US2985712A - Colour reproduction systems for correcting colour separation printer images - Google Patents

Description

May 23, 1961 D. H. MAWBY 2,

COLOUR REPRODUCTION SYSTEM FOR CORRECTING COLOUR SEPARATION PRINTER IMAGES Filed July 15, 1959 l8 Fig. 5. 5 8 U I I8 Fig. 4 R 1/ l8 F4 5 R u 6 I Inventor 29M flbwfi. flaw:

Attorney nitecl States COLOUR REPRODUCTION SYSTEMS FOR COR- figglgg; COLOUR SEPARATION PRINTER David Harry Mawby, London, England, assignor to J. F.

Crosfield Limited, London, England, a British com- P y This invention relates to colour reproduction systems of the kind in which a black printer is used in addition to the colour printer plates. When a black printer is used it is necessary to remove some of the colour from the colour printer plates, the amount removed being dependent on that provided by the black printer. The present invention has for its object to provide a system for undercolour removal which is simple but flexible.

In electronic colour reproduction systems, the electronic computer has previously been designed to compute as accurately as possible the amounts of the cyan, magenta and yellow inks which are required to simulate in the reproduction the red, green and blue components of the original. Usually, the design has been such that the computer solves the Neugebauer equations for colour reproduction. For a four-colour reproduction system (i.e. one including a black printer) these equations express each additive primary colour component of the original as the sum of sixteen colours, namely white, the three ink colours, the three colours resulting from an overlap of two coloured inks, the colour resulting from an overlap of the three coloured inks and each of the previous eight colours together with black. The coefficient of each colour except the triple-overlap colour includes expressions defined by the difference of two terms representing amounts of coloured inks. The computers have therefore included means for adding the correction signals in opposite polarity or phase. However, the results obtained from such computers have not been consistently good, and the applicant believes that this results from the presence of the difference terms in the correction signal produced in the computer. In theory, using difference terms in the solution of the Neugebauer equations should enable all the unwanted density to be removed from the colour separation printer image. In practice, a serious loss in the quality of the reproduction occurs because contamination is introduced due to small defects in the optical system, and noise which is always present in valves and, in particular, in photomultipliers. For instance, particles of dust which would normally not be noticed may cause small signals in one channel and may not be present in the other, or due to the optical arrangement the same dust particles may, in fact, be present in the other channel but occur at a different time. In order to achieve a high degree of correction, the signal in one channel is divided into a subtracted from the signal in the other, and the resultant quotient or difference signal which is often very small in comparison with the channel signals (and in which variations due to dust are of appreciable magnitude), has to be considerably amplified to produce a large change in illumination. Where these small difierences have to be expanded, the presence of dust particles is very noticeable in the printed result due to the action of the correction circuits. Although dust particles have been used in the above example, grain or other blemishes on any plate or diffusing surface on or near the object or image planes or on any reflecting sur- 2,985,712 Patented May 23, 1961 face produce the same result. In addition, when a light beam is split through two or more lenses the inherent aberration of the lenses can cause density drifts which are increased in a similar way. Added to this there is always present in any circuit some noise, especially where photomultipliers are involved, and this too can be amplified to add to the interference of the result.

The applicant has found that these defects are substantially avoided if all the colour correction and undercolour removal signals modulating the exposure of the colour separation printer image are of the same polarity, the polarity being such that each tends to reduce the density of a positive image resulting from the exposure. Moreover, the applicant has found that although in theory the smallest of the colour channel signals should be selected as being representative of the neutral component present in the scanned element, and should be used for undercolour removal, in practice such as system introduces further difiiculties. When the signals are small, a slight drift in the relative operating potentials of the three channels can result in the Wrong signal being selected. Additional circuits to stabilise voltage levels can be cumbersome and detract from the reliability of the device as a whole.

According in the present invention therefore the colour separation printer images are obtained by scanning at least one image constituted by or derived from the original to obtain a plurality of electric signals varying with the transmission values of the scanned elements, at least one of the signals including a component representing undercolor to be removed from the colour separation printer images, deriving from the said electric signals correction signals representing the colour correction to be applied to the colour separation printer images, modulating a light source by means of an electric signal which varies continuously with each of the undercolour removal signals from the scanner and which also includes the colour correction signal, and exposing each of the photographic surfaces from which the colour separation printer images are to be produced to light which originates at the light source and which at each point on the photographic surface represents the density of the colour to be corrected modified in accordance with the colour correction and undercolour removal signals, each of which acts in a sense such as to reduce the density of a positive image resulting from the exposure.

This method has been found in practice to produce acceptable and reliable results. The signal designed to efiect the removal of undercolour is derived throughout the exposure from the same source and defects arising from relative changes due to potential drifts do not occur. Moreover, the interference resulting from the presence of difference terms does not exist. As a result a small misadjustment of the correction circuit does not produce a completely unacceptable result, as in other systems where the adjustment is necessarily more critical.

When a single image is scanned to provide the undercolour removal signal, it may be obtained by exposure to white light transmitted through an original colour transparency, or it may be obtained as a colour separation plate from the original, the colour separation plate being produced by a single exposure to light representing the presence of a single colour in the original, or by a number of exposures in which the light rays represent respectively the presence of a number of different colours in the original.

According to a subsidiary feature of the invention, a black printer is obtained by scanning an image or images to obtain a colour-component signal or signals, and exposing a photographic surface from which the black printer is to be produced to light which at each point on.

the surface represents the density of a multi-coloured image of the original modified in accordance with the values of the colour-component signal or signals so as to reduce the density of a positive image resulting from the exposure.

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

Figure 1 shows diagrammatically a form of imagereproducing apparatus which may be used to carry out the method according to the present invention; and

Figures 2 to 5 are diagrams illustrating the manner in which the apparatus of Figure l is used.

In carrying into efiect this example of the method according to the invention an undercolour removal negative is first made by exposure of a photographic layer to substantially white light transmitted through or reflected from a coloured original, or by exposure to the coloured original through a number of colour filters (for example, through red, green and blue filters in succession). In practice, it may in some cases be desirable for the exposing light source to have some colour bias. The undercolour removal negative is then used, for the purpose of removing undercolour from the colour separation plates, in the apparatus of Figure 1. This apparatus includes a cathode ray tube 6 on the face of which is formed a scanning raster 8 which is imaged on to three transparencies 10-. Light which passes through the three transparencies is collected by the three multipliers 12, the output signals from which are fed to a computer 14. The computer 14 provides four signals representing the corrections to be applied in producing the photographic plates from which the three colour printers and the black printer are made. The switch 16 enables one of these output signals to be applied to the control electrode of the cathode ray tube 6, with the result that a correction light mask is formed on the face of the tube. The photographic plate 18 to be exposed is placed immediately be hind and substantially in contact with the middle one of the three transparencies 10, and is consequently exposed with light values representing the transmission values of the middle transparency 10 corrected by the light values of the corresponding elements of the correction light mask 8 on the face of the cathode ray tube. Diifusion plates 20 are placed behind the outside transparencies 10, the difiusion produced by these plates being equal to that produced by the photographic plate 18, and the latter is provided with a backing filter 22 which prevents exposure of the plate 18 to light of a colour to which it is sensitive While permitting the passage of light of a colour to which the plate 18 is insensitive. In producing the three colour printer plates each separation transparency in turn is placed in the centre position, and the transparencies which are to be used to correct the printer plate are placed in the outer positions.

In the example which is being described, to produce the magenta printer plate, the green filter separation negative G is placed in the centre position in contact with the plate 18 to be exposed, as shown diagrammatically in Figure 2. On one side is placed the red filter (cyan printer) separation negative R which alone is used to provide colour correction for the magenta printer. On the other side is placed the undercolour removal negative U, which is used to produce undercolour removal for the magenta printer. By adjusting the amounts of cyan and undercolour removal signal which are added in the computer 14 to provide the modulating signal for the light source 6, satisfactory colour correction and undercolour removal can be produced for the magenta. The cyan correction signal may be increased above the normal colour correction value (which may in a typical case be about 30%) to provide, some undercolour removal as well as colour correction. In terms-of percentage signal measured on the grey scale, there may be 4 40% subtraction of the cyan signal and 20% subtraction of the undercolour removal signal.

In the case of the yellow printer, the blue filter separation negative B is placed inthe centre position, the green filter (magenta printer) separation negative G is placed on one side for the purpose of "colour correction, .and the white light undercolour removal negative U is placed on the other side to provide undercolour removal, as shown diagrammatically in Figure 3. Again, there may be 40% substraction of the magenta signal and 20% subtraction of the signal from the negative U.

The cyan printer does not need to have a large colour correction signal from one of the other colours, and it is therefore used to correct itself. The red filter (cyan printer) negative R is placed in the centre position (Figure 4) and a signal'is fed back from the centre photomultiplier (cyan channel) to reduce itself to approximately the same extent as the magenta and yellow were reduced by their correcting colours. The white light negative U is again used for undercolour removal. Suitable subtraction values are again 40% from the cyan negative R and 20% from the negative U.

In cases where the feedback required for the correction of the yellow and magenta plates is different, compensation for the plate requiring the smaller correction can similarly be obtained by utilising the centre photomultiplier to feed back a signal derived from this plate in such a sense that the signal reduces itself. If the total correction is maintained the same for each plate, then the separations produced can all be given the same photographic development treatment.

To produce the black printer plate, the white light negative U is used in the centre position (Figure 5) and the red filter (cyan printer) and blue filter (yellow printer) negatives R and B are used in the two side positions. These two signals are fed back to the cathode ray tube together to correct the black printer, that is to say to remove density from the black printer in the coloured areas of the picture. Suitable subtraction values are 30% from each of the negatives R and B.

In one modification of this method, the cyan printer separation negative R is used for undercolour removal instead of the white light negative U in the production of the magenta and yellow printers. This means that in the case ofthe magenta printer (Figure 2) a correction signal, sufiicient to give colour correction, is produced from the cyan negative, and then a further percentage of cyan channel signal is added to the correction signal to give the required amount of undercolour removal, the righthand position being unused. In the case of the yellow printer, the magenta signal is still used for colour correction, but its subtraction value may be increased to produce some undercolour removal, the remainder being effected by the cyan signal. The amount of colour removed by the cyan signal is, of course, greater than the amount which would be removed if the cyan signal were used only for further colour correction of the magenta printer in a three-colour system in which no undercolour removal is carried out. The cyan printer and black printer plates are produced in the same was as before.

It will be clear that if desired the apparatus of Figure 1 can be modified by the provision of a fourth separation negative position, and that two of these four positions can then be used for colour-correcting separation negatives while another is used for the white light negative for undercolour removal. The remaining position is, of course, that which receives the separation negative to be corrected in contact with the plate to be exposed.

The method according to the invention can also be used with other known forms of scanning and exposure apparatus, for example rotating-drum and oscillatingtable scanners, and apparatus in which separate analysing and reproducing scanners are used.

As well as colour correction and undercolour removal for each plate, modification to the tonal characteristic curve of the plate can simultaneously be made by feeding signals through a separate channel as described in copending application No. 826,764, filed July 13, 1959. In the case of the colour separation positive plates, when undercolour removal is being applied, the required tonal modification is made to a scale of densities representing the total range of the colour (after undercolour removal) to be printed from that positive. In the case of the black printer positive, the modification is adjusted so that when it is combined with the colour positives in the printing process, the scan of greys for the subject is also correct.

I claim:

1. In a colour reproduction system of the type using a black printer, the method of producing at least one of the colour printers which includes the following steps, placing an unexposed photographic emulsion from which the colour printer is to be derived behind a transparency representing the image, scanning the transparency by means of a light source in an electro-optical scanner to expose the photographic emulsion to light variations representing the corresponding colour component image, simultaneously scanning with light from the same light source at least one transparency representing the image to obtain a scanner output signal representing colour correction to be applied to the colour printer and a scanner output signal representing undercolour to be removed from the colour printer, the undercolour removal signal being always derived from the same transparency, adding the said scanner output signals to produce a resultant correcting signal, and modulating the light source with the said resultant signal, whereby the said photographic emulsion is exposed to light values which include, in addition to the modulation due to the transparency behind which the photographic emulsion is placed, variations representing colour correction and undercolour removal.

2. A method according to claim 1, in which the transparency which is scanned to provide the undercolour removal signal is obtained by exposure of an original colour transparency to light which is substantially white.

3. A method according to claim 1, in which the photographic emulsion to be exposed is placed behind and substantially in contact with the transparency through which it is to be exposed.

4. A method according to claim 1, in which the transparency which is scanned to provide the undercolour removal signal is the cyan printer separation transparency.

5. In a colour reproduction system of the type using a black printer, the method of producing at least one of the colour printers which includes the following steps, placing an unexposed photographic emulsion from which the colour printer is to be derived behind a transparency representing the image, scanning the transparency by means of a light source in an electro-optical scanner to expose the photographic emulsion to light variations representing the corresponding colour component image, simultaneously scanning with light from the same light source a transparency representing the image to obtain a colour channel signal, amplifying the colour channel signal to an extent greater than that required for colour correction and modulating the light source with the amplified signal, the photographic emulsion being thereby exposed to light values which include, in addition to the modulation due to the transparency behind which the photographic emulsion is placed, variations due to the light source modulation which represent colour correction and a degree of undercolour removal and which are always derived from the scanning of the same transparency.

6. A method according to claim 5, in which the transparency which is scanned to provide a signal which, when amplified, serves both for colour correction and for undercolour removal, is the cyan printer separation transparency.

7. A method according to claim 5, in which the transparency which is scanned to provide a signal which, when amplified, serves both for colour correction and for undercolour removal, is the magenta printer separation transparency.

8. In a colour reproduction system of the type employing a black printer, the method of obtaining the black printer for the image to be reproduced including the following steps, placing an unexposed photographic emulsion from which the black printer is to be derived behind a transparency representing the colour image, scanning the transparency by means of a light source in an electrooptical scanner to expose the photographic emulsion to light variations representing the transmission values of the multi-coloured image, simultaneously scanning with light from the same light source at least one transparency representing the image to obtain at least one scanner output signal representing a colour component of the multicolour image, and modulating the light source with the said colour-component signals, whereby the said photo graphic emulsion is exposed to light variations which include, in addition to the modulation due to the transparency behind which the photographic emulsion is placed, variations acting in a sense such as to reduce the density of a positive image resulting from the exposure.

Hall July 15, 1941 Rydz May 5, 1959

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