GB1605076A - Colour printers - Google Patents

Abstract

The printer analyses the red, green and blue densities of the original and then, by means of a linear combination of these densities, determines the red, green and blue exposure times. The printer transforms the first reference space (R, G and B) into a second reference space (N, GC and I) so as to be able to define and make easier corrections to the printing with a view to obtaining better prints. Application to the production of automatic printers for colour prints. <IMAGE>

Description

(54) IMPROVEMENTS IN OR RELATING TO COLOUR PRINTERS (71) We, EASTMAN KODAK COMPANY, a Company organized under the laws of the State of New Jersey, United States of America of 343 State Street, Rochester, New York 14650, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to automatic colour photographic printers having variable matrix exposure control computers.

U.S. Patent No. 3,120,782 discloses an exposure determination system for a colour photographic printer that is based on a linear combination of red, green, and blue large area transmission densities (LATD's). For an explanation of LATD and a general introduction to the discussion that follows, see the SPSE Handbook of Photographic Science and Engineering, Wiley and Sons 1973, pages 461--468. In such a printer, the red exposure, for example, is computed as a function of all three (i.e. red, green and blue) LATD's rather than being based on a measurement of the red LATD alone.

If the characteristics of the printing filters are similar to the responses of the colour channels of the densitometer portion of such a printer, the printing densities relate linearly to the measurements made by the densitometer portion of the 'printer and can be expressed in matrix form as follows: [D]=[p] [C]([LATDi-[LATD]) ( I ) where: [D] - is a 3x 1 column vector having elements d, representing Printing Densities in a Red, Green, Blue co-ordinate system; [P1 -- is a 3x3 matrix of elements p,l relating Integral Densities to Printing Densities ; [C] -- is a 3x3 diagonal matrix of elements c,i relating Densitometer Voltages to Integral Densities; [LATDl - is a 3x I column vector having elements LATD, representing the Densitometer Voltages generated by the Red, Green, and Blue colour channels of the densitometer portion of the printer; and [LATD] -- is a 3x 1 column vector having elements LATD representing the Densitometer Voltages of a calibration original, the calibration original representing some standard such as the center of the population of originals to be printed.

The exposure equations for such a printer take the following form: [log E]=[K]+[A][D] (2) where: [log El - is a 3x I column vector having elements log E, representing the Log Source Exposures of the exposure portion of the printer, i.e. the exposures impinging upon the original, expressed in a Red. Green, Blue co-ordinate system; [K] - is a 3x I column vector having elements k, representing the average Log Exposures or aim points for the population of originals being printed; [Di - is a 3x 1 column vector having elements d, representing the Printing Densities calculated in equation (1); and [A] - is a 3x3 matrix of elements a,; relating the Log Exposures to the Printing Densities.

all a12 a13 [A] = a21 a22 a23 a31 a32 a33 The matrix [A], which is a linear matrix, is called the correction matrix of the printer and will generally have off-diagonal elements. A linear matrix is one which offers solutions to linear systems of equations i.e. equations not involving power terms. Matrix [A] is physically embodied in the printer described in U.S. Patent No. 3,120,782 as a matrix of operational amplifiers and variable resistors. The elements or correction values a11 of the correction matrix are adjustable by changing the values or responses of the electronic components comprising the matrix. For this reason, such a printer is called a settable matrix printer. In another type of settable matrix printer, the matrix is stored in digital form in the memory of a digital computer associated with the printer. The exposure control equations are solved by the digital computer, and the results are used to control the exposure portion of the printer. An example of such a printer is the Eastman Kodak Company 2610 Colour Printer.

In a settable matrix printer, the correction matrix [A] is said to be a zero correction matrix if measured differences in LATD's, from original to original, result in corresponding changes of comparable magnitude in the resulting prints.

Thus if matrix [A1 is zero correction matrix, the log exposures will be equal to the aim points k, of the exposure equations (2), and the correction matrix [A] will approach a null matrix, i.e., all elements approach zero.

A full correction matrix is defined as one that results in all originals being printed so that there is no variation in the average overall densities in the resulting prints. In such a case, the correction matrix [A] would approach a unit diagonal matrix, i.e., elements al,, a22, and a33 would approach unity, and all other elements would approach zero.

Full correction is desirable to compensate for unwanted variations in LATD's caused by such things as film keeping, incorrect exposure, and improper match between illuminant and film balance (e.g., exposing daylight film under tungsten lighting conditions). However, for those cases where the variations in LATD are due solely to the content of the original scene (commonly called subject failure scenes because they disobey the basic assumption that all scenes integrate to a shade near grey), a zero correction matrix is desirable. The zero correction matrix is needed so that unwanted colour shifts in the resulting print will not be introduced by the corrective action of the printer.

One approach to resolving these conflicting requirements for zero and full correction has been to adjust the values of matrix elements razz in correction matrix [A1 so that the printer will operate at some compromise correction between full and zero. For a discussion of optimum correction for photographic printers in general, see the Article published by Bartelson and Huboi in the Journal of the Society of Motion Picture and Television Engineers, Vol. 65, pages 205-215, 1956.

As is pointed out in the Bartelson and Huboi article, the optimum correction level is a function of the population of negatives being printed and varies for different negative populations.

In order to explain the prior art methods for adjusting the matrix elements in the printer correction matrix [A], it would first be helpful to expand upon the concept of printer correction that has been discussed above.

In general, "correction level" may be defined as the rate of change of a given component of source exposure with respect to some related change in a component of the density of the original. If the exposure equation (2) is rewritten to represent changes, it appears as follows: [h log E]=IA]tA D] (3) It is seen that in the general sense, the matrix elements a" are correction levels since they relate changes in components of log source exposure ( log Ej) to some changes in components of the measured density of the original (å D,). For example, element a" relates the change in red log exposure to a change in red density; a21 relates the change in green log exposure to a change in red density, etc. The relationships or correction levels defined by the elements of matrix [A], however, do not coincide with any intuitive concept of correcting the exposure of a print.

The traditional subjective criteria for evaluating colour prints have been neutral density (i.e., is the overall print too light or too dark), colour balance or hue (i.e., do the colours in the print appear as the scene is remembered) and saturation or pureness of the colour (i.e., are the colours bright and pure, or are they grey and muddy).

The primary additive colours can be thought of in terms of an orthogonal 3 dimensional co-ordinate system with axes labeled R, G and B (see Figure 1). The traditional subjective criteria for judging colour prints may be described in a different co-ordinate system defined in the following way: A Neutral axis N is defined as R=G=B (red=green=blue). At any given point on the Neutral axis N, a plane perpendicular to the axis N is called a chromaticity plane and the projection of the R, G, B axes onto the chromaticity plane are called the Red, Green, and Blue chromaticity axes. The radial distance from the point n on the chromaticity plane represents saturation and the location in the chromaticity plane relative to the chromaticity axes represents hue. This different co-ordinate system is shown relative to the R, G, B co-ordinate system in Figure 1.

Tradiationally, correction levels have been discussed in terms of changes along the Neutral axis N and changes along axes in a chromaticity plane.

Strictly speaking, there is only one Overall Neutral Correction Level and it represents the rate of change in the neutral component of the combined R, G, B Log Source Exposures with respect to a given change in the neutral density of an original. The Overall Neutral Correction Level is defined as: A log EN ONC= (4) A DN where: ONC is the Overall Neutral Correction Level A log EN=l/3 (A log ER+A log EG+A log E,,) and A DN=l/3 (d DR+A DG+A DB) However, the Overall Neutral Correction Level is commonly broken down into its Red, Green, and Blue components which are traditionally referred to as slopes and are expressed as: A log E1 NC,- (5) A D, where i takes on the values R, G, or B.

Any printer may be considered to have a unique chromaticity correction level for any arbitrary axis in a chromaticity plane. However, the most commonly considered chromaticity correction levels have been those occurring along the Red, Green, and Blue chromaticity axes and are defined as: A log ECI CC,= (6) A Dci where: i takes on the values R, G, and B and log Eci and Dc, are changes in log exposure and density along the respective R, G and B chromaticity axes.

In addition, changes along an axis that lies substantially midway between the minus Red (or Cyan) and Blue chromaticity axes in the chromaticity plane have been considered. This chromaticity axis will be called the Illuminant-chromaticity axis since it is generally along this axis that colour shifts occur when scene illumination is changed from tungsten to daylight. Since these colour shifts are less frequently representative of scene content, it is usually desirable to have a high correction level along the Illuminant-chromaticity axis. Figure 2 represents a chromaticity plane showing the relationship of the Red, Green, and Blue chromaticity axis to the Illuminant-chromaticity axis. Extensions of the R, G, and B axes are labeled C, M, and Y respectively to represent the subtractive primary colours cyan, magenta, and yellow.

The chromatic correction levels along the Illuminant-chromaticity axis can be calculated from the correction matrix [A] of a settable matrix printer, but are usually determined empirically by printing a test patch or set of patches that vary from an average along the Illuminant-chromaticity axis. The patches are printed in a "locked beam" (i.e., correction matrix disabled) mode and in a "normal" (correction matrix in effect) mode and the resulting prints compared by densitometry to see how much correction has taken place.

Since most amateur photographic cameras operate with a fixed aperture and shutter speed i.e., they do not include automatic exposure control, a large percentage of the pictures taken with such cameras are either under or over exposed. Thus, in any given population of originals taken by amateur photographers, by far the largest number of unwanted variations in the originals occur along the Neutral axis. For this reason, a relatively high Overall Neutral Correction Level is generally desirable. The next most frequent unwanted variation in originals taken by amateur photographers occurs along the Illuminantchromaticity axis and is due to the fact that when film that is balanced for daylight exposure is exposed indoors under tungsten illumination, the colours in the original will be shifted along the illuminant axis with the resulting print appearing too warm.

The reverse of this problem occurs when film balanced for tungsten illumination is exposed in daylight. Thus, a relatively high Illuminant-chromaticity correction level is generally desirable.

According to prior art methods, the Red-neutral, Green-neutral, and Blueneutral and Red-chromaticity, Green-chromaticity and Blue-chromaticity correction levels have been computed from the elements of the correction matrix [A] in the following manner:

where NC, are the Red, Green, and Blue Neutral Correction Levels, ONC is the Overall Neutral Correction Level and a, are elements of correction matrix [A].

and CC,=2/3 airy/3 (a,j+a,k+al,+aK)+l/6 (ai+akk+alk+ak;) (9) where: a4 are the correction matrix elements, injok, and CC, are the Red, Green, and Blue Chromaticity Correction Levels.

When it was desired to modify the correction matrix [A] to change correction levels, a change matrix [Q] was constructed according to some empirically derived rules outlined below. The old correction matrix [A] was multiplied on the right by the change matrix [Q] to yield the new correction matrix.

[A]=[A]cid[Q] where [Q] is a 3x3 matrix of elements q"

911 q12 q13 q21 q22 q23 (10) q31 q32 q33 For independent adjustment to all Red, Green, and Blue correction levels, the following rules were applied to generate the elements q,j of the change matrix [Q]: diagonal elements q"=.222 (5X,-X,-X,-1.5Y,) (Il) Y1-q11 off-diagonal elements q,=q,,= 2 (12) where: X, axis is the ratio of the new chromaticity correction level to the old chromaticity correction level CC1 new xi CC1 old Y1 - is the ratio of the new neutral correction level to the old neutral correction level NC Y1= NC1 old and ijok, each taking on the value of R, G, or B. Some control over the Illuminant-chromaticity correction level was exercised by relating the Illuminantchromaticity correction level to the Red, Green, and Blue chromaticity correction levels by the formula: CC,=2/3 (CC,+CC,)-1/3 CCa where CC, is the Illuminant-chromaticity correction level.

From the above, it can be seen that, at most, 6 parameters (the 3 neutral correction levels NC, and 3 chromaticity correction levels CC,) were used to define changes to all nine elements of the correction matrix [A].

There is an inherent limitation in the prior art method of modifying the correction matrix [A] since the combination of 6 correction levels does not define a 3x3 correction matrix [A]. Different correction matrices having different relationships between the off-diagonal elements (a11, ii) may produce identical values for the six correction levels defined above. These differences in the off- diagonal elements of different correction matrices having identical correction levels result in hue shifts in the resulting prints. These hue shifts have been intuitively discussed in the prior art in terms of colour "rotations". However, in the prior art methods known to the inventor, there is not a means for precisely quantifying and exactly controlling these colour rotations when changing correction levels in variable matrix printers.

A second problem faced by the photofinisher is that he does not intuitively think in terms of red-chromaticity and blue-chromaticity when examining a large group of prints. It is easier for him to think in terms of brightness-darkness and warmness-coldness.

It is the object of the present invention to provide a colour printer in which the correction levels can be changed without introducing colour rotation and/or can be changed by making adjustments to parameters that are easier for the photofinisher to relate to the visual inspection of prints.

According to the present invention there is provided a photographic printer comprising means for producing first signals representative of the colour components of an original and expressed in a first co-ordinate system, correction means having adjustable correction values for correcting the first signals to produce second signals, and exposure means for exposing a photosensitive copy material to controlled amounts of light as a function of the second signals expressed in the first co-ordinate system, wherein the printer includes either means for transforming the first signals from the first co-ordinate system to a second coordinate system and for retransforming the transformed signals, after correction by the correction means, to the first co-ordinate system or for transforming the correction values to the second co-ordinate system and for retransforming the transformed values, after adjustment, to the first co-ordinate system such that, in both alternatives, any adjustment of the correction values occurs in the second coordinate system.

A preferred example of a second co-ordinate system usuable in the invention is one in terms of neutral, illuminant-chromaticity and green-chromaticity.

According to a preferred embodiment of the present invention, the printer includes means for converting signals representative of the colour components of the original in the first co-ordinate system to signals representative of colour components of the original in the second co-ordinate system. These signals are then used to determine signals which are a function of exposure in terms of the second co-ordinate system, using a correction matrix in the second co-ordinate system.

These second co-ordinate system exposure signals are then converted back to the first co-ordinate system for printing.

An advantage of the present invention is that the correction matrix in the second co-ordinate system can be changed either manually or automatically, without introduction of unwanted colour rotations to the extent presently experienced when a matrix in red, green and blue is changed.

The manual change would ordinarily be effected when the overall performance of the printer needs changing e.g. a new series of originals, or a new printing paper. In such circumstances, a change is made directly to the second coordinate system matrix in the printer. The printer then is operated with the new matrix until changed again.

However, changes in the matrix can also be made automatically on a print to print basis in response to sensing a particular characteristic in the original. For example, it is known to scan the original and vary exposure as a result of detection of high contrast or the like. With the present invention, instead of directly varying exposure, the matrix can be varied by scaling portions of it according to the particular characteristic detected. This characteristic can be a characteristic detected by a scan of the original or it can be one or more of the measured LATD's.

According to another embodiment of the present invention, the printer includes a correction matrix in the same co-ordinate system as the exposing and original measuring portions of the printer, for example, red, green and blue.

However, the printer also has means for transforming that matrix into the second co-ordinate system when it is desired to change it. The change is then made, manually or automatically, in the second co-ordinate system and the changed matrix is converted to the first co-ordinate system for use in printing.

The invention also provides a method of producing a coloured photographic copy of an original, including the steps of 1) measuring the original to produce first signals which are a function of the colour components of the original in a red, green and blue co-ordinate system; 2) applying correction signals, representing a variable linear matrix formula having adjustable correction values, to the colour component signals to produce second signals representing a function of exposure for producing the copy; 3) exposing the original onto a photosensitive copy material in red, green and blue light as a function of received second signals; and, including either a) transforming the first signals into signals in a neutral, green-chromaticity and illuminant-chromaticity co-ordinate system and, retransforming the transformed signals after correction to produce the second signals representing a function of exposure in the red, green and blue co-ordinate system, or b) transforming the correction values into the neutral, green-chromaticity and illuminant-chromaticity co-ordinate system, adjusting the correction values and retransforming the transformed values to the red, green, blue co-ordinate system.

Another advantage of a printer according to the present invention is that the co-ordinate system in terms of neutral, illuminant-chromaticity and greenchromaticity is easier for a photofinisher to relate to characteristics of a group of prints.

The invention will be described further, by way of example with reference to the accompanying drawings, in which: Fig. I is a diagram showing the relationship between a conventional orthogonal 3 dimensional coordinate system having axes labelled R, G, B and another coordinate system defined by a Neutral axis N and a chromaticity plane; Fig. 2 is a diagram of a chromaticity plane showing the Red, Green, and Blue positive and negative chromaticity axes, and an Illuminant-chromaticity axis; Fig. 3 is a diagram showing how the conventional Red, Green, Blue orthogonal coordinate system may be transformed into a Neutral, Greenchromaticity, Illuminant-chromaticity coordinate system by a series of rotations; Fig. 4 is a diagram showing the effect of changing, in a positive sense, each of the matrix elements of the correction matrix expressed in the Neutral, Greenchromaticity, Illuminant-chromaticity coordinate system; Fig. 5 is a schematic diagram of a variable-matrix, digital computer-controlled, photographic printer suitable for use with the present invention; Fig. 6 is a flow chart illustrating the prior art programme for calculating exposures for the digital computer of Fig. 5; Fig. 7 is a flow chart illustrating how the digital computer associated with the photographic colour printer of Fig. 5 may be programmed to enable the operator to effect changes to the correction matrix according to the present invention; Fig. 8 is a flow chart illustrating an alternative exposure calculating programme, according to the present invention, for the digital exposure-control computer associated with the colour printer shown in Fig. 5; Fig. 9 is a flow chart illustrating the programme for the digital exposurecontrol computer shown in Fig. 5 to be used with the exposure calculation programme illustrated in Fig. 8 to enable the printer operator to effect changes to the correction matrix according to the present invention; Fig. 10 is a schematic diagram of a variable matrix, analogue computercontrolled, colour photographic printer according to the present invention; Fig. 11 is a schematic circuit diagram of the matrix exposure computation portion of the colour printer of Fig. 10; Fig. 12 is a schematic diagram of a portion of a printer similar to the printer shown in Fig. 10; Fig. 13 is a schematic diagram of a correction level selector; Fig. 14 is a schematic diagram of a preferred correction level selector; Fig. 15 is a drawing of a saturation contour; and Fig. 16 is a perspective drawing of an analogue function generator for saturation contours.

As pointed out above, the elements or correction values a,, of correction matrix [A] do not directly represent any traditionally defined or easily visualized correction levels. Therefore, any changes in the correction levels of a settable matrix printer must be effected through a procedure that relates the desired changes in correction levels to the values of the elements of the correction matrix.

The methods and apparatus of the prior art do not yield a unique correction matrix since certain effects called "rotations" are not handled in a quantitative manner.

In the course of the search for a solution to this problem, it was noted that since a plane can be completely described in terms of any two orthogonal axes, the chromaticity plane with its Red, Green, and Blue chromaticity axes is overdefined.

If any two orthogonal axes in the chromaticity plane are chosen, such as the Illuminant-chromaticity axis and the Green-chromaticity axis, the chromaticity plane will be completely defined. If the Neutral axis is chosen as the third axis, a new orthogonal coordinate system is defined in which any point in the R, G, B coordinate system may be expressed.

As shown in Fig. 3, this new Neutral, Green-chromaticity, Illuminantchromaticity (N, GC, I) coordinate system can be generated from the Red, Green, Blue coordinate system by means of two successive rotations about coordinate axes, these coordinate rotations are not to be confused with the colour "rotations" discussed above. Since the R, G, B coordinate system shown in Fig. 1 is right handed, the first rotation is a negative rotation of 45" about the Green axis producing two new axes R' and B'. B' now corresponds to what is commonly referred to as the Illuminant-chromaticity axis in a chromaticity plane passing through the origin of the R, G, B coordinate system. A second negative rotation of 54" 44' about this Illuminant-chromaticity axis will shift the axis G to produce a first axis G' that is equidistant from the original R, G, and B axes and which therefore corresponds to the Neutral axis. The second rotation also produces a second axis R" which corresponds to the negative extension of a Greenchromaticity axis GC. The illuminant-chromaticity axis and the Greenchromaticity axis define a plane perpendicular to the Neutral axis. This coordinate transformation can be expressed in the form of a coordinate transformation matrix [WI.

The printing densities from equation (1) in this new N, GC, I coordinate system may be defined as: [D']=[W] [D] (13) where: [D'] - is a 3xl column vector having elements d', representing the Printing Densities in the N, GC, I coordinate system; [D] - is a 3x 1 column vector having elements d, representing the Printing Densities in the R, G, B coordinate system; and [W] - is a 3x3 matrix of elements w,l representing the coordinate transformation from R, G, B to N, GC, I.

Equation (13) can be written in a form representing changes in printing density: [AD']=[W] [AD] (14) Similarly, the source exposures in the new coordinate system can be written as: [logE']=[W] [log E] (15) and likewise changes in the source exposures relate as: [AlogE']=[W] [AlogE] (16) solving equation (15) for [logE]: [logE]=[W]-' [logE'] (17) If a correction matrix in the new coordinate system is designated as [B], the changes in source exposure relate to changes in printing density in the following way: [AlogE']=[B] [AD3] (18) To solve for [B] in terms of [W] and [A], we substitute [A] [AD] for [AlogEl in equation (16) so that: [AlogE'l=[W] [A] [AD] (19) solving for [AD] in equation (14) we get: [AD]=[W]-1 [AD'] (20) and substituting [W]-1 [AD'] for [AD] in equation (19) we get: [AlogE']=[W] [A] [W]-' [AD'] (21) comparing equation (18) to equation (21) it is seen that: [B]=[W] [A] [W]-1 (22) and by similar argument, it can be shown that: [AlAW]-1 [B] [W] (23) If [W] is orthogonal, [W]-1=[W]T

and tW]T becomes:

It will be noted that the matrix elements w are the cosines of the angles between the R, G, B coordinates and the corresponding Neutral, Greenchromaticity, Illuminant-chromaticity coordinates.

In the new coordinate system, the diagonal elements bi of the correction matrix [B] correspond exactly with the traditionally defined and intuitively comprehensible correction levels along the N, GC, and I axes; thus: b11 - is identical to the neutral correction level NC computed in equation (8) above; b22 - is the Green-chromaticity correction level computed from matrix elements a by means of equation (9) above; and b22 - is the Illuminant-chromaticity correction level which was most often derived from test patches in the prior art.

The remaining off-diagonal terms can best be described by referring to Fig. 4 where a three dimensional vector space having axes labeled N, GC, and I, is represented. The vectors B1j, along the axes show the effect of increasing (in a positive sense) each of the elements b,i of the matrix. For instance, greens become more neutral (or magenta) along the vector B22 if matrix element b22 is made more positive. In addition, the two rotational effect vectors B,2 and B32 show the neutral rotation for green-magenta scenes and illuminant rotation for green-magenta scenes respectively. Green scenes, for example, can be made to print heavier by increasing b12 and warmer by increasing b22. Each of these latter two elements, b,2 and b32 has the opposite effect on magenta scenes as is shown by the vectors in Fig.

4. In summary, the effect of adjusting each of the coefficients b,; in a positive sense can be tabulated as in Table 1: TABLE 1 b" Neutral Correction b,2 N/GC rotation b,3 N/I rotation whites darker greens darker cold scenes darker blacks lighter magentas lighter warm scenes lighter b2, GC/N rotation b22 GC Correction b22 GC/I rotation whites more magncta greens more magenta cold scenes more magenta blacks greener magentas greener warm scenes greener b3, I/N rotation b32 VGC rotation b23 I Correction whites warmer greens warmer cold scenes warmer blacks colder magentas colder warm scenes colder Thus in order to modify any particular correction level of a settable matrix printer, requires that correction matrix [A] be transformed to the [B] form, the particular elements or correction values b,; be modified as required, and the modified matrix [B] be transformed back into the R, G, B coordinate system to yield the new correction matrix [A].

For example, a photofinisher may find that he is receiving a higher than usual number of originals that were improperly exposed, resulting in a larger than normal neutral density spread in his population of originals. In this case, the photofinisher may wish to increase the Neutral Correction Level of his settable matrix printer. In order to implement such a change, after transforming the printer correction matrix from the [A] form to the [B] form, the photofinisher would increase the value of element b1, of the transformed matrix to the desired new Neutral Correction Level.

To cancel any colour rotations that may have been introduced by changing the Neutral Correction Level, elements b21 and b21 would also be increased by a proportional amount. The new [B] matrix would then be inversely transformed to yield a new [A] matrix for the settable matrix printer.

Similarly, if a photofinisher is located in an area where customers' film is ordinarily subjected to high heat and humidity such as may occur in a tropical climate, or is routinely kept for long periods of time and sent in for finishing after the expiration date of the film, unwanted colour shifts along the Greenchromaticity axis may occur and the photofinisher may wish to increase the Green chromaticityCorrection Level of his settable matrix color printer without affecting the illuminant-chromaticity Correction Level or the Neutral Correction Level. The Green-chromaticity Correction Level is increased by increasing the value of element b22 of the transformed correction matrix. Any colour rotation that may have been induced by this change is removed by increasing the value of element b32 by a proportional amount. Although the neutral rotation that is introduced by changing a chromatic correction level is not ordinarily observable in the resulting prints, the neutral rotation may be removed by changing the value of element b1, in proportion to the amount by which element b22 was changed.

Likewise, a photofinisher whose population of originals exhibits more or less than the usual amount of illuminant failure may want to increase or decrease the Illuminant-chromaticity Correction Level of his settable matrix printer. This adjustment is accomplished by changing the value of element b33 in the desired direction. The colour rotation thus induced is removed by changing element b23 a proportional amount in the same direction. The neutral rotation may be removed as before by similarly changing the value of element b,3 by the same proportions.

Of course, it will be understood that any two or all three of the Neutral, Greenchromaticity, and/or Illuminant-chromaticity Correction Levels may be changed at the same time by observing the procedure outlined above. Especially, all chromaticity correction levels may be changed by the same amount by changing b22 and b, by the same proportion at the same time.

It should be noted that this method provides a direct, quantitative control of the Neutral, Green-chromaticity, and Illuminant-chromaticity correction levels and associated colour rotations. A means for direct manipulation of the Red and Blue chromaticity correction levels is not provided, however manipulation of these correction levels can still be accomplished using the prior art methods.

In an unmodified settable matrix printer of the type disclosed in U.S. Patent No. 3,120,782, the above disclosed method of adjusting correction levels would be accomplished by noting the settings or correction values of the variable matrix elements of matrix [A], mathematically transforming the matrix [A] to the [B] form according to equation (22), effecting the desired changes in the matrix elements b,p mathematically transforming the new [B] matrix back to the [A] form according to equation (23), noting the new values a, of the new matrix [A], and implementing the changes by setting the variable matrix elements in the printer to correspond to the values of the newly calculated matrix elements. Of course, the above calculations can be accomplished by a programmed digital computer, and in the case where the exposure control computation portion of the settable matrix printer comprises a digital computer, the exposure control computer may be programmed to accomplish the matrix transformation, to output the [B] matrix on a suitable display device such as a teletypewriter, to accept the desired changes via a suitable input device, to perform the inverse transformation to yield the new [A] matrix, and to substitute the new values for matrix elements a,j in the printer correction matrix.

The implementation of such a programme will now be described with references to Figs. 5, 6, and 7. Fig. 5 illustrates schematically a digital computercontrolled photographic colour printer of the type known in the prior art. The photographic printing apparatus shown schematically in Fig. 5 includes a photographic colour printer, generally designated 100, for processing originals such as photographic negatives and exposing photographic prints therefrom, an analogue-to-digital converter 102 for converting analogue signals to digital signals, a digital exposure control computer 104, and an input/output device 106 such as a teletypewriter connected to the digital computer. The printer 100 includes a densitometer portion 108 having Blue, Green, and Red channels 110, 112, and 114 for measuring the Blue, Green, and Red LATD's of originals 116 and producing analog output signals LATDB, G, R representative of the respective LATD's on lines 118, 120, and 122. The analogue output signals LATDB X3 R are supplied via lines 118, 120, and 122 to the inputs of A-D converter 102. The A-D converter is responsive to the analogue signals to produce digital signals representing the respective LATD's. The digital LATD signals are supplied via lines 124, 126, and 128 to digital exposure-control computer 104.

The digital exposure-control computer 104 is programmed according to the flow chart shown in Fig. 6 to execute the following operations. The computer 104 receives the digital LATD data via lines 124, 126, and 128 (Block 301 in Fig. 6). The LATD values are converted to printing densities according to equation (1) (Block 302 in Fig. 6) The printing densities are then used to compute log exposures duly corrected according to matrix A as shown by equation (2) (Block 303 in Fig. 6). The log exposures are next converted to exposures by means of a subroutine (Block 304 in Fig. 6) according to the equation: E=10 log Ei (26) where - i=R, G, B E1-are the exposures; and log E1-are the log exposures.

The computer then issues the exposure commands E, (Block 305 in Fig. 6) on output lines 130, 132, and 134 as shown in Fig. 5.

The printer 100, further includes an exposure portion 138 having Red, Green, and Blue channels 140, 142 and 144 responsive to the red, green, and blue exposure signals ER, 0. B produced by the digital exposure-control computer for controlling the exposure of the originals 116 onto a photosensitive medium such as photographic paper 136.

According to one embodiment of the present invention, the digital computer 104 illustrated in Fig. 5 includes a programme as outlined in the flowchart shown in Fig. 7 which, in this case, will constitute a sub-flow chart of block 303 of Fig. 6.

When the programme is executed, the computer first transforms the correction valves of the matrix [A] expressed in the R, G, B coordinate system (which is available in block 303 of Fig. 6) to the [B] form expressed in the N, GC, I coordinate system according to equation (22). (Block 401 of Fig. 7). The programme then causes the correction matrix - [B] to be displayed on the input/output device 106 shown in Fig. 5 (Block 402 of Fig. 7). The operator can input the desired changes in the N, GC, and I values to the correction matrix [B], according to Table 1 via the input/output device (Block 403 of Fig. 7). The computer then modifies the correction matrix [B] according to the operator input (Block 404 of Fig. 7) and performs an inverse transformation on the correction matrix according to equation (23) to return the modified correction matrix to the R, G, B coordinate system (Block 405 of Fig. 7). Finally, the computer replaces the old [A1 matrix in the computer memory in block 303 of Fig. 6 with the new [A] matrix for use in future exposure calculations (Block 406 of Fig. 7). Thereafter the steps of blocks 304 and 305 of Fig. 6 can be carried out.

According to a preferred embodiment of the present invention, the correction matrix resides in the exposure control computer in the [B] form (i.e., in the N, GC, I coordinate system) so that it is not necessary to perform a coordinate transformation on the correction matrix each time the correction matrix is to be changed.

A flowchart for programming the digital exposure control computer 106 of Fig. 5 to calculate exposures according to this preferred embodiment is shown in Fig. 8. The steps there shown replace the steps shown in Fig. 6. Referring now to Fig. 8, the computer receives the digital LATD information from the A-D converter (Block 501) and converts the LATD data to printing densities according to equation (I) (Block 502). The density data is next transformed from the R, G, B coordinate system to the N, GC, I coordinate system according to equation (13) (Block 503). The density data expressed in the N, GC, I coordinate system is obtained by transformation in block 503 and is used to compute log exposures [logE'] expressed in the N, GC, I coordinate system according to equation (18) (Block 504), and the log exposures are transformed back into the R, G, B coordinate system (Block 505) according to equation (17). Next, the aim points kj in the R, G, B coordinate system are added to the log exposures to complete the log exposure calculations (Block 506). The log exposures [logE1] are converted to exposures according to equation (26) (Block 507) and signals representing the exposures are fed out via lines 130,132 and 134 shown in Fig. 5.

To enable the operator to adjust the correction matrix, a routine is programmed in the computer according to the flowchart shown in Fig. 9. When the routine is called, the printer displays the correction matrix [B] via the input-output device 106 (Block 601). The operator inputs the desired changes via the input/output device (Block 602) and the computer modifies the correction matrix accordingly (Block 603). This modified correction matrix is used in block 504 of Fig. 8.

Thus, according to the preferred embodiment of the present invention, the correction matrix always resides in the exposure control computer in the [B] form (i.e., in the N, GC, I coordinate system) and can be adjusted according to the rules of Table 1 without first having to perform the transformation.

The preferred embodiment outlined above may also be incorporated in a settable matrix printer having an analogue exposure control computer. Referring to Fig. 10, a schematic of such a photographic colour printer is shown. The elements of the photographic colour printer, generally designated 100, are similar to the elements of the colour printer 100 of Fig. 5 and have been described previously; similar elements have been similarly numbered.

The signals generated by the densitomer portion 108 of the printer and representing the Blue, Green, and Red LATD's of the original being measured are supplied respectively via lines 118, 120, and 122 to an analogue exposure control computer 146. The analogue computer 146 includes a circuit 148 responsive to the LATD signals for producing signals representative of printing densities DR G B according to equation (1). Such circuits are well known in the art and will not be described in detail. The signals representing red, green, and blue printing densities produced by circuit 148 are fed to a log exposure computer 150 that is responsive to those signals for producing signals representing red, green, and blue log exposures according to equation (2), but prior to adding the aimpoint constants kj. The red, green, and blue log exposure signals produced by computer 150 are applied to the inputs of a circuit 152 which is responsive to the log exposure signals for producing the final exposure signals that are used to control the exposure portion 138 of the colour printer 100 via lines 130, 132 and 134. Circuit 152 includes means for adding the constants k, of equation (2) to the red, green, and blue log exposure signals supplied by log exposure computer 150 and means for converting the resulting log exposure signals to signals representing red, green, and blue exposures. Circuits for performing these functions are well known in the art and will not be described further.

Log exposure computer 150 includes a first fixed matrix 154 which is responsive to the signals representing the Red, Green, and Blue printing densities supplied by circuit 148 to its inputs for producing signals representative of Neutral, Green-chromaticity, and Illuminant-chromaticity printing densities at its output.

The output of fixed matrix 154 is connected to the input of a variable matrix 156.

Variable matrix 156 is responsive to the Neutral, Green-chromaticity, and Illuminant-chromaticity printing density signals for producing signals representing Neutral, Green-chromaticity, and Illuminant-chromaticity log exposures at its output. Variable matrix 156 corresponds to the correction matrix [B] expressed in the N, GC, I coordinate system. Adjustable elements labelled bit enable the operator to control the value of corresponding matrix elements bij of the printer correction matrix [B1. The input of a second fixed matrix 158 is connected to the output of variable matrix 156. Fixed matrix 158 is responsive to the signals representing Neutral, Green-chromaticity, and Illuminant-chromaticity log exposures for producing at its outputs, signals representing Red, Green, and Blue log exposures.

The log exposure computer 150 will now be described in greater detail with reference to Fig. 11.

The signals representing Red, Green, and Blue printing densities are applied to the first fixed matrix 154 on lines 160, 162, and 164 respectively. The fixed matrix 154 includes a first channel having an operational amplifier 166 to which the Red, Green, and Blue densities are supplied via matrix resistors 168, 170, and 172 to produce the Neutral printing density according to equation (13). The values of matrix resistors 168, 170, and 172 are selected in relation to the value of a feedback resistor 174 such that the ratio of the feedback resistor to the value of the matrix resistor equals the value of the corresponding matrix element of equation (24). For example, if the value of feedback resistor 174 is 1000 ohms, the values of resistors 168, 170, and 172 would all be 1,732 ohms (i.e., axe000). Operational amplifier 166 has a normal output 176 and an inverted output 178 upon which are impressed signals respectively corresponding to +DN and DN.

Similarly, the first fixed matrix 154 includes a second channel having an operational amplifier 180 to which the Red, Green, and Blue printing densities are supplied via matrix resistors 182, 184, and 186 to produce normal and inverted Green-chromaticity printing density signals (+DGC) according to equation (13) and (24). The signals applied through matrix resistors 182 and 186 are first inverted by inverting amplifiers 188 and 190 since the signs of the values for the corresponding matrix elements w2, and w22 of the transformation matrix [W] equation (24) are negative.

A third channel includes operational amplifier 192 to which the signals representing the Red and Blue printing densities are supplied via matrix resistors 194 and 196 respectively to produce signals representing the normal and inverted Illuminant-chromaticity densities (+7) according to equation (13). The red printing density signal is inverted by inverting amplifier 198 prior to supplying it to resistor 194 since the corresponding matrix element w,, of equation (24) is negative.

No Green printing density signal is supplied to amplifier 192 since the corresponding matrix element w32 is zero.

The variable matrix section 156 of the log exposure computer includes nine potentiometers 200208, nine buffer amplifiers 210--218, and three summing amplifiers 220, 221, and 222.

The normal and inverted output signals of operational amplifiers 166, 180, and 192 are applied across potentiometers 200--208 in parallel groups of three. The adjustable tap of each potentiometer then senses a proportional part, either positive or negative depending upon the adjustment of the potentiometer, of the signals representing printing display expressed in the N, GC, I coordinate system.

Each signal from the adjustable tap of each potentiometer is buffered by the respective buffer amplifier to isolate the impedance of the potentiometers from the impedence of the summing resistors in the summing amplifiers. The buffered signals are then fed to the summing amplifiers in groups of three, one signal to each amplifier from each channel of fixed matrix 154.

The summing amplifiers combine the appropriate terms to supply signals representing Neutral, Green-chromaticity, and Illuminant-chromaticity log exposures on lines 224, 226, and 228 respectively according to equation (18).

The second fixed matrix portion 158 of the log exposure computer includes a first channel having an operational amplifier 230 to which the Neutral, Greenchromaticity, Illuminant-chromaticity log exposure signals are applied via lines 224, 226, and 228 through matrix resistors 232, 234, and 236 to produce a signal on line 237 representing the Red log exposure according to equation (17). The inputs of matrix resistors 234 and 236 have been inverted by inverting amplifiers 238 and 240 since the signs of the corresponding matrix elements w2, and w,, of equation (25) are negative.

Similarly, a second channel including operational amplifier 242 produces a signal corresponding to the Green log exposure on line 243 in response to receiving the signals representing Neutral and Green-chromaticity log exposures via matrix resistors 244, and 246. No signal representing Illuminant-chromaticity log exposure is applied to this second channel since the corresponding matrix element is zero.

Finally, a third channel including operational amplifier 248 is responsive to Neutral, Green-chromaticity, and Illuminant-chromaticity log exposure signals supplied via matrix resistors 250, 252, and 254 respectively to produce a signal representing Blue log exposure on line 255. The input to matrix resistor 252 has been inverted by an inverting amplifier 256 since the corresponding matrix element w22 of equation (25) is negative.

The operator may adjust the Neutral, Green-chromaticity, and Illuminantchromaticity correction levels and associated colour rotations of the printer directly by adjusting the potentiometers 200--208.

In a particularly advantageous embodiment of the invention, the transformed matrix [B] is varied automatically, print by print, in response to sensed density values of the original. This feature is made practical by the introduction of the transformed matrix, because automatic variation of a matrix in normal red, green and blue coordinates is likely to introduce unwanted colour rotation.

Fig. 12 shows a portion of a printer similar to the printer shown in Fig. 10.

According to Fig. 12, the printing density signals DR. DG and De are fed into a correction level selector 320 which produces a colour scaling factor signal fc and a neutral density scaling factor signal In for varying the variable matrix 156. The signals fc and In represent factors which preferably range between 1.0 and 0.0 and are used to scale the terms of the variable matrix 156 as follows:

The colour scaling factor signal fc is used to scale the coefficients which relate the chromaticity densities to chromaticity exposures. Such scaling serves to adjust the degree of correction toward neutral without introducing hue or neutral shifts, i.e. essentially, only saturation is corrected. This selection of coefficient signals for scaling involves a recognition that those contributions to neutral saturation exposure that are introduced as a function of chromaticity densities are not highly correlated to desirable saturation correction and, hence, fixed values (not scaled ones) are preferable for signals b12 and b,3 of equation (27).

Neutral correction is customized by scaling a separate set of correction matrix coefficients. All coefficient signals which are intended to multiply a neutral density signal are scaled by a common factor In Such scaling recognises that the chromaticity exposures based on interactions with neutral density are desirably correlated to the diagonal term neutral exposure component.

Referring to Fig. 13, there is shown a relatively simple correction level selector 320 for determining the scaling factor signals, fc and In. Corresponding factor and density values (fc and In versus DR. a. B) are stored in a memory 350. A signal corresponding to a factor of 1.0 is, for example, identified with density zones where the normal correction is satisfactory (the unaltered matrix coefficients commonly yield a correction of around 90 /O for neutral and 85% for colour saturation). In zones which are empirically identified as having a high likelihood of subject failure, the level of fc is reduced to correspond to a factor less than 1.0. The table for signal In, on the other hand, might provide for unity scaling generally, with a change toward reduced correction for zones corresponding to low light level photography.

A table-look-up logic device 352 responds to the density signals DR, 0. B and cooperates with memory 350 to determine the signals fc and In for the corresponding density zone.

Referring to Fig. 14 a more sophisticated form for the correction level selector 320 utilizes a fixed matrix [W] 360 to transform the density signals DR.O. B to a set of transformed density signals DN. ec. I corresponding to the presently preferred second coordinate system as above described. Saturation and hue representative signals (S and H respectively) are produced by a function generating device 362, for example, based on the relationships:

H = Tan'l (D'CC/D'I) A discrete valued, hue-representative signal (H) may, as an alternative, be developed based on the signs and relative magnitudes of the DIGC and D', signals.

Such a signal can be produced using known logic circuit techniques or by a series of tests programmed into a digital computer. A function generator 364 provides a cutoff signal (So) representing a colour saturation level for zero correction. The definition of the cutoff signal is related to the hue signal H is developed empirically recognizing that the likelihood of subject failure is strongly correlated to hue.

An example of an "So contour" is shown in Fig. 15. Referring to Fig. 16, analogue apparatus suitable for defining the relationship of the signal H to the signal So has a servo driven cam 380 which is shaped in accordance with the desired relationship of the signals So and H, and a cam follower 382 which is coupled to the moveable contact of a voltage divider 384. Preferably, however, the relationship is defined using a table-lock-up device cooperating with a memory having tables of corresponding values (So versus H).

The factor signal fc is produced by a function generator 366, which may be a divider circuit and a summer, according to the relationship: fc= IS/Sp This relationship provides a linear decrease in the factor signal fc as saturations increases, with a zero value at the saturation SO. A limiter 368 prevents the factor signal fc from going negative. With such a definition, the factor fc reduces correction as saturation increases. This is in recognition of a generally higher likelihood of subject failure with increasing saturation.

A factor signal In may be produced by a non-linear function generator 370 and, for example, might have a value of 1.0 (nominal correction) over the usual neutral density range and a reduced value for neutral densities below a threshold level (indicated as K1 in Fig. 14.) The threshold level might, for example, be selected so that density levels falling below such level have a high likelihood of being existing ligh

Claims (17)

**WARNING** start of CLMS field may overlap end of DESC **. cutoff signal (So) representing a colour saturation level for zero correction. The definition of the cutoff signal is related to the hue signal H is developed empirically recognizing that the likelihood of subject failure is strongly correlated to hue. An example of an "So contour" is shown in Fig. 15. Referring to Fig. 16, analogue apparatus suitable for defining the relationship of the signal H to the signal So has a servo driven cam 380 which is shaped in accordance with the desired relationship of the signals So and H, and a cam follower 382 which is coupled to the moveable contact of a voltage divider 384. Preferably, however, the relationship is defined using a table-lock-up device cooperating with a memory having tables of corresponding values (So versus H). The factor signal fc is produced by a function generator 366, which may be a divider circuit and a summer, according to the relationship: fc= IS/Sp This relationship provides a linear decrease in the factor signal fc as saturations increases, with a zero value at the saturation SO. A limiter 368 prevents the factor signal fc from going negative. With such a definition, the factor fc reduces correction as saturation increases. This is in recognition of a generally higher likelihood of subject failure with increasing saturation. A factor signal In may be produced by a non-linear function generator 370 and, for example, might have a value of 1.0 (nominal correction) over the usual neutral density range and a reduced value for neutral densities below a threshold level (indicated as K1 in Fig. 14.) The threshold level might, for example, be selected so that density levels falling below such level have a high likelihood of being existing light exposures for low light levels. An adjustment based on detailed scan information (the signal Sn mentioned above) may be included at a summer 372, for example, to reduce correction for high contrast negatives. The factor signals fc and In produced by the correction level selector 320 are supplied to variable matrix 156 as discussed above. While matrix customisation according to the invention generally aids in avoiding hue changes, selective scaling of coefficient signals may be employed to introduce wanted hue changes. For example, the coefficients relating chromaticity densities to chromaticity exposures may be scaled by related but different factors to shift "warm" originals e.g. low light level originals toward blue. The above apparatus has been described in terms of known types of circuit elements, but the prescribed signal processing may involve either analogue signals, digital signals or a hybrid of analogue and digital signals. Although the invention has been described with particular reference to preferred embodiments thereof, it will be readily understood that variations and modifications can be effected within the scope of the invention as described above and as defined in the following claims. WHAT WE CLAIM IS:

1. A photographic printer comprising means for producing first signals representative of the colour components of an original and expressed in a first coordinate system, correction means having adjustable correction values for correcting the first signals to produce second signals, and exposure means for exposing a photosensitive copy material to controlled amounts of light as a function of the second signals expressed in the first coordinate system, wherein the printer includes either means for transforming the first signals from the first coordinate system to a second coordinate system and for retransforming the transformed signals, after correction by the correction means, to the first co-ordinate system or for transforming the correction values to the second coordinate system and for retransforming the transformed values, after adjustment, to the first coordinate system such that, in both alternatives, any adjustment of the correction values occurs in the second coordinate system.

2. A printer as claimed in Claim I wherein the first coordinate system is an orthogonal, three dimensional coordinate system having axes representing red, green and blue, and the second coordinate system is an orthogonal, three dimensional coordinate system having an axis defined as red=green=blue.

3. A printer as claimed in Claim 2 wherein the second coordinate system has second and third axes respectively representing illuminant-chromaticity and greenchromaticity.

4. A printer as claimed in Claim 1, 2 or 3 wherein the first coordinate system is

an orthogonal, three-dimensional, coordinate system having axes representing red, green, and blue, and the second coordinate system is generated from the first coordinate system by rotating the first coordinate system substantially 45" in a negative sense around the green axis to produce a first new axis substantially midway between the minus-red and blue axes and a second new axis perpendicular to the first new axis, and rotating the resulting coordinate system in a negative sense about the first new axis substantially 54" 44', to produce a third new axis substantially equidistant from the red, green, and blue axes of the first coordinate system, and a fourth new axis substantially perpendicular to the third new axis and the first new axis, the second coordinate system having axes, corresponding to the third new axis, the negative extension of the fourth new axis, and the first new axis.

5. A printer as claimed in any preceding Claim wherein the correction means comprises a variable linear matrix.

6. A printer as claimed in Claim 5 wherein the variable linear matrix includes coefficient varying means for selectively varying coefficients by which the signals, when expressed in the second coordinate system, are modified before retransforming the signals to the first coordinate system.

7. A printer according to Claim 6 wherein the coefficient varying means includes means responsive to a sensed characteristic of an original.

8. A printer according to Claim 6 or 7 wherein the coefficient varying means includes means for producing one or more factor signals in response to a sensed characteristic of an original, and means for applying the factor signal to the variable linear matrix means to vary selected coefficients of the matrix.

9. A photographic colour printer which includes a variable linear matrixresponsive to signals representing colour components of an original for producing signals representing a function of exposure, the signals and the coefficients of the matrix being expressed in terms of a first coordinate system, and wherein the printer includes means for transforming the coefficients of the variable linear matrix into coefficients expressed in terms of a second coordinate system, means for varying the coefficients expressed in terms of the second coordinate system, and means for retransforming the varied coefficients into the first coordinate system and for substituting the varied transformed coefficients for the coefficients in the variable linear matrix.

10. A method of producing a coloured photographic copy of an original, including the steps of I) measuring the original to produce first signals which are a function of the colour components of the original in a red, green and blue coordinate system; 2) applying correction signals, representing a variable linear matrix formula having adjustable correction values, to the colour component signals to produce second signals representing a function of exposure for producing the copy; 3) exposing the original onto a photosensitive copy material in red, green and blue light as a function of received second signals; and, including either a) transforming the first signals into signals in a neutral, green-chromaticity and illuminant-chromaticity coordinate system, and retransforming the transformed signals after correction to produce the second signals representing a function of exposure in the red, green and blue coordinate system, or b) transforming the correction values into the neutral, green-chromaticity and illuminant-chromaticity coordinate system, adjusting the correction values and retransforming the transformed values to the red, green, blue coordinate system.

11. A method as claimed in Claim 10 including the step of varying the variable linear matrix formula in response to a measured characteristic of the original.

12. A colour printer substantially as hereinbefore described with reference to and as illustrated in Figs. 5 and 6 as modified by Fig. 7, or as modified by Fig. 8 or Fig. 8 as modified by Fig. 9 or Figs. 10 and 11 or Figs. 10 and 11 as modified by Figs.

12, 13, 14 and 16 of the accompanying drawings.

13. A method of producing a coloured copy of an original substantially as hereinbefore described with reference to and as illustrated in Figs. 2, 3,4 and Fig. 6 as modified by Fig. 7, 8 or Fig. 8 as modified by Fig. 9 or Fig. 15 of the accompanying drawings.

14. An apparatus for use in conjunction with a photographic colour printer having an analyser for determining the colour components of an original to be reproduced, the apparatus comprising means for transforming information receivable from the analyser relating to the colour content of the original, the information being expressed in a first coordinate system, into a second coordinate system, a variable means for correcting the information in the second coordinate system, and a means for re-transforming the corrected information back to the first coordinate system, whereby it may be fed to a colour printer to cause exposure of a photosensitive material in accordance with the corrected information.

15. An apparatus as claimed in Claim 14 wherein the variable linear matrix is settable in accordance with variations in the second coordinate system an axis of which is a neutral axis.

16. An apparatus as claimed in Claim 14 or 15 wherein the second coordinate system is an orthogonal three dimensional coordinate system having illuminant and green chromaticity axes.

17. In a method of reproduction of coloured originals by photography' including the step of analysing the original to determine the red, green and blue colour components thereof, the steps of transforming the components into a coordinate system having neutral, illuminant and green chromaticity coordinates, correcting the transformed components as necessary, and transforming the corrected components back into red, green and blue values which latter are to be used in the exposure of a photosensitive material.