US1990396A - Method and apparatus for half-tone, color, or similar photography - Google Patents

Description

Feb. 5, 1935. E. s. BALLARD 1,990,396

METHOD AND APPARATUS FOR HALF-TONE, COLOR, OR SIMILAR PHOTOGRAPHY Filed July 31, 1930 2 Sheets-Sheet l 7L 27 17 "16/ 12 r t 16' 1/ I g J2 J1 Z9 swam 9430c Feb. 5, 1935. I E, s BALLARD 7 1,990,396

METHOD AND APPARATUS FOR HALF-TONE, COLOR, 0R SIMILAR PHOTOGRAPHY Filed July 31, 1930 2 Sheets-Sheet 2 EA. Zia/lard Patented Feb. 5,, 1935 PATENT OFFICE METHOD AND APPARATUS FOR HALF-TONE, COLOR, on SIMILAR PHOTOGRAPHY Ernest S. Ballard, Berkeley, Calif., assignor to Standard Process Corporation, Chicago, 111., a corporation of Delaware Application July 31, 1930, Serial No. 4'l2,075

7 Claims.

This invention-relates to a method and apparatus for substantially insuring accurate results in the photographic preparation of reproductions of copy for use in the preparation of the plates or blocks employed in photo-engraving, either planographic or intaglio in character. The invention is particularly adapted for successful application to the preparation of half-tone or similar reproductions and lends itself admirably to the reproduction of accurate photographs of vari-colored copy, each color of which reflects light of varying intensity.

In half-tone or similar photography numerous factors of a .variable character necessitate adjustments and changes in the steps essential to the successful reproduction of copy for engraving purposes. The most diflicult variable to compensate for-is the intensity of the light entering the camera. Such variations may be the result of changes in position of the light source or sources which in half-tone work are generally readily movable. The light source or sources also vary because of inherent structural characteristics thereof. Arc lamps are frequently used for this purpose, and, as is well known their intensity varies from time to time in actual use. The latter factor is ordinarily very difllcult to properly compensate for. Another factor causing variation inthe intensity of the light entering the camera arises from the fact that the copy reflects light from the source or sources into the camera, and the amountof light reflected varies widely with the nature of the copy. Somecopy is bright with exceedingly white high lights. Other copy, as in the case of matt-surface photographs, is low in reflecting quality. In color work involvingthe use of filters and requiring a multiplicity of exposures, of the copy the ratios or relative light intensities of the reds, blues and 0 yellows in the copy are generally only very ap proximately known, and so far as known no etfort has heretofore been made to determine accurately such ratios. The latter vary, as a matter of course, in various specimens of color work dependent upon their nature and origin.

The variability in the amount of light entering the camera from the above causes is understood by skilled photographers and such operators attempt to minimize the effect thereof upon reproductions intended for photogravure or similar purposes, by the exercise of Judgment. operators can compensate to a degree for differencesin copy and'variations in light intensity from other causes, though the results by such hit or miss methods are by no means certain Skilled Another cause for changes in the intensity of light entering the camera arises from the need for making enlargements or reductions of copy in the same apparatus. As is well known, therelative size'of the reproduction compared to the copy is determined by the relative distance of'the lens from the plate. Inasmuch as the light afiecting the plate varies inversely with the square of the distance of the lens, through which it enters the camera, from the plate, it is apparent that when such distance is varied adjustment must be made of the lens aperture or stop in order that light of the same intensity may affect the plate irrespective of the camera extension, which determines the distance of the lens from the plate. Heretofore, it has been the general practice to resort to calculations or the use of tables or slide rules to mathematically determine the correct lens opening for an enlargement or reduction and while the results obtainable by this method are generally more satisfactory than the results that can be secured by the hit or miss methods employed in an eflort to compensate for the above referred to empirical variations in light intensity, they require mental effort and involve the ever present liability of error.

The primary object of this invention is to provide an accurate method and apparatus for insuring that the correct amount of light shall atfeet the photographic plate irrespective of all. of theiabove stated conditions that tend to cause a variance in light intensity, and to accomplish this purpose without the use of elaborate computations and without relying upon the hit or miss methods of determining light intensity heretofore generally used. 4

A further object of the invention is to provide a method and apparatus for making halftone' or similar reproductions for photogravure purposes utilizing a photo-electric cell, which exhibit a sensitivity to variations in light intensities approximately twenty times greater than the human eye, for the purposes of determining with accuracy such important elements in half-tone work as screen distance, stop size, the condition of the sources of light and the correct time of each exposure regardless of variations in conditions of a character not readily observable by even the most skillful operators. 7

A still further object of this invention resides in the provision of a method and apparatus at fording an adjustable standard of light intensity with which the light intensity under operating conditions may readily be compared, and corrections made to compensate for variations from said standard utilizing the sensitive photo-electric cell to measure such variations.

Another object of the invention isito provide a. method and apparatus for determining the proper stop size or screen distance in the production of half-tone reproductions utilizing a photo-electric cell to this end.

A still furtherbbject of the invention is to provide a method and apparatus for use in half-tone or similar work utilizing a standard light intensity for comparison with the intensity of the source or sources of light used in the normal operation of the apparatus, and employing a photo-electric cell to make the comparison between said standard light and the normal light source or sources.

A still further object is to provide a simple method of making half-tone or similar repro- 'ductions in which the period of exposure is corrected in accordance with any variations from a standard of the light conditions affecting the photograph to the end that the stop opening may remain at the point'of adjustment thereof necessary to secure the best results. 9

Still further objects of the invention will appear as a description thereof proceeds, with reference to the accompanying drawings in which:

Figure 1 is a horizontal sectional view through a camera designed for making half-tone or similar reproductions in accordance with this invention.

Figure 2 is an enlarged horizontal section through the pilot lamp housing applied by the camera extension.

Figure 3 is .a fragmentary horizontal sectional view on an enlarged scale showing. the manner in which'the photo-electric cell is slidably mounted in the camera.

Figure 4 is a side elevation of the parts shown in Fig. 3. I

Figure 5 is a diagrammatic view showing the interconnections between the diaphragm that controls the lens opening, and that which controls the intensity of the light entering the camera from the pilot lamp.

Figure 6 is a view of the focusing glass showing a picture in position thereon, and indicating in dotted lines the location of the electric cell with respect to a high light therein..

Like reference characters indicate like parts throughout the several figures;

Referring to Figure 1, a photograph enlarging camera of any well known form may be employed consisting of alens support 10 provided with lenses 11 of any well known form, and an iris diaphragm 12 of usual construction to control the lens opening. Connected to the lens support is the usual bellows 13 permitting the camera to be extended to secure the desired enlargement or reduction of the copy. A plate and focusing glass supporting frame 14is connected to the end of the bellows 13, that'is remote from the lens supporting frame 10. Said frame 14 supports a focusing glass 15 in well known manner. The frames 10 or 14 may be adjustably mounted ,on tracks 16 in any. known manner. Supported on the tracks 16, or'adjacent thereto, is a copy mounting board 1'7 upon which the copy to be reproduced is mounted, in accordance with well known practices. v,

A source or sources of lightL are disposed in front of reflectors 18 arranged adjacent each side of the copy board 17, the sources of light being usually arc lamps mounted so that their position with respect to the copy board-may be varied to suit the needs of the operator. The apparatus thereof. The sides of the frame defining said slot are provided with tongues 19 and 20, Figure 3, to serve as guides for a slidably mounted closure plate 21. Said closure plate is provided with extensions 22 and 23 extending above and below the "frame 14 so that in all positions of said slidably mounted-plate the frame is closed to extraneous light. A photoelectric cell support, preferably in the form of a hollow tube 24, extends slidably through an opening in theclosure plate 21. ',Tube 24 is turned upwardly adjacent its external end to provide a handle 25, the arrangement being such that the support 24 may be adjusted so as to vary the position of the photo-electric cell 26 carried on the end thereof. That is, so that said cell may be adjusted freely toward and from the closure plate 21, while the closure plate may be shiftedto anyone of a number of vertically'spaced positions in the slot defined by the tongues 19 and 20. By this arrangement the photo-electric cell 26. may be brought to any point in front of the focusing glass 15.

The photo-electric cell 26 is arranged in well known manner in circuit with a milliammeter I showing by the deflection of the indicator thereof the intensity of the light affecting said cell. Such photo-electric cells and indicators are in themselves well known and constitute no part of the present invention.

When the camera is used in making half tone reproductions from copy for any purpose, it is provided with a half-tone screen 27 indicated in Figures 1 and 3. Said screen is preferably ar-.

The lens support 10 of the camera is provided with a tubular extension forming a casing or housing 28 opening at one end into the camera and closed at its opposite end as indicated at 29. Disposed within the casing 28 is a pilot lamp 30. The portion of the casing containing the pilot lamp 30 is separatedfrom that immediately ad- .jacent the camera by means of a difiusing glass the vertical plane containing the iris diaphragm 12 of the camera, and so that the ring 36 control,-

ling the size of opening in the latter, is directly opposite the ring 35 controllingthe extent of the opening of the iris diaphragm 34 in casing 28.

The rings 35 and 36 are'interconnected for simultaneous operation by providing said rings with arms 37 and 38 respectively, (Figure 5). The arm 38 preferably is continued so as to form a pointer 39 moving over scale 40 supported in any suitable manner beneath the lens of the camera. The scale 40 carries indicia, as indicated in Figure 5, to show the size of the opening in the iris diaphragm 12. In order to insure the simultaneous operation of the diaph'ragms 34 and 12 as above stated, the arms 37 and 38 are interphragm 34 is arranged so that it is disposed in connected by a link 42 for a purpose hereinafter stated.

The apparatus just described is used in the following manner in making half-tone reproductions of copy with extremely uniform and consistent results. The copy is mounted on the board 17 in well known manner. and the camera is extended to give the desired enlargementor reduction as the ca:e may be. In half-tone work, for good results, long practice has shown that it is essential that a definite ratio be maintained between the lens aperture, the screen size or mesh, the screen distance from the plate and the lens distance from the plate, the latter factor being ordinarily termed the camera extension. If A represents the diameter of the lens aperture, B the diameter of the screen or mesh thereof, C the distance of the screen from the plate and E the camera. extension, thisrelation may be expresse in the form of a proportion, as follows:

From a consideration of this necessaryrelation, and bearing in mind that the screen size or mesh and the screen distance are in ordinary practices not varied, the only factors that may be varied are the lens aperture and the camera extension. But in order to preserve the relation essential to the attainment of the best results the lens aperture must always bear a definite relation to the camera extension. The latter factor is determined in each instance by the size of the enlargement or reduction as the case may be. Ac-

cordingly, for any particular enlargement or reduction three of the four factors become fixed and the fourth factor, that is, the lens opening may be computed. Heretofore tables or computations have been used to determine the correct lens opening. These are burdensome and this invention provides a ready means of avoiding their use.

The reason necessitating that the lens opening be varied with each variation in camera extension will be readily appreciated from a consideration of the fact that the intensity of light varies inversely with the distances. Accordingly when the camera is extended, unless there is a corresponding increase in the stop size the intensity of the light affecting the plate is out down propontionately as the lens is moved away from the plate. And since the area of lens opening varies with the square of the diameter. just as the area of the plate affected by the light varies with the square of the camera extension, any change in the lens opening that varies with the camera extension will result in a maintenance of the same light intensity per unit area of plate.

This invention provides a ready means of deter- 'mining the correct stop size or lens opening, withlamps L. The camera is then adjusted to give an image of the proper size and definition on the focusing glass. The photo-electric cell 26 is then adjusted'in front of the glass until it is arranged opposite a high light 41 of'the picture as indicated in Figure 6 of the drawings. The reading of the milliammeter is taken for the intensity of light that has been found to give the best results, and such reading is utilized as the standard. Now, assuming that the light remains invariable, and the camera extension is varied in accordance with the need for a reproduction of a different size compared to the copy, the operator can readily determine the correct stop 'size by simply reading his milliammeter and opening or closing the diaphragm until the correct or standard reading is obtained. Such opening or closing is readily accomplished by adjustment of the iris diaphragm. The operator is thus assured that light of the correct intensity will affect the plate and he need resort to no calculations, and need consult no tables. This method however, does not take into account the fact that the intensity of the source or sources of illumination will vary with changes in the position of the arc-lights, with their condition, and with the nature of the copy.

In order to enable correction to be made to take care of these variable factors, I provide the apparatus with the pilot lamp 30, which, in the operation just described, was not used. Such lamp is, preferably, a standard incandescent type of, say, watts power. Such lamps I find exhibit exceedingly small variation in intensity over relatively long periods of time and can, in practice, be used as a standard with which the arc lights constituting the sources of illumination may be compared from time to time, and as a standard for general comparison of the intensity of the light that enters the camera, which varies not only with the condition of the lights, but also with their position and with the nature of the copy, which reflects varying amounts of light into the camera. The iris diaphragm 32 is used to control the amount of light from pilot lamp that passes through the diffusing glass 31.

The diaphragm just referred to is adjusted so 'thatit will admit light of the same intensity as has been found to give entirely satisfactory results. This is accomplished by permitting the camera adjustments, the position of the lights and the copy,'to remain in exactly the condition at which they stood in obtaining a perfectly satisfactory reproduction of said copy. The photofor the attainment of uniformly satisfactory results, and the amount of light from the pilot lamp is now adjusted so as to provide the same intensity of illumination for future comparisons,

since as above stated the condition of the lights vary, their position is variable and the reflecting power of the copy may vary widely from time to time, and it is essential that such variations be readily determinable so that proper compensation may be made in the manner hereinafter pointed out.

With the camera adjustments the same the lens opening in the camera is now capped shutting off all lightentering the camera, and the pilot lamp is illuminated. In order to prevent inadyertent burning of the pilot lamp, it is connected in a circuit that is interlocked with the focusing glass frame, so that when the camera is loaded with a sensitive plate, and the focusing glass removed the pilot lamp circuit is certain to be open. Any suitable switch is provided to illuminate the iii) the opening in the diaphragm 34 will be exactly the same as the opening in diaphragm 12, since these diaphragms are interconnected by the link 42 as above described. It will be observd further that since the pilotlamp is carried by the lens support it is adjusted with the lens as the camera is extended, and that its position in securing the identical reading of the milliammeter just referred to, was determined by the previously made camera extension adjustment, which was preserved in securing such reading. After such reading is obtained under conditions just stated, the diaphragm 32 is locked in'its adjusted position, as by a set screw or in any convenient manner, so as to maintain the setting of diaphragm 32 that will give the reading of the milliammeter for the best results.

,The pilot lamp 30, by virtue of the fact that it is carried by the lens support, takes the place of the illuminated lens, and as it is moved away from the photo-electric cell the intensity of the light coming therefrom varies inversely with the distance just as it would if coming from the lens itself. To maintain the required unchanging intensity upon the cell the iris diaphragm 34, or the diaphragm 12, to which it is linked, are adjusted. Accordingly it is apparent that if the exterior lighting system were a fixed quantity, and the high lights of the copy always of the same reflecting power, light coming from without would always produce the same electrical reaction in the photo-electric cell as that coming from the pilot lamp, if the readings of said cell are taken separately.

.The method of establishing the correct lens aperture above described is very simple and direct, but-by the use of the pilot lamp, I am also able to readily correct the time of exposure of the copy in accordance with fluctuations in the light entering the camera, whatever the cause of such fluctuation. This is accomplished with the simplest possible manipulation in the manner now to be described.

It is well understood that, so far as half-tone photography is concerned, that for the best results the lens aperture should be maintained at a definite value for a given camera extension. The foptical relations within the camera determine the correct or best stop size, and in order to secure the best results when the degree of illumination varies, it is desirable to extend the time of exposure. This invention enables the ready determination of fluctuations in the degree of illumination and points out a simple way of correcting the time of exposure to compensate therefor.

To/the end of determining these fluctuations, copyj'is placed on the board 17 and such copy is focused to the correct size on the glass 15. Light is then thrown upon said glass by the pilot lamp 30 the camera adjustments just made being maintained. The diaphragm 34 is then adjusted until the milliammeter in circuit with the cell 26, which is positioned in front of glass 15, gives the reading that has been determined to represent the correct light intensity. When this reading is reached, the correct lens opening has been determined without calculations, but with certainty, And since the diaphragm 34 is directly linked to diaphragm 12, it will be understood that the operator, in adjusting diaphragm 34 simultaneously adjusts diaphragm 12 to the same extent. The camera lens opening is thus correctly ascertained and fixed.

The pilot lamp is then extinguished, the cap is removed from the lens, and thereading of the pointer 39 on the scale 40 is noted, so that the correct stop size may be kept in mind. Lightfrom the sources of illumination reflected from the copy again enter the camera and cause the filament of the cell 26 to emit electrons, and the milliammeter to show a reading. If there has been any change in the condition of the arc lights, their position, or if the copy presents an abnormal reflective value, the reading of the milliammeter been determined as the correct reading for light intensity ofthe correct value. Compensation for will vary from that reading which hasheretofore this condition cannot be made by increasing or decreasing the step size, since the correct lens opening is inflexibly set for proper half-tone reproductions by optical relations in the camera. As above indicated said condition is overcome by increasing the exposure time, and this invention provides a ready means to determine the increment of time by which the normal time must be increased, if the light intensity is below the proper value, to compensate therefor.

To determine this, when it is found that the light intensity is below the correct value the diaphragm 12 is opened temporarily until the milliammeter again shows the correct reading. -In opening the stop the pointer 39 has moved along the scale until it indicates a larger opening. If it be assumed that for the camera adjustment, the correct step size as determined by the pilot lamp is 18 millimeters, but that in ,order to secure a correct reading of the operating light sources, the lens opening had to be adjusted I have found that the equation' 5 :29 B3 X where A is the correct stop opening in millimeters, B is the stop opening required to maintain illumination of standard intensity from the copy under the action of the operating sources of light, TC. is the time constant in seconds, or the normal length of exposure for a light intensity "of standard value, and X is the time of exposure in seconds to a light having an intensity varying from standard intensity, gives a computed exposure time that is entirely satisfactory for exposures made under light conditions varying from standard; r 1 c Applying this equation to the specific example above referred to, and assuming that the correct exposure time for a light of standard intensity, or the time constant is 60 seconds then Solving this equation it will be found that a:==88 and a fraction seconds. The fractional second may be disregarded as too small to be of consequence, so that the correct exposure time in the assumed case is 88 seconds. A table or a graph may be employed to avoid the computation above indicated, if desired, so that the corrected time of exposure may be directly read therefrom.

The pilot lamp accordingly gives a ready means for comparison with the operating sources of light, shows whenone or both lamps are functioning improperly or are not properly positioned, indicates the relative reflective powers of varying kinds of copy, or of copy of varying colors, and finally gives a practical way of determining the time of exposure under externalconditions that vary widely from the standard. The application of the invention to half-tone reproduction work avoids the use of hit-or-miss methods commonly used by operators at the present time, and enables a relatively inexperienced photographer to secure uniformly excellent results.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description,

and all changes which come within the meaningand range of equivalency of the claims are therefore intended to be embraced therein.

What I claim as new and desire to secure by United States Letters Patent is:'- h

1. The method of producing half-tone or similar reproductions of copy for photogravure purposes, which consists in disposing a photo-electric cell in circuit with a current measuring instrument adjacent the focusing glass of the camera and the screen, adjusting said cell to a position opposite a high light of a picture to be photographed, subjecting said cell to and noting the reading of said instrument for a substantially invariable standard light intensity for said part of the picture, comparing the reading of the instrument when said part of the picture is subjected to the light to be used for said reproduction, and adjusting the lens opening until the reading of the instrument corresponds to the reading thereof for said standard light intensity.

2. The method of determining the reflective value of copy and the variations of light sources used in half-tone or similar photographic work,

which consists in subjecting the portion of the lieu of said light of standard intensity, measuring the intensity of said light at the same position in said camera, and noting the variations in said measurements.

3. A camera for making half-tone or similar reproductions including a focusing glass, a photoelectric cell and a slide carrying a shiftable cell support to adjustably mount said cell so that it may be vertically and horizontally shifted to any position in front of said focusing glass.

4. A camera for making half-tone or similar reproductions including a sensitive plate, a focusing glass, a support for the sensitive plate and focusing glass, a slide mounted in said support, a'photo-electric cell support slidably mounted in said slide, and a photo-electric cell carried by said last named support, whereby said cell may be shifted to any position in front of said focusing glass.

5. A camera for making half-tone or similar reproductions including a lens support, a lens, a focusing glass, and an adjustable diaphragm to control the lens-aperture size. a source of light carried by said lens support, an adjustable diaphragm to control the intensity of light entering the camera from said source, mechanism interconnecting said diaphragms for simultaneous adjustment, and means to measure and register the intensity of light at a point in the camera adjacent the position of the focusing glass.

a 6. A camera for making half-tone or similar reproductions, including a lens support, a lens, a

focusing glass, and an adjustable diaphragm to control the lens-aperture size, a source of light carried by said lens support, an adjustable diaphragm to control the intensity of light entering the camera from said source, mechanisms for interconnecting said diaphragms-for simultaneous adjustment, and a photo-electric cell adjustably .mounted in said camera close to said focusing glass so that it may be moved into any position adjacent the focusing glass thereof for measuring the intensity of light entering the camera through either of said diaphragms. Y

. 7. A camera for making half-tone or similar reproductions, including a lens support, a lens, a focusing glass, a source of light carried by said support, a photo-electric cell mounted close to said focusing glass for measuring the light entering said camera from said source or through the lens, and adjustable diaphragms interconnected for simultaneous movement to control the intensity of light entering the camera through said lens and from said source, whereby the lens opening of the camera may be adjusted by adjusting the diaphragm controlling the intensity of light from said source.

ERNEST s. BALLARD.

Images