US3110761A

US3110761A - Colour printer with colour correction

Info

Publication number: US3110761A
Application number: US85856A
Authority: US
Inventors: Allen Gordon Stanley James; Mawby David Harry; Gresham Donald Charles
Original assignee: Crosfield Electronics Ltd
Current assignee: Crosfield Electronics Ltd
Priority date: 1960-02-03
Filing date: 1961-01-30
Publication date: 1963-11-12
Anticipated expiration: 1980-11-12
Also published as: GB934108A

Description

2, 1963 G. s. J. ALLEN ETAL 3,110,761

COLOUR PRINTER WITH COLOUR CORRECTION Filed Jan. 30, 1961 cecla Y MAX/MUM MAX/MUM m I88 M1 mam? .SHHTOK 50\ j MIXER 38 I v v flmm fi fi mfizzm A ttorney:

United States Patent 3,110,761 COLOUR PRINTER WITH COLOUR CQRRECTION Gordon Stanley James Allen, London, David Harry Mawby, Tackleway, Hastings, and Donald Charles Gresham, London, England, assignors to Crosfield Electronics Limited, a British company Filed Jan. 30, 1961, Set. No. 85,856

Claims priority, application Great Britain Feb. 3, 1960 7 Claims. (Cl. 178-5.,2)

This invention relates to the reproduction of coloured originals and in particular to the correction of colours in the reproduction.

It is known that standard printing inks contain colour impurities so that, for example, magenta ink appears to contain some yellow. In single-stage masking a correction is made for this impurity by reducing the yellow printer density (whenever there is sufficient yellow to enable this to be done) in proportion to the magenta printer density. However, this does not provide good colour correction, firstly because of the failure of the effective blue-filter density of the superimposed magenta and yellow inks to equal the sum of the blue-filter densities of the inks when printed side by side, and secondly because the uncorrected negatives are matched to a neutral scale and too great a reduction of the yellow printer by a correcting printer will introduce serious colour distortion into the neutral tones. Such correction also reduces' the contrast of the grey scale, and this is particularly apparent when one colour channel signal (for example the yellow printer signal) is corrected from both the cyan printer and the magenta printer channels. The degree of correction obtained by this method is therefore usually limited by the amount of grey scale compression which is permissible.

According to the present invention, a coloured original or a set of separation transparencies derived from the original are electro-opt-ically scanned to provide electric signals representing the colour components of the original, and at least one of the colour component signals is corrected, for at least a :part of the range of relative values of the colour-component signals, by reducing its amplitude as a function of only the larger of the correcting signals. The selection of the larger of two signals can conveniently be achieved by means of an electronic maximum signal selector circuit consisting, for example, of a pair of diodes.

This method according to the invention is especially useful for the yellow printer channel in which correction is required from both the magenta and the cyan signals in areas corresponding to high values of magenta or cyan in the original to be reproduced. In the case of the yellow printer a predetermined percentage of magenta correction may be obtained where magenta exceeds cyan, and the same percentage of cyan correction may be obtained where cyan exceeds magenta. If these percentages are equal, the masking of the grey scale is expressed by the same percentage. If the two correcting signals were added together in the usual way, the masking of the grey scale would be equal to the sum of their two percentages.

According to a subsidiary feature of the invention, the signal representing the value of the colour to be corrected is modified in accordance with the value of the larger of two signals, the first of which represents the value of a first correcting colour and the second of which represents the difference between a second correcting colour and the colour to be corrected, or a function of this difference. Preferably, the signal representing the colour to be corrected is also modified in accordance with the value of two further signals, the first of which represents the second correcting colour and the second of which represents the difference between the first correcting colour and the colour to be corrected, or a function of this difference. The advantage of this method is that by subtracting the colour to be corrected from each of the con'ecting colours, each difference signal resulting from a subtraction has zero 'value for the neutral colour composed of the two corresponding colour components, and has a low value for colours close to this neutral colour. The gainin the circuits following the subtraction circuits can then be increased as required to increase the amplitude of each difference signal, which is then applied, together with a signal representing the other correcting colour channel signal, to a maximum signal selector circuit. The increasing of the amplitude of the difference signal adjusts the colour range within which the difference signal will exceed the correcting colour channel signal which is applied directly to the same maximum signal selector circuit. Thus, if the yellow printer is to be corrected, the correction for cyan and magenta can be increased without an increase in the reduction of the greys by adjusting the gain of the circuits following the first and second mixer circuits. However some reduction of greys may be desirable, especially in four-colour printing, when the grey scale in the colour printers is compressed to some extent to allow for the superimposition of a black printer.

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

FIGURE 1 shows diagrammatically colour correction apparatus embodying the invention;

FIGURE 2 shows a maximum signal selector circuit for use in the apparatus of FIGURE 1; and

FIGURE 3 shows diagrammatically a simplified form of apparatus embodying the invention.

In FIGURE 1, a cathode ray tube 6 constitutes the light source for scanning the separation transparencies, and is provided with conventional deflection coils and time-base circuits which produce a rectangular raster on the face of the tube. The moving light spot which pro- Vides the raster is modulated in intensity in accordance with a signal which is applied by way of conductor 7 to the grid of the tube. As is more fully described in Patent Ser. No. 654,408, the modulating signal which is to be applied to the grid of the cathode ray tube first passes through a gate circuit, not shown in the present drawings, which is controlled by a square-wave generator, so that the light spot on the face of the tube is pulsating in character, the object of this being to ensure that the signals derived from photo-electric devices which receive light from the cathode ray tube are alternating signals, as a result of which the design of the subsequent circuits is greatly simplified.

Light from the scanning spot on the face of the tube is focused by means of optical systems (not shown), which may include partially silvered mirrors, on to red, blue and

green separation negatives

8R, 8B and 8G respectively. As the light spot moves over the face of the cathode ray tube, it scans over and passes through each of the separation transparencies, the intensity of the light transmitted through each transparency depending upon the transmission factor of the transparency at the point which is being scanned. An unexposed light-transmitting photographic plate or film which is to provide the yellow printer is placed immediately behind and in contact with the separation transparency 8B, and as a result this plate or film will be exposed in accordance with the information on the blue separation transparency 8B, and also in accordance with the modulation on the face of the cathode ray tube. The film or plate 10 is backed by a filter 12 such that it passes only light to which the emulsion of the plate or film is insensitive, that is to say it prevents the transmission of light to which the emulsion is sensitive. This prevents undesired exposure of the emulsion by light reaching the latter through the back surface of the photographic plate. Assuming the emulsion to be sensitive only to the blue end of the spectrum, the backing 12 absorbs blue light but transmits light in the remainder of the spectrum. The light which passes through the emulsion of the photographic plate or film l and the backing 12 is diffused by the emulsion and backing, and consequently diffusing

plates

14R and 14G are placed behind the

separation transparencies

8R and 8G in order to diffuse the light transmitted through these transparencies in a similar manner.

The light rays which pass through the red separation transparency SR and diffusing plate 14R, the blue separation transparency SB and the plate or film 1i and its backing 12, and the green separation transparency 8G and its diffusing plate 146, are modulated in accordance with the densities corresponding to red, blue and green values of the elemental area which is being scanned at that moment. These light rays are collected, respectively by

lightintegrating units

16R, 16B and 16G and fall on

photomultipliers

18R, 18B and 186, the signals from which are the cyan, yellow and magenta printer signals, respectively. The signal from the photo-multiplier 18B, which varies with the density or transmission factor of the successively scanned elements of the blue separation negative 8B, that is to say the separation negative to be corrected, is applied to an invertor circuit 20, the output of which is applied to two

mixer circuits

22 and 24. These mixer circuits also receive the output signals from the

photomultipliers

18R and 18G, respectively, and their outputs thus represent the alegbraic sum of the red signal and the inverted blue signal, in the case of the mixer 22, and the alegbraic sum of the green signal and the inverted blue signal in the case of the mixer 24. The output signals from the

mixers

22 and 24 may thus be represented by (RB) and (G-B) respectively, in which R, B and G represent the red, blue and green channel signals. These signals are applied through

gain controls

26 and 28 to maximum

signal selector circuits

30 and 32 respectively. These two circuits also receive through

gain controls

34 and 36, signals representing the outputs of the

photomultipliers

18R and 18G, respectively. The output of the maximum signal selector circuit 30 is thus the larger of the two signals f (R) and f (GB), and the output of the maximum signal selector circuit 32 is the larger of the two signals f '(G) and f (RB), in which 3, f f and f are functions determined bythe settings of the

controls

26, 28, 34 and 36. These two output signals are applied to a further mixer circuit 38, in which they are added to provide the final correcting signal. This signal is the one which is applied by way of conductor 7 to the grid of the cathode ray tube 6.

In operating apparatus of the kind described above, the gain controls 26 and 28 following the first and

second mixer circuits

22 and 24 Would initially be set to minimum value so that the signals reaching the final mixer would be the cyan and magenta correcting signals, the apparatus then performing single-stage masking. The magenta correcting channel would modulate the light spot to reduce densities corresponding to grey, violet, magenta and red, and the cyan correcting channel would similarly reduce densities corresponding to grey, cyan, violet and green. The gain controls 34 and 36 which control the amount of the magenta and cyan signals applied to the two maxi mum

signal selector circuits

30 and 32 respectively are then adjusted to reduce these signals and the

gain controls

26 and 28 following the first and

second mixer circuits

22 and 24 are adjusted to increase the amplitudes of the difference signals applied to the maximum signal selectors. The signalfrom the mixer 24 then has a higher value for magenta and violet tones, that is to say for tones in which magenta exceeds yellow, and the signal from the mixer 22 has a higher value for cyan and violet tones, that is to say tones in which cyan exceeds yellow. For grey and red tones, the magenta component and yellow component signals have cancelled in the mixer 24, and for grey and green tones the cyan component and yellow component signals have cancelled in the mixer 22. Thus, the apparatus described enables the compression of neutral tones by any desired amount, together with a controlled amount of additional colour correction in the form of reduction of densities corresponding to magenta, cyan and violet in the yellow printer. As a result of the subtraction in the

mixers

22 and 24, the additional correction does not affect the neutral scales.

If additional correction of magenta or cyan in the yellow printer is not desired, the corresponding part of the apparatus shown in FIGURE 1 can be omitted. Thus if the additional cyan correction is not required, the mixer 22, the gain control 26 and the maximum signal selector circuit 32 could be omitted, the magenta signal from the gain control 36 being applied directly to the mixer 38. Alternatively, these parts can be left in the circuit, the gain control 26 being set to its minimum value.

In the apparatus shown diagrammatically in FIGURE 1, the'invertor circuit 20 is of conventional des'mn, and the

mixer circuits

22, 24 and 3 8 are conventional summing amplifiers. The maximum signal selector circuit is shown in FIGURE 2, from which it will be seen that the two input signals are applied over the

conductors

40 and 42 to the anodes of two

diodes

44 and 46, the cathodes of which are connected together and to a point of suitable fixed potential. It the signal on conductor 40 is more positive than the signal on conductor 4-2, the potential of the cathodes will rise to almost the potential of the conductor 40, and as a result the diode 46 will be blocked. Similarly, if the conductor 42 is at a more positi-ve potential than the conductor 40, the diode 44 will be blocked. In each case the output signal on conductor 4-8 will correspond to the more positive of the two input signals.

The apparatus shown in FIGURE 1 can be used for the correction of the cyan and magenta printer separation negatives SR and 8G, the negative to be corrected being placed in the central position in place of (the negative 8B of FIGURE 1, and the photographic film or plate to be exposed being placed behind the separation transparency to be corrected. In the case of the magenta printer separation negative however, the gain controls 28 and 36 would probably be set to a low value or zero because it is frequently unnecessary to correct a magenta positive with a yellow negative. The maximum signal selector circuit 32 would then act as a blocking circuit for negative-going signals from the mixer 22, and this channel would then in eiiect be a two stage magenta-cyan correcting loop. With the gain control 28 turned to zero the cyan signals from the photo-multiplier 18R would pass unaffected through the maximum signal selector circuit 30 to the mixer 38.

The apparatus shown diagrammatically in FIGURE 1 can also be used for the production of a corrected black printer positive. The uncorrected black printer separation negative may be produced, for example, by successively exposing the original through the red, blue and green filters, to white light. This black printer negative is then placed in the middle position in FIGURE 1, and two of the colour separation negatives, usually the yellow and cyan negatives are placed one on each side. The gain controls 34 and 36 to serve to vary the compression of the neutral scale to the required range, andat the same time reduce the unwanted colour densities. in the case of grey tones, the grey signal from the inverter 20 cancels with the yellow and cyan signals from the outer photo-multipliers in the

mixers

22 and 24. If additional colour reduction were required, the

controls

26 and 28 could be adjusted to increase the signals applied from the

mixers

22 and 24 to the maximum signal selector circuits .5 beyond the point at which the colour signals in these channels exceeded in amplitude the signals from the gain controls 34 and 36. These larger signals would then be selected by the maximum signal selector circuits and additional colour density reduction would be achieved in the black printer.

FIGURE 3 shows diagrammatically a simplified apparatus embodying the invention, which can sometimes be used to obtain colour correction. In FIGURE 3 it is again assumed that the blue separation transparency corresponding to the yellow printer is to be corrected. The signals from the photo-

multipliers

18R and 18G are in this case applied to a maximum signal selector circuit 50, the output of which is applied by way of a conductor 52 to the grid of the cathode ray tube. A given percentage of magenta correction is obtained in areas in which magenta exceeds cyan, and the same percentage of cyan correction is obtained in areas in which cyan exceeds magenta. The masking of the grey scale is then expressed by the same percentage, and not by their sum as would be the case if the two correcting signals were added together in the usual Way.

The simplified apparatus of FIGURE 3 can also be used for the correction of a black printer transparency, produced, as described above, by successive exposures through blue, red and green filters. The black printer negative is placed in the central position, with the photographic emulsion to be exposed behind and in contact with the black printer negative, and the correcting separation transparencies, for example the yellow and cyan transparencies, are placed in the router channels. The maximum signal selector circuit 50 then selects the larger of the output signals from the two outer photo-multipliers, and applies this to the cathode ray tube to reduce corresponding colour densities in the black printer. If desired, a fourth channel could be added in FIGURE 3 for the third colour separation transparency, and the maximum signal selector circuit would then be provided with three inputs.

In the simplified circuits shown in FIGURE 3, the larger of the two signals from the outer photo-multipliers is always applied to modulate the intensity of the scanning light spot on the face of the cathode ray tube. In FIG- URE 1, on the other hand, the smaller of the two correcting signals is not represented in the modulated signal applied to the cathode ray tube only when it is less than the difference signal with which it is compared in the maximum signal selector circuit. This difference signal is a function of the difference between the outer correcting signal and the signal to be corrected, the function depending upon the setting of the gain. control following the mixer circuit in which this difference signal is derived.

In FIGURE 1, the mixer circuit 38 can be replaced by a further maximum signal selector circuit or a single maximum signal selector can be used in place of the circuits 30, 3 2 and 38. If this is done, when a grey tone or a tone only a little different from grey is scanned, it is the signals Which are received directly from the photomultipl-iers 18R and 18G which are applied through the maximum

signal selector circuits

30 and 32 to the further maximum signal selector circuit, and for these tones the apparatus of FIGURE 1 is then equivalent to the apparatus of FIGURE 3 except that the gain controls 34 and 36 of FIGURE 1 would normally act to reduce the signals from the photo multipl-iers tor the reasons given earlier.

Although in the apparatus which has been described the photographic plate or film to be exposed is placed in contact with the corresponding separation negative, and is of such a kind that the scanning light rays pass through the combination to a photo-multiplier, any other form of electro-optical scanner can be used to produce the electrical signals representing the colour components of the original and to expose the photographic emulsion from which the printer is to be prepared. Similarly, other forms of light source can be used in place of the cathode ray tube, and if desired a number of light sources can be used for scanning the separation negatives and the printer positive. In a further alternative, the colour-component signal representing the colour to be corrected is added to the signal which, in the example described above, was used to modulate the intensity of the scanning light spot, the corrected colour-component signal thereby obtained being used to modulate a light source which directly exposes the plate or film which will provide the colour printer. Furthermore, the separation negatives can be replaced by the original transparency, the scanning light rays which pass through the original being separated and passed through filters to fall on photoelectric devices which provide colour component signals.

We claim:

1. Apparatus for use in the reproduction of coloured originals, comprising: an electro-optical scanner for deriving electric signals representing the colour components of successive elemental areas of a coloured original; means for determining a correction for a given one of said colour component signals including maximum signal selector means arranged to receive only signals representing the colour components for colour correcting said given component and to provide an output signal representing the larger of these colour correcting components; and means controlled by said given colour component signal and said maximum signal selector means for exposing a lightsensitive layer, which is to provide a corrected printer for the given colour component, in accordance with the uncorrected colour component values represented by said given colour component modified in accordance with the output of the maximum signal selector circuit.

2. Apparatus for use in the reproduction of coloured originals, comprising: an electro-optical scanner for providing a set of electric signals representing the colourcomponent values of successive elemental areas of a coloured original, a subtracting circuit connected to said scanner for subtracting the signal corresponding to a given colour component to be corrected from a first correcting colour channel signal; a maximum signal selector circuit which receives the second correcting colour channel signal and the output of the difference circuit, and provides an output representing the larger of these two signals; and means controlled by said given colour component signal and said maximum signal selector circuit for exposing a light-sensitive layer which is to form a corrected colour printer in accordance with the uncorrected colour-component values represented by said given colour component signal modified in accordance with both the output signal from the maximum signal selector circuit and the first correcting colour channel signal.

3. Apparatus for use in the reproduction of a coloured original, comprising: an electro-optical scanner for deriving a set of electric signals representing the colour-component values of successive elemental areas of a coloured original; a first subtracting circuit for subtracting the colour component signal representing the colour to be corrected from the first correcting colour channel signal; a second subtracting circuit for subtracting the colour component signal representing the colour to be corrected from the second correcting colour channel signal; a first maximum signal selector circuit connected to receive the first correcting colour channel signal and the output signal from the second difference circuit; a second maximum signal selector circuit connected to receive the second correcting colour channel signal and the output of the first difference circuit; and means controlled by said given colour component signal and said two maximum signal selector circuits for exposing a light-sensitive layer which is to form the corrected colour printer in accordance with the uncorrected colour-component values represented by said given colour component signal modified in accordance with each of the two output signals from the two maximum signal selector circuits.

4. Apparatus according to claim 2 including an adjustable gain. control between each difference circuit and the following maximum signal selector circuit.

5. Apparatus for use in the reproduction of a coloured original, comprising: an electro-optical scanner for deriving a set of electric signals representing the colour-component values of successive elemental areas of a coloured original, a first subtracting circuit for subtracting the colour component signal representing the colour to be corrected from the first correcting colour channel signal a second subtracting circuit for subtracting the colour component signal representing the colour to be corrected from the second correcting colour channel signal; and maximum signal selector means connected to receive the first and second correcting colourchannel signals and the signals from the first and second difference circuits and to provide an output signal representing at any moment the largest of these signals; and means controlled by said given colour component signal and said maximum signal selector circuit for exposing a light-sensitive layer which is to form the corrected colour printer in accordance with the uncorrected colour-component values represented by said given colour component signal modified in accordance with the output of the maximum signal selector means.

6. Apparatus according to claim 2 in which a light source in the electro-optical scanning means is arranged to scan three separation transparencies, and in which a lighttransrnitting and light-sensitive layer to be exposed is placed behind and substantially in contact with the separation transparency to be corrected, the apparatus including separate photo-electric devices for receiving the light rays passing through the two correcting separation transparencies and the light rays passing through the combination of the transparency to be corrected and the light-sensitive layer to be exposed, the output signals from the photo-electric devices varying with the colour-component values in the successively scanned elemental areas of the separation images.

7. Apparatus aaccording to claim 1, including a light source for directly exposing the light-sensitive layer in accordance with corrected colour-component values, and means for modulating the light source in accordance with both the colour-component signal representing the colour to be corrected and a signal representing the correction to be applied, whereby the light-sensitive layer is exposed with corrected colour-component values.

References Cited in the file of this patent UNITED STATES PATENTS 2,892,016 Hall June 23, 1959