US3672768A - Apparatus for making prints from colour negatives - Google Patents

Abstract

AN APPARATUS FOR MAKING PRINTS FROM COLOUR NEGATIVES HAS ADJUSTABLE FILTERS FOR CONTROLLING THE COLOUR COMPOSITION OF THE PRINTING LIGHT WHICH ILLUMINATES THE NEGATIVE BEING PRINTED. THE REQUIRED COLOUR COMPOSITION IS DEPENDENT ON THE COLOUR DENSITIES OF THE NEGATIVE. TO MEASSURE THE COLOR DENSITIES THE PRINTING LIGHT IS SAMPLED BOTH BEFORE AND AFTER TRANSMISSION THROUGH THE NEGATIVE AND THE SAMPLES ANALYZED IN RESPECT TO THEIR PRIMARY COLOUR COMPONENTS. THE CORRESPONDING COMPONENTS OF THE LIGHT SAMPLES BEFORE AND AFTER THE NEGATIVE ARE COMPARED TO

OBTAIN DIFFERENT SIGNALS REPRESENTING THE COLOUR DENSITIES OF THE NEGATIVE. THESE SIGNALS ARE USED IN TURN TO AUTOMATICALLY ADJUST THE FILTERS FOR THE CORRECT COLOUR COMPOSITION OF THE PRINTING LIGHT. A SHUTTER IS LOCATED BETWEEN THE NEGATIVE AND THE PRINTING MATERIAL AND IS OPENED TO EXPOSE THE LATTER ONCE THE PRINTING LIGHT HAS BEEN ADJUSTED. THE LIGHT SAMPLES ARE ALSO USED TO DERIVE BRIGHTNESS SIGNALS FOR CONTROLLING THE TIME FOR WHICH THE SHUTTER IS OPENED.

Description

June 27, 1972 H. SCHAUB HAL 3,672,768

APPARATUS FOR MAKING PRINTS FROM COLOUR NEGATIVES Filed Aug. 24, 1970 3 Sheets-Sheet 1 FIGJ June 27, 1972 H. SCHAUB ErAL 3,672,768

APPARATUS FOR MAKING PRINTS FROM COLOUR NEGATIVES Filed Aug. 24, 1970 3 Sheets-Sheet 2 U08 18 0c; 16 U u q June 27, 1972 scH u ETAL 3,672,768

APPARATUS FOR MAKING PRINTS FROM COLOUR NEGATIVES Filed Aug. 24, 1970 3 Sheets-Sheet 5 DOB 1 FIGA United States Patent Oflice 3,572,768 Patented June 27, 1972 3,672,768 APPARATUS FOR MAKING PRINTS FROM COLOUR NEGATIV ES Heiner Schaub, Wettingen, Zurich, and Kurt Thaddey and Tino Celio, Buchs, Zurich, Switzerland, assignors to Ciba-Geigy AG, Basel, Switzerland Filed Aug. 24, 1970, Ser. No. 66,416 Claims priority, application Switzerland, Aug. 28, 1969, 13,065/ 69 Int. Cl. G03b 27/78 US. Cl. 355-38 7 Claims ABSTRACT OF THE DISCLOSURE An apparatus for making prints from colour negatives has adjustable filters for controlling the colour composition of the printing light which illuminates the negative being printed. The required colour composition is dependent on the colour densities. of the negative. To measure the colour densities the printing light is sampled both before and after transmission through the negative and the samples analysed in respect of their primary colour components. The corresponding components of the light samples before and after the negative are compared to obtain difference signals representing the colour densities of the negative. These signals are used in turn to automatically adjust the filters for the correct colour composition of the printing light. A shutter is located bet-ween the negative and the printing material and is opened to expose the latter once the printing light has been adjusted. The light samples are also used to derive brightness signals for controlling the time for which the shutter is opened.

This invention relates to apparatus for making prints from colour negatives.

It is known in such apparatus to have filters arranged between a light source and the negative to be printed, by means of which filters the proportion of the primary colour components of the light of the light source can be controlled. Additionally light blending or mixing device is arranged in the light path between the filters and the negative to mix the primary colour components to produce a uniform printing light. To control the colour composition of the printing light for printing from a given negative, there is provided a colour intensity regulating system selectively sampling the printing light by means of a photoelectric device. The system so controls actuating means for the filters that the light illuminating the negative has the required spectral compositions. The known apparatus included a shutter arranged in the light path between the negative and a plane at which printing took place. After the filters have been adjusted, the shutter is opened by a time switch device for the period of time required to expose the printing material.

Heretofore, in apparatus of the kind outlined above the colour intensity regulating system is responsive only to light which is transmitted by the negative. This has the disadvantage that the measurement of the colour density of the negative also includes the influence of the light source and the filters. It cannot be ascertained Whether the measured value pertaining to the negative derives solely from the negative or includes the effects of changes in the light source or the filters.

The present invention arises from the realisation that the above-mentioned disadvantage can be mitigated by measuring the colour composition of the printing light before and after the negative. Consequently the integral density of the negative can then be accurately determined, irrespectively of fluctuations of the light source or filters. The colour components are adjusted by the colour filters.

In its broadest form the invention provides apparatus for making prints from colour negatives, comprising a light path including a source of light, a plane at which a negative to be printed is receivable, a plane at which printing material is receivable, means for imaging a received negative at the printing plane, a plurality of filters interposed in said path between said source and said negative plane, said filters being selectively actuable to control the proportions of the primary colour components in the light received at said negative plane, and means disposed between said filters and said negative plane to mix said colour components to produce a printing light for illuminating a negative received at the negative plane; first and second sampling means disposed at opposite sides of said negative plane to sample the light transmitted to said negative plane for illuminating a negative and the light transmitted from said negative after transmission through the negative respectively; means responsive to the primary colour components of the sampled light to produce a first set of signals representing the intensities of the primary colour components sampled by said first sampling means and a second set of signals representing the intensities of the primary colour components sampled by said second sampling means; a respective means for comparing each primary colour signal of the first set with the corresponding primary colour signal of the second set to derive a signal representing the difference between the intensities of that primary colour at said opposite sides of said negative plane; a respective means coupled to actuate each of said filters; and respective means coupling each comparison means to that one of the actuating means for the filter controlling the primary colour with which the comparison means is associated whereby each filter is actuated in dependence upon said difference signal of the primary colour controlled by the filter.

In order that the invention may be better understood an embodiment thereof will now be described with reference to the accompanying drawings in which:

FIG. 1 shows a schematic representation of the whole apparatus;

FIG. 2 shows a section along line 1111 of FIG. 1; and

FIGS. 3 to 6 are diagrams to illustrate the operation of the apparatus.

The apparatus of FIG. 1 comprises an opto-mechanical portion and an electrical portion. The opto-mechanical portion comprises a light path at one end of which is a printing light source 1. The source is followed by a filter system having yellow magenta and cyanogen filters 2, 3 and 4 respectively, means (not shown) for the positioning of a negative in a plane indicated at 5, an objective lens 6, a light shutter 7 and means (not shown) for the positioning of printing material in a plane indicated at 8. The printing light source 1, which preferably comprises one or more halogen lamps, is located within a reflector 9, which comprises a number of cold light mirrors. A heat filter 10 is arranged at the output end of the reflector 9 to ensure that as little infrared radiation as possible passes to the negative plane 5. The yellow magenta and cyanogen filters 2, 3 and 4 are preferably interference filters. Each can be swung into the path of rays leaving reflector 9 to a selected extent, the filters being movable by respective drive motors 11, 12 and 13. Thus the colour composition of the light received at the negative plane is determined by the relative degrees of insertion of the filters into the optical path between the source and the negative plane. The colour composition is controlled by the filters above specified in terms of the blue, green and red components. Additionally light blending or mixing means are disposed in the light path between the filters and the negative plane. After passing through the filters 2, 3 and 4 the light passes into a first mirror tunnel 14 for the mixing of the blue, green and red components.

A pyramid 15 of material with low diffusion capacity ensures a further mixing of the light. The light passes through the pyramid 15 into a second mirror tunnel 16 which is closed on the output side by a translucent ground glass screen 17. In the second mirror tunnel a further mixing of the light takes place. The second mirror tunnel is provided with corner mirrors 18, which reflect the light from the pyramid 15 in such a way that the corners of the ground glass screen 17 are also adequately illuminated, By virtue of the arrangement and special design of the second mirror tunnel, uniform illumination of the ground glass screen 17 and hence of the negative is accomplished. The light leaving the screen 17 may be conveniently referred to as the printing light. When the shutter 7 is opened, the negative illuminated by the printing light is projected through the objective 6 and imaged at the printing plane 8.

In the path of the printing light, four fibre optic, light guides 19, 20a, 20b, and 21 are arranged to sample the printing light before and after the negative plane. The light guide 19 picks up the printing light of intensity before the negative, whereas the light guides 20a, 20b, and 21 pick up the printing light of intensity after the negative being printed. The three light guides a, 20b and 21 are combined together to a common fibre optic guide 22. The light picked up by the light guides is separated into the colour components blue, green and red by means of a filter wheel arranged between the light output ends of the light guides 19 and 22 and a photo-detector 24. The filter wheel is rotatable about its axis by a motor 23. The manner in which the colour components are separated will be described with reference to FIGS. 2 and 3.

As can be seen from FIG. 2, the filter wheel 25 comprises an opaque disc in which there is provided a blue filter 26, a green filter 27, and a red filter 28 angularly (circumferentially) displaced but at the same radial distance from the wheel axis. Position-defining indicia 30 are arranged in concentric tracks 29 at the periphery of the wheel and cooperate with a scanner 33. Furthermore a window 31 is arranged in the filter wheel for periodically unmasking a reference lamp 32 arranged in its track. The window 31 is radially inwardly displaced from the colour filters and is at a different circumferential (i.e. angular) position. The position indicia 30 provide information on the instantaneous position of the filters relative to the position of the ends of the light guides 19 and 22 on the upper side of the filter wheel as the latter rotates. This information is detected by means of the scanner 33. The light guides 19 and 22 are disposed adjacent one another in the track of the colour filters so as to be successively swept by each of the filters in turn.

Upon rotation of the filter wheel 25 by the motor 23, the colour filters blue 26, green 27 and red 28 are cyclically inserted into the path of the light between the ends of the light guides 19, 22 and the photocell 24 on opposite sides of the filter disc. Consequently the output of the photocell 24 comprises two sets of signals a first of which cyclically represents the blue, green and red separation components of the light intensity 4: These primary colour component signals are interleaved with corresponding components of the light intensity 3 which constitute the second set, the light sample being picked up integrally by the light guides 20a, 20b. The light of the reference light source 32 also passes periodically through the window 31 to the photo-cell 24 at a different time in the cycle of rotation of the filter wheel. With the help of this intensity reference it is possible to compensate for drift in the subsequent measuring circuitry as is disclosed hereinafter.

The pulse sequences appearing at the output of the photo-cell 24 as a result of the arrangement described by way of example, is presented in the diagram of FIG. 3 as a function of time. On each cycle of rotation of the filterwheel 25 the following seven electrical quantities appear 4 at the output of the photo-cell in succession corresponding to the following light intensities:

U corresponding the light intensity of the blue component of the printing light before the negative;

U corresponding to the light intensity of the blue component 5 of the printing light after the negative;

U corresponding to the light intensity of the green component before the negative;

U corresponding to the light intensity of the green component 5 afterthe negative;

U corresponding to the light intensity of the red component before the negative;

U corresponding to the light intensity of the red separation after the negative; and

U corresponding to the intensity 11 of the reference lamp 32.

The outputs of the photo-cell 24 and the scanner 33 are the inputs of the electrical portion of the apparatus which will now be described. The output signals U U13, UOG, UIG, UQR, U and U Of the photocell are analogue values and are converted into logarithmic converter stage 34 to form the signals D n, D D D Don, D and D having logarithm-representing values. [It is convenient to convert the analogue values into logarithms. In this way several orders of intensity values can be processed with a constant relative error. Correction values are summated in densities and not in transmissions. After conversion to logarithmic form, the seven resulting signals D D113, D D Don, D and D pass through a distributor or scanned switching device 35 and are then stored individually in seven stores. The distributor is controlled by the position information from scanner 33, and connects the photocell 24 toits several outputs in succession in synchronism with the rotation of the wheel 25. Thus the signals of the first and second sets are passed only into the respective stores assigned to them. For simplicity only the stores for the blue channel are shown in the block diagram. The store 37 is for the blue component (D before the negative being marked 37, and store 38 is for the blue component (D after the negative. The distributor 35 has a further scanned output by which the measured value of the intensity of the reference lamp 32 is set to a further store 36. In a comparator stage 42 this measured value is compared with a selected reference value 53. If the measured value deviates from the reference value, this means a drift of the operating point of the stage 34. The comparator stage 42 then transmits an error signal to the stage 34 :which compensates this drift in such sense as to reduce the error signal. In this way drift of both the photo-cell 24 and of the stage 34 is compensated.

Before going further with the signal processing, it should be noted that the term summation stage will be used herein for stages effectively performing addition or substraction or both, since the mathematical function performed depends on the polartities of the signals applied to any such stage. These relative polarities are shown by the or sign app-lied to the various signal inputs of a summation stage.

The output of the D store 37 is led directly, and that of the D store 38 by way of a summation stage 78, to a summation stage 44 from which the difference value D is obtained. The output D of the summation stage 44 is taken to the respective inputs of a slope correction stage 47, a summation stage 46 (performing a subtraction function) and average-calculating stage 73. The output of the stage 46 is connected via an undercorrection stage 45 (explained below under the operation of the apparatus) to one of the addition inputs of summation stage 63 (performing both addition and subtraction operations). The output of the slope correction stage 47 also goes to an addition input of stage 63. The average calculating stage 73 is connected in the same way as shown for the blue channel to the D D summation stages corresponding to stage 44 of the red and green channels. A subtraction input of the stage 63 is connected to the output of the D store 37. The stage 63 has further addition inputs to signal-providing stages 58, 54 and 55, which provide inputs relating to manual corrections (58), negative parameters (54) and printing material parameters (55) respectively.

The output of the summation stage 63 is applied to a blue channel servo-amplifier 50 which drives the actuating means for positioning the yellow filter 2, the actuating means being the servo-motor. A like arrangement is used for all three colour channels and is therefore shown in the drawing only for the blue channel (indexv B).

The outputs of the D D and D -stores, only the D -store 37 being shown, are lead to an averagecalculating stage 57 which averages the three inputs. The output D of the average-calculating stage 73 is connected to the input of a brightness-correction stage 61. The output of the average calculation stage 57 is conneoted to the input of summation stage 64 (which performs addition). Stage 64 has further inputs from the brightness correction stage 61 and the correction stages 58, 54 and 55 for manual corrections (58), negative parameters (54) and printing material parameters (55). The output of stage 64 is applied a timer 74 controlling a release mechanism 74 for the shutter 7, the shutter opening being dependent on the value of signal from stage 64. The timer 74 is connected further to the output of a blocking stage 71 having three inputs one of which is connected to the outputs of the summation stage 63 of the blue channel and the other two of which are connected to the outputs of the corresponding summation stages (not shown) of the red and green channels. The blocking stage 71 releases the timer 74 for operation only when the signal at each input of the blocking stage falls below a certain value (threshold value).

The manner of operation of the apparatus so far described will now be considered in more detail with the help of the diagrams of FIGS. 4 to 6.

If a print which is correct in the colour reproduction is to be produced from a standard negative, this can be done only with certain positions of the yellow, magenta and cyanogen filters 2, 3 and 4 respectively giving a particular colour composition of the printing light before the negative.

The diagram of FIG. 4 is a plot of the values D against D wherein the logarithm of the blue intensity before the negative is plotted as function of the density of the blue separation of the negative itself. The filter position of the yellow filter 2, which controls the value D at correct printing of the standard negative, symbolized by index St, is indicated by point 100. Now, if a negative is inserted which is denser than the standard negative but which is balanced in respect of colour, the slope correction stage 47 and the regulating circuitry will ensure that the yellow filter swings out, so that the same blue intensity acts at the printing plane as in the standard negative. This correction is accomplished in the example shown in FIG. 4 by swinging out of the yellow filter to the position symbolized by point 101. Usually it is appropriate and may be even necessary, to influence this relationship, which is dependent on the density of the negative, so as to compensate for disturbing parameters, such as the Schwarzschild effect and other effects deriving, for example, from the paper development. For this purpose the straight line relationship 102 has its slope changed but still intersects the standard point 100: i.e. line 102 is turned through an angle about the point 100, so that it assumes a position such as is shown by line 103. This socalled slope-adjustment takes place in the slope-correction stage 47 and may be positive or negative or zero. Thus the slope of the straight line 103 may be set greater or smaller than and, of course also equal to, that of the straight line 102.

The difference signal fed by the summation stage 46 into the under-correction stage 45 indicates whether the negative being printed has a colour dominant or a colour cast, but it is independent of whether the negative is over or under exposed, since the brightness component D has been subtracted from the colour component D by stage 46.

By means of the under-correction stage 45 the effectiveness of the gray compensation can be adjusted in the sense of an under-correction. Without under-correction a negative is printed in colour so that on the positive a print is made which has equal blue, green and red components. The so-called grey criterium is then fully eifective. Negatives with a colour cast are correctly printed in this manner. Now if in a picture one colour component dominates (take as an example a picture of blue sky, lake and small white yacht) the regulating circuitry cannot ascertain whether the negative has a cast or whether in the subject of the picture, in fact, the blue dominates. With complete grey compensation a small white area (eg the yacht) in large blue field (sky) would be printed with a yellow cast. This deficiency can be eliminated by under-correction. An under-correction, eg in the blue, has the effect that the blue dominant of the negative in the blue is only partly entered as a parameter for the control of the yellow filter. At complete under-correction in the blue, the blue component of the printing light is equal to that of the standard negative and is, therefore, independent of the blue component of the particular negative inserted.

The slope correction and the under-correction can be adjusted independently of one another by means of stages 47 and 45.

The sum of the colour density signals appearing on the addition inputs of stage 63 each forms a specified desired value, which is a function of the density of the negative, the slope and under-correction adjustment and the further parameters adjustable in stages 54, 55 and 58. The sum on the addition inputs is in elfect compared with the D signal (a subtraction input). The servo-loop controlling the yellow filter 2 acts to reduce the output of stage 63 at least below the threshold value referred to above.

The operation described for the blue channel is the same for the green and red channels.

The diagram of FIG. 5 shows, as an example, the desired values S S S for the blue, green and red channel expressed in voltage values (mv.). In this example the desired value S of the red channel is maximum. The regulating circuitry now commands the servo-amplifier of the red channel (not shown) to turn the rotor of the servomotor 13 in such a way that the cyanogen filter 4 controlling the red component of the printing light is completely removed from the path of rays. The position of the yellow and magenta filters 2 and 3 respectively is adjusted until the desired and the actual value for each colour channel agree with one another. Only then can the shutter 7 be released by the action of blocking stage 71 on timer 74. The actual valueD for the blue channel is contained in the store 37; moreover the actual values D and D for the two other colour channels are stored (in stores not shown in the drawing).

The logarithmic signal D corresponding to the blue intensity after the negative is stored in the store 38 as voltage value; similarly, the signals D and D corresponding to the other two colour channels are stored in other stores (not shown). In the stage 57 the average value of the three signals D B, D and D is formed. After taking into account various adjustable parameters in the stages 54, 55, 58, such as enlargement factor, size of negative, objective data, paper parameters, the stage 64 produces a signal which is converted from a logrithm into an ordinary number again and converted in to a pulse of a certain duration for the control of the release mechanism 70 for the shutter 7. Fluctuations of the voltage supplied to the projection lamp 1, as well as changes of intensity caused by inserted colour filters and the negative present at planes in the light path of rays,

are virtually completely compensated by the measuring device that has been described. The time control has the advantage that with a medium lamp power an optimum exposure time is possible. A constant time control would have to be designated for densest negative, which, as experience shows, requires approximately five times the lamp power; or, at equal lamp powers, an exposure time which is five times as long. In case of equipment malfunction seizure of movement of the colour filters 2, 3 and 4), compensation is no longer possible, whereupon the exposure is automatically blocked and wrong exposures are avoided.

Some other features of the apparatus of FIG. 1 will now be described.

Frequently it is desirable to make the exposure time also dependent on the brightness of the negative. This is done in stage 61 already mentioned. In stage 73 the average value of the blue, green and red components of the densities of the negative is formed. By adjustment of the so-called brightness slope the effect of this measured average value can be adjusted before application to stage 64.

The diagram of FIG. 6 shows the function of the stage 61 (brightness slope) in which the logarithm of the exposure time t is plotted as a function of mean negative density D Let the tested standard negative, which is again symbolised by index St, be correctly printed with an exposure time t at point 113. Without the effect of the brightness slope the exposure times for negatives with deviating brightness would be on the straight line 110. With the effect of the brightness slope, e.g. according to straight line 1.11, negatives, the means density D of which is greater than the mean density D of the standard negative will be exposed for less time than would otherwise be the case, while bright negatives will be exposed for longer than without brightness slope correction.

The two light guides 20a and 20b pick up the printing light integrally for what we shall call an integral measurement. The light guide 21 serves for what we shall call a partial measurement in the printing plane and can be displaced with its input cross-section in that plane. Either the two light guides 20a and 2011 or the light guide 21 are used in making a measurement, or are connected to common optical guide 22 respectively. An auxiliary circuit, consisting of the summation stage 78, a switch 79 and an auxiliary voltage source 80, ensures that the differences of intensity arising between the integral and the partial measuring method have no effect on the measured result. Three further switches 77, 76 and 75 serve for the manual operation of the light shutter 7 and for the blocking of the servomotor 11. The shutter 7 must be opened for a measurement to be made with the light guide 21, no printing material being present at plane 8; the switch 75 is closed together with the corresponding switches in the other channels. The colour filters 2, 3 and 4 are then adjusted in their servo-loops as described above. By manual opening of the switch 75 movement of the colour filter 2 is blocked; similarly movement of the filters 3 and 4 are blocked by opening the corresponding switches in the two other colour channels. The shutter 7 is closed by the switch 76. After insertion of the printing material the shutter 7 can be released by means of the switch 77 formed as a key controlling the timer 74 for exposure. To allow measurement and exposure to take place under identical conditions, the voltage applied to the projection lamp 1 is kept constant by means of a stabilizer 81 inserted in the power supply line.

Owing to the different measuring conditions of integral and partial measurement-which occur even with a negative uniform over its area and with use of a constant light intensity -the light flux issuing from the light guide 21 via the guide 22 will be different from that obtained by use of the light guides 20a, 20b. In the store 38 different density signals will be stored depending on the method of measurement. In order to receive identical density signals in both measuring methods, the selection switch 79 will be switched on at partial measurement to apply the auxiliary voltage U to the summation stage 78. The voltage U has its polarity and absolute value so chosen that at the output signal of the stage 78 is independent of the measuring method. For dilferent enlargement factors and light guides naturally also several voltage sources 80 can be selectively switched onto the summation stage 78.

In summary it will be realised that the described apparatus provides compensation for the effects of colour changes in the printing light source or the control filters while allowing the latter to be adjusted in respect of the colour densities of each negative presented for printing.

What is claimed is:

1. Apparatus for making prints from colour negatives, comprising a light path including a source of light, a plane at which a negative to be printed is receivable, a plane at which printing material is receivable, means for imaging a received negative at the printing plane, a plurality of filters interposed in said path between said source and said negative plane, said filters being selectively actuable to control the proportions of the primary colour components in the light received at said negative plane, and means disposed between said filters and said negative plane to mix said colour components to produce a printing light for illuminating a negative received at the negative plane; first and second sampling means disposed at opposite sides of said negative plane to sample the light transmitted to said negative plane for illuminating a negative and the light transmitted from said negative after transmission through the negative respectively; means responsive to the primary colour components of the sampled light to produce a first set of signals representing the intensities of the primary colour components sampled by said first sampling means and a second set of signals representing the intensities of the primary colour components sampled by said second sampling means; a respective means for comparing each primary colour signal of the first set with the corresponding primary colour signal of the second set to derive a signal representing the difference between the intensities of that primary colour at said opposite sides of said negative plane; a respective means coupled to actuate each of said filters; and respective means coupling each comparison means to that one of the actuating means for the filter controlling the primary colour with which the comparison means is associated whereby each filter is actuated in dependence upon said difference signal of the primary colour controlled by the filter.

2. Apparatus as claimed in claim 1 further comprising means connected to all said comparison means and responsive to said colour component diiference signals to provide a signal representing the average thereof; and wherein each coupling means comprises a stage having inputs coupled to the output of the associated comparison means and to the output of said averaging means to combine the output signals therefrom, and an under-correction stage responsive to said combined signals to modify the value thereof, the output of said under-correction stage being coupled as a signal independent of the density of a received negative to control the associated filter actuating means.

3. Apparatus as claimed in claim 2 wherein said light path further comprises a shutter disposed between said negative plane and said printing plane to control the transmission of light to said printing plane; and further comprising means including a timer to open said shutter to allow light to impinge on printing material received at said printing plane for a desired period of time, said timer having an input for reeciving a signal the value of which determines said period of time, means responsive to said first set of signals to provide a signal representing the average value of the signals of said first set, and means coupled to said averaging means to combine said first set, average value signal with said signal representing the average of said colour component difference signals to produce a resultant signal, said timer input being coupled to be responsive to said resultant signal.

4. Apparatus according to claim 1 wherein said means responsive to the primary colour components of the sampled light includes a logarithmic converter for producing said first and second sets of signals in logarithmic form.

5. Apparatus as claimed in claim 1 wherein said sampled light-responsive means comprises a rotatably mounted opaque body having a set of filters for said primary colour components disposed at different angular positions therein, said body bearing angular position-indicative indicia, means arranged to detect said indicia as the body rotates, a photo cell arranged to one side of said body to respond to light transmitted by said filters therein; and switching means connected to said photoelectric device and having a plurality of outputs to which the photocell is successively connectable, said switching means being controllable by said position-indicative indicia detector to scan said outputs in synchronism with the rotation of said body; and wherein said first and second light sampling means terminate at the other side of the body to direct the sampled light toward said photoelectric device, the filters in the body being disposed to successively transmit light from each light sampling means to said photoelectric device upon rotation of the body whereby separation of the sampled light into primary colour components at the photoelectric device is effected to produce said first and second sets of primary colour components signals in a sequence therefrom, said switching means operating to di- 10 rect each primary colour component signal to a respective output thereof.

6. Apparatus as claimed in claim 5, wherein said sampled light-responsive means includes a logarithmic converter connected between said photo-electric device and said switching means to convert said primary colour component signals to logarithmic form.

7. Apparatus as claimed in claim 6 further comprising a window in said filter body, a reference light source at said other side of the body to illuminate said photoelectric device through said window at a predetermined angular position of the body, said switching means including a further scanned output at which is obtained a signal representing the illumination of said photoelectric device by said reference light source; means for comparing said reference light source signal with a predetermined signal to produce an error signal, and means in said logarithmic converter responsive to said error signal to control the conversion operation effected thereby so as to reduce said error signal.

References Cited UNITED STATES PATENTS 2,616,331 11/1952 Pavelle 35536 X 3,554,642 1/1971 Zahn 35532X SAMUEL S. MATTHEWS, Primary Examiner R. A. WINTERCORN, Assistant Examiner US. Cl. X.R.

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