WO1999019770A1

WO1999019770A1 - Light negative photographic films and preparation method

Info

Publication number: WO1999019770A1
Application number: PCT/FR1998/002167
Authority: WO
Inventors: Jean-Marie Nouel
Original assignee: Nouel Jean Marie
Priority date: 1997-10-10
Filing date: 1998-10-09
Publication date: 1999-04-22
Also published as: FR2769725A1; FR2769725B1

Abstract

The invention concerns photographic films for use in the graphic industry as light negative films and the preparation of such films. The structure of said films comprises a transparent base (10 + 50) with two substantially parallel surfaces and fine thickness, and a photosensitive layer (20) covering one of said two surfaces of said transparent base (10 + 50). The invention is characterised in that said structure further comprises, integral with said transparent base (10 + 50), on the side of said photosensitive layer (20), small opaque volumes (30), advantageously randomly arranged.

Description

Lightweight negative photographic films and their preparation

The subject of the present invention relates to light negative photographic films, for use in the graphics industry and the preparation of said films. More specifically, it relates to photographic films, for use in the graphics industry as light negative films and the preparation of said films.

Said invention is mainly located in the context of offset printing processes - dry or wet offset - and it proposes, according to its first object, films, the use of which allows in particular the preparation, industrially by the printer, of light negative offset plates; i.e. the creation of small non-encrophilic surfaces in the encrophilic parts of said offset plates. Such films can also be conveniently used in other printing techniques and in particular in flexography. The preparation of such films, which constitutes the second object of the present invention, is easy, inexpensive, especially on an industrial scale.

A few years ago the Applicant proposed an improvement in printing techniques in general. In particular to reduce the "fattening" and, in general, to facilitate printing, he recommended reducing it, more precisely creating small non-encrophilic surfaces in the encrophilic surfaces. The basic principle of this lightening technique has been described in patent application FR-A-2 660 245. A preferred implementation thereof - according to a stochastic screening - has been described in the patent application WO -A-96 02868. In these two documents of the prior art, various techniques have been described for introducing said small non-ink-absorbing surfaces into the ink-absorbing surfaces of the offset plates; techniques implemented at the level of the plates or films used to expose said plates or even further upstream in the process for manufacturing said plates. The various techniques described prove to be cumbersome to implement which is hardly compatible with said implementation on an industrial scale.

The Applicant therefore wished to develop other "lightening" techniques for easier implementation.

He recently proposed an industrial process and device for the preparation of positive, light or pre-light plates. These methods and device are described in patent application WO-A-97 35233. In this context of positive plates, lightening is implemented at the level of the plates; more precisely at the level of the photosensitive positive layer of a strip intended to be cut into said plates.

In the context of the present invention, the Applicant proposes a new lightening technique for negative plates. In this context of negative plates, it recommends the implementation of said reduction in the films intended to expose said plates and more precisely at the stage of preparation of said films. According to the invention, small opaque volumes or small opaque surfaces are not introduced a posteriori by printing or exposure to the surface of existing films (techniques described in FR-A-2 660 245 and WO-A- 96 02868) but we intervene in the preparation of said films, by providing, in their structure, under the photosensitive layer, small opaque volumes. Films thus modified, qualified as lightened films within the meaning of the invention (films which have small opaque volumes in their structure; small opaque volumes intended to generate small opaque surfaces in the transparent parts of said developed films, themselves, intended to generate small non-ink-absorbing surfaces in the ink-absorbing parts of the negative offset plates exposed with said films) constitute the first object of the present invention.

We have mentioned above the lightened negative films of the invention with sole reference to their use for exposing negative plates. We would like to point out, already, here that said films of the invention can also be used (like any negative film of the prior art) to prepare a positive film (by exposure, in contact for example, development and fixing) which in turn can be used to expose a positive plate. By means of the lightened negative films of the invention, it is therefore possible to obtain lightened (corresponding) positive films and lightened positive plates ...

The films used to date in the printing industry consist of a transparent support, generally made of polyester and with a thickness of between 10/100 and 18/100 mm, covered on the back with an anti-bending layer ( "Anti-curling") containing an anti-halo agent, and on the front, an adhesion underlay (or bonding undercoat) then the photosensitive layer, generally based on gelatin and salts silver (about 50% - 50%, by weight) and about 5 μm thick. Generally, there is finally on said photosensitive layer, to ensure its protection against scratches, a very thin film of a varnish (said varnish generally being sprayed in the form of particles). Under the action of light and after development and fixing, the exposed parts of said photosensitive layer become black and opaque.

When the photoengraver photographs and frames a document using such films, he obtains a negative film. Today, more and more "flashers" are used, whose laser insulates the photosensitive layer of films. If the light sent by the laser represents the white parts of the document, the film obtained is a negative film; if the light sent by the laser represents the opaque (black) parts of the document, the film obtained is a positive film. We are used to working for the most part in the United States,

Canada and Mexico, with films and negative plates; in other countries of the world, with positive films and plaques.

There are different types of photosensitive layer, proposed by film manufacturers to meet certain uses and / or certain insolation materials, and in particular more or less rapid, more or less contrasted, more or less opaque photosensitive layers.

There are also films called "Auto-reversal" or negative-negative, which allow to obtain a negative film directly from another negative film.

Even if certain manufacturers use special treatments during the production of their films and / or use layers of different natures in the laminated structure of said films, it is noted that the structure of said films - photographic films for use in industry graphic, in particular as negative films - includes a transparent support and a photosensitive layer (based on gelatin and silver salts: AgBr, AgCl, Agi ...).

After these details on the films of the prior art, it is proposed to describe below the films of the invention: photographic films, for use in the graphics industry as light negative films which can be qualified, for simplification , reduced negative films. Said films of the invention, intended for use in the graphics industry (printing) have a structure of the type of that of the corresponding films of the prior art; ie their structure comprises, conventionally, a transparent support - which has two substantially parallel faces and a small thickness - and a photosensitive layer which covers one of said two faces of said transparent support. Said transparent support and photosensitive layer of the films of the invention may respectively consist of transparent supports and photosensitive layers of the negative films of the prior art, films marketed to date. Thus, said transparent support for the films of the invention may consist of a support sheet or comprise such a support sheet as the main element of a laminated structure. This laminated structure generally comprises, as according to the prior art, said support sheet coated on its front side (intended to receive the photosensitive layer) with an adhesion sublayer (intended to improve the adhesion of said photosensitive layer on said support sheet) and / or on its reverse side with an anti-bending layer containing an anti-halo agent. Said support sheet is advantageously a polyester sheet as described above.

In the same way, the photosensitive layer of the films of the invention can be a photosensitive layer of the type of those known to date, based on gelatin and silver salts.

Typically, the structure of the negative films of the invention comprises, in addition to the transparent support and the photosensitive layer, as described and arranged above, small opaque volumes, secured to said transparent support, on the side of said photosensitive layer. Said small opaque volumes are advantageously, but in no way limitative, identical.

Said small opaque volumes are located under said photosensitive layer, at the level of the front face of said transparent support. In fact :

- If said transparent support comprises an untreated support sheet on its front face intended to receive the photosensitive layer, said small opaque volumes are arranged on and secured to, said front face of said support sheet;

- If said transparent support comprises a “treated” support sheet, covered on its upper face with at least one treatment layer, in particular with an adhesion sublayer, said small opaque volumes are arranged on and secured to either said front side of said support sheet (under said treatment layer (adhesion sublayer)), ie the front side of the transparent support of laminated structure (on said treatment layer (adhesion sublayer)). We do not exclude the presence of said small opaque volumes between two layers of treatment ... Whatever the embodiment of their transparent support, the negative films of the invention typically have, in their internal structure, under the photosensitive layer, small opaque volumes (at the level of the front face of the transparent support, treated or not treated). Said small opaque volumes may consist of volumes of any opaque material (not allowing light to pass through), capable of being deposited and adhering to the surface on which it is desired to stabilize them (generally the front side of the support sheet treated subsequently or untreated (generally, polyester surface) or front side of the treated support sheet (generally, upper surface of the adhesion undercoat deposited on the polyester) and inert with respect to the photosensitive layer (it is possible be less severe on the inertia required when the small volumes of said opaque material are "isolated" from said photosensitive layer by a treatment layer).

Said small opaque volumes obviously have reasonable dimensions. Their height is minimal but sufficient to ensure the desired opacity; their section is capable of generating points of relief with a reasonable surface area, generally between 140 and 2,500 μm ² (see below).

Advantageously, the opaque material involved is a material which dries very quickly under conditions of easy processing. Said opaque material used can in particular consist of an ink, black for example or a varnish.

The small volumes of said opaque material, provided according to the invention within the structure of the films (and not on their surface) are distributed, distributed, advantageously randomly. It is not excluded, within the framework of the invention, to find them, distributed in a non-random manner (for example, according to a conventional screening or screening with amplitude modulation) but it is strongly recommended to distribute them in a manner random, for example:

- according to a stochastic screening or screening with frequency modulation: a reduction is then implemented, according to a stochastic screening, as described, in principle, in application WO-A-96 02868 (with small areas of the same dimension );

- according to a mixed screening: with frequency modulation whose points have variable areas; - completely randomly: it is thus possible to generate said small opaque volumes without using a frame, for example, by simple projection.

To better understand the advantage of these small opaque volumes and their advantageous random distribution as well as that of the advantageous characteristics of the films of the invention presented below, reference may be made to the Applicant's previous work on the concept of lightening. and in particular to the texts of applications FR-A-2 660 245 and WO-A-96 02868.

According to said advantageous characteristics, to be considered independently of one another or in combination with one another:

- Said small opaque volumes are arranged (distributed) according to a stochastic screening;

- Said small opaque volumes cover between 4 and 25% of the surface of said support (front of the treated or untreated support sheet or upper surface of the treatment layer (generally adhesion sublayer) deposited on the support sheet);

- Said small opaque volumes generate opaque surfaces (dots) whose surface is between 140 and 2,500 μm ² .

The films of the invention therefore have the laminated structure of the films of the prior art (laminated structure with at least two elements: the transparent support and the photosensitive layer but which may include other elements such as:

- an anti-bending layer containing an anti-halo agent on the back of said transparent support; a treatment layer (generally an adhesion sublayer) on the front of said transparent support, or even,

- a protective film (transparent anti-scratch varnish type) on the photosensitive layer) and, typically, therein, between said support and said photosensitive layer, small opaque volumes, advantageously distributed randomly.

Said lightened negative films of the invention, called to present small opaque surfaces in their transparent parts, can be used to prepare either positive films (which will then present small transparent surfaces in their black parts), or negative offset plates ( which will have small non-encrophilic surfaces in their encrophilic parts). They can also be used, as indicated above, in other contexts and in particular in flexography.

In the context of the present invention, it has been found, quite surprisingly, possible to obtain such high-performance films, and this, industrially. An industrial process for obtaining said films is described in detail below. Indeed, a real prejudice had to be overcome by the Applicant for the design and production of such films. As a person skilled in the art, he was aware of the great fragility of photographic films in general before use (problem of the sensitivity of the photosensitive layer to many parameters (chemicals, odors, humidity, heat ...) and of its attachment to the support) or the difficulties in obtaining batches for manufacturing such films, which are homogeneous.

We now propose to describe the process recommended for obtaining, in particular industrially, the reduced negative photographic films of the invention. It is not excluded to obtain said films by other methods but, in any event, the Applicant strongly recommends, for the introduction of small opaque volumes on the transparent support, a printing technique ... rather, for example, than a projection technique. Such a printing technique is capable of ensuring the required precision, both in terms of the quantity of product deposited and in terms of the distribution of said product (it is desired to deposit small volumes, advantageously distributed randomly).

Typically, the process for preparing negative films of the invention comprises: - the deposition, by printing, of small opaque volumes, advantageously distributed randomly, at one of the faces of the transparent support;

- coating said face with a photosensitive layer.

We speak of the deposit of said small opaque volumes at one of the faces of the transparent support insofar as it is provided that said deposit can be implemented:

- on the front (on the upper face) of an untreated support sheet, which will eventually be treated (generally covered with an adhesion undercoat) or - on the front (on the upper face) of a treated support sheet (generally covered with an adhesion undercoat); or even between two layers of treatment. In other words, according to the method of the invention, the small opaque volumes on the front of the treated or untreated support sheet are deposited by printing. In the event that such a deposit is implemented on an untreated support sheet, the latter is then (after drying of said deposited volumes) either treated, before being covered with the photosensitive layer or directly covered with said photosensitive layer . It will be recalled here that said treatment generally consists of depositing an adhesion sublayer on the face of the transparent support intended to receive the photosensitive layer (on the front of said support).

The coating of the photosensitive layer is carried out in a manner known per se, on the front of the transparent support where the small opaque volumes have been deposited. Said small opaque volumes can thus be directly in contact with said photosensitive layer (if there is no treatment sublayer or if they are deposited on said (or even said) treatment sublayer (s)) or spaced therefrom, by at least one treatment sub-layer on the front side of the support. In the context of the process of the invention, provision is generally made, after the deposition of said small opaque volumes, for their stabilization before the deposition of the photosensitive layer or even that of at least one treatment sublayer and of the photosensitive layer. Such stabilization generally comprises, or even consists of, heating which, for example, ensures the drying of small volumes of ink deposited or the crosslinking of small volumes of heat-crosslinkable resin deposited. An adequate treatment can also be provided to crosslink small volumes of photocrosslinkable resin.

In fact, it is recommended, according to the invention, to prepare the original lightened films like the non-lightened films of the prior art by simply providing an additional step for the deposition of small opaque volumes.

The printing technique used for depositing said small opaque volumes is advantageously that of dry offset or gravure printing. It is particularly recommended to deposit said small opaque volumes by gravure printing. The implementation of this printing technique, in particular under the advantageous conditions recommended below [a) with a gravure gravure cylinder whose structure (the engraved surface) does not contain a rotogravure frame; and / or, advantageously and, b) with a gravure cylinder whose copper has been etched chemically] has given, in the context of the present invention, good results. This was not a priori obvious. Rotogravure uses copper-coated cylinders. Said copper is engraved by computer-controlled styli (Helioclichograph type). However, another technique is used which involves a “resist” photosensitive layer deposited on the copper. According to said technique, after exposure, development and drying, the copper is hollowed out with a bite based on iron perchloride in the places where said photosensitive layer "reserve" left during development.

Said engraved cylinder is then soaked, generally at about half of its radius, in a tank filled with very liquid ink. It is then wiped off with a thin metal blade called a doctor blade, the wiping action of which leaves ink only in the hollow parts of said cylinder. By then passing, between said inked cylinder and a pressure roller, the surface to be printed, in particular paper, the ink is transferred onto said surface, said paper, from said hollow parts.

This printing technique does not allow precise and perfect reproduction of fine lines and characters (letters) whose edges always include small hatching, due to the presence of the weft, called rotograme, used to support the doctor blade and avoid this, by plunging into the hollows, to remove the ink.

In addition, it is known that this printing technique excels at reproducing dark values but has difficulties in reproducing very light shades. However, according to the invention, for depositing small opaque volumes on the transparent support, said technique proves to be particularly effective, even if the values to be reproduced are very low (each small opaque volume corresponds to approximately a value of 1 to 5% in frame 120, that is to say at a very clear value, close to white-paper). It turns out to be all the more efficient when said gravure printing is implemented:

- with a gravure cylinder, the structure of which does not contain a so-called gravure screen (this eliminates the problems inherent in the presence of said screen (see above). We can do without said screen as far as the hollow surfaces of the cylinder are not important); and or advantageously and: - with a gravure cylinder, the copper of which has been attacked chemically. Chemical bite is better for generating in copper from the cylinder the small recesses from which the small opaque volumes are transferred to the support.

It can also be specified here that, in a manner known per se, the surfaces on which said small opaque volumes are deposited and then the photosensitive layer are advantageously subjected, before said deposits, to a surface treatment of the corona type intended to improve the adhesion on said surfaces. Treatments of this type, in the printing field, are familiar to those skilled in the art.

It is proposed to specify below an advantageous variant of implementation of the method of the invention, for the preparation of a reduced negative film, within the meaning of the invention.

Polyester reel (transparent support) is printed in gravure printing, to present on its front side (possibly covered with an adhesion undercoat) small volumes of black ink. To prepare the gravure cylinder, a film is used which has dots (small areas), advantageously distributed randomly, the area of each of which is between 140 and 2,500 μm ² and the number of which covers 4 25% of the surface area of the polyester to be printed. With said film, the photosensitive "reserve" layer of the cylinder is exposed. After development, rinsing and drying, the developed parts allow the chemical attack solution (based on iron perchloride) to pass which digs the copper from said cylinder. The “reserve” layer is then removed and the copper of the cylinder is covered by electrolysis of bright and hard chromium ... this, to improve its mechanical resistance. A rotogravure cylinder was thus prepared, without the usual gravure printing, to print small volumes of ink.

The polyester spool is then printed with said cylinder, with or without prior corona treatment, depending on the nature of the polyester and the ink used.

Said reel thus printed is unwound on a reel holder, passing through a slightly heated tunnel so as to remove any trace of solvent. It is advantageously subjected to a corona treatment to allow good adhesion of the photosensitive layer; this layer then being deposited with the usual precautions. It has been noted that such a corona treatment is particularly advantageous for allowing good attachment of said photosensitive layer to the edges of small opaque volumes deposited. Films of the invention are then obtained by cutting the reel, which, after exposure, development and fixing, may have small opaque surfaces in their transparent parts. Said films are advantageously used for exposing negative offset plates. After exposure and development, said plates have ink-absorbing surfaces which contain very small non-ink-absorbing surfaces, advantageously distributed randomly. These are lightened plates within the meaning of documents FR-A-2 660 245 and WO-A-96 02868.

Used on an offset machine, said plates provide the expected advantages of light plates:

- superior print quality,

- reduced paper waste,

- higher productivity.

Their use at the printer does not imply any change in the material and techniques used.

Only the film manufacturer has to modify its manufacturing process somewhat. The modifications to be envisaged, as is apparent from the comments above, are easy to implement and do not really disturb the process for manufacturing the negative films used to date. The films of the invention can be produced industrially, at a cost price entirely acceptable to the printer.

The invention is illustrated in Figures 2 and 3 appended to this description; its original character is apparent from the consideration of said figures 2 and 3 and of figure 1 which illustrates the technique of the prior art. Figure 4 illustrates another non-performing technique. We have shown, in section, in said Figures 1, 2,

3 and 4 respectively:

- a lightened film according to the technique of the prior art,

- a lightened film according to a variant of the invention,

- a lightened film according to another variant of the invention, - a lightened film according to another non-performing technique.

Said Figures 1 to 4 are obviously schematic. They do not represent in real size, on the same scale, the various constituent elements of films.

With reference to FIG. 1, a lightened photographic film according to the technique of the prior art is described below. Said light film of the prior art is a commercial film comprising: a support sheet 1 coated on the back with an anti-bending layer 4 (“anti-curling”) containing an anti-halo agent and on the front with an adhesion under layer 5;

- A photosensitive layer 2 coated on the front with a film of a protective varnish 6; which has been pre-treated, by printing, to present small volumes of black ink 3 on the front (on its upper face). In the same way, it could have been pre-treated, by sunstroke. Said pretreatment has been implemented, according to the prior art, on the finished commercial product. Small volumes of black ink 3 were deposited on the protective varnish film 6 of the photosensitive layer 2.

Referring to Figures 2 and 3, we now describe light films of the invention. These films also include:

a support sheet 10 coated on the back with an anti-bending layer 40 containing an anti-halo agent and on the front with an adhesion sublayer 50; a photosensitive layer 20 coated on the front with a film of a protective varnish 60; but also include, in their internal structure, small opaque volumes 30.

In the context of the variant represented in FIG. 2, said small opaque volumes 30 have been deposited on the adhesion sublayer 50 of the support sheet 10. The assembly [support sheet 10 + adhesion layer 50] constitutes, according to this variant, the transparent support of the film of the invention.

In the context of the variant shown in FIG. 3, said small opaque volumes 30 have been deposited on the support sheet 10, before the successive operations of the adhesion sublayer 50, then of the photosensitive layer 20 and finally of the protective film 60.

Said small opaque volumes 30 are, according to the two variants of the invention shown diagrammatically, advantageously distributed randomly, at the level of the upper face of the transparent support of the film. FIG. 4 represents a classic film, the laminated structure of which comprises the traditional elements:

- support sheet 1 ′ coated on the back with an anti-bending layer 4 ′ containing an anti-halo agent and on the front with an adhesion sublayer 5 ′;

- photosensitive layer 2 'coated on the front with a film of protective varnish 6'. During the development of this structure, small opaque volumes 3 'were deposited on the back of the support sheet 1'. Said small opaque volumes 3 ′ prove to be troublesome during sunshine and generally disappear during the latter. The invention is also illustrated by Examples 1 to 3 below. It was shown in Examples 4 and 5, called comparative, that the introduction of small opaque volumes at other levels of the film structure (than those recommended in the context of the present invention) was not satisfactory.

Example 1

Preparation and installation of the gravure gravure cylinder

Using a gravure cylinder covered with copper and a photosensitive "reserve" layer. Said photosensitive "reserve" layer is exposed with a transparent film, having opaque points, distributed according to a stochastic screening. The surface of each of said points is approximately 900 μm ² . In total, said points cover approximately 10% of the surface of the film. The exposure is carried out under conditions such that no part of the photosensitive “reserve” layer escapes it.

After development and drying, a chemical bite based on iron perchloride attacks and digs the copper in all the parts of said photosensitive "reserve" layer exposed by development. The said “reserve” layer is then removed and, in order to avoid any rapid wear on the copper, the said copper is covered, by electrolysis, with shiny and hard chromium.

The rotogravure cylinder thus prepared (without a so-called gravure screen) is placed on a machine. It soaks, about half of its radius, in a tray (inkwell) filled with black ink. Said ink is suitable for the film support to be treated according to the invention. Said film support is a commercial support, 10/100 mm thick, made of polyester coated on the front (on its face intended to receive the photosensitive layer) with an adhesion undercoat and on the back of a anti-bending layer containing an anti-halo agent.

_I. _mp.re. ssion.d. '. a _. delete from .fi.l _. m _. _en.po. lyes.ter

According to the invention, a reel of such a film support is treated continuously. By unwinding the reel, the (film) manufacturer continuously performs corona treatment on the front side coated with the adhesion undercoat, in order to reinforce the adhesion of small volumes of ink, then deposited by printing. said small volumes of ink on said front side, more precisely on said adhesion sublayer.

The printing is done at a speed of 10 meters per minute (the reel running between the gravure cylinder and a pressure roller) so that we then obtain, in a drying tunnel, arranged downstream of the cylinder d 'gravure printing a complete drying of the ink deposited in the form of small opaque volumes, distributed in a random manner.

_. Preparation of a _. movie _. negative "light."., _. to _.partir,. said _. support _. of..film _, printed ^ use of that _: çi The front side of the film support thus printed undergoes a new corona treatment and is coated with a photosensitive layer about 5 μm thick. Said photosensitive layer contains about 50% by weight of silver salts in gelatin. It is a material sold by the company Bergger (FR). The reel of photosensitive film obtained is dried with the usual precautions. It can then be covered, in a manner known per se, with a protective film.

This reel is cut into 240 mm x 300 mm films.

The films obtained are of the type shown schematically in Figure 2 attached.

One of these films is exposed so as to present, after development and fixing, transparent "encrophilic" parts representing lines, solid text and images, in a 120 frame.

By examining the film with a magnifying glass, we observe that it contains in its transparent parts thousands of small opaque dots.

An offset plate is produced using said film.

Said negative plate is exposed with said film. It is then developed, rinsed and dried in a conventional manner. The opaque points stopped the light during the sunshine. At the corresponding locations, there is no encrophilic layer; this being part of the development.

Put on a machine, said plate gives the printer the recognized advantages of small non-ink-absorbing surfaces in the ink-absorbing parts. Example 2

The procedure is as in Example 1, using a commercial polyester film support which is not coated on the front with an adhesion undercoat. The deposition of small volumes of ink is implemented (by rotogravure printing) on the polyester film support.

The adhesion underlayment is then put in place. The films obtained are of the type shown schematically in Figure 3 attached. Example 3 A film from the reel prepared in Example 1 is used to make a positive film with small transparent dots in its opaque parts. In fact, a conventional film is exposed by contact and, after development and fixing, the positive film is obtained with thousands of small transparent surfaces in the opaque parts. Copied onto a positive plate, said film generates small non-encrophilic surfaces in the encrophilic parts of said positive plate.

Example 4 (Comparative example)

Small ink volumes are deposited on the back of a polyester film support (polyester 10/100 mm thick), before the anti-bending layer is put in place. On the front of said support, there is, conventionally, an adhesion sublayer and the photosensitive layer.

During the use by the printer of the “lightened” film obtained (of the type shown diagrammatically in FIG. 4 appended), the thickness of the support makes it very difficult for precise reproduction on the offset plate of small non-ink-absorbing surfaces in the encrophilic parts; this coming from the diffusion of light, due to the thickness of the support.

Example 5 (Comparative example: prior art)

The deposition of small volumes of ink is carried out on the photosensitive layer (gelatin - silver salts). More specifically, said deposit is implemented by printing on the protective film of the photosensitive layer of a negative film of the prior art. The film obtained is of the type shown schematically in Figure 1 attached. During development, a large and irregular quantity of small volumes of ink detaches and pollutes the development solution.

Claims

- Claims -

1. Photographic films, for use in the graphics industry as light negative films, the structure of which comprises a transparent support (10 + 50) which has two substantially parallel faces and a small thickness, and a photosensitive layer (20) which covers the 'one of said two faces of said transparent support (10 + 50), characterized in that said structure further comprises, secured to said transparent support (10 + 50), on the side of said photosensitive layer (20), small opaque volumes (30 ).

2. Films according to claim 1, characterized in that said small opaque volumes (30) are distributed randomly.

3. Films according to one of claims 1 or 2, characterized in that said small opaque volumes (30) are distributed according to a stochastic screening.

4. Films according to any one of claims 1 to 3, characterized in that said small opaque volumes (30) cover between 4 and 25% of the surface of said support (10 + 50).

5. Films according to any one of claims 1 to 4, characterized in that said small opaque volumes (30) generate opaque surfaces whose surface is between 140 and 2,500 μm ² .

6. Method for the preparation of films according to any one of claims 1 to 5, characterized in that it comprises:

- Depositing, by printing, small opaque volumes (30), advantageously distributed randomly, at one of the faces of said transparent support (10 + 50); - coating said face with a photosensitive layer (20).

7. Method according to claim 6, characterized in that said small opaque volumes (30) are deposited by printing in dry offset or rotogravure; advantageously by gravure printing.

8. Method according to claim 7, characterized in that said small opaque volumes (30) are deposited by gravure printing; said gravure printing being implemented with a gravure cylinder, the structure of which does not contain a so-called gravure screen.

9. Method according to one of claims 7 or 8, characterized in that said small opaque volumes (30) are deposited by gravure printing; said gravure printing being implemented with a gravure cylinder in which the copper has been attacked chemically.