GB1592839A - Method of obtaining a target colour on colour photosensitive photographic material - Google Patents

Abstract

The target colour is illuminated with a light source, the ratio of three primary colours - red, green and blue - being determined. The illuminated target colour is compared with a data bank which comprises a plurality of colours. A reference colour which is closest to the target colour is read out from the data bank. A group of one or more colour filters is identified, these being used for producing the reference colour. The exposure times for the colour filters which are necessary for reproducing the target colour on the colour-photographic material when it is developed are determined. Undeveloped colour-photographic material is illuminated through the colour filter during periods of time which have been determined for each respective colour filter. The exposed colour-photographic material is developed in order to reproduce the target colour thereon. The simple, cost-effective method is suitable for the synthesis of a target colour by step-wise exposure of a photographic recording material with light-variable colour. The method can be used both on positive and on negative photographic recording materials. Once a defined colour has been synthesised it can be reproduced repeatedly. <IMAGE>

Description

(54) METHOD OF OBTAINING A TARGET COLOR ON COLOUR PHOTOSENSITIVE PHOTOGRAPHIC MATERIAL (71) We, MARJORIE DENISE INGALLS and RICHARD DAVID INGALLS, both of E.1104--57th Street, Spokane, State of Washington 99203, 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 a method of obtaining a target color on color photosensitive photographic material, utilizing a light source and up to three color filters.

The target color producing method of the present invention relates in general to the field of synthesizing a color on photographic media by the successive exposure thereof to an illuminant source through colored filters. More particularly, the method of the present invention relates to the art of preparing and using a colored, reference data base as a basis for determining the exposure time needed for each filter used, and then utilizing such exposure times to produce the target color.

In a classic experiment which was performed by Newton as early as the year 1730, light from a test lamp was shown on a white screen and viewed by an observer. An adjacent part of the screen was illuminated by three different light sources equipped to give light of widely differing colors, typically, red, green and blue. Interestingly, it was found that by adjusting the intensities of the three colored light sources, they could be made to produce a combined light on the screen which would match that of the test lamp. The importance of this basic experiment is that it teaches that the light from three different colored light sources, commonly referred to as the primary lights, can combine to reproduce a test color by a technique known as additive color matching.

Once color photography was introduced it was an easy step to take to note that by shining primary lights of different colors on the same piece of photographic media, an image of their combined color would be obtained which was different from the color of the primary lights themselves. Such a concept is disclosed in U.S.

Patent No. 3,741,649 issued June 26, 1973, to Podesta et al. However, Podesta discloses merely an empirical system whereby, through trial and error, light is projected through a multitude of combinations of filters onto photographic media which is then developed to discover what usable color, if any, has been obtained.

By noting which filters were used, some rudimentary repeatability of results was attainable. However, the method disclosed by Podesta make no provision for reproducing a randomly selected target color and makes no provision for duplicating such a target color when a light source having a different color temperature is used, or when filters are used which are of a color different from those already used.

U.S. Patent No. 3,322,025 issued May 30, 1967 to Dauser, discloses a somewhat more sophisticated, but still empirical, method which involves the production of a cylindrical color solid wherein each colored segment is identified by its hue, value and intensity. The color solid is generated, apparently, by varying the voltages to three primary light sources, red, green and blue, to thereby regulate their intensities and thus produce the various colored segments comprising the color solid. Repeatability is obtained by noting the voltage input or light output of each of the respective lamps needed to produce each colored segment in the color solid.

However, such a system is severely limited since it provides onlv for producing the cylindrical color solid by the use of three colored lights which together are able to form achromatic light. If it is desired to use three lights which are unable to form achromatic light, what techniqe to use is undisclosed. Further, if a different set of achromatic lights is used, it is clear that the first produced color solid is useless and thus a new color solid must be prepared for each different set of lights employed. In addition, the method's ability to faithfully reproduce a color segment is questionable since it is well known that the color properties of photographic media vary from manufacturer to manufacturer and from lot to lot. How Dauser would compensate for such variations without regenerating a complete color solid for each batch of photographic paper used is not disclosed. In addition, it is well known that the color temperature of even standardized light sources changes with age and hence the colors they produce similarly change. Again, how such aging is compensated for, without the periodic reconstruction of a complete color solid is a problem not addressed by Dauser. Further, it is well known that varying the voltage to a light source is a crude way to regulate its intensity since as the bulb dims or brightens, the color produced by the bulb also changes. Apparently Dauscr failed to consider this variable which would tend to make his color solid non-linear.

Lastly, if both positive and negative photographic media were to he used, apparently a separate color solid would have to he generated for each, a costly and time consuming process.

Many other attempts have been made over the years to systematically organize the various colors visible to the human eye. One of the most successful of the prior systems has been the CIE system (Commission International de L'Eclairage, or the International Commission on Illumination). As seen in Fig. 1, color as described in the CIE system may be plotted graphically in a plane Cartesian coordinate system wherein the x values are plotted horizontally on the abscissa 10, and the y values are plotted vertically on the ordinate 12. The result ofsuch a plot is the standard CIE 1931 chromaticity diagram, generally designated at 14, showing a horseshoe-shaped spectrum locus on which the point representing the chromaticities of the spectrum colors are plotted according to their wavelengths in nanometers. A straight line 18, upon which the purples and magentas are plotted, forms the base of the chromaticity diagram and connects the lower ends 20, 22 of the spectrum locus.

The standard CIE diagram 14 may be obtained from recognized works on the subject such as the Handbook of Colorimetr} by A. C. Hardy, published in 1936 by the Massachusetts Institute of Technology.

The CIE chromaticity diagram 14 is frequently used as a reference system, as in U.S. Patent No. 2,850,563, issued September 2, 1958 to Gretener. Gretener uses the CIE diagram 14 as a theoretical basis for explaining his method of more accurately reproducing images in color through the use of photography or television. Primarily, his method achieves accurate color reproduction by properly adjusting the spectral response of the recording apparatus. For reproducing flesh tones, the spectral response is adjusted in such a manner that the flesh tones are recorded with color components having an intensity of equal size, i.e., having a unitary ratio, or in the preferred case, with only the red and green components having substantially equal intensities.

In explaining his process, Gretener discusses the use of a color or filter triangle 24 plotted on the CIE diagram 14 which has the primary reproduction colors 26, 28, 30 plotted at its vertices. He notes that the color triangle 24 delineates the area on the CIE diagram within which a desired color may be reproduced by the additive mixture of a suitable ratio of intensities of the primary reproduction colors 26, 28, 30. Gretener further explains that if a color is to be reproduced by the additive mixture of three colored lights, the required ratio of intensities may be derived in a well known manner from the location of the particular target color within the filter triangle 24 formed by the primary colors. It is noted that the thrust of the Gretener patent is directed towards a method of unitizing the ratio of at least two of the primary colors.

The Gretener patent, however, fails to disclose a method by which a target color may be produced on positive photographic media by calculating the exposure time needed for each of the primary colored lights. In addition, no method is disclosed by which the proper exposure times for each of the three primary light sources may be calculated such that when negative photographic media is exposed thereto, the target color will result on the developed media. Further, no indication is given by Gretener of how, when attempting to achieve a particular target color, the target color may be accurately located on the CIE diagram as a starting point for the calculation of the exposure time needed for each of the primary light sources.

The present invention now provides a method of photographically substantially reproducing a target color on color photosensitive photographic material, utilizing a light source and up to three color filters, said method comprising a) preparing a color reference data base expressing the primary color component make up of a plurality of colors illuminated by a given light source b) determining from the data base the primary color component make up of the color in the data base most similar to the target color under illumination by the given light source c) selecting a group of from one to three filters capable of reproducing the target color by appropriate sequential exposures and development of the photographic material d) determining from the primary color component make up of each filter under illumination by the given light source and from the sensitivity of the photographic material to light from the given source filtered by each filter the color filter exposure times appropriate to substantially reproduce the target color on the photographic material when developed e) exposing the undeveloped color sensitive photographic material to the illuminant source in sequence through each color filter for each respective filter exposure time thus determined, to substantially reproduce the target color on the photographic material when developed and f) developing the exposed photographic material.

Preferably, the group of filters selected contains three filters.

The color reference data base expresses the primary color component make up-that is the relative amounts of each of a set of three primary colors which combine to make up the color in question-for each of a plurality of colors. By comparing a target color with the colors in the color reference data base one may pick the color in the data base most similar to the target color and find the appropriate primary color component make up.

Preferably primary color component make ups are each expressed as a set of co-ordinates x, y, and z where x represents the content of a first primary color in the group, y represents the content of a second primary color in the group and z represents the content of the third primary color in the group such that x+y+z=l.

Preferably, the values of the x and y co-ordinates for each filter are determined by reference to the manufacturers reference data.

Preferably, the capability of the group of three filters to reproduce the target color is judged by plotting the primary colour component make up of each filter and of the target color in Cartesian co-ordinates and verifying that the plot of target color lies in or on a triangle formed by the plots of the filters.

The photographic material may be a positive photographic material.

In such a case filter proportions may be determined such that the sum of the first primary color components of each filter each multiplied by the filter proportion for that filter equals the first primary colour component of the target color the sum of the second primary color components of each filter each multiplied by the filter proportion for that filter equals the second primary color component of the target color, and the sum of the third primary color components of each filter each multiplied by the filter proportion for that filter.

The filter exposure times may then be calculated by multiplying each filter proportion by factor representing the sensitivity of the photographic material to light from the given light source filters by that filter.

The factor may be the saturation time for the photographic material using light from the given source filtered by each filter, or may be proportional to the saturation time for the photographic material using light from the given source filtered by each filter.

Equally the photographic material may be negative photographic material, in which case filter proportions may be determined such that the sum of the first primary color components for each filter each multiplied by the filter proportion for that filter equals the first primary color component of a color complementary to the target color, the sum of the second primary color components for each filter each multiplied by the filter proportion for that filter equals the second primary color component of the said complementary color and the sum of the third primary color components for each filter each multiplied by the filter proportion for that filter equals the third primary color component of the said complementary color.

Filter exposure times may then be calculated by multiplying each filter proportion by factor representing the sensitivity of the photographic material to light from the given light source filters by that filter.

The factor may be the saturation time for the photographic material using light from the given source filtered by each filter, or may be proportional to the saturation time for the photographic material using light from the given source filtered by each filter.

The primary color component make up of the complementary color may be determined by plotting the x and y co-ordinates as defined above of the target color on a working diagram, plotting the x and y co-ordinates of the given light source on the working diagram, plotting a standard CIE diagram on the working diagram, extending a straight line through the plotted co-ordinates of the light source and target color to opposite sides of the standard CIE diagram determining the percentage of the distance on the straight line that the plotted point for the target color lies between the plotted point for the illuminant source and the nearest side of the standard CIE diagram; and noting the x, y co-ordinates of a point located on the straight line a percentage of the distance from the plotted point for the illuminant source to the far side of the standard CIE diagram which is the same as said percentage of the distance for the target color.

The color reference data base may comprise a multitude of swatches of colored material mounted on a substrate and associated with means indicating the primary component color make up of the color of each swatch.

The said means may indicate the co-ordinates x and y, defined above, for each color.

The colored swatches may be mounted on a colored reference diagram, each in a position indicated its x and y co-ordinates.

The invention includes colored photographic material produced by a method as described above.

The invention further includes a method of producing a color reference diagram for use in a method as described above, which method comprises selecting a plurality of sets of primary color components each set of components corresponding to a unique color, for each set preparing a colored swatch on photographic material having primary color components corresponding to those of the set, and mounting each colored swatch in a position on a substrate indicative of its primary color component make up.

The sets may be selected by superimposing a grid on a standard CIE type diagram and reading off the x and y co-ordinates of each grid intersection.

Each colored swatch may be prepared by a method as defined in steps (c) to (f) above, treating the color corresponding to each set as the target color.

The invention includes a color reference diagram produced by such a method.

The invention further includes a method for producing a colored swatch for use in making a color reference diagram which method comprises selecting a set of primary color components corresponding to a unique color, and preparing a swatch of that color by a method as defined in paragraphs (c) to (f) above, treating the said color as the target color.

The invention includes a colored swatch when prepared by such a method.

Further procedures are described hereafter to be followed when the target color is a tint or tone of the most closely matching color on the color reference data base.

The present invention will be illustrated by the following detailed description of preferred embodiments thereof in which reference is made to the accompanying drawings, in which: Fig. 1 is a pictorial representation of a standard CIE diagram.

Fig. 2 is a block diagram outlining one of the methods of the present invention.

Fig. 3 is a pictorial representation illustrating the use of a working diagram in the present invention.

Figs. 4 and 5 are block diagram outlines of other methods of the present invention.

Fig. 6 is a pictorial representation illustrating the use of a working diagram in preparing a color reference data base.

It should be noted that applicants' present method has a great advantage in that it can be utilized to produce the target color with an illuminant source having any color temperature. However, for the most accurate results, certain constraints in the color temperature of the illuminant source must be observed.

As is explained in greater detail subsequently, the color temperature of the particular illuminant source used affects the location on the working diagram of many of the plotted points utilized in the present method. Among the plotted points affected are those for the illuminant source, the filters, the target color and the complement of the target color. In addition, it will be seen that the pattern of colors appearing on the color reference data base will also change as the color temperature of the illuminant source changes, as will the saturation times of the colored filters.

Thus, it will become apparent that for best results in practicing the method of the present invention, the illuminant source used in preparing the color reference data base and in determining the filters' saturation times should have the same color temperature as that used to produce the target color. This same color temperature must also be used in determining the coordinates of the illuminant source, the filters, the target color, and the complement of the target color. The color temperature of the illuminant source may be easily determined in many ways, as for example, with a Kelvin meter.

In addition, the illuminant source(s) should not only have the same color temperature, but they should also have the same spectral distribution. The spectral distribution of an illuminant source may be obtained from a variety of sources, including its manufacturer. Naturally, even though the properties of a particular illuminant source slowly change as it ages, one can insure to a considerable degree the desired constancy of color temperature and spectral distribution merely by using the same illuminant source throughout the methods of the present invention.

Failure to use illuminant sources of the same color temperature and spectral distribution will cause some inaccuracy in the results obtained which may be acceptable if the particular user does not need a high degree of accuracy.

For the purposes of the present invention it may be assumed that most incandescent illuminant sources of the same color temperature have substantially the same spectral distribution, and thus an independent determination of spectral distribution need not be made. Of course, if inaccurate results obtain, then the spectral distributions must be checked. Care must be taken since non-incandescent illuminant sources, such as fluorescent light, seldom have the same spectral distribution as an incandescent illuminant source of the same color temperature.

In order to properly understand and carry out the method of the present invention diagrammed in Fig. 2 and shown in Fig. 3, a brief summary of the method is set forth below, along with a discussion of the method's underlying principles.

Preliminarily, for purposes of discussion of the present invention, it is assumed that every color visible to the human eye is composed entirely of red, green and/or blue components. Thus, it follows that any such color may be synthesized by the proper additive mixing of red, green and blue lights. However, the problem remains of how to determine what are the red, green and blue components of a given target color, and how to select and use colored filters which will extract the proper amounts of red, green and blue light from a given illuminant source to reproduce that color on the particular photographic media being used.

Since, as has been explained, the color temperature of the illuminant source is a basic factor which must be considered in the present method, the starting point for the present method, as shown in Fig. 2, is to determine the color temperature of the illuminant source which will be used to produce the target color. Having determined the color temperature of the illuminant source, it may then be plotted on the working diagram 15 (Fig. 3) for whatever use may be made of it. For example, this plotted point is useful in determining such information as the dominant wavelength of the target color and the complement of the target color.

At this time the spectral distribution may also be determined, as by reference to its manufacturer's specifications.

Next, as shown in Fig. 3, the proper location is determined on the working diagram for the color we are trying to achieve-the target color 32. This is done by comparing the target color to a color reference data base which was prepared with an illuminant source having the same color temperature and spectral distribution as the illuminant source being used to produce the target color. An example of a color reference data base whose preparation is described in detail hereafter with reference to Fig. 6 is a multitude of individual differently colored swatches mounted on to paper as if plotted as points on a graph so that for each color the components of two primary colors can be read off the axes of the graph and the third component calculated from these two. The primary color components of a target color are thus determined by the position of the closest matching color in the data base. The coordinates on the colored reference data base of the closest matching color will be taken to be those of the target color. These coordinates are then used to plot the target color 32 on the working diagram 15, shown in Fig. 3.

Then, a set of three filters, 34, 36, 38 are selected which can be used to produce the particular target color 32. In order to determine a set of three filters which can be used to achieve a particular target color, all of the colored filters being considered must first be plotted on the working diagram. As has been explained, since the position of each filter depends upon the color temperature and the spectral distribution of the illuminant source used, the coordinates chosen for each filter must be those determined for an illuminant source having a color temperature and a spectral distribution which is the same as that used to produce the target color.

Filter triangles are then drawn among the plotted filters, and any set of three filters may be used which forms a filter triangle coincident with the plotted point for the target color. This is because, as is recalled, a set of three filters may be used to produce any color located within or on the filter triangle they form. It is understood that if none of the filter triangles satisfies this requirement, additional or different filters would be necessary in order to produce the target color.

Now, in order to determine what are the red, green and blue components of the target color 32 and of each of the colored filters 34, 36, 38, applicants make use of the same working diagram 15 upon which the target color and the three colored filters have been plotted. For the purposes of the present invention, the x coordinate of the plotted point for the target color 32 and for each filter 34, 36, 38 is taken to represent its red component, and its y coordinate is taken to represent its green component. Since any color is understood to equal the sum of its red, green and blue components, it follows that each color's blue component, which we designate as z, may be calculated according to the equation: zl-(x+y).

Accordingly, once the 'position of the target color 32 and of each of the colored filters 34, 36, 38 is known on the working diagram, the size of their red and green components may be read directly from the working diagram, while their blue components may be easily computed by the use of the formula given. Using this procedure, the x, y, z coordinates of each colored filter are determined, which represent, of course, their red, green, and blue components, respectively.

At this point it should be noted that, as seen in Fig. 2, somewhat different procedures are used depending on whether positive or negative photographic media are to be utilized to produce the target color. The method for positive photographic media is first discussed below.

Since the goal is to reproduce the target color photographically, and not by matching colored lights on a screen as used to produce a standard CIE diagram, it is necessary to find the exposure time needed for each of the three colored filters being used. This is done by first discovering the saturation time for each colored filter 34, 36, 38 for the particular positive photographic media being utilized.

Briefly, this may be accomplished for each colored filter by making a series of successively longer exposures through the filter onto the photographic media, until lengthening the exposure time produces no observable change in the color found in the developed media. The first exposure time which achieves this result is considered to be the saturation time for that particular colored filter.

Next, as seen in Fig. 2, the red, green and blue components of the target color are obtained by means of the previously described procedure which starts with noting the target color's x, y coordinate on the working diagram and then using the formula z=l-(x+y) to compute its blue component.

Once the red, green and blue components of the target color and of each of the three colored filters are known, it must then be discovered how much of each colored filter is needed in order to produce the target color. This is done by determining what percentage of each colored filter is needed to achieve a result such that the sums of their red, green and blue contributions equal, respectively, the red, green and blue components of the target color. This determination of the filter percentage needed for each colored filter may be made by conventional mathematical techniques.

Having obtained the filter percentage needed for each colored filter, each filter's exposure time is obtained by multiplying its respective saturation time by its respective filter percentage. Finally, the particular, positive photographic media being used is exposed sequentially to the illuminant source through each colored filter for its respective exposure time. Upon developing the photographic media in accordance with the manufacturer's instructions, the target color will be reproduced on the developed positive photographic media.

It should be again noted that the method just described for positive photographic media must be altered somewhat to make it suitable for use with negative photographic media, which is the media preferred by most of those skilled in the photographic art. However, as seen in Fig. 2, both methods have many steps which are similar and these steps have been connected by dotted lines. One of the similar steps is the determination of the saturation time for each of the colored filters on the particular negative photographic media being used. This step is performed analogously to the step heretofore described for determining the saturation time for each of the colored filters on positive photographic media.

However, the differences in the methods, which are seen in Fig. 2, may be accounted for by the fact that, as is known, when a negative photographic medium is exposed to light of a color that is the same as the target color, the target color will not result when the negative photographic medium is developed. Instead, a color will result which is the complement of the target color. Apparently, if the target color is to result when the negative photographic medium is developed, it is necessary to expose the negative photographic media to light whose color is the complement of the target color. In a procedure which is described in more detail subsequently, the location on the working diagram for the complementary color 46 is found through use of the plotted point for the target color 32, the plotted point for the illuminant source 48, and the CIE diagram 14.

temperature and a spectral distribution the same as that of the illuminant source being used to prepare the target color. Then, the target color was compared to the color, reference data base and it was noted that the x, y coordinates of the closest matching color thereon were x(red)=.211 and y(green)=.3424. Using these coordinates, a point for the target color 32 was plotted on a working diagram 15 as shown in Fig. 3. It is to be understood that the working diagram 15 need not include the CIE diagram 14, except for certain steps as is explained subsequently, but may instead be any Cartesian coordinate system having units which are proportional to those used in a standard, CIE diagram.

Other plotting systems may be used without departing from the scope of the present invention.

Next, three colored filters 34, 36, 38 were selected to be plotted on the working diagram 15. Of course, any colored transparent material could have been used as a filter. But preferably, the filters used should have excellent optical quality so that they will not diffuse the light transmitted therethrough, and should be of uniform color throughout to avoid color variations in the results produced. Finally, since the dyes used to produce precision filters gradually change color with age, it is preferred that the filter be selected to be as color stable as possible. Filters found to be particularly suitable are produced by the Eastman Kodak Company and are generally designated as being Kodak Wratfen Photomechanical Filters. In this example, the filters chosen were the Kodak Wratten Photomechanical filters numbered 25 (red), 58 (green) and 47B (blue), and have been designated as filters 34, 36, 38, respectively. "Wratten" is a Registered Trade Mark.

In order to plot the filters on the working diagram, it is necessary to determine their x, y coordinates. This may be done in two ways. either by referring to the manufacturer's reference data, or by direct comparison of the colored filters to the color reference data base to find the coordinates of the closest matching color thereon.

When a filter's coordinates are found by comparison with the color, reference data base, the filter should, of course, be illuminated by an illuminant source whose color temperature and spectral distribution are the same as that of the illuminant source which was used to produce the target color and the color reference data base. Similarly, when the filter's coordinates are being found by referring to the manufacturer's reference data, the coordinates listed therein must be chosen which were prepared using an illuminant source whose color temperature and spectral distribution are as close as possible to that of the illuminant source being used to produce the target color and the color reference data base.

As is recalled, the illuminant source 48 used in this example was a tungsten light whose color temperature was found, through the use of a Kelvin meter. to be approximately 3,0500 Kelvin. To find the x, y coordinates off the filters 34, 36, 38, reference was then had to the booklet, "Kodak Filters for Scientific and Technical Use", published by Eastman Kodak Company of Rochester, New York. This booklet lists the x, y coordinates of each filter produced by the Eastman Kodak Company for illuminant sources of several color temperatures. The data selected were that listed for Standard Illuminant A, as specified by the International Commission on Illumination, which is a tungsten light having a color temperature of 2,856 Kelvin. Since the color temperature differed by only 194" Kelvin, a fairly close match was found for the illuminant source being used. Of course, more accurate results can be obtained through interpolation of the reference data supplied by the manufacturer.

The three filters were then plotted on the working diagram shown in Fig. 3 using the following coordinates which were supplied by the manufacturer for Standard Illuminant A: Filter x (red) y (green) z (blue) 25 (red) .6850 .3147 .0003 58 (green) .2693 .6831 .0476 47B (blue) .1554 .0220 .8226 The z cooridnates Were, of course, obtained through the use of the formula z= -(x+y).

After the filters were plotted, straight lines 40, 42, 44 were drawn interconnecting the red and green filters, the green and blue filters, and the blue and red filters, respectively, to form a filter triangle 24, as seen in Fig. 3. It was observed that the target color lay within this filter triangle and thus these three filters are suitable ones to use to produce the target color.

As has been noted, the method of Fig. 2 when used to produce the target color on positive photographic media, differs somewhat from that used to produce the target color on negative photographic media. Each method is therefore described separately below, with the method for positive photographic media being addressed first. The positive print paper Kodak Ektachrome R.C. paper type 1993, manufactured by Eastman Kodak Company was used.

First, the saturation time for each colored filter was determined for the particular, positive photographic paper being used. This is an important step since the color qualities of photographic paper may vary from batch to batch, as is reflected by the color guide numbers found printed by the manufacturer on each box of photographic paper. In this step also, it is important that the color temperature and spectral distribution of the illuminant source used to obtain the saturation time be the same as that used to produce both the color reference data base and the target color.

To determine each filter's saturation time, as has been explained, a series of sequential exposures of lengthening time are taken through it on the photographic media being used, and when no change in color is observed in the developed paper, the first exposure time to produce that color is the saturation time for that filter.

Accurate checks on the saturation time are made by making successive exposures with only a small increase in exposure time and by checking the change in color found in the developed paper with a densitometer. In the example selected, the saturation times for the red, green and blue filters were found to be, respectively, 35.3 seconds, 37 seconds and 20.1 seconds.

After the saturation times have been determined for each filter, a final check on the accuracy of the result can be made by trial production of a gray at the plotted point for the illuminant source 48. This can be done by employing the method set out in Fig. 2, and then, in this method, specifying the target color to have the coordinates of the plotted point for the illuminant source. Whether a gray has been achieved may be checked by comparing the resulting color with a standard reference work such as the True color Process Guide, published by Krug Litho Art Co., in Kansas City, Missouri. If it is found that a gray has not been achieved, the dominant color(s) are observed and the exposure time(s) for the filter(s) are adjusted, as is known to those skilled in the art, until a gray is obtained.

One of the advantages of the method of the present invention is that once the saturation times for particular colored filters are known for an illuminant source of a certain color temperature, variations in the color guide numbers on different boxes of paper may be compensated for, as is also known to those skilled in the art, without experimentally redetermining the saturation times for the colored filters.

One further factor that should be emphasized, in order to ensure the accuracy of the results obtained by the method of the present invention, is that care must be taken to ensure that the intensity of the illuminant source used to determined the saturation times is the same as the intensity of the illuminant source used to expose the photographic paper when achieving the target color. The constancy of the intensity may be insured by measuring the illuminant source with a light meter; or by keeping the voltage applied to the illuminant source and the distance from the illuminant source to the photographic paper invariant. Alternatively, changes in the intensity may be compensated for as is known to those skilled in the art.

After the saturation times were determined, the x, y coordinates of the target color were noted from the working diagram to be x(red)=.211 and y(green)=.3425.

The blue coordinate was found to be z=.4465 using the formula z=l-(x+y).

Next, the filter percentages for the red, green and blue filters were determined to be, 0%, .481% and .519% respectively.

This determination was made on the basis that the sum of the respective primary color components of the filters equals the respective color components of the target color. By multiplying the red, green and blue filters' respective filter percentages by their respective saturation times, the exposure time for each filter was found to be 0 seconds, 17.8 seconds and 10.4 seconds, respectively.

The last steps involve sequentially exposing the photographic paper to the illuminant source through each of the three colored filters for its respective saturation time. After the last exposure is made, the positive photographic medium is developed in accordance with the manufacturer's instructions with the result that the target color is obtained on the developed positive photographic paper.

Turning now to the method to be used to produce the target color on negative photographic media, it is again notable that several of its steps differ from those used in the method to produce the target color on positive photographic media.

However, the substantial similarities which have been previously mentioned should be kept in mind. The negative photographic media used in the selected example was Kodak Ektacolor 37RC, type 2290, negative printing paper manufactured by the Eastman Kodak Company of Rochester, New York. "Ektacolor" is a Registered Trade Mark.

As in the method for positive photographic media, the method for negative photographic media begins by determining the color temperature of the illuminant source and then plotting it on the working diagram. Similarly, the target color 32 is also plotted on the working diagram. Three suitable colored filters 34, 36, 38 are selected, as before, by plotting the three colored filters on the working diagram.

and drawing straight lines 40, 42, 44 to join them to form a filter triangle 24. It is determined, as before, that the red, green and blue filters 34, 36, 38 are suitable for use. The x, y, z coordinates of each are determined, as has been described, to be the same as set forth in the method for positive photographic media.

Next, the saturation time for each colored filter is determined for the negative photographic media in a manner analogous to that previously described for positive photographic media. The results are that the red, green and blue filters 34, 36, 38 had saturation times of 38.4 seconds, 21.3 seconds and 28.8 seconds. respectively, on the negative photographic paper.

It is recalled that, in order to obtain the target color on the negative photographic media, the negative photographic media must be exposed to a light whose color is complementary to that of the target color. Thus it is necessary to determine the coordinates of this complementary color 46 since one must determine how to synthesize it using the three colored filters. In this step, it is necessary to use the CIE diagram 14 which is now drawn on the working diagram 15. Referring now to Fig. 3, a straight line generally designated as 52 is drawn between the plotted points for the target color 32 and the illuminant source 48 as the initial step. The straight line 52 is then extended to intersect opposite sides of the working diagram 15 at points 54, 56.

The percentage of the distance the plotted point for the target color 32 lies on the straight line 52 between the illuminant point 48 and intersection point 54 is now determined. In the selected example, it was found that, as shown in Fig. 3, the target color lay 56% of this distance. The coordinates of the complementary color 46 of the target color 32 are then determined by proceeding along the straight line 52 from the plotted point for the illuminant source 48 towards the intersection point 56 on the far side 40 of the working diagram 15. The distance traveled is such that if the plotted point for the target color 32 lies 56 Vs of the distance along the line between the plotted points for the illuminant source 48 and the near side 42 of the filter triangle, then the complementary color 46 lies the same percentage of the distance between the plotted point for the illuminant source 48 and the far side 40 of the filter triangle. From Fig. 3 it is shown that the coordinates corresponding to the complementary color 46 were x=.5050, and y=.425. The z coordinate, calculated in the usual manner, was found to be z=.07.

Next, the filter percentages needed to produce the complementary color are determined in the usual manner, using the x, y, z coordinates determined for the complementary color 46, and for the three colored filters 34, 36, 38. As a result, the red, green and blue filters 34, 36, 38 were found to have filter percentages of 58 36 , and 6%, respectively. By multiplying each filter's respective filter percentage by its respective saturation time, each filter's exposure time needed to produce the complementary color was found to be 22.3 seconds, 7.7 seconds and 1.7 seconds, respectively.

When the negative photographic media was exposed to each of the colored filters for its respective exposure time, and developed in accordance with the manufacturer's instructions, it was found that, as expected, the target color had been achieved on the developed paper.

Turning now to other considerations. it may be that when the target color is compared to the color reference data base in accordance with the method set forth in Fig. 2, for both positive and negative photogcaphic media, a close match for the target color is not found. In that case, it is apparent that the target color is either a tint (lighter than) or a tone (darker than) of the closest matching color found on the color, reference data base. It will be seen that by using the methods that are diagrammed in Figs. 2 and 4, any target color may be accurately reproduced, which is one of the significant advantages of the present invention.

However, in order to obtain accurate results when the target color is a tint or a tone of the closest matching color, it is necessary to use the method as set forth in Fig. 4 which is based on, but is a refinement of the method of Fig. 2. In fact, it has been observed that the steps of the method set forth in Fig. 4 which are enclosed by a dotted line are directly analogous to the corresponding steps of the method set forth in Fig. 2. When using positive photographic media in the method of Fig. 4, the only difference is that instead of determining the exposure time for each colored filter for the target color, the exposure time for the closest matching color is determined. When using negative photographic media in the method set forth in Fig. 4, the only difference is that instead of determining the exposure time for the complement of the target color, the exposure time for the complement of the closest matching color is determined.

Once the exposure time for each filter for the closest matching color or its complement is determined, different methods are needed, as shown in Fig. 4, depending on whether positive or negative photographic media is to be used to produce the target color.

It is found that the target color is a tint of the closest matching color, and if a positive photographic.medium is used, a series of exposures is made, using as a basis the exposure time for the closest matching color. In the series of exposures, the exposure times for all of the filters are proportionally increased, resulting in a hue of lighter and lighter tint on the developed photographic medium until the target color is reached. If a negative photographic medium is used, the exposure time for each of the filters that was found for the complement of the closest matching color is proportionately decreased and again, a series of exposures is taken. The result is a hue of a lighter and lighter tint on the developed negative photographic medium until the target color is reached.

If the target color is found to be a tone of the closest matching color, again different methods are used depending on whether positive or negative photographic media are employed, as seen in Fig. 4. In the case of use of positive photographic media, the exposure time for each of the colored filters found for the closest matching color is proportionately decreased and a series of exposures are made, each with a shorter and shorter exposure time. The result in the developed positive photographic medium is a hue of darker and darker tone until the target color is reached. If a negative photographic medium is used, the exposure time for each of the colored filters found for the complement of the closest matching color is proportionately increased. A series of exposures is made, each having a longer exposure time, until the target color is finally obtained in the developed, negative photographic medium.

Since it is a basic step of the methods shown in Figs. 2 and 4 to plot the target color on the working diagram 15, it is apparent that an accurate, detailed, color reference data base must be available against which the target color can be matched, in order to find its coordinates. At present, applicants are aware of no existing color reference data base which fulfills the requirements necessary for the proper execution of applicants present invention. At best, the only color reference data base of which applicants are aware consist of relatively crude painted reproductions of the colors appearing on a standard CIE diagram. The reason for the lack of a color reference data base may be that the CIE system is not associated with any particular set of physical samples. Instead, the system is based on the concept of additive color mixing as derived from experiments in which a colored light from a test lamp is shown on a white screen and is than matched by mixing three primary colored lights on the screen adjacent the test light.

Briefly, according to the method of the present invention, a color reference data base is prepared, as shown in Fig. 5, by first selecting a plurality of points located on a working diagram 15 which again has units proportional to those appearing on a standard CIE diagram. As before, working diagram 15 need not include the CIE diagram 14, but it is preferred that the CIE diagram be included to aid in the aforesaid selection of points, since no color visible to the human eye will appear outside the CIE diagram 14. As has been explained, an illuminant source is selected whose color temperature and spectral distribution are the same as that being used to produce the target color. Then, a colored swatch is prepared photographically for each point selected, and the finished color reference data base is formed by mounting each colored swatch over its respective point on the working diagram 15. Since it is known that on a standard CIE diagram the colors merge imperceptibly into one another, one must keep in mind that the accuracy of the colored, reference diagram thus fabricated depends upon the number of discrete sample points selected. A relatively few sample points give a rather inaccurate result under the present method, while a relatively large number of sample points will, of course, give much better results. The number of sample points taken is thus determined by the degree of accuracy required by the user. Through the use of a large number of sample points, a color reference data base is generated which has an accuracy sufficient to meet any need of a particular user. Of course, a countervailing factor is that, as the number of sample points increases, the cost of preparing the colored, reference diagram also increases. Thus, some balancing must be done by the user between his need for accuracy, and the cost of preparing a color reference data base capable of meeting that degree of accuracy.

The plurality of points may, of course, be selected by hand. However, referring now to Fig. 6, in order to provide a uniform and systematic method by which a plurality of points may be selected on the working diagram for use in preparing the color reference data base, applicants prefer that a grid 58 of uniform spacing be superimposed over a blank, standard CIE diagram 14. Each intersection 68 on the grid is taken as a sample in the preparation of the color reference CIE data base.

Thus, for example, the grid shown in Fig. 6 has a spacing of .02 units, wherein the units are arbitrary. Of course, a larger or smaller spacing can be chosen by the user depending on the order of accuracy needed. Or, more frequent points could be chosen for those areas on the CIE diagram in which the user is particularly interested. In addition, it should be emphasized that the grid 58 shown in Fig. 6 is merely used as an example of an organizing chart, and that any other form of organizing chart to help select sample points, is within the scope of the present invention.

After the plurality of points are selected, a comparison of the methods shown in Figs. 2 and 5 reveals that all of the steps shown encompassed by the dotted line in Fig. 5 are analogous to the corresponding steps shown in Fig 2. The only difference is that in Fig. 5, the colored swatch and complement of the colored swatch are used, respectively, instead of the target color and complement of the target color of Fig. 2. Inasmuch as the method shown in Fig. 2 has been explained previously in considerable detail, no further discussion of the manner of photographically preparing each colored swatch 62 need again be given.

As an aid in selecting which filters 34, 36, 38 to use in the preparation of the colored swatches 62, it should be recalled that any set of three colored filters produces only those colors located within the filter triangle they define. Thus, if a set of filters of widely differing colors is selected, such as the red, green and blue filters 34, 36, 38 shown in Fig. 6, a relatively large number of the colored swatches can be produced by this single set of filters. Thus, by utilizing this technique of selecting filters of widely differing colors, it is seen that the number of filters needed to cover large portions of the working diagram 15 may be greatly reduced.

Of course, the use of a set of filters forming a filter triangle covering a large portion of the data base also makes it more probable that any given target color will lie within this filter triangle. Thus, the same set of filters may be used to produce both the needed portion of the color reference data base and the target color.

Here again, it should be noted that the color temperature of the illuminant source used to produce the color reference data base is an important factor which must be considered. This is because as the color temperature of the illuminant source changes, its color also changes. Accordingly, the illuminant source's location on a standard CIE diagram also changes as its color temperature changes.

The color temperature-dependent shifting of the location of the illuminant source on a standard CIE diagram is well known per se and is shown in Fig. I as the before described locus of black body light sources 50. In addition, it is known that, once the illuminant source being used has been plotted on a standard CIE diagram a straight line drawn from this point to the periphery of a standard CIE diagram identifies all colors of a single hue having a particular dominant wavelength. Each hue varies in saturation from zero at the illuminant source to one hundred percent at the periphery of a standard CIE diagram. Thus, the pattern of colors displayed on a standard CIE diagram is centered around the plotted point for the illuminant source. In general, the colors are gray or white around the plotted point for the illuminant source and grow more pronounced as one travels outward from the illuminant source on a straight line until one encounters the pure spectrum colors on the periphery of a standard CIE diagram.

Accordingly, since the pattern of colors on a standard CIE diagram is centered around the plotted point for the illuminant source, it is apparent that as the location of plotted point for the illuminant source changes according to its color temperature, the pattern of colors on a standard CIE diagram also changes. Thus, we are led to the important conclusion that the pattern of colors on a standard CIE diagram is different for each illuminant source of a different temperature.

Consequently, in order to achieve accurate results, it is important that the color reference data base used be produced with an illuminant source whose color temperature is as close as possible, and preferably the same as, the color temperature of the illuminant source being used to produce the target color. Of course, their spectral distributions should also be as close as possible, and preferably the same.

However, one of the outstanding features of the present invention is that once a color reference data base is prepared for an illuminant source of a particular color temperature and spectral distribution, it may be used again as a reference in the future at any time when an illuminant source having the same or similar color temperature and spectral distribution is being used to produce a target color.

Further, once a color reference data base has been prepared for an illuminant source of a particular color temperature, and spectral distribution, it can be easily reproduced by conventional photographic or printing means for use by others in carrying out the present invention. Thus, it is within the scope of the present invention that a series of color reference data bases be prepared, each for an illuminant source of a different color temperature and spectral distribution and particularly for those commonly used in the photographic art. These may then be distributed as an aid to others practicing applicants present invention.

From the foregoing, various further applications, modifications and adaptations of the methods disclosed by the foregoing preferred embodiments of the present invention will be apparent to those skilled in the art to which the present invention is addressed, within the scope of the following claims.

WHAT WE CLAIM IS: 1. A method of photographically substantially reproducing a target color on color photosensitive photographic material utilizing a light source and up to three color filters, said method comprising a) preparing a color reference data base expressing the primary color component make up of a plurality of colors illuminated by a given light source b) determining from the data base the primary color component make up of the color in the data base most similar to the target color under illumination by the given light source c) selecting a group of from one to three filters capable of reproducing the target color by appropriate sequential exposures and development of the photographic material d) determining from the primary color component make up of each filter under illumination by the given light source and from the sensitivity of the photographic material to light from the given source filtered by each filter the color filter exposure times appropriate to reproduce the target color on the photographic material when developed e) exposing the undeveloped color sensitive photographic material to the illuminant source in sequence through each color filter for each respective filter exposure time thus determined, to substantially reproduce the target color on the photographic material when developed and f) developing the exposed photographic material.

2. A method as claimed in claim I, wherein the group of filters selected contains three filters.

3. A method as claimed in claim 2, wherein primary color component make ups are each expressed as a set of co-ordinates x, y, and z where x represents the content of a f

Claims (39)

**WARNING** start of CLMS field may overlap end of DESC **. temperature, the pattern of colors on a standard CIE diagram also changes. Thus, we are led to the important conclusion that the pattern of colors on a standard CIE diagram is different for each illuminant source of a different temperature. Consequently, in order to achieve accurate results, it is important that the color reference data base used be produced with an illuminant source whose color temperature is as close as possible, and preferably the same as, the color temperature of the illuminant source being used to produce the target color. Of course, their spectral distributions should also be as close as possible, and preferably the same. However, one of the outstanding features of the present invention is that once a color reference data base is prepared for an illuminant source of a particular color temperature and spectral distribution, it may be used again as a reference in the future at any time when an illuminant source having the same or similar color temperature and spectral distribution is being used to produce a target color. Further, once a color reference data base has been prepared for an illuminant source of a particular color temperature, and spectral distribution, it can be easily reproduced by conventional photographic or printing means for use by others in carrying out the present invention. Thus, it is within the scope of the present invention that a series of color reference data bases be prepared, each for an illuminant source of a different color temperature and spectral distribution and particularly for those commonly used in the photographic art. These may then be distributed as an aid to others practicing applicants present invention. From the foregoing, various further applications, modifications and adaptations of the methods disclosed by the foregoing preferred embodiments of the present invention will be apparent to those skilled in the art to which the present invention is addressed, within the scope of the following claims. WHAT WE CLAIM IS:

1. A method of photographically substantially reproducing a target color on color photosensitive photographic material utilizing a light source and up to three color filters, said method comprising a) preparing a color reference data base expressing the primary color component make up of a plurality of colors illuminated by a given light source b) determining from the data base the primary color component make up of the color in the data base most similar to the target color under illumination by the given light source c) selecting a group of from one to three filters capable of reproducing the target color by appropriate sequential exposures and development of the photographic material d) determining from the primary color component make up of each filter under illumination by the given light source and from the sensitivity of the photographic material to light from the given source filtered by each filter the color filter exposure times appropriate to reproduce the target color on the photographic material when developed e) exposing the undeveloped color sensitive photographic material to the illuminant source in sequence through each color filter for each respective filter exposure time thus determined, to substantially reproduce the target color on the photographic material when developed and f) developing the exposed photographic material.

2. A method as claimed in claim I, wherein the group of filters selected contains three filters.

3. A method as claimed in claim 2, wherein primary color component make ups are each expressed as a set of co-ordinates x, y, and z where x represents the content of a first primary color in the group, y represents the content of a second primary color in the group and z represents the content of the third primary color in the group, such that x+y+z=l.

4. A method as claimed in claim 2, wherein the values of the x and y coordinates for each filter are determined by reference to the manufacturers reference data.

5. A method as claimed in any preceding claim, wherein the capability of the group of three filters to reproduce the target color is judged by plotting the primary colour component make up of each filter and of the target color in Cartesian coordinates and verifying that the plot of target color lies in or on a triangle formed by the plots of the filters.

6. A method as claimed in any preceding claim, wherein the photographic

material is a positive photographic material.

7. A method as claimed in claim 6, wherein filter proportions are determined such that the sum of the first primary color components of each filter each multiplied by the filter proportion for that filter equals the first primary colour component of the target color. the sum of the second primary color components of each filter each multiplied by the filter proportion for that filter equals the second primary color component of the target color, and the sum of the third primary color components of each filter each multiplied by the filter proportion for that filter.

8. A method as claimed in claim 7 wherein filter exposure times are calculated by multiplying each filter proportion by factor representing the sensitivity of the photographic material to light from the given light source filters by that filter.

9. A method as claimed in claim 8, wherein the factor is the saturation time for the photographic material using light trom the given source filtered by each filter.

10. A method as claimed in claim 8 wherein the factor is proportional to the saturation time for the photographic material using light from the given source filtered by each filter.

A I. A method as claimed in any one of claims 1 to 5 wherein the photographic material is a negative photographic material.

12. A method as claimed in claim 11, wherein filter proportions are determined such that the sum of the first primary color components for each filter each multiplied by the filter proportion for that filter equals the first primary color component of a color complementary to the target color. the sum of the second primary color components for each filter each multiplied by the filter proportion for that filter equals the second primary color component of the said complementary color and the sum of the third primary color components for each filter each multiplied by the filter proportion for that filter equals the third primary color component of the said complementary color.

13. A method as claimed in claim 12, wherein filter exposure times are calculated by multiplying each filter proportion by factor representing the sensitivity of the photographic material to light from the given light source filters by that filter.

14. A method as claimed in claim 13, wherein the factor is the saturation time for the photographic material using light from the given source filtered by each filter.

15. A method as claimed in claim 13, wherein the factor is proportional to the saturation time for the photographic material using light from the given source filtered by each filter.

16. A method as claimed in any one of claims 12 to 15 wherein the primary color component make up of the complementary color is determined by plotting the x and y co-ordinates as defined in claim 3 of the target color on a working diagram, plotting the x and y co-ordinates of the given light source on the working diagram, plotting a standard CIE diagram on the working diagram, extending a straight line through the plotted co-ordinates of the light source and target color to opposite sides of hhe standard CIE diagram determining the percentage of the distance on the straight line that the plotted point for the target color lies between the plotted point for the illuminant source and the nearest side of the standard CIE diagram; and noting the x, y co-ordinates of a point located on the straight line a percentage of the distance from the plotted point for the illuminant source to the far side of the standard CIE diagram which is the same as said percentage of the distance for the target color.

17. A method as claimed in any preceding claim wherein the color reference data base comprises a multitude of swatches of colored material mounted on a substrate and associated with means indicating the primary component color make up of the color of yach swatch.

18. A method as claimed in claim 17, wherein the said means indicates the coordinates x and y defined in claim 3 for each color.

19. A method as claimed in claim 18 wherein the colored swatches are mounted on a colored reference diagram, each in a position indicating its x and y coordinates.

20. A method as claimed in claim 1 substantially as hereinbefore described with reference to any one of Figures 1, 2, 3 or 4 of the accompanying drawings.

21. A colored photographic material produced by a method as claimed in any preceding claim.

22. A method of producing a color reference diagram for use in a method as claimed in claim 1, which method comprises selecting a plurality of sets of primary color components each set of components corresponding to a unique color, for each set preparing a colored swatch on a photographic material having primary color components corresponding to those of the set, and mounting each colored swatch in a position on a substrate indicative of its primary color component make up.

23. A method as claimed in claim 22 wherein the colored swatches are mounted on the substrate in position determined by a Cartesian plotting of their x and y co-ordinates as defined in claim 3.

24. A method as claimed in claim 22 or claim 23 wherein the sets are selected by superimposing a grid on a standard CIE type diagram and reading off the x and y co-ordinates of each grid intersection.

25. A method as claimed in any one of claims 22 to 24 wherein each colored swatch is prepared by a method as defined in steps (c) to (f) of claim 1 treating the color corresponding to each set as the target color.

26. A method as claimed in claim 25, wherein the photographic material is a positive photographic material.

27. A method as claimed in claim 26 wherein filter proportions are determined such that the sum of the first primary color components of each filter each multiplied by the filter proportion for that filter equals the first primary color component of the target color the sum of the second primary color components of each filter each multiplied by the filter proportion for that filter equals the second primary color component of the target color, and the sum of the third primary color components of each filter each multiplied by the filter proportion for that filter, and exposure times are calculated for each filter by multiplying the filter proportions by a factor related to the sensitivity of the film to light from the given light source filtered by that filter.

28. A method as claimed in claim 27, wherein the factor is the saturation time of the film for light from the given light source filtered by that filter.

29. A method as claimed in claim 25, wherein the photographic material is a negative photographic material.

30. A method as claimed in claim 29 wherein, filter proportions are determined such that the sum of the first primary color components for each filter each multiplied by the filter proportion for that filter equals the first primary color component of a color complementary to the target color, the sum of the second primary color components for each filter each multiplied by the filter proportion for that filter equals the second primary color component of the said complementary color and the sum of the third primary color components for each filter each multiplied by the filter proportion for that filter equals the third primary color component of the said complimentary color.

31. A method as claimed in claim 30 wherein the factor is the saturation time of the film for light from the given light source filtered by that filter.

32. A method as claimed in claim 30 or claim 31, wherein the primary color component make up of the complementary color is determined by plotting the x and y co-ordinates as defined in claim 3 of the target color on a working diagram, plotting the x and y co-ordinates of the given light source on the working diagram, plotting a standard CIE diagram on the working diagram, extending a straight line through the plotted co-ordinates of the light source and target colour the sum of the second primary color components of each filter each multiplied by the filter proportion for that filter equals the second primary color component of the target color, and the sum of the third primary color components of each filter each multiplied by the filter proportion for that filter.

33. A color reference diagram produced by a method as claimed in any one of claims 22 to 31.

34. A method for producing a colored swatch for use in making a color reference diagram as claimed in claim 33, which method comprises selecting a set of primary color components corresponding to a unique color, and preparing a swatch of that color by a method as defined in paragraphs (c) to (f) of claim 1, treating the said color as the target color.

35. A method as claimed in claim 34 wherein the photographic material is a positive photographic material and wherein filter proportions are determined such that the sum of the first primary color components of each filter each multiplied by the filter proportion for that filter equals the first primary color component of the target color the sum of the second primary color components of each filter each multiplied by the filter proportion for that filter equals the second primary color component of the target color, and the sum of the third primary color components of each filter each multiplied by the filter proportion for that filter and exposure times are calculated for each filter by multiplying the filter proportions by a factor related to the sensitivity of the film to light from the given light source filtered by that filter.

36. A method as claimed in claim 35 wherein the factor is the saturation time of the film for light from the given light source filtered by that filter.

37. A method as claimed in claim 34 wherein the photographic material is a negative photographic material and wherein filter proportions are determined such that the sum of the first primary color components for each filter each multiplied by the filter proportion for that filter equals the first primary color component of a color complementary to the target color, the sum of the second primary color components for each filter each multiplied by the filter proportion for that filter equals the second primary color component of the said complementary color and the sum of the third primary color components for each filter each multiplied by the filter proportion for that filter equals the third primary color component of the said complementary color and exposure times are calculated for each filter by multiplying the filter proportions by a factor related to the sensitivity of the film to light from the given light source filtered by that filter.

38. A method as claimed in claim 37, wherein the factor is the saturation time of the film for light from the given light source filtered by that filter.

39. A colored swatch when prepared by a method as claimed in any one of claims 34 to 38.