US3680956A

US3680956A - Apparatus for controlling contrast during reproduction of photographic images

Info

Publication number: US3680956A
Application number: US99106A
Authority: US
Inventors: Peter Custer
Original assignee: Individual
Current assignee: Individual
Priority date: 1970-12-17
Filing date: 1970-12-17
Publication date: 1972-08-01
Anticipated expiration: 1989-08-01

Abstract

A method and apparatus for controlling contrast during the reproduction of photographic images from a negative or transparency. A mask is formed in a sheet of phototropic material by placing the sheet in the focal path of the image and transmitting the image to the sheet by a sheet-activating radiation source. Phototropic materials characteristically darken on exposure to a radiation source of certain wavelengths. The intensity of the radiation source may be selectively varied to produce a masking image of the desired density. The reproduction exposure is then made through the original image and the mask formed on the phototropic sheet prior to clearing of the masking image.

Description

United States Patent Custer [54] APPARATUS FOR CONTROLLING CONTRAST DURING REPRODUCTION OF PHOTOGRAPHIC IMAGES [72] Inventor: Peter Custer, Box 100, Newtown,

[22] Filed: Dec. 17, 1970 v [21] Appl. No.: 99,106

52 vs. C! 355/71, 355/80 51 Int. Cl. -0031 27/76 [58] Field of Search ..355/35, 71, 77, so, 132, 133; 350/160; 250/237 [56] References Cited FOREIGN PATENTS OR APPLICATIONS 891,992 3/1962 Great Britain ..355/71 873,380 7/1961 Great Britain ..350/l60P 1451 Aug. 1,1972

Primary Examiner-Samuel S. Matthews Assistant Examiner-Russell E. Adams, Jr. Attorney-Howson and Howson 5 7 ABSTRACT A method and apparatus for controlling contrast during the reproduction of photographic images from a negative or transparency. A mask is formed in a sheet of phototropic material by placing the sheet in the focal path of the image and transmitting the image to the sheet by a sheet-activating radiation source. Phototropic materials characteristically darken on exposure to a radiation source of certain wavelengths. The intensity of the radiation source may be selectively varied to produce a masking image of the desired density. The reproduction exposure is then made through the original image and the mask formed on the phototropic sheet prior to clearing of the masking image.

8 Claims, 11 Drawing Figures PATENTED H973 3.680.956

sum 1 BF 2 PETER CUSTER ATTYS INVENTORI PATENTEUAUB I ma SHEET 2 BF 2 mvcn'roaz CUSTE FIG] FIQG UL MA 1004:? uawr PETER ATTYS.

APPARATUS FOR CONTROLLING CONTRAST DURING REPRODUCTION OF PHOTOGRAPHIC IMAGES The present invention relates generally to photographic reproduction processes and relates more particularly to a method and apparatus for controlling contrast in the reproduction of photographic images from negatives or transparencies.

A well known problem in the printing of photographic transparencies and negatives is the inability of the copy to reproduce the density range of the negative or transparency. ln black and white printing, this necessitates the use of a technique known as dodging in which light passing through the clearer areas of the negative representing shadows or dark objects in positive is physically restricted from some part of the exposure to the photographic paper. In a more sophisticated and controllable method, a shadow mask a thin diffuse positive image made on a second piece of photographic film by contact exposure with the negative, black and white or color is used to add density to the shadow or dark areas of the negative. The effect of this mask is to selectively restrict the light passing through these areas of the negative to the paper thus reducing the contrast range of the image, seen by transmitted light to one which the paper seen by reflected light, is able to reproduce.

Color transparencies similarly have inherent high contrast and in making duplicate transparencies or color separation negatives for engravings or prints, highlight masking is an absolute necessity. Made in the same manner as a-shadow mask, a highlight mask is a thin negative black and white image of the positive color transparency which adds density to the highlight of lighter areas which would otherwise bum out in reproduction.

The production of the described shadow and highlight masks is a tedious and time consuming process. Care must be taken to insure an exact registration of the processed mask when returned to' the negative or transparency and various techniques and devices are employed to facilitate such registration. The proper exposure and processing of the mask is in the firs instance a matter of judgment and in many cases the mask must be remade to provide the desired contrast correction. Accordingly, in view of its trial and error nature, the time required for processing and drying of the mask, the cost of materials and processing, and the problems of precise registration, it can be understood that present masking techniques are time consuming and expensive.

In the present invention, the contrast control during the reproduction exposure is effected by means of a masking image produced in a sheet of phototropic material. The sheet of phototropic material is placed in the focal path of the negative or transparency and serves the same masking function as the conventional shadow mask or highlight mask referred to above. Registration problems and processing delays are completely eliminated with the present method and apparatus and the masking effect is observed or measured and immediately correctable.

Phototropic or photochromic materials are those light-transmitting materials exhibiting the phenomena wherein an exposure to radiant energy produces a reversible change in their optical density and/or color.

The effect can be produced by the inclusion of a variety of organic and inorganic materials in a transparent medium such as glass or plastic. Depending upon the materials utilized to produce the effect, the phototropic reaction may be produced by wave lengths throughout the spectrum including infra-red, visible and ultraviolet radiation. Reactions may be produced in which the material becomes darker upon exposure to the activating radiation, or in which the material becomes lighter during such exposure. In many instances, a colorless material becomes deeply colored under the radiation influence while conversely a colored material may change color or become colorless during irradiation. The phototropic effects typically are produced rather quickly and reverse somewhat more slowly although the speed of activation and reversal can be varied by the variation in the composition of the phototropic materials employed, the intensity of the activating radiation, and the use of radiation of a different wave length or heat to accelerate the clearing of the material.

The physicochemical theories involved in the phototropic reactions and the various classes of materials producing such effects are known and will not be dealt with in the present disclosure. A discussion of materials exhibiting phototropic reactions may be found in U. S. Pat. No. 3,436,353. Other US. Pat. Nos.

which disclose phototropic materials include 3,197,296; 3,397,023; 3,407,145;- 3,436,144; 3,454,778 and 3,476,459.

There has to date been little commercial usage of phototropic material, its most common use being for eye glass lenses wherein the density of the lenses increases upon exposure'to ultraviolet radiation. Other suggested applications include vehicle and building windows, photographic lenses and filters, and miscellaneous applications such as television tubes, instruments, etc.

The primary suggested use of phototropic materials for photographic purposes has been as a filter to control the amount of light reaching the film during the photographing of a subject. In US. Pat. No. 3,436,353, phototropic filters are suggested as a substitute for the mechanically operated or photocell-operated iris commonly in use. It is further suggested in this patent that phototropic material may also be utilized as a coating on the photographic film itself.

The present invention does not involve camera lenses, filters or films utilized in recording a subject image. The invention is specifically concerned with the density range correction required in the reproduction of photographic images from the typical high contrast negative or transparency.

In the present invention, a mask is formed in a sheet of phototropic material by placing the sheet in the focal path of the negative or transparency to be reproduced and exposing the sheet to an activating source of radiation. Since the phototropic reaction of .certain phototropic materials including the particular material used to date is dependent upon the intensity of the radiation striking the phototropic sheet, the radiation passing through lighter areas of the negative or transparency produces a more pronounced reaction than the radiation passing through the darker areas thereof. In the case of a negative, the reacn'on forms a positive image in the phototropic material which is in perfect registration with the negative since it was formed in place. The mask created in this fashion is immediately utilized in the exposure of the printing paper or film be fore the phototropic reaction can reverse. Since these exposure times are conventionally far shorter than the clearing time of the phototropic material, there is only a negligible fading of the mask during the exposure. Mask density may, alternately, be sustained by continuous exposure of the mask forming material, which tends to achieve a steady state of density relative to the intensity of the exposing light.

Due to the reversible nature of phototropic materials, the same sheet may be repeatedly used. In a preferred embodiment of the invention, a sheet of phototropic material is employed in place of the lower glass element in the negative carrier of an enlarger. The phototropic sheet may alternately be positioned on the enlarger easel/camera back for formation of the masking image, the film or photographic paper being placed therebeneath without changing the position of the sheet relative to the projected image. The invention may similarly be employed in the masking of motion picture film utilizing the general method described which may effectively be carried out by apparatus disclosed herebelow.

It is accordingly a first object of the present invention to provide an improved method and apparatus for controlling image contrast during all methods of reproduction of images from photographic negatives, transparencies, and motion picture film.

A further object of the invention is to provide a method and apparatus as described which while using a masking technique eliminates the conventional processing and registration procedures necessary with conventional shadow or highlight masks.

An additional object of the invention is to provide a masking method and apparatus as described in which the formation of the mask can be observed and its density regulated as determined by the printers judgment or instrumentation.

Another object of the invention is to provide a method and apparatus as described which can be carried out using conventional equipment with a minimum amount of modification.

A still further object of the invention is to provide a method and apparatus as described which may be effectively utilized with both black and white and color photographic reproductions.

An additional object of the invention is to provide a method and apparatus as described which is adapted for use in darkroom work or in automated printing such as employed in bulk color printing of amateur color negatives.

Still another object of the invention is to provide a method and apparatus as described which is simpler, more economical and less time consuming thant'hose heretofore employed.

Additional objects and advantages of the invention will be more readily apparent from the following detailed description of embodiments thereof when taken together with the accompanying drawings wherein:

FIG. 1 is a schematic front elevational view of an enlarger constructed in accordance with the present invention;

FIG. 2 is an enlarged view partly in section taken along line 2-2 of FIG. 1 showing details of the enlarger lamphouse;

FIG. 3 is an enlarged view partly in section taken along line 33 of FIG. 1 showing the enlarger negative carrier;

FIG. 4 is a greatly enlarged sectional view taken along line 4-4 of FIG. 3 showing the phototropic sheet mounted in the negative carrier;

FIG. 5 is an exploded perspective view showing the photographic negative and the masking image thereof produced on the adjacent phototropic sheet;

FIG. 6 is a perspective view showing schematically the manner in which the mask can be produced exteriorly of the enlarger by exposure of the phototropic sheet to an external radiation source;

FIG. 7 is a schematic front elevational view of an enlarger similar to that shown in FIG. 1 but having the phototropic element mounted in the easel rather than the negative carrier;

FIG. 8 is a greatly enlarged sectional view taken in the circled area of FIG. 7;

FIG. 9 is a schematic side elevational view showing apparatus for masking motion picture film in accordance with the present invention;

FIG. 10 is a sectional view taken along line l0--l0 of FIG. 9; and

FIG. 11 is a sectional view similar to FIG. 10 of a modified form of the invention utilizing a phototropic film.

As indicated above, the present method and apparatus utilize a phototropic or photochromic material to form a contrast correcting mask. Although it is possible and within the scope of the present invention to use a phototropic material as a masking medium which upon activation becomes less dense for purposes of increasing contrast in a negative or transparency, by far the more common correction required is the decreasing of the range of the image density. Accordingly, in each of the described embodiments a phototropic material is employed which becomes increasingly dense upon activation by a suitable radiation source to reduce the contrast of the transmitted image.

The embodiments illustrated were designed to employ a commercially available phototropic glass sold by Corning Glass Works, Corning, New York under the name PI-IOTOGRAY". This material is essentially a silicate glass which includes silver halide crystals which are considered to provide the phototropic effects. This glass is sensitive to radiation in the near ultraviolet region of the spectrum having wave lengths of approximately 3,000 to 4,000 angstroms. Although for purposes of the invention the phototropic material may be a type sensitive to any suitable radiation source such as light in the visible range, it is convenient to utilize a material sensitive only to ultraviolet radiation to effect control of the mask density.

A preferred form of apparatus for carrying out the present invention is illustrated in FIGS. 14 and comprises a photographic enlarger generally designated 12 which is largely of conventional design. In the schematic illustration, the enlarger head 14 is shown disposed above the baseboard 16, the usual head supporting column and height adjusting means having been omitted to simplify the drawings. The enlarger head includes the casing 18 enclosing the condenser lenses (not shown) and supporting the lamphouse 20. Secured to the lower end of the casing is an apertured plate 22. An adjustable bellows 24 extends downwardly from the plate 22, terminating in a lens board 26 on which is mounted the enlarging lens 28. A negative carrier 30 is slidably received in a slot 31 in the plate 22, as shown most clearly in FIG. 3.

Mounted in the lamphouse is a conventional incandescent enlarging lamp 32 provided with suitable socket means 34 connected with a power source 36 by conduits 38. A timer 40 is included in the lamp circuit as is a manually controlled switch 42. Surrounding the incandescent lamp 32 is a circular gas discharge lamp 44, for example, a mercury vapor lamp, adapted to produce and transmit radiation in the near ultraviolet region of the spectrum. A suitable socket 46 for the bulb 44 is connected by conduits 48 to the power source 36. A timer 50 and manual switch 52 are provided in the ultraviolet lamp circuit as well as a voltage regulator 53 for control of the intensity of the lamp output.

An easel 54 is disposed on the baseboard 16 and is of a conventional construction including a base member 56 and a framing assembly 58 attached thereto by hinge 60. Photographic paper is inserted on the base member 56 beneath the framing elements of the framing assembly which are adjusted to suit the size of the projected image by means of the adjustment knobs 62.

The negative carrier 30 as shown in the enlarged views of FIGS. 3 and 4 includes a carrier plate 64 and a cover plate 66 of a generally rectangular shape adapted to be superposed as illustrated with the aid of aligning pins 68. As shown in FIG. 3, a tongue portion 70 of the carrier extends from the enlarger when the carrier is seated within the slot 31 of plate 22 to permit the ready removal of the carrier from the enlarger to change or adjust the subject negative in the carrier.

Aligned apertures

72 and 74 respectively in the

carrier plates

64 and 66 permit light from the lamphouse to pass through a negative or transparency held by the carrier to project an image thereof onto the easel 54. To support the negative or transparency in a flat plane perpendicular to the optical axis of the enlarger, it is customarily sandwiched between sheets of glass which are received by a groove 76 in the carrier sheet 64 adjacent aperture 72. In the preferred embodiment of the invention, the lower glass sheet comprises a phototropic glass sheet 78 which may, for example, be a sheet of the Coming PI-IOTOGRAY glass referred to above. A negative 80 to be enlarged is secured in flat disposition on the phototropic sheet by the cover glass sheet 82. The sheet 83 for convenience is attached to the carrier plate 66 while the phototropic glass is mounted on the plate 64, thereby facilitating the removal or adjustment of the negative 80. The described negative carrier is entirely conventional with the exception of the employment of phototropic glass for the negative supporting sheet.

For operation of the apparatus shown in FIGS. 1-4, the negative 80 to be enlarged is positioned in the negative carrier 30 in the manner illustrated and the carrier inserted in the enlarger as shown in FIGS. 1 and 3. Prior to insertion of the printing paper into the easel, the incandescent bulb 32 is turned on and the image of the negative is accurately focused on the surface of the easel by adjustment of the bellows 24. The density range or contrast of the negative is observed and may be gauged by means of a densitometer to determine whether a mask is needed and if so, the density of the mask required to bring the contrast range of the transmitted image down to that of the paper.

Following the determination of the mask requirement, the ultraviolet bulb 44 is turned on and adjusted in intensity by means of voltage regulator 53 to activate the ultraviolet-sensitive phototropic sheet 78 which is of a type which achieves steady state density relative to the intensity of the activating light source. As will be noted with reference to the schematic illustration of FIG. 5, the light areas of the negative permit passage of the ultraviolet light which on striking the phototropic material activates the material to produce a positive image of the subject negative image. As is necessary in any type of masking technique, the masking image must be considerably less dense than the negative or the two will tend to cancel each other out when light is transmitted through the combined negative and mask. Any tendency of the phototropic material to produce a slightly diffused image is highly desirable as explained herebelow. The reverse effect is true in the case of transparencies (positive images) wherein density is added to the lighter (highlight) areas.

After exposure of the phototropic material to the ultraviolet light to produce the desired density of the masking image, the photosensitive material is inserted into the easel and the incandescent lamp 32 is turned on to effect the exposure of the photosensitive material through the negative 80 and phototropic masking image. In view of the fact that most phototropic materials reverse their reaction at a rate considerably slower than the rate of activation, the ultraviolet light may be extinguished just prior to the exposure of the photosensitive material since the masking image persists for a longer period than the usual exposure time of photosensitive materials.

If, following development of the first exposure, it should be determined that the masking image was not of the proper density to correct the contrast range of the negative, the procedure may be promptly repeated, varying the density of the mask by changing the intensity of the activating radiation. The intensity of the lamp 44 is adjusted by means of a voltage control device 53 such as a variable transformer. As indicated above, the phototropic material selected produces a steady state of image density dependent on the intensity of the activating radiation source and the density of the image produced may thus be readily varied to suit the contrast of the negative.

The rate of activation and clearing of the phototropic glass are dependent on a number of factors including the composition and heat treatment of the glass. These rates are further changed significantly by the temperature of the glass. The activation can be accelerated by use of a high intensity source, such as pulsed, xenon, to a small fraction of a second. Similarly, the clearing rate can be accelerated by exposure of the glass to heat or to light of wave lengths longer than those used to activate the glass. In the present case for example, radiant heat or sodium vapor lighting can be used to significantly speed up the clearing rate of the glass.

Although for convenience the apparatus of FIGS. 1 and 2 wherein the ultraviolet source is located within the enlarger is preferred, it will be apparent that the masking image in the phototropic glass may alternately be produced outside of the enlarger by removal of the negative carrier and exposure of the negative and phototropic glass to an external ultraviolet source as schematically illustrated in FIG. 6. After the masking image has been formed, the negative carrier is immediately returned to the enlarger and the exposure of the photographic paper is carried on as described above.

A modified apparatus utilizing the same phototropic masking principle is illustrated in FIGS. 7 and 8 and comprises an enlarger 12' similar to that described in FIG. 1. The enlarger l2 differs only in the placement of the phototropic sheet. Instead of mounting the sheet in the negative carrier, a large phototropic glass sheet 86 of a size sufficient to cover the photosensitive material 88 is secured to the framing assembly 58'. The conventional clear sheets of glass are used in the negative carrier to support the negative.

In use, the embodiment of FIGS. 7 and 8 is operated in substantially the manner explained above, the phototropic sheet 86 being exposed to the ultraviolet lamp following an initial determination of the density of the mask required. The photosensitive material 88 is positioned in the ease] beneath the sheet 86 and the exposure of the paper is made through the masking image on the phototropic cover glass. Care must be taken not to move the easel after making the mask since the masking image would then not register with the projected negative image.

In view of the fact that most glass does not transmit ultraviolet radiation, the condenser lenses of the enlargers 12 and 12' must be made of an ultraviolet trans- 'mitting material such as quartz glass. The cover glass sheet 82 in the negative carrier 30 of enlarger 12 must similarly be of an ultraviolet transmitting material. In the enlarger 12 the condenser lenses, both negative carrier glass sheets, and the enlarging lens must be capable of transmitting ultraviolet radiation.

Since some phototropic materials will reach a steady state density relative to the intensity of the activating radiation, the ultraviolet lamp may remain on during the exposure of the paper if an ultraviolet filter layer is provided on the lower side of the sheet of phototropic material. This will eliminate the possibility of fading of the masking image during white light exposure of the photosensitive materials. On the other hand, it may be convenient to make the print exposure (black and white) utilizing the ultraviolet light source. This can be done, assuming the enlarger optical system is adapted to transmit ultraviolet light, by first exposing the phototropic material to form the mask, briefly interrupting the transmission of ultraviolet to the ease] during insertion of the paper, and resuming the ultraviolet transmission to effect the print exposure. In the event that the print exposure is made with the ultraviolet light, only the ultraviolet bulb need be included in the enlarger lamphouse.

As indicated above, any tendency of the phototropic masking image to be somewhat diffuse is a desirable attribute of the present method and will enhance the sharpness of the transmitted masked image by adding density but not exactly matching detail. The desirability of a difiused masking image is well known in the art and it is common practice to use a diffusion sheet between the image to be masked and the film to be exposed as a mask.

Depending on the characteristics of the phototropic masking material, the mask exposure may be varied in length as well as intensity to achieve the proper mask density. Until the material reaches a steady state density commensurate with the light intensity, the image density is a function of the time of exposure to the sensitizing light source.

The apparatus described in the above embodiments are only examples of devices which could be employed to carry out the present method. It will be evident that different types of enlargers such as the well known diffuser enlarger could be effectively employed to carry out the method. Different types of enlarger light sources may also be employed, although it is highly desirable that the phototropic sheet activating source and the printing light source be on the same optical axis.

From the foregoing, it will be apparent that the present method can also be employed in masking contact exposures, for example in making duplicates of color transparencies or negatives. The procedure is similar to that described above except that the mask is placed above the transparency when the duplicating exposure is made. This can most readily be explained by a simple example describing the preparation of a duplicate transparency. First the transparency is affixed to the phototropic sheet with its emulsion side facing away from the sheet. A glass cover sheet is placed over the transparency and the assembly is exposed to an activating light source to produce the desired masking image in the phototropic material. The cover sheet is then removed and the duplicating material placed in contact with the emulsion side of the transparency. The duplicating exposure is then made from the opposite side of the phototropic material, the exposing light passing through the mask first, and then the transparency to the copy. Because of the position of the mask, the masking effect may be more diffuse than in the previously described applications, but as indicated, this is a desirable effect.

Due to the rapid density changes possible in the phototropic material when activated by high intensity ultraviolet radiation such as obtained with a capacitor discharge light, the present process is particularly adapted to the masking of motion picture film. The process could be utilized for the high speed printing of motion picture film master prints to release copies color negative to color positive, black and white negative to positive, color reversal to color reversal and black and white reversal positive to reversal positive. It could also be used when film is enlarged or reduced in size. Registration, which has been a problem in masking motion picture film, is not a problem with the present method since the mask remains in a fixed position relative to the master after the masking image is secured to a shaft 94 which is supported by and driven at a constant speed by gear motor 96 mounted on supporting frame 98. A film supporting sheel rim 100 made of a phototropic material includes a flange portion 102 which is secured, to the sheel 92 by bolts 104. A cylindrical film supporting surface 108 on the rim includes

shoulders

110 and 112 respectively along the inner and outer surface edges to guide the film along a predetermined path on the rim.

In this instance, a master color negative film 114 to be printed is led from reel 1 16 over a guide roll 1 18 and through a density sensing device 120 which senses the density extremes. The need for a mask is proportional to these extremes. The density information for each frame is briefly stored in a memory device 122 which is coupled to and directs a high intensity ultraviolet source 124 positioned closely adjacent the wheel rim 100. The film 114 after passing through the density sensor 120 passes around the guide roll 126 and'onto the surface 108 of the wheel rim with the emulsion side facing away from the wheel surface. As the film passes beneath the high intensity light source 124, the source is energized to an intensity proportional to the film density at that frame to expose the phototropic wheel rim thereby creating a masking image in the rim portion beneath the surface 108. A film 130 to be exposed to the duplicate image is supplied from a reel 132 and passed around a guide roll 134 which leads the film in superposed relation to the film 114 on the surface 108 with the emulsion surface thereof against the emulsion surface of the film 114. As the wheel rotates, the films are moved at a predetermined uniform rate past the read, green and blue film exposure lights 136, 138 and 140 which are mounted on the stationary bracket 142 positioned radially interiorly of the rim portion 100. The exposure of the film 130 is thus made through the masking image in the rim 100 and the negative film 114. The exposed film 130 is led around the guide rolls 144 and 146 and is collected on the spool 148. The negative film 114 is similarly led around guide roll 150 and collected on the spool 152. Suitable means are provided to control the rotation of the

spools

116, 132, 148 and 152 in accordance with the speed of movement of the films with respect thereto.

The size of the wheel 92 is selected so that the masking image formed in the phototropic material of which the wheel is made will fade completely away in one wheel revolution. Should this not prove feasible, heat or a light source of relatively long wave length can be employed to accelerate the erasure of the masking image.

In FIG. 11 a modification of the apparatus of FIGS. 9 and is illustrated. In this embodiment, instead of the phototropic wheel rim 100, a rim 160 of clear glass extends as an integral part of the glass wheel 162. The masking function in this embodiment is carried out by a thin film 164 of phototropic material which is introduced onto the wheel rim before and removed after the film 114. The phototropic film should be at least as long as the film being reproduced and accordingly there will be no problem of the fading or erasing of the masking image which can be accomplished after the reproduction exposure has been completed in any convenient manner. The operation of the modified embodiment of FIG. 11 is essentially the same as that of FIGS. 9 and 10.

It will be apparent that the wheelarrangement illustrated is not essential to the method described and that the several exposing steps can be carried out utilizing a linear movement of the juxtaposed films. For example, the masking may be accomplished by moving the master film together with a phototropic mask forming film past a series of slot gates, the first sensing the density of the mask necessary, the second exposing the phototropic film through the master, and the third gate or gates exposing the copy through the mask and master.

In three light color printing systems the density of the mask appropriate to each exposing color could be sensed and used to print that particular color. Three separate masks of course could not be employed with either of the wheel type arrangements illustrated but could readily be adapted to the linear slot gate system described above. In using separate masks for each color it may be possible to utilize phototropic materials which upon activation develop colored images.

A further application of the invention is in the socalled camera back masking process. Typically carried out in a process camera, the phototropic mask is exposed to the photographic image entering the camera prior to insertion of the film which is placed directly in contract with the phototropic masking material for the film exposure.

The present invention would be particularly suited for the production printing of color prints from amateurs color negatives. Each negative would be automatically scanned to measure the extremes from light to dark and, if such scanning determined that masking would make a better print, a phototropic mask is exposed immediately prior to the print exposure (or a steady state density mask exposed during the print exposure), the intensity of the activating light being controlled by information provided by the scanning sensors. If the photographic materials are sensitive to the ultraviolet light which is preferably used to expose the mask, the undersurface of the mask could be coated with an ultraviolet absorbing filter which would not pass the ultraviolet through to the photosensitive emulsions.

If the density achieved in a given phototropic material is not adequate to serve the desired masking function despite variations in activating light intensity, a phototropic masking sheet which achieves a different density may be used. In a commercial operation, an operator might for example have four or five phototropic sheets available which would be graded in terms of their attainable density range.

As indicated above, it may be possible to utilize the phototropic mask to increase image contrast by utilization of phototropic materials which become lighter in those areas exposed to the activating light source.

It will be clear from the foregoing that the use of phototropic masks is equally applicable to black and white or color negatives as well as transparencies (color positives). Accordingly where the term negative" is used, it should be understood that the process involved is equally applicable to positive and negative images, color or black and white. With a negative, the phototrophic mask will function as a shadow mask to hold back the darker areas of the images, while with a transparency the mask will function as a highlight mask to add density to lighter areas of the image.

Manifestly, changes in details can be effected by those skilled in the art without departing from the spirit and the scope of the invention.

I claim:

1. Apparatus for masking motion picture film and the like comprising means for advancing the film successively through first, second and third stations, the first station comprising density sensing means for sensing the image contrast range of each frame of the film, the second station comprising a mask forming station, said mask forming station comprising means for projecting the images of said film onto a phototropic material contiguous said film to activate said phototropic material to form masking images thereon, said latter means being controlled by said density sensing means to provide masking images commensurate with the film image density, the third station comprising a printing station, means for presenting a photosensitive material to said film for passage through said printing station on the opposite side thereof from said phototropic material, and means in said printing station fro exposing said photosensitive material to light passing through said phototropic material and the' film whereby masked images are presented to said photosensitive material.

2. Apparatus for masking motion picture film comprising means for advancing the. film successively,

through first, second and third stations, the first station comprising density sensing means for sensing the image contrast range of each frame of the film, the second station comprising a mask forming station, means for presenting a sheet of phototropic material to said film for passage through said mask forming station in contact therewith, said mask forming station comprising means for projecting the images of said film onto said phototropic sheet to activate said sheet to form masking images thereon, said latter means being controlled of phototropic material compr1ses a film having a by said density sensing means to provide masking images commensurate with the film image density, the third station comprising a printing station, means for presenting a photosensitive film to said film for passage I through said printing station in contact therewith on the opposite side thereof as said phototropic material, and means in said printing station for exposing said photosensitive film to light passing through said phototropic material and the film whereby masked images are presented to said photosensitive film.

3. Apparatus as claimed in claim 2 wherein said phototropic material characteristically achieves a steady state density proportional to the intensity of the activating means.

Apparatus as claimed in claim 3 wherein said means in said mask forming station for activating said sheet comprises a light source variable in intensity, the intensity thereof being varied by said density sensing means in accordance with the film image density.

5. Apparatus as claimed in claim 4 comprising a memory device operatively connected with said density sensing means and said light source to correlate the density reading with the mask formation in accordance with the rate of advance of the film and phototropic material.

6. Apparatus as claimed in claim 4 wherein said phototropic material is subject to activation by ultraviolet light, and wherein said variable intensity light so rce com rises a urce f ult avi let li t. Appar tus as claimed i n claim fwher eiin said sheet length at least as great as the film to be masked.

8. Apparatus as claimed in claim 4 wherein said phototropic material defines a cylindrical surface, and means for rotating said surface at the same speed as

Claims

2. Apparatus for masking motion picture film comprising means for advancing the film successively through first, second And third stations, the first station comprising density sensing means for sensing the image contrast range of each frame of the film, the second station comprising a mask forming station, means for presenting a sheet of phototropic material to said film for passage through said mask forming station in contact therewith, said mask forming station comprising means for projecting the images of said film onto said phototropic sheet to activate said sheet to form masking images thereon, said latter means being controlled by said density sensing means to provide masking images commensurate with the film image density, the third station comprising a printing station, means for presenting a photosensitive film to said film for passage through said printing station in contact therewith on the opposite side thereof as said phototropic material, and means in said printing station for exposing said photosensitive film to light passing through said phototropic material and the film whereby masked images are presented to said photosensitive film.
3. Apparatus as claimed in claim 2 wherein said phototropic material characteristically achieves a steady state density proportional to the intensity of the activating means.
4. Apparatus as claimed in claim 3 wherein said means in said mask forming station for activating said sheet comprises a light source variable in intensity, the intensity thereof being varied by said density sensing means in accordance with the film image density.
5. Apparatus as claimed in claim 4 comprising a memory device operatively connected with said density sensing means and said light source to correlate the density reading with the mask formation in accordance with the rate of advance of the film and phototropic material.
6. Apparatus as claimed in claim 4 wherein said phototropic material is subject to activation by ultraviolet light, and wherein said variable intensity light source comprises a source of ultraviolet light.
7. Apparatus as claimed in claim 4 wherein said sheet of phototropic material comprises a film having a length at least as great as the film to be masked.
8. Apparatus as claimed in claim 4 wherein said phototropic material defines a cylindrical surface, and means for rotating said surface at the same speed as that of said film advancing means.