EP0000450B1 - Photographic reproduction apparatus for transparent documents, especially of the photographic enlarger type - Google Patents

Description

The present invention relates to photographic reproduction apparatus for transparent documents in black or in color such as enlargers for amateur, professional photography, and more particularly apparatus providing halftone or continuous background photographs for printing in four colors.

The optical principle of these devices is that of a photographic enlarger. The optical elements include a light source (which can be associated with a parabolic mirror and a diffusing glass), a condenser and interchangeable objectives of various focal lengths.

Depending on the type of device, the object plane is fixed and the image plane is mobile, or the object plane is mobile and the image plane is fixed. Each of these two types has advantages and disadvantages.

In the case of the fixed object plane, the image plane is moved, the object plane being at breast height. The documents and masks are on a fixed work surface at eye level, which facilitates handling. On the other hand, given the small maximum difference which is thus imposed between the two object and image planes, it is possible to reach field angles of 60 °.

In the case of a moving object plane and the fixed image plane, the field angle is generally less than 60 °. On the other hand, the documents are sometimes not very accessible, which makes handling the masks difficult.

Apparatuses providing printing plates for printing also include an auxiliary exposure source intended to flatten the contrasts in the film to be impressed. This auxiliary system can be constituted by a device scanning the image plane before the shooting by means of a tubular source or by an annular source or a crown of optical fibers placed around the objective.

The object of the invention is to propose an apparatus making it possible to increase the maximum usable formats to reach for example a format of 20 x 25 cm for the original and 50 x 65 cm for the reproduction; this invention also offers the possibility of photometricizing any point of the image plane when the main and auxiliary sources are switched on simultaneously.

In the course of the description given below, the photometric problems raised by apparatuses of the enlarger type will be recalled.

The goal is also to obtain uniform illumination in the reproductive plane.

We can summarize the essential conditions for this obtaining as follows:

With regard to the source, this must be assimilated to a plane source and placed perpendicular to the optical axis of the system and preferably have a uniform luminance while radiating according to LAMBERT's law.

Regarding the condenser, it must be large to cover the maximum formats imposed, correct the aberrations, in particular spherical and chromatic, in order to rigorously obtain in the plane of the pupil of the objectives an excellent image of source.

Regarding the system for adjusting the dimensions of the image of the source in the plane of the pupil of the objectives, it must be possible to choose at will, according to the characteristics or the defects of the original, a lighting of the so-called diffuse, semi-directed or directed light type, the image of the source in the plane of the pupil having to be respectively greater than, equal to or less than the diameter of the objective used.

It is necessary to provide a screen correcting the influence of the inclination of the useful brush on the axis of the system which is unique if possible, whatever the objectives and enlargements required, and which works in normal incidence for all the points of field in order to facilitate the calculation and measurement of the absorption at the various points of the correction screen for the factor cos ⁴ a.

With regard to the auxiliary lighting source, this must produce a uniform illumination taking into account the factor cos ⁴ a, whatever the location of the reproduction plane, and allow the plane to be photometric when it irradiates simultaneously with the main source.

In order to meet these various requirements, the subject of the invention is a photographic reproduction apparatus for transparent documents in black or in color of the type comprising a main light source, whether or not colored, a condenser, interchangeable objectives, object and image planes. movable relative to each other, characterized in that the main source is a planar source developed in the plane of the pupil of the objective and is constituted by a light source, a glass disc with faces plane-parallel or plane-concave, one or both sides of which are frosted, the condenser consists of two stepped lenses working for their focus and infinity, between which is disposed a screen correcting for variations in lighting in the image plane due to the inclination of the useful brush on the axis of the system, this screen being crossed perpendicularly by the useful brush and the constituent elements of the source, the condenser and the objective ut ilisés are fixed relative to each other and movable in one piece relative to the document to be reproduced according to the focal length of said objective and the format of the document.

According to another embodiment of the invention, a diaphragm, comprising several holes, colored filters and glasses for correcting chromatic aberration of the stepped lenses, are placed against the ground glass disc.

According to another embodiment of the invention tion, the apparatus comprises an auxiliary exposure source constituted by an annular fluorescent source with adjustable luminance without changing its color temperature, housed in a housing, in the bottom of which is disposed an element illuminated by said annular fluorescent source, a objective formed by a meniscus forming the image of the element illuminated in the mean reproduction plane, a coaxial tubular screen at the meniscus interposed between the annular source and said element and a trunk of cone reflecting internally to suppress the effect of cat's eye at the exit pupil of the lens.

In another embodiment, the auxiliary exposure source is constituted by an annular fluorescent source with adjustable luminance without changing its color temperature, housed in a housing in the bottom of which is disposed an element illuminated by said annular fluorescent source, a lens formed by a meniscus forming the image of a corrector screen of the term cos ° α placed against and before a field lens whose focus is coincident with the image of the front pupil of the meniscus, this front pupil consisting of the small base of a trunk of cone reflecting internally. The illuminated element is either a white diffusing, flat or spherical screen, the position of which can be adjusted along the axis of the annular fluorescent source, or a transparent opaline screen at the rear of which is disposed a white opaque screen with radial gradient comprising its center an opening for the installation of a photoelectric cell for controlling the intensity of the annular fluorescent source. The auxiliary source further comprises in the plane of the pupil before a variable aperture diaphragm followed by colored screens.

The possibility of photometering, the system for adjusting the dimensions of the image of the light source and the auxiliary source are not compulsory in order to achieve the aim of the present invention, namely "to make large formats usable by improving uniformity of the lighting and the handiness of the camera but they rather offer additional advantages for reproduction in photoengraving.

• Figs. 1 et 2 illustrent les problèmes de variations d'éclairement dans le plan-image dues à l'inclinaison a du pinceau utile sur l'axe du système.
• Figs. 3 à 6 illustrent les problèmes de variations d'éclairement dans le plan-image liées à celles de la pupille de l'objectif de projection.
• Fig. 7 représente le schéma optique d'un appareil selon l'invention.
• Figs. 8 et 9 illustrent les possibilités de correction de la mise au point de l'appareil selon l'invention.
• Figs. 10 et 11 représentent schématiquement deux systèmes de correction du chromatisme des lentilles minces utilisées dans l'appareil de l'invention.
• Fig. 12 représente en coupe un mode de réalisation d'une source principale de l'appareil selon l'invention.
• Fig. 13 représente une variante de réalisation de la source principale.
• Fig. 14 représente une coupe suivant la ligne XIV-XIV du dispositif de la figure 13.
• Fig. 15 représente en coupe une autre variante de réalisation de la source principale.
• Fig. 16 représente en coupe la source principale de la figure 15 modifiée.
• Fig. 17 représente en coupe une variante préférée de réalisation de la source principale.
• Fig. 18 représente schématiquement la disposition de la source auxiliaire de l'appareil selon l'invention.
• Fig. 19 représente en coupe un mode de réalisation de la source auxiliaire.
• Fig. 20 représente schématiquement une variante de réalisation de la source auxiliaire.
• Fig. 21 représente une autre variante de réalisation de la source auxiliaire.

Other advantages will emerge from the following description of embodiments of an apparatus according to the invention, description given by way of example only and with reference to the appended drawings in which:

• Figs. 1 and 2 illustrate the problems of variations in illumination in the image plane due to the inclination of the useful brush on the axis of the system.
• Figs. 3 to 6 illustrate the problems of variations in illumination in the image plane linked to those of the pupil of the projection objective.
• Fig. 7 shows the optical diagram of an apparatus according to the invention.
• Figs. 8 and 9 illustrate the possibilities of correcting the focusing of the apparatus according to the invention.
• Figs. 10 and 11 schematically represent two systems for correcting the chromatism of the thin lenses used in the apparatus of the invention.
• Fig. 12 shows in section an embodiment of a main source of the device according to the invention.
• Fig. 13 shows an alternative embodiment of the main source.
• Fig. 14 shows a section along line XIV-XIV of the device in FIG. 13.
• Fig. 15 shows in section another alternative embodiment of the main source.
• Fig. 16 shows in section the main source of FIG. 15 modified.
• Fig. 17 shows in section a preferred alternative embodiment of the main source.
• Fig. 18 schematically represents the arrangement of the auxiliary source of the apparatus according to the invention.
• Fig. 19 shows in section an embodiment of the auxiliary source.
• Fig. 20 schematically represents an alternative embodiment of the auxiliary source.
• Fig. 21 shows another alternative embodiment of the auxiliary source.

We will first of all recall the photometric problems which arise in the case of photographic enlargers of which the apparatus according to the invention is a part.

First, at the level of the lighting device, the image of a source is formed in the pupil of a projection objective by means of a condenser.

In conventional devices, the distance between the original and the condenser is small and constant.

The condenser is generally composed of two plano-convex lenses with spherical or aspherical faces. In some projection systems, such as "overhead projectors", these lenses are replaced by molded stepped lenses ("FRESNEL" type). The performance of these devices is however insufficient for the production of photographic images.

Thus, US Patent 3,834,813 mentions the use of a condenser consisting of two stepped lenses working for their focus and the infinite in a photographic reproduction apparatus.

In all these cases, during variations in magnifications, the adjustment of the image of the source in the pupil of the objective is carried out by displacement of the source along the axis of the system with or without change of condenser, the latter being constantly placed against the original.

With conventional lighting devices, variations in lighting in the image plane are observed due to the inclination a of the useful brush on the axis of the system (FIG. 1).

In the case of a circular planar source diffu _- health (similar to the pupil of a projection lens, uniformly illuminated), one could compensate for the decrease in illumination due to the inclination using a correction screen 1 arranged at a certain distance below the pupil 2 of the projection objective, so as to have in the image plane 3 or 3 ′ uniform illumination.

This corrective screen 1 must be calculated for a given distance from the pupil 2 of a given objective. It modifies the aberrations (spherical aberrations, chromatism, astigmatism, distortion) and provides stray light. In addition, it must be performed with great precision. Indeed, it is obtained by vacuum deposition, in order not to diffuse, and must be deposited on a plane-parallel plate of “optical” quality and protected by an identical blade given the fragility of the deposits of very low absorption.

Due to the variable thickness of the absorbent deposit from the center to the periphery of the screen and due to the variable oblique incidences of the light rays, the calculation, the realization of this screen and its calibration in transmission are difficult, in so it is an expensive and hardly conceivable solution. The use of a radial gradient screen is indicated in US Patent 3,922,085.

In these devices, there are also variations in the illumination in the image plane, linked to those of the pupil of the projection objective and due to the poor quality of the image of a light source, moreover not planar.

FIGS. 3 and 4 illustrate the variations in illumination (without taking into account the attenuation in cos ⁴ a) in the case of a planar source of uniform and non-uniform luminance respectively, the source being developed in the plane of pupil 4. The round and oval in Figures 3 to 6 show schematically the appearance of the pupil seen from the center or the edge of the field.

In the case of FIG. 3, a uniform illumination is obtained at the level of the image. The same is true in the case of FIG. 4. In fact, if the pupils are covered non-uniformly, however, the same law of distribution of the luminance is observed from the center or from the edge of the field in the plane of the pupil.

FIGS. 5 and 6 illustrate the case of a source developed outside the plane of the pupil with respectively a uniform luminance source and a non-uniform luminance source.

In the case of FIG. 5, there is a uniform illumination if the image of the source is of diameter much greater than that of pupil 4. This has the drawback that one cannot choose between diffuse lighting and lighting directed from the original. In this case, the lighting of the original is of the diffuse type in the center of the field and of the type directed at the edge of the field.

In the case of FIG. 6, it is a source of non-uniform luminance and the illumination is also non-uniform.

In the case of a non-planar source, of helical shape for example, the image of the source is not uniform. It does not form in a single plane, that of the pupil for example. We then find ourselves in a case analogous to that illustrated in FIG. 6 (non-uniform illumination).

In summary, the essential conditions for obtaining uniform illumination in the reproduction plan are as follows:

1. Source

It must be assimilated to a plane source, having the largest possible surface in order to allow the lighting of the original document in diffuse light; it is placed perpendicular to the optical axis of the system and must preferably have a uniform and high luminance, while radiating at each point according to LAMBERT's law in a solid angle equal to that of the condenser.

2. Condenser

It must be of large dimensions to cover the maximum formats imposed and must be well corrected for aberrations, in particular spherical and chromatic, in order to rigorously obtain in the pupil an excellent image of the source.

3. Adjustment system

It is necessary to provide a system for adjusting the dimensions of the image of the source in the pupil, in order to choose at will diffuse, semi-directed or directed lighting, according to the characteristics or defects of the original.

4. Correct screen for the term cos ⁴ a

This screen must be unique, if possible, whatever the objectives and magnifications required and must work in normal incidence for all points of the field.

5. Auxiliary light source

It must be planned in order to produce a uniform and adjustable illumination taking into account the term cos ⁴ α, whatever the location of the reproduction plane. It must make it possible to photometricize this plane when it irradiates it simultaneously with the main source, the two sources being able to be provided or not with colored screens.

FIG. 7 represents the optical diagram of an apparatus according to the invention, capable of satisfying the requirements recalled above.

This device consists of a light source 10, a frosted glass 11 on its flat outer face to obtain a more uniform luminance source than the source itself, and a diaphragm 12 in contact with the frosted surface. This diaphragm 12 is constituted by a rotary disc around an axis 13 parallel to the optical axis 14 of the system. In FIG. 7, two holes 15 and 16 have been shown in the diaphragm 12. The hole 16 is in the axis 14 of the system and has a diameter substantially greater than that of the hole 15.

The diaphragm 12 is interposed between the frosted glass 11 and a glass 17 intended to stop the infrared.

Behind the glass 17 are arranged the colored screens 18 mounted on a rotating disc 19.

The condenser of the apparatus consists of two stepped lenses 20 and 21 working for their focus and infinity and between which is disposed a screen 22 correcting the term cos ⁴ a, working perpendicular to its surface. These lenses are preferably used at magnification 1 and are then identical in order to limit the risks of moire.

The condenser forms the image of the source in the plane of the objective pupils 23 of various focal lengths.

There are shown at 24 and 24 'two different positions of the document to be reproduced and at 25 and 25' the two corresponding positions of the reproduction plane.

The hole 15 of the diaphragm 12 has a diameter less than the pupils of the objectives 23 and corresponds to directed lighting, while the hole 16, which is of larger diameter, corresponds to diffuse lighting.

The arrows 26 indicate the directions of movement of the document 24 according to the format. Indeed, in accordance with the invention, it is document 24 which moves relative to the assembly formed by the light source, the condenser and the objectives.

The screen 22 correcting the term cos ⁴ α can be constituted in different ways. It can be formed by depositing thin absorbent layers, by depositing a more or less intensely absorbent gelatin layer depending on whether one is more or less close to the center of the screen, or by a non-photographic film. screened or screened with a random frame, exposed and then developed. The optical quality of this screen is of little importance since it is placed before the lens and the original.

In the case of the production of this screen 22 using absorbent layers, since this screen is arranged between the two lenses 20 and 21 in the parallel beam, the deposition of the layers and the calibration of the screen in transmission are much easier to calculate and measure than in the case of the screen in FIG. 2. The deposition can be carried out on a slide without "optical" quality.

Figures 8 and 9 illustrate the possibilities of correcting the focusing of the apparatus according to the invention according to the different formats. In these figures, there is shown at 30 the work surface of the document holder and at 31 the ground on which the apparatus rests. Depending on the format of the document (32 or 32 '), the assembly formed by the source shown diagrammatically at 33, the condenser 34 and the objective 35 is moved in a single block.

The source-pupil conjugation of the objective is carried out during manufacture, and the displacement of the source-condenser-objective assembly in the case of the fixed document holder can be carried out manually or automatically.

FIGS. 10 and 11 schematically represent two systems for correcting the chromaticism of lenses 20 and 21. These corrections are essential in the case of lighting in directed light, but are not necessarily so in the case of lighting in diffused light . In the case of FIG. 10, this chromaticism correction system consists of a colored screen 40 embedded between two thick plane-parallel glasses 41 disposed behind the diaphragm 12, the over-correction of the glasses 41 compensating for the chromatic under-correction of the lenses 20 and 21.

When using, for example, red light, the focus is made directly on the diaphragm 12 (0 red with the screen 18). In the case of blue light, the focusing is carried out on the image 0 'of the diaphragm through the corrector assembly 40-41.

In the case of FIG. 11, the correction system consists of a plane-parallel glass 42 of suitable dispersion and thickness, a colored screen 18 which can be placed in front of the corrective glass 42. The latter is responsible for correct the under-correction of lenses 20 and 21.

In FIG. 11, the planes 43 and 44 are the red and blue images of the diaphragm 12 through the corrective glass 42 and correspond to the focal points of the lenses 20 and 21.

The light source is a standard pulsed XENON source of helical shape 50. The source 50 is surrounded by a trunk of cone reflecting internally 51, while in the axis of the turns of the source is disposed a cone reflecting externally 52.

The device is completed by a frosted divergent lens 53 and a diaphragm 54.

The purpose of the divergent lens 53 is to decrease the apparent thickness of the helical shape 50 and to bring its image closer to the frosted surface in order to simulate as much as possible a plane source, while the truncated cone 51 and the cone 52 are intended to '' increase the luminance and its uniformity on the frosted surface of the lens 53.

Figures 13 and 14 illustrate an alternative embodiment of the main source.

According to this variant, a pulsed XENON source 50 ′ is used, the winding of the tube of which follows another arrangement. The turns are elongated, parallel to each other and arranged in a staggered fashion in a cross section (Figure 14). The system is completed by a mirror 55 with a trough-shaped section, by a frosted glass 56 and a diaphragm 57. This variant makes it possible to obtain greater uniformity, greater brightness of the frosted glass as well as cooling by look easier.

Figure 15 illustrates another embodiment of the main source.

For this purpose, there is a standard pulsed XENON source of helical shape 80 disposed behind a frosted glass 81. To improve the illumination at the frosted level 81, there is behind the source a concave spherical mirror 82 reflecting internally so as to obtain a system of revolution, that is to say that the axis XX 'of the helical tube constituting the XENON source is merged with the axis of the optical system and that of the spherical mirror whose center C is inside said helical tube.

With a main source of gender repre _- sented in Figure 15, there may still be the frosted glass of the center 81 a dark area.

This area can be attenuated by using the assembly shown in Figure 16. As with the assembly in Figure 15, there is a XENON pulsed source of helical shape 80, of a spherical mirror 82 of center C and of a frosted glass 81. But, in this case, the mirror 82 is slightly off-center, which shifts the image of the source laterally but removes the symmetry of revolution. To restore this symmetry, the mirror 82 is rotated around the axis XX 'of the XENON tube as indicated by the arrow using a motor 83 or any similar device. This gives better uniformity of illumination of the frosted glass 81. Such a construction is more difficult to achieve mechanically. In addition, the illumination obtained is not of revolution at a given time.

The main source arrangement in FIG. 17 makes it possible to avoid such rotation of the spherical mirror. For this purpose, the movable off-center spherical mirror of FIG. 16 is replaced by a concave fixed toric mirror 84 reflecting internally so as to obtain a system of revolution. The centers C, and C ₂ respectively of the torus parts 85 and 86 have been chosen slightly offset from the axis XX 'of the XENON tube of standard helical shape 87 and are located in such a way that the images supplied by the various regions of the mirror reform slightly enlarged on either side of the plane of the frosted glass 81 and of the axis XX '.

We can further improve this main source assembly with a toroidal mirror by providing a second mirror M, spherical and concave, reflecting internally. The center C ₃ of this second mirror is on the axis XX 'of the XENON tube. This mirror M is placed opposite the toric mirror 84 and surrounds the source 87.

The presence of the mirror M allows a further increase in the illumination of the “flat source by recovery of light beams located outside the field of the toric mirror. This spherical mirror M has two other advantages: on the one hand it facilitates the circulation of air around the helical tube, therefore its cooling, and, on the other hand, it intercepts beams which, without said mirror, would provide stray light around the device.

FIG. 18 shows the location of an auxiliary source 60 in the apparatus according to the invention with a view to uniformly illuminating the reproduction plane 61.

Figure 19 shows in section an embodiment of this auxiliary source.

It comprises a housing 62 in which is enclosed an annular fluorescent source 63 held in place by centering joints 64 and 65.

In the bottom of the housing 62 is removably mounted a white screen diffusing 66 plane or spherical. The screen 66 is integral with a plug 67 screwed into the housing 62, which makes it possible to adjust the distance between the screen 66 and the annular fluorescent source 63, the screen 66 being arranged in the axis of this source. In the housing, opposite the screen 66, is disposed, in a barrel 68 screwed into the housing 62, a lens constituted by a meniscus 69.

Between the source 63 and the screen 66 is interposed a tubular screen 70 adjustable along the axis of the optical system so as to modify the distribution of the illuminations on the diffusing plane.

On the side of the meniscus 69 opposite the screen 66, there is provided, on the edges of this meniscus, a reflecting truncated cone 71 intended to suppress the cat's eye effect at the exit pupil of the meniscus 69.

In Figure 19, there is shown in A and B the extreme reproduction planes and dashed the average plane of development.

According to a variant of this device shown in Figure 20, the tubular screen 70 (Figure 19) which changes the light distribution in the plane of the diffusing screen 66, can be replaced by a field lens 72 and a corrective screen 73 _COS ⁴ a. The focal point of the field lens 72 is combined with the image of the front pupil (small base of the truncated cone 71) of the meniscus 69. The corrective screen 73 is placed in a parallel beam against the field lens 72 between it ci and the broadcasting screen. The diffusing screen 66 is conveniently located relative to the annular source 63 so that its illumination is uniform. The front pupil of the objective can be completed by an adjustable diaphragm 74 and by a disc 75 carrying colored glasses 76-77.

These last two devices can be provided in the embodiment of FIG. 19.

According to a variant of the device in FIG. 20, the white diffusing screen 66 can be replaced by a transparent, translucent or opaline disc 90 (FIG. 21) made of synthetic glass, such as a poly (methyl methacrylate), for example that put on the market under the name "Altuglas by the company Altulor (France). At the rear of the disc 90 is disposed a white opaque screen 91 carrying the reproduction of a radial gradient intended to modify the distribution of the luminance of the disc 90. This screen 91 is obtained by photography or by any other suitable process. There is further provided in this auxiliary source arrangement a photoelectric cell 93 placed behind an opening 92 formed in the center of this degraded screen. This cell is used to control and control the intensity of the light tube 63. Furthermore, all the other elements of the auxiliary source assembly of FIG. 21 are identical to those of FIG. 20. This latter device can be provided in the mode Fig. 19.

The apparatus which is the subject of the present invention makes it possible to obtain a certain number of advantages from the practical point of view. It allows in particular to reduce the exposure times up to 10 times compared to the devices of the prior art. It also makes it possible to obtain very good reproductions with sources 4 to 5 times less powerful than those of the known art, for example with sources of 5 KW. It should be added, as already mentioned above, that it is possible, thanks to the device according to the invention, to increase the maximum usable formats to reach for example a format of 20 x 25 cm for the original. and 50 x 65 cm for reproduction. The apparatus according to the invention finds an application in the industrial field: to carry out a reproduction, only the main source is obligatory (FIG. 7), but, to make photo-engraving, it is essential to provide in addition a source auxiliary (Figures 18 to 21).

Claims (14)

1. Apparatus for photographic reproduction of transparent documents in black or colours, of the type comprising a coloured or uncoloured main light source (33), a condenser (34), interchangeable objectives (23, 35), object planes (24, 24') and image planes (25, 25') movable with respect to one another, characterised in that the main source (33) is a plane source focused in the plane of the pupil of the objective (35) and comprised of a light source (10), a glass disc (11) with plane- parallel or plane-concave faces one or both of which are frosted, the condenser (34) is comprised of two echelon lenses (20, 21) operating at their focus and infinity, between which are disposed a corrector screen (22) for variations in illumination in the image plane due to the inclination of the effective pencil on the axis of the system, this screen being traversed perpendicularly by the effective pencil, and the component parts of the source (33), the condenser (34) and the objective (35) used are fixed with respect to one another and displaceable as a unit relative to the document to be reproduced as a function of the focal point of said objective (23, 35) and the format of the document.

2. Apparatus according to claim 1, characterised in that a diaphragm (12) comprising a plurality of holes (15, 16), coloured filters (18, 40) and chromatic aberration corrector lenses (41, 42) for the echelon lenses (20, 21) are placed against the frosted glass disc (11).

3. Apparatus according to claim 1, characterised in that said light source (10) is comprised of a pulsed XENON source of helical shape (50) surrounded by an inwardly reflecting truncated cone (51) and inside of which is disposed an outwardly reflecting cone (51), the frosted glass (53) being comprised of a divergent lens.

4. Apparatus according to claim 1, characterised in that said light source (10) is comprised of pulsed XENON source (50') with elongated turns parallel to one another and arranged in a quin- cunx along a sectional plane transverse to the turns, the source being housed in a mirror (55) of trough-shaped section.

5. Apparatus according to claim 1, characterised in that said light source (10) is comprised of a XENON source (87) of helical shape behind which is disposed an inwardly reflecting concave toric mirror (84) whose centres C, and C₂ are slightly offset with respect to the axis of the helix comprising the Xenon source (87).

6. Apparatus according to claim 5, characterised in that the XENON source (87) of helical shape is surrounded by an inwardly reflecting concave spherical mirror M whose centre C₃ is located on the axis of the helix comprising the XENON source.

7. Apparatus according to any one of claims 1 to 6, characterised in that it comprises a device for correcting the chromatism of the echelon lenses (20, 21) comprised of a glass plate (41) in which is mounted a coloured screen (40).

8. Apparatus according to any one of claims 1 to 6, characterised in that it comprises a device for correcting the chromatism of the echelon lenses (20, 21) comprised of a suitable dispersion plate (42) compensating for the chromatism for the entire spectrum.

9. Apparatus according to any one of claims 1 to 8, characterised in that the two echelon lenses (20,21) are identical and operate with a magnification equal to 1.

10. Apparatus according to any one of claims 1 to 9, characterised in that it comprises an auxiliary exposure source (60) comprised of an annular fluorescent source (63) with adjustable luminance without changing its colour temperature, housed in a housing (62) in the bottom of which is disposed an element (66) lighted by the said annular fluorescent light source (63), a device formed by a meniscus (69) forming the image of the element (66) lighted in the middle reproduction plane, a tubular screen (70) coaxial to the meniscus (69) interposed between the annular source (63) and the said element (66) and an inwardly reflecting truncated cone (71) for eliminating the cat's eye effect at the level of the output pupil of the objective (69).

11. Apparatus according to any one of claims 1 to 9, characterised in that it comprises an auxiliary exposure source (60) comprised of an annular fluorescent source (63) with adjustable luminance without changing its colour temperature, housed in a housing (62) in the bottom of which is disposed an element (66) lighted by the said annular fluorescent source (63), a objective formed by a meniscus (69) forming the image of a corrector screen (73) for the term cos4 a placed against and in front of a field lens (72) whose focal point coincides with the image of the front pupil of the meniscus (69), this front pupil being comprised of the small base of an inwardly reflecting truncated cone (71).

12. Apparatus according to either one of claims 10 or 11, characterised in that the uniformly lighted element is a plane or spherical diffusing white screen (66) whose position is adjustable along the axis of the annular fluorescent source (63).

13. Apparatus according to either one of claims 10 or 11, characterised in that the uniformly lighted element is a transparent opaline screen (90) at the back of which is disposed a radially graduated white opaque screen (91) comprising at its centre an opening (92) for accommodating a photoelectric cell (93) for controlling the intensity of the annular fluorescent source.

14. Apparatus according to any one of claims 10 to 13, characterised in that it further comprises in the plane of the said front pupil a variable opening diaphragm (74) followed by coloured screens (76, 77).

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