Apparatus for the photographic reproduction of transparent documents

The apparatus comprises a principal light source, condenser, lenses, object and image planes movable with respect to one another and an auxiliary light exposure source. The condenser is constituted by two thin stepped lenses between which is arranged a corrector screen for the variations in illumination in the image plane due to the inclination of the useful beam to the axis of the system. This screen operates perpendicularly to its surface, the constituent elements of the source, of the condenser and of the pupil of the lenses used being fixed with respect to one another and movable in a single unit relative to the document to be reproduced according to the focal length of said lens determining the size of the document. The apparatus is useful for the reproduction of transparent documents in black and white or in color.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to apparatus for the photographic 
reproduction of transparent documents in black and white or in color, such 
as enlargers for amateur or professional photography and more particularly 
apparatus providing half-tone or continuous-tone negatives for four-color 
printing. 
2. Description of the Prior Art 
The optical principle of these apparatuses is that of a photographic 
enlarger. The optical elements comprise a light source, a parabolic mirror 
condenser and interchangeable lenses of various focal lengths. 
According to the type of apparatus, the object plane is fixed and the image 
plane is movable or the object plane is movable and the image plane is 
fixed. Each of these two types have advantages and drawbacks. In the case 
of the fixed object plane, the image plane is moved, the object plane 
being at man's height. The documents and the masks are in a fixed working 
plane at the height of a man, which facilitates manipulation. On the other 
hand, according the maximum separation which is thus imposed between the 
two object and image planes, field angles of 60.degree. may be reached. 
In the case of a movable object plane and of the fixed image plane, the 
field angle is generally less than 60.degree.. On the other hand, the 
documents are sometimes scarcely accessible, which renders manipulation of 
the masks difficult. 
Apparatus providing negatives for printing also comprise an auxiliary 
exposure source designed to flatten the contrasts at the level of the film 
to be printed. This auxiliary system may be constituted by a device 
scanning the image plane before taking of photograph by means of a tubular 
source or by an annular source or a ring of optical fibers placed around 
the objective. 
It is an object of the invention to provide an apparatus enabling increase 
of the maximum useful sizes to reach, for example, the size of 20.times.25 
cm for the original and 50.times.65 cm for the reproduction, and offering 
the possibility of photometering any point of the image plane when the 
principal and auxiliary sources are simultaneously illuminated. 
In the course of the description which follows, the photometric problems 
relating to apparatus of the enlarger type will be recalled. 
It is also an object of the invention to provide an apparatus which has 
uniform lighting in the plane of reproduction. 
It is possible to summarize the conditions indispensible to this end in the 
following manner: 
As regards the source, the latter must be assimilated to a plane source and 
placed perpendicularly to the optical axis of the system and have 
preferably a uniform luminance whilst radiating according to the Lambert 
law. 
As regards the condenser, the latter must be of large dimensions to cover 
the maximum sizes imposed, to correct the aberrations well, in particular 
spherical and chromatic aberrations, in order to obtain strictly in the 
plane of the pupil of the objectives an excellent image of the source. 
As regards the adjusting system for the sizes of the image of the source in 
the plane of the pupil of the objectives, it is necessary for the latter 
to permit selection at will according to the characteristics or the 
defects of the original, an illumination of the so-called semi-directed or 
directed, diffused 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 corrector screen for the influence of the 
inclination .alpha. of the useful beam to the axis of the system which is 
single if possible, whatever the objectives and enlargements required and 
which operates at normal incidence for all the field points in order to 
facilitate calculation and measurement of the absorption at different 
points of the corrector screen of the cos.sup.4 .alpha. factor. 
As regards the auxiliary illumination source, the latter must produce a 
uniform illumination taking into account the cos.sup.4 .alpha. factor, 
whatever the position of the reproduction plane and enable photometering 
of the plane when it radiates simultaneously with the principal source. 
GENERAL DESCRIPTION OF THE INVENTION 
In order to respond to these various requirements, according to the 
invention there is provided an apparatus for photographic reproduction of 
transparent documents in black and white or in color of the type 
comprising a principal light source, colored or not, a condenser, 
interchangeable objectives, object and image planes movable with respect 
to one another, and an auxiliary light exposure source, characterized in 
that the condenser is constituted by two thin stepped lenses operating for 
their focus infinity, between which is arranged a corrector screen for the 
variations in illumination in the image plane due to the inclination of 
the useful beam to the axis of the system, this screen operating 
perpendicularly to its surface, the constituent elements of the source, of 
the condenser and of the pupil of the objective used being fixed with 
respect to one another and movable in a single unit relative to the 
document to be reproduced according to the focal length of said objective 
determining the size of the document. 
According to another feature of the invention, the principal source is a 
plane source focused in the plane of the pupil of the objective and is 
constituted by a source of light, a disc with plane-parallel or 
plane-concave surfaces, for example of glass or of silica, wherein one or 
both faces can be frosted and against which can be placed different 
diaphragms, then colored filters and corrector glasses for chromatic 
aberrations of the stepped lenses. 
According to another feature of the invention, the apparatus includes an 
auxiliary exposure source constituted by an annular fluorescent source 
with adjustable luminance without changing its color temperature, housed 
in a box, in the bottom of which is arranged an element uniformly lighted 
by said annular fluorescent source, an objective formed by a meniscus 
forming the image of the lighted element in the average reproduction 
plane, a tubular screen coaxial with the meniscus interposed between the 
annular source and said element and an internally reflecting truncated 
cone for eliminating the cat's-eye effect at the level of the output pupil 
of the objective. 
According to another embodiment, the auxiliary exposure source is 
constituted by an annular fluorescent source with adjustable luminance 
without change in its color temperature, housed in a box in the bottom of 
which is arranged an element uniformly lighted by said annular fluorescent 
source, an objective formed by a meniscus producing the image of a 
corrector screen placed against and before a field lens whose focus is 
coincident with the image of the front pupil of the meniscus, this front 
pupil being constituted by the small base of an internally reflecting 
truncated cone. 
The uniformly lighted element is either a plane or spherical, diffusing 
white screen, the position of which being adjustable with respect to the 
annular fluorescent source or an opaline transparent screen behind which 
is disposed a white opaque screen with radial degraded comprising in its 
centre an opening to place a photoelectric cell for controlling of the 
intensity of the annular fluorescent source. 
The auxiliary source comprises further, in the plane of the front pupil, a 
diaphragm of variable aperture followed by colored screens. 
Other features and advantages will emerge from the description which 
follows of particular embodiments of an apparatus according to the 
invention, which description is given purely by way of example and with 
reference to the accompanying drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The photometric problems which occur in the case of photographic enlargers 
to which the apparatus according to the invention provides a solution, 
will now be recalled. 
In the first place, at the level of the lighting device, an 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 plane-convex lenses with 
spherical or aspherical surfaces. In some projection systems, such as 
retroprojectors, these lenses are replaced by thin stepped lenses, molded 
("FRESNEL" type). The performance of these apparatuses is however 
insufficient for the production of photographic negatives. 
In all these cases, on variation in enlargement, the adjustement of the 
image of the source in the pupil of the objective is effected by movement 
of the source along the axis of the system with or without change in 
condenser, the latter being constantly placed against the original. 
With conventional lighting devices, variations in illumination are observed 
in the image plane due to the inclination .alpha. of the useful beam to 
the axis of the system (FIG. 1). 
In the case of a diffusing circular plane source (comparable to the pupil 
of a uniformly lighted projection objective), it would be possible to 
compensate for the decreasing illumination due to the inclination .alpha. 
by means of a corrector screen 1 arranged at a certain distance below the 
pupil 2 of the projection objective, so as to have uniform lighting in the 
image plane 3 or 3'. 
This corrector screen 1 must be calculated for a given distance of the 
pupil 2 from a given objective. It modifies the aberrations thereof 
(spherical aberrations, chromatism, astigmatism, distortion) and 
introduces interfering light. Moreover, it must be constructed with great 
accuracy. In fact, it is obtained by vacuum deposition, in order not to 
diffuse, and must be deposited on a plane-parallel sheet of "optical" 
quality and protected by an identical sheet, owing to the fragility of 
deposits of very low absorption. 
Due to the fact of the variable thickness of the absorbing deposit from the 
center to the periphery of the screen and due to the fact of the variable 
oblique incidences of the light rays, the calculation, the construction of 
this screen and its rating (or grading) in transmission are difficult, so 
that it becomes an expensive and hardly to be envisaged solution. 
It is also observed in these apparatuses that there are variations in 
illumination in the image plane, connected with those of the pupil of the 
projection objective and due to the poor quality of the image of a 
luminous source, non-planar in addition. 
FIGS. 3 and 4 illustrate variations in lighting (not taking into account 
the lessening of the cos.sup.4 .alpha.) in the case of a plane source of 
uniform and non-uniform luminance respectively, the source being focused 
in the plane of the pupil 4. The rounds and ovals in FIGS. 3 to 6 show 
diagrammatically the appearance of the pupil seen from the center or from 
the edge of the field. 
In the case of FIG. 3, uniform illumination is obtained. It is the same in 
the case of FIG. 4. In fact, if the pupils are covered non-uniformly, 
there is observed however the same law of luminance distribution in the 
plane of the pupil. 
FIGS. 5 and 6 illustrate the case of a source focused outside of the plane 
of the pupil with respectively a uniform luminance source and a 
non-uniform luminance source. 
In the case of FIG. 5, uniform lighting is present if the image of the 
source is of a diameter much greater than that of the pupil 4. This has 
the drawback that it is not possible to select between diffused lighting 
and lighting directed from the original. In this case, the lighting of the 
original is of the diffused type at the center of the field and of the 
directed type at the edge of the field. 
In the case of FIG. 6, it relates to a non-uniform luminance source and the 
lighting is also non-uniform. 
In the case of a non-planar source, spiral for example, the image of the 
source is not uniform. It is not formed in a single plane, that of the 
pupil for example. We have then a similar case to that illustrated by FIG. 
6 (non-uniform lighting). 
In summary, the conditions indispensable for obtaining a uniform lighting 
in the plane of reproduction are the following: 
1. Source 
It must be comparable with a plane source having the greatest possible 
surface, so as to permit a diffused lighting of the original document, it 
is placed perpendicularly to the optical axis of the system and must have 
preferably a uniform and high luminance, whilst radiating in each point 
according to the Lambert law in a solid angle .alpha. equal to that of the 
condenser. 
2. Condenser 
It must be of large size to cover the maximum sizes imposed and must be 
well corrected by aberrations, in particular spherical and chromatic, in 
order to obtain an excellent image of the source strictly in the pupil. 
3. Adjusting system 
An adjusting system for the dimensions of the image of the source in the 
pupil must be provided, in order to select at will a diffuse illumination, 
a semi-directed or a directed illumination, according to the 
characteristics or defects of the original. 
4. Corrector screen for the factor cos.sup.4 .alpha. 
This screen must be single, if possible, whatever the objectives and 
enlargements required and must operate at normal incidence for all points 
in the field. 
5. Auxiliary source of lighting 
It must be provided in order to produce uniform and adjustable lighting, 
taking into account the factor cos.sup.4 .alpha. whatever the position of 
the reproduction plane. It must permit photometering of this plane when it 
illuminates simultaneously with the principal source, the two sources 
being able to be provided optionally with colored screens. 
FIG. 7 shows the optical diagram of an apparatus according to the 
invention, capable of satisfying the requirements set out above. 
This apparatus is constituted by a source of light 10, a glass 11 frosted 
on its outer flat surface to obtain a more uniform source of luminance 
than the source itself, and a diaphragm 12 in contact with the frosted 
surface. This diaphragm 12 is constituted by a disc rotatable around an 
axis 13 parallel to the optical axis 14 of the system. In FIG. 7, are 
shown two holes 15 and 16 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 
designed to stop infrared. 
Behind the glass 17 are arranged the colored screens 18 mounted on a rotary 
disc 19. 
The condenser of the apparatus is constituted by two thin stepped lenses 20 
and 21 and between which is arranged a cos.sup.4 .alpha. corrector screen 
22, which is fixed so that its surface is perpendicular to the parallel 
beam. These lenses are preferably utilized at enlargement 1 and are then 
identical in order to limit the risks of crepe marking. 
The condenser forms the image of the source in the plane of 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 a directed lighting, whilst the hole 16, 
which is of larger diameter, corresponds to diffused lighting. 
The arrows 26 indicate the directions of movement of the document 24 
according to the size. 
Indeed, according to the invention, it is the document 24 which is moved 
with respect to the assembly formed by the light source, the condenser and 
the objectives. 
The cos.sup.4 .alpha. corrector screen 22 may be constituted in different 
ways. It may be formed by a deposit of absorbent thin layers, by the 
deposit of a layer of more or less intensely absorbent gelatine according 
as it is close or further from the center of the screen, or by a non-ruled 
or well-ruled with a random ruling, photographic film, exposed and then 
developed. The optical quality of this screen is of little importance 
given that it is placed before the objective and the original. 
In the case of the production of this screen 22 by means of absorbent 
layers, due to the fact that this screen is disposed between the two 
lenses 20 and 21 in the parallel beam, the deposit of the layers and the 
grading of the screen in transmission are distinctly easier to calculate 
and to measure than in the case of the screen of FIG. 2. The deposit may 
be effected on a plate without "optical" quality. 
FIGS. 8 and 9 illustrate the possibilities of correcting the focusing of 
the apparatus according to the invention as a function of the different 
sizes. 
In these figures, there is shown at 30 the working plane of the document 
holder and at 31 the floor on which the apparatus rests. According to the 
size of the document (32 or 32'), the assembly formed by the source shown 
diagrammatically at 33, the condenser 34 and the objective 35, are moved 
as a single unit. 
The source-pupil of the objective conjugation is effected on manufacture 
and the movement of the source-condenser objective unit in the case of the 
fixed document holder can be done manually or automatically. 
FIGS. 10 and 11 show diagrammatically two correction systems for the 
chromatism of the thin lenses 20 and 21. These corrections are 
indispensable in the case of lighting with directed light, but are not so 
necessarily in the case of lighting with diffused light. In the case of 
FIG. 10, this correction system for the chromatism is constituted by a 
colored screen 40 mounted between two thick plane-parallel glasses 41 
arranged behind the diaphragm 12, the over-correction of the glasses 41 
compensating for the chromatic under-correction of the lenses 20 and 21. 
In the use, for example, of red light, the focusing is done directly on the 
diaphragm 12 (red O with screen 18). In the case of blue light, the 
focusing is carried out on the image O' of the diaphragm through the 
corrector assembly 40-41. 
In the case of FIG. 11, the correction system is constituted by a 
plane-parallel glass 42 of suitable dispersion and thickness, a colored 
screen 18 being placeable before the corrector glass 42. The latter is 
intended for correcting the under-correction of the lenses 20 and 21. 
In FIG. 11 the planes 43 and 44 are red and blue images of the diaphragm 12 
through the corrector glass 42 and correspond to the focal point of the 
lenses 20 and 21. 
FIG. 12 shows in section an embodiment of a principal source according to 
the invention. 
This source is a spiral standard pulsed XENON source 50. The source 50 is 
surrounded by an internally reflecting truncated cone 51, whilst in the 
axis of the turns of the source is arranged an externally reflecting cone 
52. 
The device is completed by a frosted divergent lens 53 and a diaphragm 54. 
The divergent lens 53 is for the purpose of reducing the apparent thickness 
of the spiral source 50 and to bring the image close together to the 
frosted surface in order to simulate a flat source as much as possible 
whereas the truncated cone 51 and the cone 52 are for the purpose of 
increasing the luminance and the uniformity of the frosted surface of the 
lens 53. 
FIGS. 13 and 14 illustrate a modification of the principal source. 
In this embodiment, a spiral pulsed XENON source 50' in another arrangement 
is used. Indeed, the spiral is flattened, the turns being elongated 
parallel to each other and arranged in quincunx in cross-section (FIG. 
14). The system is completed by a mirror 55 with trough-shaped section, by 
a frosted glass 56 and a diaphragm 57. This modification enables greater 
uniformity, and greater luminosity of the frosted glass to be obtained as 
well as easier air cooling. 
FIG. 15 shows another embodiment of the principal source. 
In this case, a spiral pulsed standard XENON source 80 is used; this source 
is placed behind the frosted glass 81 in order to have the XENON source 
image on this glass. To improve the lighting of the source 80, one 
disposes behind the source a concave spherical internally reflecting 
mirror 82 so as to obtain a revolution system, that is to say that the 
axis XX' formed by the spiral constituting the XENON source is confounded 
with the axis of the optical system and the curvature radius of the 
spherical mirror is such that the center C of said mirror is on said axis 
substantially inside said spiral. 
With a principal source of the type showed on FIG. 15, there can still 
exist in the center of the frosted glass 81 a dark zone. This zone can be 
attenuated in using a device represented on FIG. 16. Like for the device 
of FIG. 15, one uses a spiral pulsed XENON source 80, a spherical mirror 
82 of center C and a frosted glass 81. But in this case, the mirror 82 is 
slightly decentered, which shifts the image of the source laterally but 
suppresses the revolution symmetry. To restore this symmetry, one causes 
the mirror to rotate around the axis XX' of XENON source as indicated by 
the arrow by means of a motor 83 or similar device. A better uniformity of 
lighting of frosted glass 81 is thus obtained. Such a construction is more 
difficult to realize mecanically. Further, the obtained lighting is not of 
revolution at a given moment. 
The device of principal source according to FIG. 17 permits avoiding such a 
rotation of the spherical mirror. In this case, the mobile decentered 
spherical mirror of FIG. 16 is replaced by a fixed concave 
toroidal/internally reflecting mirror 84 so as to obtain a revolution 
system. The centres C.sub.1 and C.sub.2 respectively of the torus parts 85 
and 86 have been chosen slightly decentered with respect to the axis XX' 
of the spiral XENON source 87 and are located so that the images formed by 
the different areas of the mirror form again slightly greater on both 
sides of the plane of the frosted glass 81 and of axis XX'. 
The device of principal source with torodial mirror can be further improved 
in disposing a second mirror M, spherical and concave, internally 
reflecting. The center C.sub.3 of this second mirror is on the axis XX' of 
the XENON source. The mirror M is placed in front of the toroidal mirror 
84 and surrounds the source 87. 
The presence of mirror M permits a supplementary increase of the lighting 
of the "whole source" by recovery of the light beams placed out of the 
field of the toroidal mirror. This spherical mirror M presents two further 
advantages: one, it facilitates the circulation of air around the spiral 
source, thus its cooling, and second, it shuts out beams, which, without 
mirror, would give parasite light around the apparatus. 
FIG. 18 shows the location of an auxiliary source 60 in an apparatus 
according to the invention for uniformly illuminating the reproduction 
plane 61. 
FIG. 19 shows in section one embodiment of this auxiliary source. 
It includes a box 62 in which an annular fluorescent source 63 is enclosed 
kept in place by centering seals 64 and 65. 
In the bottom of the box 62 is movably mounted a plane or spherical 
diffusing white screen 66. The screen 66 is fast to a plug 67 screwed into 
the box 62, which permits adjustment of 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 box, opposite the screen 66, is arranged, in a 
cylinder 68 screwed into the box 62, an objective 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 lighting over the diffusing plane. 
On the side of the meniscus 69 opposite the screen 66, is provided, on the 
edges of this meniscus, a reflecting truncated cone 71 designed to 
eliminate the cat's-eye effect at the level of the output pupil of the 
meniscus 69. 
In FIG. 19, is shown at A and B the end reproduction planes, and in dashed 
lines the average focusing plane. 
According to a modification of this device shown in FIG. 20, the tubular 
screen 70 (FIG. 19), which modifies the light distribution in the plane of 
the diffusing screen 66, may be replaced by a field lens 72 and a 
corrector screen 73 for the term cos.sup.4 .alpha. factor. The focus of 
the field lens 72 is conjugated with the image of the pupil in front 
(small base of the truncated cone 71) of the meniscus 69. The corrector 
screen 73 is placed in a parallel beam against the field lens 72 between 
the latter and the diffusing screen. The diffusing screen 66 is suitably 
located with respect to the annular source 63 in order that its 
illumination may be uniform. The pupil in front of the objective may be 
completed by an adjustable diaphragm 74 and by a holder disc 75 for 
colored glasses 76-77. 
The latter devices may be provided in the embodiment of FIG. 19. 
According a modification of device of FIG. 20, the diffusing white screen 
66 can be replaced by a disc 90 transparent, translucent or opaline (FIG. 
21) in synthetic glass, such as a methylpolymethacrylate, for example that 
is sold in France by the Society Altulor under the commercial name 
"Altuglas". Behind disc 90 is placed a white opaque screen 91 bearing the 
reproduction of a radial degraded to modify repartition of luminance of 
disc 90. This screen 91 is obtained by photography or by any other 
suitable process. A photo-electric cell 93 is further present in this 
device of auxiliary source; this cell is placed behind an opening 92 
disposed at the center of this degraded screen. This cell needs to control 
the intensity of fluorescent source 63. 
Further, all the other elements of the device of auxiliary source of FIG. 
21 are identical to those of FIG. 20. 
This latter device can be used in the embodiment of FIG. 19. 
The apparatus according the present invention permits to obtain a certain 
number of advantages on a practical point. It permits especially to reduce 
the time of exposure up to 10 times in relation with apparatuses of the 
prior art. It permits further to obtain very good reproductions with 
sources 4 to 5 times less strong than that of prior art, for example with 
sources of 5 KW. It must be added as already mentioned above, that is is 
possible with the apparatus according the invention to increase the 
maximum useful sizes to reach, for example, the size of 20.times.25 cm for 
the original and 50.times.65 cm for the reproduction. The apparatus 
according the invention finds an application in the industrial field: to 
effect a reproduction, only the principal source is obligatory (FIG. 7) 
but for making photogravure, it is obligatory to place further an 
auxiliary source (FIGS. 18 to 21). 
Of course the invention is not limited to the particular embodiments 
illustrated and described above but covers on the contrary all 
modifications.