large screened images, which are equally sharp regardless of the degree of enlargement.

In the method mentioned in the descriptive preamble the present invention is characterized in that it incorporates utilization of at least one screen with randomly distributed, distinct dots, called a stochastic screen, which is located ahead of the said film, plate, etc., on which the said screened image of the said picture is intended to be created, and in that the tones of the said stochastic screen are located in direct contact with the light-sensitive layer of the said film, plate, etc.

More precisely, the present invention is characterized in that the said picture is exposed several times on the same film while utilizing a plurality of stochastic screens with differing permeabilities to light and in that a plu¬ rality of stochastic screens with differing permeabilities to light are used primarily for light and dark parts respectively of the said picture.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

The present invention will be described and explained in greater detail herein below, with particular reference to Fig. 1 of the accompany¬ ing drawing, which illustrates schematically how the method according to the invention is utilized in a concrete embodiment.

BRIEF DESCRIPTION OF THE INVENTION

A suitably dimensioned camera incorporates i.a. by way of example light sources 1 for illumination of a suitable and adapted picture 2 in the form of, for example, a negative picture, a positive picture, etc. Via suit¬ able and preferably adjustable lenses, 3,4, an image of the said picture 2 is exposed on a flat surface 7. This flat surface 7 is provided with a large number of thoroughgoing holes 7a-7z, to which a negative pressure is supplied from a vacuum source in a manner not specified to the left side of the flat surface 7, as seen in Fig. 1.

A light-sensitive film 6 is applied to the left side of the flat surface 7, as seen in Fig. 1, and is retained located closely against the flat surface 7 with the aid of the said negative pressure from some of the said holes 7g- 7p. The distance between the holes 7a-7z for supplying negative pres¬ sure to the left side of the flat surface 7, as seen in Fig. 1, as well as the size of the said holes are so chosen that the do not have any appreciable influence on the flatness of the light-sensitive film 6. The light-sensing layer 6a of the light-sensitive film 6 faces away from the flat surface 7, i.e. towards the light rays coming from the light sources 1 via the picture 2 and the lenses 3, 4.

A screen 5 of the type which is built up of distinct dots or pixels randomly distributed over the entire surface of the screen and called stochastic screens here is located in the beam direction seen before the light-sensitive film 6 and with its toner or blacking layer 5a facing towards the light-sensitive layer 6a of the film 6, i.e. so that there is direct contact between the light-sensitive layer 6a of the film 6 and the toner layer 5a of the stochastic screen 5. This stochastic screen 5 is chosen larger in size than the light-sensitive film 6, as is indicated sketched in the figure, and is retained also with the aid of the said negative pressure from, for example, holes 7e-7u against the flat surface 7. It should be noted here

that both the film 6 and the stochastic screen 5 are preferably soft and flexible and adapt themselves therefore both to each other and to the flat surface 7. It should be particularly observed in this context that the distinct dots 5a of the stochastic screen 5 can in themselves have identical or different shapes but that they are discrete, i.e. they are sharply delimited both individually and mutually and that these dots are randomly distributed over the screen. Moreover it is import in cases when several stochastic screens 5 are used in producing a given image that stochastic screens from different manufac¬ turers are used or which stochastic screens have been manufactured by mutually independent, randomly working screen generators in order to avoid the potential risk that moire patterns will be created, if the screens are generated by the same screen generator or by screen generators which work on the same basis, for example using the same type of random number generator.

As has been mentioned several screens 5 can be used with differing permeabilities to light after one another in order to control to a still greater degree the grey scale upon exposure of the picture 2 on the film 6. Furthermore, different permeabilities to light can be utilized for the light and dark parts respectively of the picture 2. A first stochastic screen with about 60- 70% permeability to light can be used, for example, for the dark parts of the picture 2, which dark points can for example be defined as being impermeable to light from about 99-30%. A second stochastic screen with about 70-90% permeability to light can be used, for example, for the light parts of the picture 2, which light points can for example be defined as being impermeable to light from about 30-1%. Obviously several stochastic screens can be used repeatedly as well as also exposure without screens, for example between two to four stages.