US2985086A - Photographic printing - Google Patents

Description

nited States Patent O PHOTOGRAPHIC PRINTING Dwin R. Craig, Falls Church, Va., assignor to Logetronics, Inc., Alexandria, Va., a corporation of Delaware Original application June 17, 1957, Sei. N0. 666,124,

now Patent N0. 2,921,512, dated Jan. 19, 1960. Divided and this application Nov. 25, 1959, Sex. N0. 855,380

8 Claims. (C1. 9573) This invention relates to photographic printing and particularly to methods for produeing photographic prints substantially automatically.

Obtaining a good print from a negative has traditionally been a matter of trial and error, based on the best judgment f highly experienced laboratory technicians. It is the purpose of this invention to eliminate completely the need for such judgrnent, by incorporating in an automatic contact printer not only a means for evaluating negatives but also additional means for properly eXecuting the exposure. These evaluations and execution functions are performed simultaneously by means cf feedback from a plurality of light sensing devices to a plurality of light sources, by virtue of which it becomes possible for the first time to print all black and white negatives on a single type of conventional high contrast printing material. Since all aspects of the final print are determined during exposure in the printer, it is also possible in accordance with the present invention to utilize a standardized developing technique regardless of the character of the negative being printed. The combination of standardized printing material, automatic contact printer, and standardized developing techniques, therefore rednces the production of high quality prints from a variety of negatives to a matter of routine. Exposure of the print is obviosuly the key to such an operation.

The methods contemplated by this invention combine automatically the several functions of dodging, choosing exposure level, and selecting the final print contrast. In quality printing, dodging has always been necessary 10 overcome the inherently short exposure scale and nonlinear reproduction characteristics of printing papers; choice of exposure level has been achieved in the past by selecting an exposure time corresponding to the average density of the negative being printed; and the determination of final print contrast has been eifected previously by the selection of a printing material whose exposure scale would equal that of the negative. Since all of these functions, or their equivalents, still appear to be necessary in quality printing, it is among the objects of this invention to provide methods and apparatus Whereby they will be performed automatically, and by virtue 0f which their combined eifect permits all negatives to be printed on a single emulsion, to be followed by a standardized development.

According to this invention, duration of the exposure is determined by a phototube and integrating circuit, by which light which has penetrated both the negative and photosensitized paper or other carrier Will be integmted with respect to ti1ne. When the value of integrated light reaches a predetermined level corresponding to the speed of the emulsion, an electronic switch operates to terminate the exposure by extinguishing or otherwise rendering ineffective the exposing light. Consequently, variations between negatives and variations in average light level during exposure do not aiiect final print density.

Automatic dodging is performed by using a cathode ray tube as the printing light source, and employing a second phototube for continuously measuring the brightness of the scanning spot after it has penetrated both the negative and the photosensitized paper. Variations in signal level to control brightness of the cathode ray tube are fed back inversely from the phototube so that the scanning spot becomes brighter for denser areas in the negative and dimmer for thinner areas. The amount of in- Verse feedback applied is a function of density variations within the negative and is automatically controlled to permit residual excursions in brightness at the phototube which correspond to the exposure scale of the printing material. Extreme subject contrast can produce negatives of corresponding extreme contrast which may fall beyond the limits of the dodging circuitry and according to this invention the existence of such a negative is recognized by its peak to peak amplitude in the dodging channel, whereupon the automatic contrast control becomes eliective. This automatic contrast control eomprises a second light source arranged to superimpose a uniform layer of non-image forming light on the emulsion of the printing material, which is etfected in a contact printer by transmitting light to the emulsion through the paper or other base of the printing material. This combined exposure to image forming light and non-image forming light causes a reduetion in contrast similar to that produced by flashing as used in the graphic arts, or similar to the reduetion in contrast encountered in aerial photography by the interposition of haze between the ground and the carnera.

A more complete understanding of the invention will follow from a description of the accompanying drawings Wherein:

Fig. 1 is a schematic circuit diagrarn depicting a contemplated forma of apparatus; and

Fig. 2 is a curve which will assist an understanding of the prineiples of the invention.

Referring to Fig. 2, there is depicted a curve showing the H and D characteristics of a typieal hotographie printing emulsion, and illustrating the combined etfect of the three automatic exposure functions contemplated by this invention. As commonly designated in this art, E represents exposnre and log E symbolizes the logarithm thereof, The speed of the photosensitive emulsion is defined as the value of log E necessary to produce a middle density in the final print. The exposure scale of the photosensitive print material is defined as the difference in log E which can be reproduced on the substantially linear portion of the D, log E characteristic. As illustrated in Fig. 2, the difierence in transmission of the negative between its highlight and shadow areas (S-H) exceeds the scale of the printing material. Consequently, in ordinary printirig, both the shadows and highlights would be reproduced on the flat portion of the D, log E curve, highlight contrast H would be reduced to print contrast H (underexposed) and shadow contrast S would be reduced to print contrast S (overexposed). With automatic dodging however, more light is passed through the dense areas of the negative and less light is passed through its thin areas, producing an apparent compression of the negative which brings the values of S and H closer together on the log E scale, reprodueing the highlights in the print as H and the shadows as S Highlight contrast H is here defined as density variation within an area smaller than the scanning spot but in a region of high density within the negative, and shadow contrast as density variation within an area smaller than the spot but in a thin region of the negative. These definitions provide the basis for a distinction between detail contrast separation. Accordingly, it can be seen that automatic dodging tends to reduce gross contrast, S-H, while at the Same time maximizing detail contrast, observing that S is greater than S and H is greater than H Whereas a scanning spot can exert no explicit influence on detail smaller than its 'own dimensions, it is nevertheless able to exert implicit control of detail contrast by altering its position on the D, log E curve. As clearly shown in Fig. 2, shadow contrast S is nearly equal to negative contrast in the shadows S, where as highlight contrast H in the print is still lass than highlight contrast in the negative H. This situation represents the case where gr'oss contrast in the negative exceeds that which can be controlled by automatic dodging. T correct this Situation, the printer must automatically inject some non-image forming light to further reduce contrast in the final print. If. we assume that the relative intensity of light passing through S and H is expressed by the ratio l, by the addition of one unit of non-image forming light, the new ration becomes 11 2, representing a substantial r eduction in contrast, as required. The shadow exposure on the other hand, was increased from 10 to 11, representing an increase in this value of only 10%, as compared with an increase of highlight exposure from 1 to 2, or 100%. Consequently, print contrast in the shadows S is relatively unalfected, where as print contrast in the highlights has been increased from H t o H which in turn is now equal to highlight contrast in the negative H. Frorn the foregoing it is apparent that a combination of the three functions described is capable of altering the apparent contrast of the negative, both gross and detail, to fit the exposure scale of the printing material. EX- posure duration determines average print density, automatic dodging reduces gross contrast and maximum detail contrast, and superposition of non-image forming light further reduces both gro ss and detail contrast. It is therefore necessary to choose a printing material With sufficiently high gamma to accommodate the flattest negative which will be encountered in practice, whereupon all other negatives will be reduced to this same apparent contrast.

As depicted in Fig. 1, the paper or other support 10 is coated with an emulsion providing a photosensitive surface 12 and is placed in intimate contact With a negative or other transparency 14 to be reproduced and thus supported by conventional means. The primary light source is a conventional cathode ray tube 16 provided with a screen of fluorescent material 18 which emits actinic h'ght when bombarded by a scanning beam of electrons from its cathode 20 under the control of one or more control electrodes 22. The resulting spot of light at the face of the tube is projected by means of a Iens 24 through the negative 14 exposing the photosensitive surface as it scans the area to be printed. The size of the projected spot depends on not only the electronic focus within the tube but also the optical focus and scale of magnification between the face of the tube and the negative. With perfect optics, the ratio of spot diameter to format dimensions Will always be the Same at the negative as at the face of the tube. In practice, this will produce a relatively unsharp dodging pattern as compared with the sharpness 'of images generally 'recorded on photographic materials. A spot having a diameter of approximately Ms covering a format whose dimensions are, say, 8" by '10", can be iormed at a brightness level sufficient to represent practical printing speeds.

A phototube 26 is exposed to light penetrating the support 10 through a shield 29 containing an aperture 31 which restricts the field of view to a portion only of the format. This phototube 26 is D.C. coupled by a lead 28 to an integrator 30 which is connected by a lead 32 to a triggered switch 34 to which a source of adjustable Speed reference voltage 36 is also connected by a lead 38. The triggered switch is in ti1rn connected by a lead 40 to the control electrode means 22 for terminating' the exposureiwhen the integrated l ight ac hieves a predeterrnined value. During the exposure, the pulsed light and D.C. eomp onent of light sensed by the phototube 26 are combined to discharge a capacitor constituting a part of the integrator 30. Charge removed -from this capacitor corresponds to the number of photons received by the phototube 26 which in turn corresponds to the amount of light incident on the ortion of the area of the printing material to which the phototube is exposed. When the voltage on the integrating capacitor has dropped from a fixed preset value to a reference value manually selected by adjustment of the speed referenc 36, the electronic switch 34 is triggered. The manually inserted voltage reference corresponds to the speed of the printing material consituting the photosensitive surface 12. This in turn' corresponds to the amount of light which is necessary within the field of view of the phototube 26 to expose the printing material to a mid-tone of grey, and preferably the field of view of the phototube 26 can be manually directed by the operator to' aim at any selected center of interest of the picture. This selected region Will always receive the optimum exposure and Will usually be bracketed by other areas of extreme highlight or shadow. When the electronic switch 34 is triggered by the integrator 30, a signal is transmitted through the lead 40 to the control grid 22 of the cathode ray tube 16 to terminate the exfposure.

During the exposure period, a separate phototube 42 is provided in the path of light penetrating the support 10 to perforrn the functions of automatic dodging control and automatic contrast control. The signal from the phototube 42 which views the entire format, is passed through a lead 44 -to a logarithmic network 46. This network converts relative transmission of light into the logarithm of relative transmission providing a more convenient notation for relating subsequent control functions to the D, log E characteristics of the photographic emulsion; it perrnits the system to cope With large variations in light level, without producing voltage limiting in the electronic circuitry; and it tends to provide a constant loop gain for signals representing variations within the negative, even though the absolute light level may be altered drastically due to changes in lens aperture, average negative density, or transmission of the printing material.

The signal from the logarithmic network 46 is fed through a lead 48 to an A.C. amplifier 50 where it is inverted, amplified, and fed through a lead 52 back to the grid 22 of the cathode ray tube 16. Response of this feedback loop is rapid compared to the velocity of the spot in order to establish instantaneous and continuous control of spot brightness. I.f this loop were allowed to operate at inverse feedback at which a change in spot brightness exactly matches and cancels a change in negative transmission, the gross contrast in the final print would be reduced to zero. Such prints are highly informative in that they provide a maximum of detail contrast throughout, a real asset to doc'umentary photography such as aerial and scientific, but they are not pleasing from a pictorial standpoint. T0 avoid this severe reduction in gross'contrast, it is necessary to reduce the value of loop gain and allow some variation in brightness at the phototube 42.

The signal from the logarithmic network 46 is also fed through a lead 54 to a eak to peak 'detector 56, such peak to eak signal, Without feedback, representing the scale of the negative which is represented in Fig. 2 as SH. A manually inserted scale orvoltage reference 58, connected to the peak to peak detector 56 by a lead 60 corresponds to the exposure scale of the Photographie emulsion 12. When the peak to peak signal exceeds this reference voltage, a signal is passed along a lead 62 to the A.C. amplifier,increasing its gain frorh Zero to a positive value, which in turn provides inverse feedback via the lead 52 to the control electrode 22 ofthe cathode tube 16. When, after'a' few'swee'psfthis in- Verse feedback has reduced the peak to peak Signal back to the value of the scale reference 58, the gain of the A.C. amplifier is then allowed to drift back towards zero. Following this, the peak to peak signal recovers, once again introducing inverse feedback to the grid 22. This type cf hunting action thus maintains the peak to peak signal at a level corresponding to the scale of the emulsion.

When a negative is encountered whose density scale exceeds that which can be brought within acceptable range by the dodging control, it becomes necessary to introduce suitable contrast control in addition. A secondary light source 64 is arranged to illurninate the format uniforrnly with actinic light, passing through the back Of the paper to the emulsion 12. The brightness of this light is controlled by the value of the peak to peak voltage frorn the phototube 42 and the logarithmic network 46. An argon glow larnp is suitable for this application since it provides highly actinic emission and is readily controlled by 10W power electronic circuitry.

A contact printer embodying these features can be combined With mechanical rneans for positioning negatives and paper automatically so as to be capable of operation without supervision or need for operator decisions. After an initial calibration to establish the references for speed and scale of the printing emulsion, no further attention would be required, even though the negatives to be printed varied as widely as those encountered in ordinary amateur photography.

T0 illustrate the three exposure functions and their interdependence, it will be appropriate to consider the cause and effect of three general types cf negatives which might be encountered. For convenience, the three classes will be designated respectively, thin, normal, and contrasty. The thin negative is usually the result of either underexposure or underdevelopment. lf underexposure, it will have 10W contrast, being reproduced on the toe of the characteristic curve of the negative material. If thin because of underdevelopment, it will likewise exhibit 10W contrast because of the developing characteristics of films. In either case, both the gross contrast and the detail contrast will be 10W. Therefore, the phototube 42 will sense very little peak to peak signal and there would be little, if any, inverse feedback for dodging. Furthermore, the peak to peak signal would fail to exceed the scale reference 58 and would produce no light from the secondary source 64 for contrast control, as result of which, the print produced would be characterized by maximum contrast.

The normal negative usually exhibits gross contrast which exceeds that capable of reproduction by high contrast printing materials. Consequently, during exposure the peak to peak signal would exceed the scale reference and would produce inverse feedback for dodging, in which case the amount of dodging would be just suflicient to prevent either the shadows or highlights from being reproduced 011 non-linear portions of the curve.

Contrasty negatives are usually the result of subject matter possessing a high degree of contrast, such as those illuminated by bright sunlight or photo flash sources, but they may be the resu1t of overdevelopment of the film. In either case both the detail contrast and the gross contrast would be extremely high. During the printing operation according to this invention, the peak to peak signal would exceed by far the scale reference tending to produce inverse feedback approaching 100%, a condition which is seldom if ever achieved due to prac- 6 ticai limitations. Under these conditions however, the contrast control would be operating to energize the secondary light source 64 and thereby reduce not only the gross contrast but also the detail contrast to a point where the image could be reproduced by a standardized type of photosensitive coated paper made possible by this invention.

Whereas only one form of the invention has been shown and described, it should be construed as illustrative and should not be restrictive beyond the scope of the appended claims.

The apparatus disclosed herein has been claimed in application Serial N0. 666,124, filed Inne 17, 1957, now Patent N0. 292l5 12, dated Jan. 19, 1960, of which this is a division.

I claim:

1. A hotographie printing method comprising ener gizing a cathode ray tube to direct a light spot of finite area through a transparency in one direction on a photosensitive surface to be exposed, photoelectrically sensing the intensity of light from said tube through said surface, photoelectrically controlling the exposing effect of said spot jointly as an inverse function of the sensed intensity and as a function of the density scale of said transparency, and controlling the duration of exposure cf said photosensitive surface as a function cf the total light impinging on said surface.

2. A hotographie printing method according to claim 1 wherein said spot is controlled as a function of the peak to peak values of intensity sensed photoelectrically through said photosensitive surface.

3. A hotographie printing method according to claim l wherein said photosensitive surface is exposed to light from a source whose intensity is a function of peak to peak values of intensity sensed photoelectrically through said photosensitive surface.

4. A photographic printing method according to claim 3 wherein said spot and light from said source are directed oppositely towards said photosensitive surface.

5. A photographic printing method according to claim 1 wherein said duration of exposure is controlled as a function of the total light impinging on a fractional portion of said surface.

6. A photographic printing method comprising energizing a scanning light source to direct a light spot o! finite area through a transparency in one direction on a photosensitive surface to be exposed, photoeleetrically sensing the intensity of light from said source through said surface, flooding said surface with actinic light from a second source, controlling the intensity of light from said second source as a function of the degree of contrast of said transparency, and controlling the exposure of said photosensitive surface as a function of aggregate light impinging on said surface.

7. A photographic printing method according to claim 6 wherein the amount of light frorn said second source is controlled as a function of the peak to peak value of said sensed intensity.

8. A Photographie printing method according to claim 6 wherein the exposure of said photosensitiva surface in terminated as a function of the integrated value cf said sensed intensity.

References Cited in the file of this patent UNITED STATES PATENTS 2691917 Curry Oct. 19, 1954

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