CA1207374A - Contact photo printing device - Google Patents
Contact photo printing deviceInfo
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- CA1207374A CA1207374A CA000445042A CA445042A CA1207374A CA 1207374 A CA1207374 A CA 1207374A CA 000445042 A CA000445042 A CA 000445042A CA 445042 A CA445042 A CA 445042A CA 1207374 A CA1207374 A CA 1207374A
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- exposure
- light
- prescan
- density
- light source
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Abstract
ABSTRACT
Automatic control of exposure level and contrast correction for a photographic printer is disclosed.
The printer uses dynamic masking achieved by modulating the intensity and velocity of a CRT scanning spot. A prescan is employed to evaluate the negative and establish exposure level and contrast grade excursion limits for the subsequent exposure. Different control systems are disclosed for contact printing and projection printing.
Automatic control of exposure level and contrast correction for a photographic printer is disclosed.
The printer uses dynamic masking achieved by modulating the intensity and velocity of a CRT scanning spot. A prescan is employed to evaluate the negative and establish exposure level and contrast grade excursion limits for the subsequent exposure. Different control systems are disclosed for contact printing and projection printing.
Description
J 2~
This is a division of application No. 392,563, filed on December 17, 1981.
This invention is an improvement in scanning light source photographic printers used to make reproductions on film or paper from transparencies. Photographic prints are usually produced with the objective of employing the full tonal scale of the reproducing emulsion, in order to render a visually pleasing image. To this end, a variety of black and white photographic materials are available for choice, such as papers exhibiting graded or variable contrast, as well as long-scale, low-contrast or short-scale, high contrast films. Additionally, the contrast of such materials can be further controlled by appropriate adjustments to time and temperature during the processing development step.
The operation of print exposure determination is usually one in which the density range existi.ng in the original is matched, by some means, to the contrast limits of the reproducing medium so that the intensity-timè product (lxT=E) of light passing through the original exposes the desired high and low density points on the reproducing emuls]on. Pictorial and artistic photography, particularly as performed by professionals, usually allows for experimen-tal adaptation and the choice of emulsion contrast and exposure time. ~owever, in the fields of scientific, indus-trial, medical, military and photogrammetric photography, .~ ., .,, .... ~
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among others, it is impraetieal for the darkroom technieian to attempt to assess the individual characteristics of each original photograph in a situation where thousands of images may have to be copied uniformly.
Seanning light souree photographie reproduetion devicesl specifically those of the type called d-e coupled dodging printers, provide automatic exposure control by compensating for local density differenees in the Griginal transpareney. As the light source scans a particular point in the transparency, the optical transmission of that point is sensed photoelectrieally and the loeal intensity-time product of the printer exposure is eontrolled inversely.
An exeellent mathematieal analysis of dodging printers (also ealled luminous masking printers) is provided in U.S. Patent No. 3,400,632 to Wahli. ~s is noted by Craig, in U.S. Patent No. 2,921,512 assigned to the assignee of this applieation, reproduetions where the gross eontrast is substantially reduced as a result of luminous masking are frequently seientifieally informative but pietorially unpleasing. McIntosh, in U.S. Patent No. 4,265,532 also ; assigned to the assignee of this application, indicates that electronic dodging is a tone distorting reproduction process and describes a method enabling less than maximum dodging to be obtained.
Z5 In general, for the most pleasing appearance of the reproduetion, the degree of dodging seleeted should be that lowest amount which will enable all necessary informa-tion to be displayed with the greatest allowable tonal '~ 2 ~LZ~7'37~
separation, or contrast. Heretofore, such a criterion has required knowledge of the exposure range of the receiving emulsion, an individual examination of each transparency, classification of the type of photography, and recognition of the characteristics of the reproduction process, either by contact or by projection printing.
The examination of an individual black and white transparency should produce at least five elements of information:
1. DmaX
This is a division of application No. 392,563, filed on December 17, 1981.
This invention is an improvement in scanning light source photographic printers used to make reproductions on film or paper from transparencies. Photographic prints are usually produced with the objective of employing the full tonal scale of the reproducing emulsion, in order to render a visually pleasing image. To this end, a variety of black and white photographic materials are available for choice, such as papers exhibiting graded or variable contrast, as well as long-scale, low-contrast or short-scale, high contrast films. Additionally, the contrast of such materials can be further controlled by appropriate adjustments to time and temperature during the processing development step.
The operation of print exposure determination is usually one in which the density range existi.ng in the original is matched, by some means, to the contrast limits of the reproducing medium so that the intensity-timè product (lxT=E) of light passing through the original exposes the desired high and low density points on the reproducing emuls]on. Pictorial and artistic photography, particularly as performed by professionals, usually allows for experimen-tal adaptation and the choice of emulsion contrast and exposure time. ~owever, in the fields of scientific, indus-trial, medical, military and photogrammetric photography, .~ ., .,, .... ~
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among others, it is impraetieal for the darkroom technieian to attempt to assess the individual characteristics of each original photograph in a situation where thousands of images may have to be copied uniformly.
Seanning light souree photographie reproduetion devicesl specifically those of the type called d-e coupled dodging printers, provide automatic exposure control by compensating for local density differenees in the Griginal transpareney. As the light source scans a particular point in the transparency, the optical transmission of that point is sensed photoelectrieally and the loeal intensity-time product of the printer exposure is eontrolled inversely.
An exeellent mathematieal analysis of dodging printers (also ealled luminous masking printers) is provided in U.S. Patent No. 3,400,632 to Wahli. ~s is noted by Craig, in U.S. Patent No. 2,921,512 assigned to the assignee of this applieation, reproduetions where the gross eontrast is substantially reduced as a result of luminous masking are frequently seientifieally informative but pietorially unpleasing. McIntosh, in U.S. Patent No. 4,265,532 also ; assigned to the assignee of this application, indicates that electronic dodging is a tone distorting reproduction process and describes a method enabling less than maximum dodging to be obtained.
Z5 In general, for the most pleasing appearance of the reproduetion, the degree of dodging seleeted should be that lowest amount which will enable all necessary informa-tion to be displayed with the greatest allowable tonal '~ 2 ~LZ~7'37~
separation, or contrast. Heretofore, such a criterion has required knowledge of the exposure range of the receiving emulsion, an individual examination of each transparency, classification of the type of photography, and recognition of the characteristics of the reproduction process, either by contact or by projection printing.
The examination of an individual black and white transparency should produce at least five elements of information:
1. DmaX
2. Dmin
3. ~D=DmaX - Dmin
4. Davg = (Dmax + Dmin)/2
5- Dsum = l (Dl + D2 + D3 ... Dn) The ~D information must be compared to the emulsion exposure range so that when ~D is less, no dodging is necessary;
when ~D is slightly greater, some dodging is required;
and when ~D is considerably greater, maximum dodging may be needed. The Davg information represents the arithmetic average of the density e~tremes and it, or some function kDaVg of it, is useful when reproducing pictorial subjects.
Aerial photography, however, and various types of medical ~ and scientific photography, is reproduced best when DSum ; is employed, because all elements of the imagery may well - be of equal visual importance. Thus, the greater the number of points measured and averaged, the greater will be the probability that most photographic areas will be reproduced at or near their optimum exposure.
The Callier effect exerts an important influence over the contrast of the printing process used. Contact ~2~73~
printers require density measurements to be made by measuring the diffused light after it has passed through the negative.
Projection printers, however, require measurement of specu]ar or projected density in order to achieve correct contrast evaluation. Applying the diffuse density measuring technique to projection printing will result in invalid measurements.
Thus, it is necessary to use a different measuring technique for a contact printer than for a projection printer.
From all of the foregoing, it is clear that extraction of such a mass of image information by manual methods, although conceptually possible, is quite impracticàl, and that rapid preexposure evaluation of the applicable parameters of photographic transparencies re~uires recourse to an automatic device.
PRIOR ART STATEMENT
The prior art for the present invention falls into two distinct categories.
The first category includes traditional "white light" printers that prescan the negative for a variety of reasons, but are unable to alter exposure contrast.
Some of these printers modify the effective contrast range of the paper by "flashing" the paper prior to (or durilig) the main exposure, but the imagè contrast during exposure remains unchanged.
Examples of this type of reference are:
Biedermann et al, U.S. Patent No. 3,03~,400; Bickl et al, U.S. Patent No. 4,082,465; Knor et al, U.S. Patent No.
4,176,964; and Treiber, U.S. Patent No. 4,239,384. These ` 4 ~2'0 ~3~7~
references all teach prescanning of a negative before exposure.
Biedermann et al scan for the express purpose of measuring contrast and do determine an equivalent to applicants ~D
signal, but do not determine Dmin, DmaX~ or Dsum Biedermann, however, is unable to alter the contrast excursion range of the exposure because a conventional light source is used. Bledermann et al, therefore, depend upon pre-exposure or "non-image flashing" of the reproduction material to improve the subsequent exposure of detail in the toe region of the emulsion.
The second category of prior art includes electronic printers that employ dynamic mas~ing during exposure. These devices are able to atler the image contrast excursion range, but do not normally employ a prescan.
These re~erences may be further divided into two subgroups:
(a) Bestenreiner et al, U.S. Patent No. 3,984,187; and Kelly, V.S. Patent No. 4,145,709, wh:ich do employ a prescan;
and (b) McIntosh, U.S. Patent No. 4,265,532; Causley, British Patent No. 1,106,629; Elphick, British Patent No. 859,895;
and Nottall, British Patent No. 859 r 017, all of which disclose electronic print~rs without prescan. ~
Bestenreiner develops an "intensity indicating signal" that is used to avoid "stripes or striations" in the reproduced image. However, he does not develop the Dmin~ Dmax~ ~D, Dsum and DcOmb signals obtained by appli-cant's invention to assist in matching the exposure of the negative to the contrast grade and exposure index of the reproduction material.
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Kelly uses a prescan to achieve "automatic localized micro-contrast of incremental areas of negative images." He employs a multicontrast paper, and divides his density signals into categories which then cause changes in the spectral content of a CRT light source, thereby modifying the effective contrast of the reproduction material to meet the requirements of the image LO be reproduced.
Applicant's invention, to the contrary, modifies the exposure level and contrast excursion range of the image to be reproduced in order to achieve gross contrast control, and relies on the use of a single grade of paper (usually of a higher than normal contrast) for "micro-contrast" control.
Kelly's system employs a vidicon tube for scanning the image as it moves through the reading station, and a CRT for line exposure of the contact sandwich as it moves through the printing station. Applicant scans and prints "in situ" with a single CRT.
- Kelly does not determine a DSum or Dcomb as does applicant. Kelly determines Dmin and five density ranges, while applicant determines Dmin and DmaX for the negative and then calculates ~. The concept of Kelly's system is to fit the paper to the negative, while applicant alters the effective contrast range of the negative to fit the contrast acceptance range of the paper.
Kelly uses specular light for density scanning, and diffuse light for printing. Under certain conditions, the Callier effect will cause such a system to generate ~2~737~
erroneous density readings. Applicant uses identical illumi-nation for both reading and exposing.
Finally, there is one anomalous reference.
Neale, ~ritish Patent No. 906,146 uses an expensive "flood gun" type of CRT which prescans the negative to develop a luminous image "mask" which is applied to the image receiving material when the "flood gun" is activated. However, Neale does not compute Dmin, DmaX~ ~D, or DSum for use in alter-ing the various images in order to enable them to fit the brightness acceptance range of the printing material, as does applicant.
SUMMA~ OF THE INVENTIO~
It is one object o the invention ko measure, in a scanning electronic dodginy photoprinter and by means of a raster scan (hereafter called the prescan) which occurs prior to the exposing scan, the maximum and minimum densities present in the photography to be reproduced.
It is another object of this invention to obtain an integrated density signal (DSum) representative of all of the prescan densities measured in the photography to be reproduced.
It is a third object to derive a signal representing the difference between the maximum and minimum prescanned densities.
It is an additional object to employ the density difference signal to select an appropriate exposing system response.
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It is a further object to use an intensity-modulated beamed CRT spot as a part of the raster prescan density-measuring technique.
Another object is to enable correct density measurements to be obtained even though the prescan raster is adjustable in area.
Yet another object is to enable the prescan raster to be smaller in dimensions than the subsequent exposing raster so that adjacency effects, such as physical masking of the printing stage, will not interfere with the prescan readings.
It is a related object to combine the prescan measured densities Dmin, DmaXl and DSum in a manner such that these signal elements can be preerentially weighed lS and employed as an exposure level control in a wide variety o photographic reproduction techniques.
~et another object of the invention is to provide a method whereby the prescan occurs automatically, but only when required.
~ related general object of the invention is to increase the flexibility with which any spot-type scanning electronic dodging light source may be used in photographic reproduction processes.
All of these objectives, and others that ;will become apparent from the following description of the preferred embodiment, are achieved through electronic control circuits which inhibit normal photographic exposure during the information-extracting prescan operation.
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BRIEF DESCRIPTION OF THE DRAWINGS
Figures lA and lB form a block diagram of an IM-VM CRT Dodging Printer showing an embodiment employed for the reproduction of original radiographs in reduced form, in which the prescan circuits are placed in juxtaposi-tion to the exposing scan circuits.
Figure 2 is a partial schematic showing particular density-combining elements.
Figure 3 shows, in diagrammatic form, the disposition of rotary solenoid-actuated filters in the optical paths of a scanning electronic contact printer.
Figure 3A depicts the positions adopted by the ilters during the prescan mode.
Figure 3B illustrates the filter positions used during the exposing mode.
.
DESCRIPTION OF THE PREFERRED EMBODIMENT
It is well know that the cont~ol of printing contrast by means of electronic dodging tends to produce results that are scientifically superior in their information content, but pictorially unpleasing. McIntosh, in U.S.
Patent No. 4,265,532, ac~nowledges this deficiency and provides for its alleviation by means of a manually adjustable fine exposure level control, and the manual selection of dodging yrade.
In furtherance of these objectives, the circuits disclosed in Figures lA and lB, and in Figure 2, illustrate a method whereby each photograph to be reporduced .
~~
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is presca~ned automatically wlthin a time period of about 200 milliseconds or less. Where even this loss of time is objectionable or if the user prefers to make his own choice of exposure parameters, these automatic features can be disabled. The prescan action can also enable the user to manuall~ select the fine exposure level, while the dodging grade is determined automatically or, vice versa.
The present invention provides a prescan system in conjunction with a printer of the type disclosed in McIntosh, U.S. Patent No. 4,265,532. During the prescan`
the system evaluates the negative to generate a Dmin signal representative of the minimum density in the negativel a DmaX signal representative of the maximum density in the negative, and a QD signal representati~e o the difference - between Dmln and Dmax After QD has been determined by the prPscan, it is compared, in a dodging grade camparative circuit 50, to a preset value that corresponds to the contrast acceptance range of the image receiving media in the printer. The camparator circuit seeks a contrast excursion range that will cover the range from Dmin to DmaX~ The contrast excursion range selected by the comparator circuit takes the form of a diode pair chosen from among a group of diode pairs located in the dodging grade module 34. These diode pairs and their mode of selection are similar to the diode pairs 133-138 shown in Fig. 6 of McIntosh, U.S. Patent No. 4,265,532, except that seven different excursion ranges are provided for in this applicatic~n.
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The dodging grade converter circuit 50 thereby enables the system to automatically select the diode pair which will provide the least amount of dodging required to maintain the exposing image within the contrast acceptance limits of the image receiving material, as determined by the preset value entered by the operator by means of variable resistance means 51, the contrast grade adjustment.
The system may also be equipped with indicator means mounted near the Dodging Grade Selector 35 which could then provide the operator with densitometric readouts of Dmin, DmaX~ ~D and the contrast excursion range that the prescan syste~ has determined to be most appropriate for reproduction oE the negative just analyzed, on the chosen printing material. The operator can then, if desired, compare the recommended amount of dodging with the densitometric data and make an independent choice as to the actual amount of dodging to be employed. If the operator determines that a different range of dodging is desirable, that range can be entered on Dodging Grade Selector 35, whereupon the exposure cycle may be initiated at switch 3~.
The prescan system also determines two separate exposure levels automatically. The first such signal is ~DSum which is derived by: DSum = l(Dl + D2 -- Dn) by circuits 45, 46 and 47. This is an averaged sum of all density points sampled during the prescan, and can be used to determine the proper exposure level for each negative evaluated by the prescan system. The present invention, however, also provides a means for generating a DComb signal, 12~73t7~
as follows: Dcomb = ~l-K) Dsum + (l-k)K Dmin + K(k)DmaX.
K and k are variable impedance means 71 and 72 (illustrated in Fig. 2) that enable the operator to bias the exposure level to Dsum~ to Dmin~ to Dmax or to Davg~ as desired.
When means 72 is set to 0, Dsum is selected as DComb~
As impedance 72 is increased, more emphasis is placed on the Dmin~ Dmax and Davg functions represented by impedance 71.
As illustrated, Davg is a median figure since only Dmin and DmaX are summed and divided. By manipulation of controls 71 and 72 together, the operator can bias the exposure level to Dmin or to DmaX~ if desiredO The DComb feature makes this prescan system very versatlle and adaptable to many difficult types of photoprinting.
Aerial reconnaissance photography might well be biased to DSum but pictoral photography is more likely to be biased towards Dmin or DmaXl depending on whether the photograph~ is of a "high key" or "low key"
image.
As was the case with dodging grade selection, this system can provide a readout enabling the operator to choose between the automatic exposure level determined by the system, or one of the exposure levels determined by manual adjustment of Exposure Level Selector 36.
The exposure level automatically determined by DComb (or manually entered by the operator at selector 36) is then combined with the contrast excursion range selected by the comparator circuit means 50 and 34 (or by the operator at selector 35) to provide an exposure midpoint and excursion range for the current replicator 20, through lo~ging circuit 17.
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From this point forward, the exposure mode is as descriked for Fig. 6 of McIntosh, U.S. Patent No. 4,265,532. By means of logging circuit 17, the output current of PMT
11 is modified to maintain it within the contrast excursion range and exposure level requirements determined by the DG/EL circuit 34 and is replicated by current replicator 20 to automatically modulate the CRT beam intensity through cathode drive circuit 18, and the beam sweep velocity through fast-axis current inverter 22 and deflection geneLator 4.
In the present invention, both intensity modulation (IM) and velocity modulation (VM) are employed, although the invention is applicable also to an IM only system, or to a ~M only system. As will be discussed hereinafter, the prescan mode employs IM only, wh:ile the exposure mode employs both IM and VM control of the CRT.
The block diagram of Figures lA and lB, depicts, a CRT 1 having a fast-axis deflection yoke 2 and a slow-axis deflection yoke 3, which are associated with a fast-axis deflection generator 4 and a slow-axis deflection generator 5, repsectively. The CRT has a flourescent screen 6 having well-known phosphor limitations, inlcuding persiste~ce characteristics which are a non-linear function of the current density of CRT electron beam 7. This beam forms a luminous spot 8 on the phosphor screen 6 whlch is located on one side of a radiograph 9 (or other type of photographic transparency) while on the opposite side of transparency 9 a pellicle 10, photodetector 11 and camera 12 are disposed in a functional array. Camera 12 contains an apertured lens 13 and an electrically actuated light shutter (not shown).
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Photodetector 11 is a photomultiplier tube in one preferred embodiment of the invention, powered by a high voltage supply (not shown). However, many other types of photosensors may also be applicable, depending upon the light level available to actuate the sensing system.
The photodetector ouptut current Ipmt is an input signal to current amplifier 14, which produces a replicated output current Ipmt which can range between limits set by I~aX
and Imin controls 15 and 16.
In general, circuits which employ semiconductor junctions to obtain a logarithmic current output tend to be slow in response at low currents. Therefore, current Ipmt is arranged to be o a preferential amplitude at its point of injection into logging circuit 17. It should be noted that while logarithmic operation is conceptually simpler in this application than might be true of other non-linear forms of response, a square-law or other comparable function could be provided with little change of complexity or accuracy. Logging circuit 17 receives three input signals and provides three outputs, each of which will be discussed hereafter. It is appropriate at this pointr however, to delay consideration of the functions of the DENSITY output line and the EXPOSURE LEVEL, DODGING GRADE input line, until the basic scan circuits have been fuIly identified.
Input current Ipmt to logging circuit 17 produces an output signal Vlog which is applied, via cathode drive circuit 18, to the cathode 19 of CRT 1 as an intensity-modulating beam control current Ik. In its preferred and idealized `f'' i 14 ~LZ'~3~
form, current Ik is about 2000 microamperes maximum, and 126 microamperes minimum, varying continuously in response to the density range (typically, 0-2.4 density units) existing in the photography to be reproduced. Note that the range of current Ik is 2000/126, or about 15.87,and that the logarithm of 15.87 is 1.2. Under these conditions the intensity-modulated CRT cathode current portion of the IM-VM system provides one-half of the total range of density control of the exposing system.
The configuration consisting of CRT 1, radiograph 9, PMT 11, current amplifier 14, logging circuit 17, catho*e drive circuit 18, and CRT cathode 19, forms an intensity-modulated negative feedback loop. Wherever the radiograph density is 0, the output current Ipmt of photomultiplier tube 11 will be approximately 46 microamperes~ current amplifier 14 output will be 1000 microamperes, logging circuit 17 output voltage Vlog will be -9.0 volts,and the CRT cathode drive circuit 18 output current Ik will be 126 microamperes.
Conversely, wherever radiograph 9 has a density of 2.4, the output of PMT 11 will be 2.9 microamperes, current amplifier 14 output will be 63 microamperes, logging circuit 17 output voltage Vlog will be 0.0 volts, and the CRT cathode drlve circuit 18 output current Ik will be 2000 microamperes.
Under the preceding conditions, the swing of voltage Vlog is -Erom -9.0 volts to 0.0 volts for a density range in radlograph Y of 2.4 units. Thus, Vlog exhibits a change of 3.75 volts per decade change in density. It should be noted that the exposing intensity correction ~2~73~4 represented by the luminous mask on the faceplate of CRT 1 is only 15.87 to 1, corresponding to a density range of 1.2, yet the output voltage Vlog from logging circuit 17 represents a density range of 2.4.
Current f (Ipmt) constitutes a second output from logging circuit 17 and bears a non-linear relationship to current Ipmt derived from the output of current amplifier 14. Current f(Ipmt) produces, in current replicator 20, . two corresponding currents each identified as Ipmt where Ipmt is equal to, but of opposite polarity from, Ipmt.
One such current is fed back to logging circuit 17 for comparison to current Ipmt. The other current Ipmt is fed forward to the scan rate set current circuit 21, where it can be passed on (or be replaced by yet another current, depending upon the operating cycle requirements) as input to fast-axis current inverter 22. This current inverter : produces an output current ~Ipmt or ~Ipmt in response to a directional signal rom fast~axis toggle 23.
The circuit formed b.y ~ast-axis deflection generator 4, in conjunction with left and right raster edge controls 24 and 25 and fast-axis deflection yoke 2, is of a type well known in the art, as disclosed by Folse in U.S. Patent No. 3,036,497. Said generator produces a deflection current in fast-axis yoke 2 which results : 25 in a lateral translation of the CRT electron beam 7 at a velocity which is linearly related to the amplitude of . Ipmt and hence, by regression, to the output current Ipmt ; of` photomultiplier tube 11 and the densities existing in ~ radiograph 9.
~'f ~ 1 6 ~``
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In other words, a velocity modulation contrast control circuit is formed, comprised of CRT 1, radiograph 9, PMT ll, current amplifier 14, logging circuit 17, current replicator 20, scan-rate set current circuit 21, fast-axis current inverter 22, fast-axis deflection generator 4 and fast-axis deflection yoke 2. This circuit is not a negative feedback loop, inasmuch as the parameter controlled, electron beam deflection rate, is not the parameter sensed, namely, illuminance at the photocathode of PMT ll. Thus~ as will become evident, the fast-axis deflection function can be modified or replaced without interfering with the action of the intensity-modulated loop described previously.
The output of fast-axis deflection generator 4 i5 sensed by fast axis toggle 23 which, at each limit of deflection, experiences a state change from ~Vmax to -Vma~, or vice versa. When the output is ~Vnlax the fast-axis current inverter 22 produces a current +Ipmt while, when the output is ~Vma~ the output current of inverter 22 becomes ~Ipmt. At each state change of toggle 23, the one-shot multivibrator 26 provides an input pulse to slow-axis deflection generator 5, resulting in a step change in the output current through slow-axis yoke 3, as determined by the settings - of front and back raster edge controls 27 and 28, respectively.
In this way, the elements just discussed cooperate to produce a slow-axis deflection staircase current of a known form.
The slow-axis toggle 29 provides outputs to reset circuit 3Q and to stop-start network 31. When slow-axis toggle 29 output is negative ~-EmaX) the reset circuit 30 is operational, causing slow-axis deflection ,~ 17 12~7;~74 generator 5 to reset to its starting condition. The start-stop network 31 counts the negative transitions of slow-axis toggle 29 and, in conjunction with the exposure index control 32, produces 1-2-4-8 etc. cycles of slow-axis CRT de~lection.
When the output of slow-axis toggle 29 is positive, the start-stop network 31, through CRT blanking network 33, enables the cathode drive circuit 18 to drive CRT 1 with a cathode current Ik.
Adjustment of the exposure index control 32, in addition to affecting the number of frames of the exposing cycle also modifies the step size of the current staircase from slow-axis de~lection generator 5. These two factors, taken in combination, control the coarse exposure level of the printer, which is precalibrated to the sensitivity (or photographic speed) of the image-receiving emulsion in camera 12.
As stated earlier, loyging circuit 17 also receives an additional input, EXPOSURE LEVEL, DODGING GRADE, and provides an additional ouput signal, identified as DENSITY. The EXPOSURE LEVEL, DODGING GRADE (ELDG) input signal originates ln the dodging grade and exposure level module 34, which is similar to that illustrated in Figure
when ~D is slightly greater, some dodging is required;
and when ~D is considerably greater, maximum dodging may be needed. The Davg information represents the arithmetic average of the density e~tremes and it, or some function kDaVg of it, is useful when reproducing pictorial subjects.
Aerial photography, however, and various types of medical ~ and scientific photography, is reproduced best when DSum ; is employed, because all elements of the imagery may well - be of equal visual importance. Thus, the greater the number of points measured and averaged, the greater will be the probability that most photographic areas will be reproduced at or near their optimum exposure.
The Callier effect exerts an important influence over the contrast of the printing process used. Contact ~2~73~
printers require density measurements to be made by measuring the diffused light after it has passed through the negative.
Projection printers, however, require measurement of specu]ar or projected density in order to achieve correct contrast evaluation. Applying the diffuse density measuring technique to projection printing will result in invalid measurements.
Thus, it is necessary to use a different measuring technique for a contact printer than for a projection printer.
From all of the foregoing, it is clear that extraction of such a mass of image information by manual methods, although conceptually possible, is quite impracticàl, and that rapid preexposure evaluation of the applicable parameters of photographic transparencies re~uires recourse to an automatic device.
PRIOR ART STATEMENT
The prior art for the present invention falls into two distinct categories.
The first category includes traditional "white light" printers that prescan the negative for a variety of reasons, but are unable to alter exposure contrast.
Some of these printers modify the effective contrast range of the paper by "flashing" the paper prior to (or durilig) the main exposure, but the imagè contrast during exposure remains unchanged.
Examples of this type of reference are:
Biedermann et al, U.S. Patent No. 3,03~,400; Bickl et al, U.S. Patent No. 4,082,465; Knor et al, U.S. Patent No.
4,176,964; and Treiber, U.S. Patent No. 4,239,384. These ` 4 ~2'0 ~3~7~
references all teach prescanning of a negative before exposure.
Biedermann et al scan for the express purpose of measuring contrast and do determine an equivalent to applicants ~D
signal, but do not determine Dmin, DmaX~ or Dsum Biedermann, however, is unable to alter the contrast excursion range of the exposure because a conventional light source is used. Bledermann et al, therefore, depend upon pre-exposure or "non-image flashing" of the reproduction material to improve the subsequent exposure of detail in the toe region of the emulsion.
The second category of prior art includes electronic printers that employ dynamic mas~ing during exposure. These devices are able to atler the image contrast excursion range, but do not normally employ a prescan.
These re~erences may be further divided into two subgroups:
(a) Bestenreiner et al, U.S. Patent No. 3,984,187; and Kelly, V.S. Patent No. 4,145,709, wh:ich do employ a prescan;
and (b) McIntosh, U.S. Patent No. 4,265,532; Causley, British Patent No. 1,106,629; Elphick, British Patent No. 859,895;
and Nottall, British Patent No. 859 r 017, all of which disclose electronic print~rs without prescan. ~
Bestenreiner develops an "intensity indicating signal" that is used to avoid "stripes or striations" in the reproduced image. However, he does not develop the Dmin~ Dmax~ ~D, Dsum and DcOmb signals obtained by appli-cant's invention to assist in matching the exposure of the negative to the contrast grade and exposure index of the reproduction material.
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Kelly uses a prescan to achieve "automatic localized micro-contrast of incremental areas of negative images." He employs a multicontrast paper, and divides his density signals into categories which then cause changes in the spectral content of a CRT light source, thereby modifying the effective contrast of the reproduction material to meet the requirements of the image LO be reproduced.
Applicant's invention, to the contrary, modifies the exposure level and contrast excursion range of the image to be reproduced in order to achieve gross contrast control, and relies on the use of a single grade of paper (usually of a higher than normal contrast) for "micro-contrast" control.
Kelly's system employs a vidicon tube for scanning the image as it moves through the reading station, and a CRT for line exposure of the contact sandwich as it moves through the printing station. Applicant scans and prints "in situ" with a single CRT.
- Kelly does not determine a DSum or Dcomb as does applicant. Kelly determines Dmin and five density ranges, while applicant determines Dmin and DmaX for the negative and then calculates ~. The concept of Kelly's system is to fit the paper to the negative, while applicant alters the effective contrast range of the negative to fit the contrast acceptance range of the paper.
Kelly uses specular light for density scanning, and diffuse light for printing. Under certain conditions, the Callier effect will cause such a system to generate ~2~737~
erroneous density readings. Applicant uses identical illumi-nation for both reading and exposing.
Finally, there is one anomalous reference.
Neale, ~ritish Patent No. 906,146 uses an expensive "flood gun" type of CRT which prescans the negative to develop a luminous image "mask" which is applied to the image receiving material when the "flood gun" is activated. However, Neale does not compute Dmin, DmaX~ ~D, or DSum for use in alter-ing the various images in order to enable them to fit the brightness acceptance range of the printing material, as does applicant.
SUMMA~ OF THE INVENTIO~
It is one object o the invention ko measure, in a scanning electronic dodginy photoprinter and by means of a raster scan (hereafter called the prescan) which occurs prior to the exposing scan, the maximum and minimum densities present in the photography to be reproduced.
It is another object of this invention to obtain an integrated density signal (DSum) representative of all of the prescan densities measured in the photography to be reproduced.
It is a third object to derive a signal representing the difference between the maximum and minimum prescanned densities.
It is an additional object to employ the density difference signal to select an appropriate exposing system response.
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It is a further object to use an intensity-modulated beamed CRT spot as a part of the raster prescan density-measuring technique.
Another object is to enable correct density measurements to be obtained even though the prescan raster is adjustable in area.
Yet another object is to enable the prescan raster to be smaller in dimensions than the subsequent exposing raster so that adjacency effects, such as physical masking of the printing stage, will not interfere with the prescan readings.
It is a related object to combine the prescan measured densities Dmin, DmaXl and DSum in a manner such that these signal elements can be preerentially weighed lS and employed as an exposure level control in a wide variety o photographic reproduction techniques.
~et another object of the invention is to provide a method whereby the prescan occurs automatically, but only when required.
~ related general object of the invention is to increase the flexibility with which any spot-type scanning electronic dodging light source may be used in photographic reproduction processes.
All of these objectives, and others that ;will become apparent from the following description of the preferred embodiment, are achieved through electronic control circuits which inhibit normal photographic exposure during the information-extracting prescan operation.
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~ 8 ~7'379~
BRIEF DESCRIPTION OF THE DRAWINGS
Figures lA and lB form a block diagram of an IM-VM CRT Dodging Printer showing an embodiment employed for the reproduction of original radiographs in reduced form, in which the prescan circuits are placed in juxtaposi-tion to the exposing scan circuits.
Figure 2 is a partial schematic showing particular density-combining elements.
Figure 3 shows, in diagrammatic form, the disposition of rotary solenoid-actuated filters in the optical paths of a scanning electronic contact printer.
Figure 3A depicts the positions adopted by the ilters during the prescan mode.
Figure 3B illustrates the filter positions used during the exposing mode.
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DESCRIPTION OF THE PREFERRED EMBODIMENT
It is well know that the cont~ol of printing contrast by means of electronic dodging tends to produce results that are scientifically superior in their information content, but pictorially unpleasing. McIntosh, in U.S.
Patent No. 4,265,532, ac~nowledges this deficiency and provides for its alleviation by means of a manually adjustable fine exposure level control, and the manual selection of dodging yrade.
In furtherance of these objectives, the circuits disclosed in Figures lA and lB, and in Figure 2, illustrate a method whereby each photograph to be reporduced .
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is presca~ned automatically wlthin a time period of about 200 milliseconds or less. Where even this loss of time is objectionable or if the user prefers to make his own choice of exposure parameters, these automatic features can be disabled. The prescan action can also enable the user to manuall~ select the fine exposure level, while the dodging grade is determined automatically or, vice versa.
The present invention provides a prescan system in conjunction with a printer of the type disclosed in McIntosh, U.S. Patent No. 4,265,532. During the prescan`
the system evaluates the negative to generate a Dmin signal representative of the minimum density in the negativel a DmaX signal representative of the maximum density in the negative, and a QD signal representati~e o the difference - between Dmln and Dmax After QD has been determined by the prPscan, it is compared, in a dodging grade camparative circuit 50, to a preset value that corresponds to the contrast acceptance range of the image receiving media in the printer. The camparator circuit seeks a contrast excursion range that will cover the range from Dmin to DmaX~ The contrast excursion range selected by the comparator circuit takes the form of a diode pair chosen from among a group of diode pairs located in the dodging grade module 34. These diode pairs and their mode of selection are similar to the diode pairs 133-138 shown in Fig. 6 of McIntosh, U.S. Patent No. 4,265,532, except that seven different excursion ranges are provided for in this applicatic~n.
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The dodging grade converter circuit 50 thereby enables the system to automatically select the diode pair which will provide the least amount of dodging required to maintain the exposing image within the contrast acceptance limits of the image receiving material, as determined by the preset value entered by the operator by means of variable resistance means 51, the contrast grade adjustment.
The system may also be equipped with indicator means mounted near the Dodging Grade Selector 35 which could then provide the operator with densitometric readouts of Dmin, DmaX~ ~D and the contrast excursion range that the prescan syste~ has determined to be most appropriate for reproduction oE the negative just analyzed, on the chosen printing material. The operator can then, if desired, compare the recommended amount of dodging with the densitometric data and make an independent choice as to the actual amount of dodging to be employed. If the operator determines that a different range of dodging is desirable, that range can be entered on Dodging Grade Selector 35, whereupon the exposure cycle may be initiated at switch 3~.
The prescan system also determines two separate exposure levels automatically. The first such signal is ~DSum which is derived by: DSum = l(Dl + D2 -- Dn) by circuits 45, 46 and 47. This is an averaged sum of all density points sampled during the prescan, and can be used to determine the proper exposure level for each negative evaluated by the prescan system. The present invention, however, also provides a means for generating a DComb signal, 12~73t7~
as follows: Dcomb = ~l-K) Dsum + (l-k)K Dmin + K(k)DmaX.
K and k are variable impedance means 71 and 72 (illustrated in Fig. 2) that enable the operator to bias the exposure level to Dsum~ to Dmin~ to Dmax or to Davg~ as desired.
When means 72 is set to 0, Dsum is selected as DComb~
As impedance 72 is increased, more emphasis is placed on the Dmin~ Dmax and Davg functions represented by impedance 71.
As illustrated, Davg is a median figure since only Dmin and DmaX are summed and divided. By manipulation of controls 71 and 72 together, the operator can bias the exposure level to Dmin or to DmaX~ if desiredO The DComb feature makes this prescan system very versatlle and adaptable to many difficult types of photoprinting.
Aerial reconnaissance photography might well be biased to DSum but pictoral photography is more likely to be biased towards Dmin or DmaXl depending on whether the photograph~ is of a "high key" or "low key"
image.
As was the case with dodging grade selection, this system can provide a readout enabling the operator to choose between the automatic exposure level determined by the system, or one of the exposure levels determined by manual adjustment of Exposure Level Selector 36.
The exposure level automatically determined by DComb (or manually entered by the operator at selector 36) is then combined with the contrast excursion range selected by the comparator circuit means 50 and 34 (or by the operator at selector 35) to provide an exposure midpoint and excursion range for the current replicator 20, through lo~ging circuit 17.
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From this point forward, the exposure mode is as descriked for Fig. 6 of McIntosh, U.S. Patent No. 4,265,532. By means of logging circuit 17, the output current of PMT
11 is modified to maintain it within the contrast excursion range and exposure level requirements determined by the DG/EL circuit 34 and is replicated by current replicator 20 to automatically modulate the CRT beam intensity through cathode drive circuit 18, and the beam sweep velocity through fast-axis current inverter 22 and deflection geneLator 4.
In the present invention, both intensity modulation (IM) and velocity modulation (VM) are employed, although the invention is applicable also to an IM only system, or to a ~M only system. As will be discussed hereinafter, the prescan mode employs IM only, wh:ile the exposure mode employs both IM and VM control of the CRT.
The block diagram of Figures lA and lB, depicts, a CRT 1 having a fast-axis deflection yoke 2 and a slow-axis deflection yoke 3, which are associated with a fast-axis deflection generator 4 and a slow-axis deflection generator 5, repsectively. The CRT has a flourescent screen 6 having well-known phosphor limitations, inlcuding persiste~ce characteristics which are a non-linear function of the current density of CRT electron beam 7. This beam forms a luminous spot 8 on the phosphor screen 6 whlch is located on one side of a radiograph 9 (or other type of photographic transparency) while on the opposite side of transparency 9 a pellicle 10, photodetector 11 and camera 12 are disposed in a functional array. Camera 12 contains an apertured lens 13 and an electrically actuated light shutter (not shown).
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Photodetector 11 is a photomultiplier tube in one preferred embodiment of the invention, powered by a high voltage supply (not shown). However, many other types of photosensors may also be applicable, depending upon the light level available to actuate the sensing system.
The photodetector ouptut current Ipmt is an input signal to current amplifier 14, which produces a replicated output current Ipmt which can range between limits set by I~aX
and Imin controls 15 and 16.
In general, circuits which employ semiconductor junctions to obtain a logarithmic current output tend to be slow in response at low currents. Therefore, current Ipmt is arranged to be o a preferential amplitude at its point of injection into logging circuit 17. It should be noted that while logarithmic operation is conceptually simpler in this application than might be true of other non-linear forms of response, a square-law or other comparable function could be provided with little change of complexity or accuracy. Logging circuit 17 receives three input signals and provides three outputs, each of which will be discussed hereafter. It is appropriate at this pointr however, to delay consideration of the functions of the DENSITY output line and the EXPOSURE LEVEL, DODGING GRADE input line, until the basic scan circuits have been fuIly identified.
Input current Ipmt to logging circuit 17 produces an output signal Vlog which is applied, via cathode drive circuit 18, to the cathode 19 of CRT 1 as an intensity-modulating beam control current Ik. In its preferred and idealized `f'' i 14 ~LZ'~3~
form, current Ik is about 2000 microamperes maximum, and 126 microamperes minimum, varying continuously in response to the density range (typically, 0-2.4 density units) existing in the photography to be reproduced. Note that the range of current Ik is 2000/126, or about 15.87,and that the logarithm of 15.87 is 1.2. Under these conditions the intensity-modulated CRT cathode current portion of the IM-VM system provides one-half of the total range of density control of the exposing system.
The configuration consisting of CRT 1, radiograph 9, PMT 11, current amplifier 14, logging circuit 17, catho*e drive circuit 18, and CRT cathode 19, forms an intensity-modulated negative feedback loop. Wherever the radiograph density is 0, the output current Ipmt of photomultiplier tube 11 will be approximately 46 microamperes~ current amplifier 14 output will be 1000 microamperes, logging circuit 17 output voltage Vlog will be -9.0 volts,and the CRT cathode drive circuit 18 output current Ik will be 126 microamperes.
Conversely, wherever radiograph 9 has a density of 2.4, the output of PMT 11 will be 2.9 microamperes, current amplifier 14 output will be 63 microamperes, logging circuit 17 output voltage Vlog will be 0.0 volts, and the CRT cathode drlve circuit 18 output current Ik will be 2000 microamperes.
Under the preceding conditions, the swing of voltage Vlog is -Erom -9.0 volts to 0.0 volts for a density range in radlograph Y of 2.4 units. Thus, Vlog exhibits a change of 3.75 volts per decade change in density. It should be noted that the exposing intensity correction ~2~73~4 represented by the luminous mask on the faceplate of CRT 1 is only 15.87 to 1, corresponding to a density range of 1.2, yet the output voltage Vlog from logging circuit 17 represents a density range of 2.4.
Current f (Ipmt) constitutes a second output from logging circuit 17 and bears a non-linear relationship to current Ipmt derived from the output of current amplifier 14. Current f(Ipmt) produces, in current replicator 20, . two corresponding currents each identified as Ipmt where Ipmt is equal to, but of opposite polarity from, Ipmt.
One such current is fed back to logging circuit 17 for comparison to current Ipmt. The other current Ipmt is fed forward to the scan rate set current circuit 21, where it can be passed on (or be replaced by yet another current, depending upon the operating cycle requirements) as input to fast-axis current inverter 22. This current inverter : produces an output current ~Ipmt or ~Ipmt in response to a directional signal rom fast~axis toggle 23.
The circuit formed b.y ~ast-axis deflection generator 4, in conjunction with left and right raster edge controls 24 and 25 and fast-axis deflection yoke 2, is of a type well known in the art, as disclosed by Folse in U.S. Patent No. 3,036,497. Said generator produces a deflection current in fast-axis yoke 2 which results : 25 in a lateral translation of the CRT electron beam 7 at a velocity which is linearly related to the amplitude of . Ipmt and hence, by regression, to the output current Ipmt ; of` photomultiplier tube 11 and the densities existing in ~ radiograph 9.
~'f ~ 1 6 ~``
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In other words, a velocity modulation contrast control circuit is formed, comprised of CRT 1, radiograph 9, PMT ll, current amplifier 14, logging circuit 17, current replicator 20, scan-rate set current circuit 21, fast-axis current inverter 22, fast-axis deflection generator 4 and fast-axis deflection yoke 2. This circuit is not a negative feedback loop, inasmuch as the parameter controlled, electron beam deflection rate, is not the parameter sensed, namely, illuminance at the photocathode of PMT ll. Thus~ as will become evident, the fast-axis deflection function can be modified or replaced without interfering with the action of the intensity-modulated loop described previously.
The output of fast-axis deflection generator 4 i5 sensed by fast axis toggle 23 which, at each limit of deflection, experiences a state change from ~Vmax to -Vma~, or vice versa. When the output is ~Vnlax the fast-axis current inverter 22 produces a current +Ipmt while, when the output is ~Vma~ the output current of inverter 22 becomes ~Ipmt. At each state change of toggle 23, the one-shot multivibrator 26 provides an input pulse to slow-axis deflection generator 5, resulting in a step change in the output current through slow-axis yoke 3, as determined by the settings - of front and back raster edge controls 27 and 28, respectively.
In this way, the elements just discussed cooperate to produce a slow-axis deflection staircase current of a known form.
The slow-axis toggle 29 provides outputs to reset circuit 3Q and to stop-start network 31. When slow-axis toggle 29 output is negative ~-EmaX) the reset circuit 30 is operational, causing slow-axis deflection ,~ 17 12~7;~74 generator 5 to reset to its starting condition. The start-stop network 31 counts the negative transitions of slow-axis toggle 29 and, in conjunction with the exposure index control 32, produces 1-2-4-8 etc. cycles of slow-axis CRT de~lection.
When the output of slow-axis toggle 29 is positive, the start-stop network 31, through CRT blanking network 33, enables the cathode drive circuit 18 to drive CRT 1 with a cathode current Ik.
Adjustment of the exposure index control 32, in addition to affecting the number of frames of the exposing cycle also modifies the step size of the current staircase from slow-axis de~lection generator 5. These two factors, taken in combination, control the coarse exposure level of the printer, which is precalibrated to the sensitivity (or photographic speed) of the image-receiving emulsion in camera 12.
As stated earlier, loyging circuit 17 also receives an additional input, EXPOSURE LEVEL, DODGING GRADE, and provides an additional ouput signal, identified as DENSITY. The EXPOSURE LEVEL, DODGING GRADE (ELDG) input signal originates ln the dodging grade and exposure level module 34, which is similar to that illustrated in Figure
6 of U.S. Patent Mo. 4,265,532~ The ELDG input is capable of controlling logging circuit 17. In one extreme, the ELDG input may override the Ipmt input to circuit 17 from current ampli~ier 14, so that its various outputs are responsive~
only to the ELDG input. This condition exists when dodging grade selector 35 is set to its DG=0 condition. In this .
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case, logging circuit 17 is responsive only to the setting of exposure level selector 36, via dodging grade and exposure level module 34. When dodging grade selector 35 is at its other e~treme, DG=7, the dodging grade and exposure S level module 34 is totally inhibited so that logging circuit 17 is responsive only to current Ipmt from current amplifier 14. For settings of DG=l through DG=6 of selector 35, both inputs Ipmt and ELDG affect logging circuit 17 in reciprocally related amounts. The ouput signal from module 35 to module 34 is in octal form.
In a preferred embodiment the exposure level selector 36 has 15 manually selectable positions, equivalent to density segments from 0.3 to 1.7 in radiograph 9, and provides its output signal to module 34 in he~adecimal form. When the dodging grade selector 35 is at DG=0 these signal levels, via the dodging grade and exposure level module 34, produce at the outputs of logging circuit 17 a voltage Vlog and a current f(Ipmt) equal to those which would be produced by photographic densities of the same values in radiograph 9 or other similar transparency.
Correspondingly, the cathode current Ik of CRT 1, and the deflection rate of electron beam 7, are the same as those which would be produced by the appropriate photographic densities.
In order to evaluate the operation of the circuit of Figure 1 in its prescan mode, it will be assumed that dodging grade switch 35 and exposure level switch 36 have each been adjusted to the selectable position identified ~ ~ 19 ., ~2q~737gL
as "A". Each selector has an ouput line to inhibit the prescan, via module 37. If either dodging grade selector 35, or exposure level selector 36, or both, are set to the "A" position, meaning that automatic prescan is desired, it will take place when the exposure is initiated by momentary closure of start exposure switch 38. In the circumstance that neither selector is set to "A", the inhibit module 37 will activate and the prescan mode will be bypassed.
Upon activation of start switch 38, an inhibit signal is removed from slow-axis deflection generator 5 and, via reset circuit 30, said generator ouput signal causes the CRT electron beam 7 to begin to retrace toward the preset kack raster edge position. Simultaneou~ly, prescan logic circuit 39 is actuated and provides signals to the dodging grade and exposure ~evel module 34, the prescan ramp generator 40, exposure index 32, prescan shrink circuit 41, one-shot multivibrator 26 scan rate set current circuit 22 and the camera and shutter control 42. The prescan shrink circuit 41 reduces the effective front, back, left and right raster edge limits by about 15%, thereby confining measurement to the central 70% of the image area.
The prescan ramp generator 40 will inject a continuous current into the slow-axis deflection generator 5, the exposure index 32 and one-shot multivibrator 26 will be inhibited, and the scan rate set current 21 will be switched from a current Ipmt to some other constant current having ~ a nominal value of l.0 milliampere. Lastly, the camera ; and shutter control 42 wlll be inhibited so that the s~utter solenoid (not shown) will remain deaFtivated, and the , -73~
dodging grade and exposure level module 34 will be inhibited to prevent errors in sensin~.
When slow-axis toggle 29 detects the prescan raster edge limit current, its output will change state to +EmaX and the CRT cathode drive circuit 18 will be allowed to unblank, via blanking generator 33 and start/stop network 31.
Therefore, the loop consisting of CRT 1, radiograph 9, P~T 11, current amplifier 14, logging circuit 17 and the cathode drive circuit 18 producing current Ik, will become unrestrained. Thus, the voltage Vlog at every instant during prescan will represent the sensed specular density of radiograph 9. The prescan ramp generator 40 and the switched scan rate set current circuit 21 cooperate to produce, in conjunction with ast-axis deflection generator 4 and slow-axis deflection generator 5, a raster of predetermined characteristics, unaffected by any density attributes of radiograph 9. Note that a constant scan rate current will cause the fast-axis deflection of electron beam 7 to occur at a constant velocity which, in the preferred embodiment, is about 20,000 inches/second.
As mentioned earlier, prescan occurs in about 200 milliseconds. Thus, a 4000 linear inch pattern is traced out for the largest usable prescan raster. A
raster bounded by X and Y dimensions such as 15" X 18", for example, has an area of 270 square inches. If prescan covers only 85% of each axis, then the monitored area is about 200 square inches, and each square inch evaluated .
~ 21 :12~73~74 - photoelectrically is equivalent to 20 linear inches of scan. This factor is important because it represents the conversion of area determination from an XY product function to a simple time-keeping, pulse-counting activity.
In the prescan mode, the output Vlog of logging circuit 17 represents the density of radiograph 9 at every point scanned (where such density is equal to or less than the maximum permitted by known CRT faceplate contrast limitatlons). It follows, therefore, that the output line identified as DENSITY must carry some linear function of Vlog. In fact, in the preferred case, V1og and DENSIT~ are the same. The high scan rate of 20,000 inches per second is practical only of phosphor persistence, which is a non-Linear function of screen loading in terms oE beam current per square centimeter of phosphor scanned, is negligible, i~e. about 5 microseconds or less. Therefore, the use oE an intensity-modulated prescan, with maximum density, may well produce a light persistence of 5 microseconds on the screen or DmaX~ with a beam current Ik of 2000 microamperes, while 50 microseconds may be the phosphor decay time for Dmin at Kk = 126 microamperes. Because, in the IM-VM combination, all densities sensed are of twice their total value, a phosphor persistence to the 10% decay point resulting from cathode current of 126 microamperes will appear electronically as if it has been reduced to .
a persistence of 1%. This effect is not true for a prescan ;~ in which both intensity and velocity remain constant, because ~ the measurement performed would necessarily require sensing `
~ 22 ~Z~73'~4 of the ull dynamic change. Furthermore, if the intensity-modulated prescan were for an IM only system, the CRT beam current change and screen loading range would have to be much greater, as also would be the resulting persistence. Similarly, if the prescan were velocity modulated only and PMT sensitivity were kept constant, then once again the measurement performed would be over the full dynamic range of densities. The persistence considered would be to the 1% level and in order to prevent phosphor burning, would have to be arranged at a constant but moderate screen loading.
In short, the intensity-modulated prescan in an IM-VM scanning electronic dodging printing system enables more accurate density measurements to be made with any given phosphor, and at higher scanning rates.
In addition to its other signals, prescan logic circuit 39 also controls clock 42, DmaX peak detector 43, Dmin peak detector 44 and analog digital density integrator 45. During the standby mode of the printer, prior to prescan, peak detectors 43 and 44 are in reset or clear condition;
clock 42 is off, and density integrator 45 is clamped to ;~ zero. Once prescan begins, clock 42 cycles until the end - ~ of the prescan period and provides two outputs, one to density integrator 45 and the other to area integrator 46. As explained earlier, area integration of data from a raster-traced`scan can be achieved digitally, in this case with a 12-bit rippple counter of a known type. Denslty integrator 45 consists of a conventional integrator coupled , ~ to such a 12-bit~counter. The output of density integrator ~ ~ :
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45 is divided, in divider circuit 47, by the output from area integrator 4fi, producing an ouput DSum which is fed to density combiner 48. In the present embodiment, Dsum is an analog voltage level of 9.0 volts for 0.0 density and 0.0 volts for density 2-4- The DmaX and Dmin peak detectors 43 and 44, respectively, are well-known sample-and-hold circuits which monitor the DENSITY output from logging circuit 17. They are reset prior to prescan and sample the excursions of the DENSITY line during prescan. At the end of prescan, the output from the Dmin peak detector 44 should be a voltage approaching +9.0 volts as an upper limit, and from the DmaX peak detector 43 a voltage approaching 0.0 volts as a lower limit.
Dmax and Dmin~ as voltages, are applied to density combining circuit 48, detailed in Figure ~, which illustrates a method o combining thes,e signals to form DComb~ This combination is presented in such a manner that the prescanned exposure level DCc,mb can be applicable to any known class o~ photograph~. The mixing equation 20 ~ for density combiner 48 is:
comb = ~l-K) D5Um + (l-K~ K Dmin + K(k) Dmax Where: Dmin is the peak detected minimum input density.
DmaX is the peak detected maximum input density.
DSum is the integrated input density.
DComb is the combined output density.
K is a fraction representing the positional displacement o~ the arm of potentiometer 72 (Figure 2).
k is a fraction representing the positional ' displacement of the arm of potentiometer 71 ~Figure 2).
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It is evident from Figure 2 and the combining equation that any exposure level desired, within the bounds of Dmin and DmaX~ is accessible by means o~ two potentiometer settings. The output DComb from density combiner 48 is one input to dodging grade and exposure level module 34 and, if exposure level selector 36 is in the "A" position, DComb will be the exposure level used in the exposing scan following the prescan. The outputs DmaX and Dmin of peak detectors 43 and 44 are applied as inputs to ~ detector 49, which has its output ~V applied to the ~D-to-dodging-gràde converter 50 (hereafter called "dodging grade converter"~.
the signal ~D has as one limit an output of 0.0 volts, and as another limit~ a predefined maximum voltage. In the preferred embodiment DmaX is a vo]tage equivalent to a density of 2.4. Dodging grade converter 50 converts the analog signal expressing the density range of radiograph 9 to a numeric dodging grade level within the range 0-7, and furnishes it as an octal code to module 34.
The basis for dodging grade conversion is a comparison of the density range of the photographic transparency to be reproduced to a known density range acceptable to the reproducing emulsion. The latter range is entered into the prescan system via the contrast grade adjust potentiometer 51, ~associated with the dodging grade converter 50. As an example, if the reproduction capability of the receiving emulsion extends over a density range of 1.5, and the sensed ~D of radiograph 9 is 0.9 in a first case) 1.7 in a second .
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case, and 3.0 in a third case, then the first case requires no dodging, the second case requires a dodging grade DG=2, and the third case requires a DG=7 dodging grade.
While tha foregoing system has been described in detail for a black and white photographic printer, the principles disclosed are equally applicable to a printer for color photography, where the "color-masked" image must also be matched to the printing paper being used, and where the control of image contxast is made more difficult because color printing papers are not provided in various contrast grades. When equipped with a CRT having suitable spectral emission, the present invention is capable of generating electronic luminous masks which are adjustable with respect to each of the primary colors involved in color reproduction, as well as the electronic contrast correction of the neutral component of the image to be reproduced.
As was previously discussed, the Dmin and DmaX detectors 43 and 44 are well kno~n sample and hold circuits. Likewise, density integrator 45 and area integrator 46 hold this total accumulated count throughout the exposure mode, before being reset to zero.
Thus, the densitometric data obtained during the prescan is maintaLned throughout the exposure mode for reference by the DG/EL module 34. If desired, separate ~ holding circuits could be provided for peak detectors 43 and 44, and for divider circuit 47. This would afford the operator as much time as desired to review the densitometric display and to choose either manual or automatic operation of the printer.
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The foregoing description has reviewed conditions which relate mainly to non-contact printing, where either a reduction or an enlarging operation requires projection of an image through a lens, associated with a shutter to prevent photographic exposure until a desired instant.
This condition does not exist in contact printing, where a shutter connot be interposed between the original transparency and the raw stock on which the reproduction will be made.
Thereore, prescan in contact printing requires some modification of the general concept, as is illustrated in Figure 3.
Immediately above projection lens 13a there `
is a lens filter rotary solenoid assembly 82, 83, a clear stage plate 84, the photographic transparency 9, the image receiving emulsion 12a, a PMT filter solenoid assembly 80, 81, and a PM~ 11. In filter solenoid assemblies 80, 81 and 82, 83 there are optical Eilters 81a and 83a respectively, cut from the same filter stock. If photographic emulsion 12a is not panchromatic, than filters 81a and 83a might be selected for a spectral passband outside the sensitivity range of emulsion 12a but within the spectral range of PMT 11. If, however, photographic emulsion 12a is panchromatic, then the selected filters must be neutral and of such density values as to minimize the amount of exposure attributable to the prescan cycle.
Figures 3A and 3B illustrate the disposition of filter assemblies 81 and 83 in the prescan (Fig. 3A) and exposing (Fig. 3B) modes. During prescan, the lens filter assembly 83 covers the lens and reduces the actinic . ~ .
~ 27 ~Z~73~ -light at photographic emulsion 12a to a low level. Because PMT 11 is not covered by filter assembly 81, its optical sensitvity is high and low level light passing throuqh transparency 9 and receiving emulsion 12a, in combination, results in a PMT output current K. After prescan, lens filter assembly 83 is rotated to the position shown in Fig. 3B, so that the actinic light level at photographic emulsion 12a is high. Conversely, PMT filter assembly 81 reduces the light level to PMT 11 photocathode, producing a low net PMT sensitivity. The low PMT sensitivity and high light level, however, combine to produce an output current K. In short, the alternating disposition of filters ~la and 83a in the optical paths shown in Figs. 3A and 3B enables an essentially constant measuring sensitivity to be maintained, together with essentially zero prescan ex~osure of receiving emulsion 12a. Note that the prescan measurements of density are diffuse, by virtue of the location of ph~tographic emulsion 12a, and that the subsequent scanning exposure is performed in a contact mode. Hence, there is no Callier effect.
In summary, the operation of prescanniing, as here disclosed in a scanning electronic dodging photographic printer, can be caused to occur automatically and in situ relative to a subsequent main exposure. The Callier effect is compensated for in projection printing, and is not present in contact printing. Two types of density information are derived by prescanning, a summation Dsum f all prescanned points, and measurement of the density extremes, Dmin and . ~
~2~3~
DmaX~ This density data can be combined, at the users option, to produce an appropriate exposure level matched to the receiving emulsion and the class of photography being reproduced. The prescan, which is adjustable to accommodate a wide range of formats, is normally limited to a central part of the image area and usually occurs in millisecond time periods. Printing contrast range is evaluated automatically, and a dodging grade appropxiate to the parameters of the photographic emulsion in use is selected. This prescan system is adaptable to contact and projection printing, and can be activated or inhibited at the users option. Other techniques will be obvious to those skilled in the art and all fall within the spirit of this inventic)n.
~15 ~ ~ .
.
only to the ELDG input. This condition exists when dodging grade selector 35 is set to its DG=0 condition. In this .
t~ 18 lZ~737~
case, logging circuit 17 is responsive only to the setting of exposure level selector 36, via dodging grade and exposure level module 34. When dodging grade selector 35 is at its other e~treme, DG=7, the dodging grade and exposure S level module 34 is totally inhibited so that logging circuit 17 is responsive only to current Ipmt from current amplifier 14. For settings of DG=l through DG=6 of selector 35, both inputs Ipmt and ELDG affect logging circuit 17 in reciprocally related amounts. The ouput signal from module 35 to module 34 is in octal form.
In a preferred embodiment the exposure level selector 36 has 15 manually selectable positions, equivalent to density segments from 0.3 to 1.7 in radiograph 9, and provides its output signal to module 34 in he~adecimal form. When the dodging grade selector 35 is at DG=0 these signal levels, via the dodging grade and exposure level module 34, produce at the outputs of logging circuit 17 a voltage Vlog and a current f(Ipmt) equal to those which would be produced by photographic densities of the same values in radiograph 9 or other similar transparency.
Correspondingly, the cathode current Ik of CRT 1, and the deflection rate of electron beam 7, are the same as those which would be produced by the appropriate photographic densities.
In order to evaluate the operation of the circuit of Figure 1 in its prescan mode, it will be assumed that dodging grade switch 35 and exposure level switch 36 have each been adjusted to the selectable position identified ~ ~ 19 ., ~2q~737gL
as "A". Each selector has an ouput line to inhibit the prescan, via module 37. If either dodging grade selector 35, or exposure level selector 36, or both, are set to the "A" position, meaning that automatic prescan is desired, it will take place when the exposure is initiated by momentary closure of start exposure switch 38. In the circumstance that neither selector is set to "A", the inhibit module 37 will activate and the prescan mode will be bypassed.
Upon activation of start switch 38, an inhibit signal is removed from slow-axis deflection generator 5 and, via reset circuit 30, said generator ouput signal causes the CRT electron beam 7 to begin to retrace toward the preset kack raster edge position. Simultaneou~ly, prescan logic circuit 39 is actuated and provides signals to the dodging grade and exposure ~evel module 34, the prescan ramp generator 40, exposure index 32, prescan shrink circuit 41, one-shot multivibrator 26 scan rate set current circuit 22 and the camera and shutter control 42. The prescan shrink circuit 41 reduces the effective front, back, left and right raster edge limits by about 15%, thereby confining measurement to the central 70% of the image area.
The prescan ramp generator 40 will inject a continuous current into the slow-axis deflection generator 5, the exposure index 32 and one-shot multivibrator 26 will be inhibited, and the scan rate set current 21 will be switched from a current Ipmt to some other constant current having ~ a nominal value of l.0 milliampere. Lastly, the camera ; and shutter control 42 wlll be inhibited so that the s~utter solenoid (not shown) will remain deaFtivated, and the , -73~
dodging grade and exposure level module 34 will be inhibited to prevent errors in sensin~.
When slow-axis toggle 29 detects the prescan raster edge limit current, its output will change state to +EmaX and the CRT cathode drive circuit 18 will be allowed to unblank, via blanking generator 33 and start/stop network 31.
Therefore, the loop consisting of CRT 1, radiograph 9, P~T 11, current amplifier 14, logging circuit 17 and the cathode drive circuit 18 producing current Ik, will become unrestrained. Thus, the voltage Vlog at every instant during prescan will represent the sensed specular density of radiograph 9. The prescan ramp generator 40 and the switched scan rate set current circuit 21 cooperate to produce, in conjunction with ast-axis deflection generator 4 and slow-axis deflection generator 5, a raster of predetermined characteristics, unaffected by any density attributes of radiograph 9. Note that a constant scan rate current will cause the fast-axis deflection of electron beam 7 to occur at a constant velocity which, in the preferred embodiment, is about 20,000 inches/second.
As mentioned earlier, prescan occurs in about 200 milliseconds. Thus, a 4000 linear inch pattern is traced out for the largest usable prescan raster. A
raster bounded by X and Y dimensions such as 15" X 18", for example, has an area of 270 square inches. If prescan covers only 85% of each axis, then the monitored area is about 200 square inches, and each square inch evaluated .
~ 21 :12~73~74 - photoelectrically is equivalent to 20 linear inches of scan. This factor is important because it represents the conversion of area determination from an XY product function to a simple time-keeping, pulse-counting activity.
In the prescan mode, the output Vlog of logging circuit 17 represents the density of radiograph 9 at every point scanned (where such density is equal to or less than the maximum permitted by known CRT faceplate contrast limitatlons). It follows, therefore, that the output line identified as DENSITY must carry some linear function of Vlog. In fact, in the preferred case, V1og and DENSIT~ are the same. The high scan rate of 20,000 inches per second is practical only of phosphor persistence, which is a non-Linear function of screen loading in terms oE beam current per square centimeter of phosphor scanned, is negligible, i~e. about 5 microseconds or less. Therefore, the use oE an intensity-modulated prescan, with maximum density, may well produce a light persistence of 5 microseconds on the screen or DmaX~ with a beam current Ik of 2000 microamperes, while 50 microseconds may be the phosphor decay time for Dmin at Kk = 126 microamperes. Because, in the IM-VM combination, all densities sensed are of twice their total value, a phosphor persistence to the 10% decay point resulting from cathode current of 126 microamperes will appear electronically as if it has been reduced to .
a persistence of 1%. This effect is not true for a prescan ;~ in which both intensity and velocity remain constant, because ~ the measurement performed would necessarily require sensing `
~ 22 ~Z~73'~4 of the ull dynamic change. Furthermore, if the intensity-modulated prescan were for an IM only system, the CRT beam current change and screen loading range would have to be much greater, as also would be the resulting persistence. Similarly, if the prescan were velocity modulated only and PMT sensitivity were kept constant, then once again the measurement performed would be over the full dynamic range of densities. The persistence considered would be to the 1% level and in order to prevent phosphor burning, would have to be arranged at a constant but moderate screen loading.
In short, the intensity-modulated prescan in an IM-VM scanning electronic dodging printing system enables more accurate density measurements to be made with any given phosphor, and at higher scanning rates.
In addition to its other signals, prescan logic circuit 39 also controls clock 42, DmaX peak detector 43, Dmin peak detector 44 and analog digital density integrator 45. During the standby mode of the printer, prior to prescan, peak detectors 43 and 44 are in reset or clear condition;
clock 42 is off, and density integrator 45 is clamped to ;~ zero. Once prescan begins, clock 42 cycles until the end - ~ of the prescan period and provides two outputs, one to density integrator 45 and the other to area integrator 46. As explained earlier, area integration of data from a raster-traced`scan can be achieved digitally, in this case with a 12-bit rippple counter of a known type. Denslty integrator 45 consists of a conventional integrator coupled , ~ to such a 12-bit~counter. The output of density integrator ~ ~ :
', ~LZ~73~7~
45 is divided, in divider circuit 47, by the output from area integrator 4fi, producing an ouput DSum which is fed to density combiner 48. In the present embodiment, Dsum is an analog voltage level of 9.0 volts for 0.0 density and 0.0 volts for density 2-4- The DmaX and Dmin peak detectors 43 and 44, respectively, are well-known sample-and-hold circuits which monitor the DENSITY output from logging circuit 17. They are reset prior to prescan and sample the excursions of the DENSITY line during prescan. At the end of prescan, the output from the Dmin peak detector 44 should be a voltage approaching +9.0 volts as an upper limit, and from the DmaX peak detector 43 a voltage approaching 0.0 volts as a lower limit.
Dmax and Dmin~ as voltages, are applied to density combining circuit 48, detailed in Figure ~, which illustrates a method o combining thes,e signals to form DComb~ This combination is presented in such a manner that the prescanned exposure level DCc,mb can be applicable to any known class o~ photograph~. The mixing equation 20 ~ for density combiner 48 is:
comb = ~l-K) D5Um + (l-K~ K Dmin + K(k) Dmax Where: Dmin is the peak detected minimum input density.
DmaX is the peak detected maximum input density.
DSum is the integrated input density.
DComb is the combined output density.
K is a fraction representing the positional displacement o~ the arm of potentiometer 72 (Figure 2).
k is a fraction representing the positional ' displacement of the arm of potentiometer 71 ~Figure 2).
:
~2~73~
It is evident from Figure 2 and the combining equation that any exposure level desired, within the bounds of Dmin and DmaX~ is accessible by means o~ two potentiometer settings. The output DComb from density combiner 48 is one input to dodging grade and exposure level module 34 and, if exposure level selector 36 is in the "A" position, DComb will be the exposure level used in the exposing scan following the prescan. The outputs DmaX and Dmin of peak detectors 43 and 44 are applied as inputs to ~ detector 49, which has its output ~V applied to the ~D-to-dodging-gràde converter 50 (hereafter called "dodging grade converter"~.
the signal ~D has as one limit an output of 0.0 volts, and as another limit~ a predefined maximum voltage. In the preferred embodiment DmaX is a vo]tage equivalent to a density of 2.4. Dodging grade converter 50 converts the analog signal expressing the density range of radiograph 9 to a numeric dodging grade level within the range 0-7, and furnishes it as an octal code to module 34.
The basis for dodging grade conversion is a comparison of the density range of the photographic transparency to be reproduced to a known density range acceptable to the reproducing emulsion. The latter range is entered into the prescan system via the contrast grade adjust potentiometer 51, ~associated with the dodging grade converter 50. As an example, if the reproduction capability of the receiving emulsion extends over a density range of 1.5, and the sensed ~D of radiograph 9 is 0.9 in a first case) 1.7 in a second .
~ ' , .
~ ~ 25 3~Z~173~7~
case, and 3.0 in a third case, then the first case requires no dodging, the second case requires a dodging grade DG=2, and the third case requires a DG=7 dodging grade.
While tha foregoing system has been described in detail for a black and white photographic printer, the principles disclosed are equally applicable to a printer for color photography, where the "color-masked" image must also be matched to the printing paper being used, and where the control of image contxast is made more difficult because color printing papers are not provided in various contrast grades. When equipped with a CRT having suitable spectral emission, the present invention is capable of generating electronic luminous masks which are adjustable with respect to each of the primary colors involved in color reproduction, as well as the electronic contrast correction of the neutral component of the image to be reproduced.
As was previously discussed, the Dmin and DmaX detectors 43 and 44 are well kno~n sample and hold circuits. Likewise, density integrator 45 and area integrator 46 hold this total accumulated count throughout the exposure mode, before being reset to zero.
Thus, the densitometric data obtained during the prescan is maintaLned throughout the exposure mode for reference by the DG/EL module 34. If desired, separate ~ holding circuits could be provided for peak detectors 43 and 44, and for divider circuit 47. This would afford the operator as much time as desired to review the densitometric display and to choose either manual or automatic operation of the printer.
.
;~
~2~73~
The foregoing description has reviewed conditions which relate mainly to non-contact printing, where either a reduction or an enlarging operation requires projection of an image through a lens, associated with a shutter to prevent photographic exposure until a desired instant.
This condition does not exist in contact printing, where a shutter connot be interposed between the original transparency and the raw stock on which the reproduction will be made.
Thereore, prescan in contact printing requires some modification of the general concept, as is illustrated in Figure 3.
Immediately above projection lens 13a there `
is a lens filter rotary solenoid assembly 82, 83, a clear stage plate 84, the photographic transparency 9, the image receiving emulsion 12a, a PMT filter solenoid assembly 80, 81, and a PM~ 11. In filter solenoid assemblies 80, 81 and 82, 83 there are optical Eilters 81a and 83a respectively, cut from the same filter stock. If photographic emulsion 12a is not panchromatic, than filters 81a and 83a might be selected for a spectral passband outside the sensitivity range of emulsion 12a but within the spectral range of PMT 11. If, however, photographic emulsion 12a is panchromatic, then the selected filters must be neutral and of such density values as to minimize the amount of exposure attributable to the prescan cycle.
Figures 3A and 3B illustrate the disposition of filter assemblies 81 and 83 in the prescan (Fig. 3A) and exposing (Fig. 3B) modes. During prescan, the lens filter assembly 83 covers the lens and reduces the actinic . ~ .
~ 27 ~Z~73~ -light at photographic emulsion 12a to a low level. Because PMT 11 is not covered by filter assembly 81, its optical sensitvity is high and low level light passing throuqh transparency 9 and receiving emulsion 12a, in combination, results in a PMT output current K. After prescan, lens filter assembly 83 is rotated to the position shown in Fig. 3B, so that the actinic light level at photographic emulsion 12a is high. Conversely, PMT filter assembly 81 reduces the light level to PMT 11 photocathode, producing a low net PMT sensitivity. The low PMT sensitivity and high light level, however, combine to produce an output current K. In short, the alternating disposition of filters ~la and 83a in the optical paths shown in Figs. 3A and 3B enables an essentially constant measuring sensitivity to be maintained, together with essentially zero prescan ex~osure of receiving emulsion 12a. Note that the prescan measurements of density are diffuse, by virtue of the location of ph~tographic emulsion 12a, and that the subsequent scanning exposure is performed in a contact mode. Hence, there is no Callier effect.
In summary, the operation of prescanniing, as here disclosed in a scanning electronic dodging photographic printer, can be caused to occur automatically and in situ relative to a subsequent main exposure. The Callier effect is compensated for in projection printing, and is not present in contact printing. Two types of density information are derived by prescanning, a summation Dsum f all prescanned points, and measurement of the density extremes, Dmin and . ~
~2~3~
DmaX~ This density data can be combined, at the users option, to produce an appropriate exposure level matched to the receiving emulsion and the class of photography being reproduced. The prescan, which is adjustable to accommodate a wide range of formats, is normally limited to a central part of the image area and usually occurs in millisecond time periods. Printing contrast range is evaluated automatically, and a dodging grade appropxiate to the parameters of the photographic emulsion in use is selected. This prescan system is adaptable to contact and projection printing, and can be activated or inhibited at the users option. Other techniques will be obvious to those skilled in the art and all fall within the spirit of this inventic)n.
~15 ~ ~ .
.
Claims (4)
PROPERTY OR PRIVILEGE IS CLAIMED IS DEFINED AS FOLLOWS:
1. A contact printer with automatic exposure control, said printer comprising:
(a) a light source, said light source having a pre-exposure measuring mode and an exposure mode;
(b) means for supporting an image receiving material and an image carrying material in intimate emulsion to emulsion contact, with said image carrying material placed between said light source and said image receiving material;
(c) photo sensor means for measuring the light modulated by said image carrying media during the pre-exposure mode and control means to adjust the intensity or duration of said light source during the exposure mode;
(d) a pair of identical filters with opposing movement, said first filter being positioned between said light source and said image receiving material to attenuate said light during the pre-exposure measuring mode, and positined out of light path when said light source is in its exposure mode; said second filter being mounted between said image receiving media and said light sensor means to attenuate the light falling on said sensor during the exposure mode, and out of the light path during the pre-exposure measuring mode; whereby only the first of said filters attenuates the light between said light source and said photosensor during the pre-exposure measuring mode, and only the second of said filters attenuates the light between said light and said photosensor during the exposure mode.
(a) a light source, said light source having a pre-exposure measuring mode and an exposure mode;
(b) means for supporting an image receiving material and an image carrying material in intimate emulsion to emulsion contact, with said image carrying material placed between said light source and said image receiving material;
(c) photo sensor means for measuring the light modulated by said image carrying media during the pre-exposure mode and control means to adjust the intensity or duration of said light source during the exposure mode;
(d) a pair of identical filters with opposing movement, said first filter being positioned between said light source and said image receiving material to attenuate said light during the pre-exposure measuring mode, and positined out of light path when said light source is in its exposure mode; said second filter being mounted between said image receiving media and said light sensor means to attenuate the light falling on said sensor during the exposure mode, and out of the light path during the pre-exposure measuring mode; whereby only the first of said filters attenuates the light between said light source and said photosensor during the pre-exposure measuring mode, and only the second of said filters attenuates the light between said light and said photosensor during the exposure mode.
2. A contact printer with automatic exposure control as claimed in Claim 1 wherein said light source is a cathode ray tube (CRT) which generates a scanning spot to expose the image receiving media.
3. A contact printer with automatic exposure control as claimed in Claim 2 wherein said printer also comprises circuit means for establishing a Dmin signal representative of the minimum density of said image carrying media, a Dmax signal representative of the maximum density of said image carrying media, and a .DELTA.D
signal representative of the difference between Dmax and Dmin; said exposure control also including a comparative circuit to compare .DELTA.D with a preset value representative of the contrast range of the image receiving media to determine a contrast excursion range for said CRT light source.
signal representative of the difference between Dmax and Dmin; said exposure control also including a comparative circuit to compare .DELTA.D with a preset value representative of the contrast range of the image receiving media to determine a contrast excursion range for said CRT light source.
4. A contact printer with automatic exposure control as claimed in Claim 3 wherein said CRT is intensity modulated during the pre-exposure mode and intensity and velocity modulated during the exposure mode.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000445042A CA1207374A (en) | 1981-06-05 | 1984-01-10 | Contact photo printing device |
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US270,758 | 1981-06-05 | ||
| US06/270,758 US4344699A (en) | 1981-06-05 | 1981-06-05 | Exposure and contrast determination by prescanning in electronic photoprinting systems |
| CA000392563A CA1177411A (en) | 1981-06-05 | 1981-12-17 | Exposure and contrast determination by prescanning in electronic photoprinting systems |
| CA000445042A CA1207374A (en) | 1981-06-05 | 1984-01-10 | Contact photo printing device |
Related Parent Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000392563A Division CA1177411A (en) | 1981-06-05 | 1981-12-17 | Exposure and contrast determination by prescanning in electronic photoprinting systems |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1207374A true CA1207374A (en) | 1986-07-08 |
Family
ID=25669512
Family Applications (2)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000445042A Expired CA1207374A (en) | 1981-06-05 | 1984-01-10 | Contact photo printing device |
| CA000445041A Expired CA1207373A (en) | 1981-06-05 | 1984-01-10 | Dynamic exposure level control of crt light source |
Family Applications After (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000445041A Expired CA1207373A (en) | 1981-06-05 | 1984-01-10 | Dynamic exposure level control of crt light source |
Country Status (1)
| Country | Link |
|---|---|
| CA (2) | CA1207374A (en) |
-
1984
- 1984-01-10 CA CA000445042A patent/CA1207374A/en not_active Expired
- 1984-01-10 CA CA000445041A patent/CA1207373A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| CA1207373A (en) | 1986-07-08 |
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