In classical black-and-white photography a photographic element containing a silver halide emulsion layer coated on a transparent film support is imagewise exposed to light. This produces a latent image within the emulsion layer. The film is then photographically processed to transform the latent image into a silver image that is a negative image of the subject photographed. Photographic processing involves developing (reducing silver halide grains containing latent image sites to silver), stopping development, and fixing (dissolving undeveloped silver halide grains). The resulting processed photographic element, commonly referred to as a negative, is placed between a uniform exposure light source and a second photographic element, commonly referred to as a photographic paper, containing a silver halide emulsion layer coated on a white paper support. Exposure of the emulsion layer of the photographic paper through the negative produces a latent image in the photographic paper that is a positive image of the subject originally photographed. Photographic processing of the photographic paper produces a positive silver image. The image bearing photographic paper is commonly referred to as a print.
In a well known, but much less common, variant of classical black-and-white photography a direct positive emulsion can be employed, so named because the first image produced on processing is a positive silver image, obviating any necessity of printing to obtain a viewable positive image. Another well known variation, commonly referred to as instant photography, involves imagewise transfer of silver ion to a physical development site in a receiver to produce a viewable transferred silver image.
In classical color photography the photographic element contains three superimposed silver halide emulsion layer units, one for forming a latent image corresponding to blue light (i.e., blue) exposure, one for forming a latent image corresponding to green exposure and one for forming a latent image corresponding to red exposure. During photographic processing, developing agent oxidized upon reduction of latent image containing grains reacts to produce a dye image with developed silver being an unused product of the oxidation-reduction development reaction. Silver is removed by bleaching and fixingduring photographic processing. The image dyes are complementary subtractive primaries--that is, yellow, magenta and cyan dye images are formed in the blue, green and red image recording units, respectively. This produces negative dye images (i.e., blue, green and red subject features appear yellow, magenta and cyan, respectively). Exposure of color paper through the color negative followed by photographic processing produces a positive color print. Again, bleaching and fixing remove developed silver and residual silver halide that would otherwise adversely affect the color print.
In one common variation of classical color photography reversal processing is undertaken to produce a positive dye image in the color photographic element, commonly referred to as a slide, the image typically being viewed by projection. In another common variation, referred to as color image transfer or instant photography, image dyes are transferred to a receiver for viewing.
In each of the classical forms of photography noted above the final image is intended to be viewed by the human eye. Thus, the conformation of the viewed image to the subject image, absent intended aesthetic departures, is the criterion of photographic success.
It is well known to those skilled in the art that the colors reproduced on, or produced from, a photographic color-imaging element generally are not colorimetric matches of the colors originally photographed by the element. Colorimetric errors can be caused by the color recording and color reproduction properties of the photographic element and system. The distinction between the color recording and color reproduction properties of a photographic element is fundamental. Color recording by a photographic element is determined by its spectral sensitivity. The spectral sensitivity of a photographic element is a measure of the amount of exposure of a given wavelength required to achieve a specific photographic response. Color reproduction by a photographic imaging system depends not only on the color recording properties of the capturing element as described above, but also on all subsequent steps in the image forming process. The color reproduction properties of the imaging element or system can vary the gamma, color saturation, hue, etc. but cannot fully compensate for problems caused by spectral sensitivities which are not correlates of the human visual system. Metamers are an example of such a problem. Metamerism occurs when two stimuli with different spectral reflectance appear identical to the eye under a specific illuminant. A photographic element whose spectral sensitivities differ from that of the human visual system record the stimuli differently. Once recorded as disparate, a photographic element's color reproduction will only amplify or minimize that difference.
In certain applications, it is desirable to form image representations that correspond more closely to the colorimetric values of the colors of the original scene recorded on the photographic color-imaging element rather than form image representations which correspond to the reproductions of those colors by the element itself. Examples of such applications include, but are not limited to, the production of medical and other technical images, product catalogues, magazine advertisements, artwork reproductions, and other applications where it is desirable to obtain color information which is a colorimetrically accurate record of the colors of the original scene. In these applications, the alterations in the color reproduction of the original scene colors by the color recording and color reproduction properties of the imaging element are undesirable.
To achieve absolute colorimetric accuracy during recording, the photographic element's spectral sensitivity must be color-matching functions. Color-matching functions are defined as the amounts of three linearly independent color stimuli (primaries) required to match a series of monochromatic stimuli of equal radiant power at each wavelength of the spectrum. A set of three color stimuli is linearly independent when none of the stimuli can be matched by a mixture of the other two. Negative amounts of a color stimulus are routine in color-matching functions and are interpreted as the amount of that color stimulus which would be added to the color being matched and not to the mixture itself. Color-matching functions for any real set of primaries must have negative portions. It is possible to functionally transform from one set of color-matching functions to any other set of color-matching functions using a simple linear transformation. By using the color-matching functions which correspond to the primaries of the intended output device or medium as the photographic element's spectral sensitivities, no additional color signal processing is necessary.
The selection of spectral sensitivities for colorimetric recording is based on the primaries of the imaging system in question. The primaries in a photographic system are defined by the imaging dyes of the element used to form the final reproduction of the recorded image, the spectral composition of which is all positive. Color-matching functions for a set of all-positive primaries contain negative responses. Within the realm of known photographic mechanisms, it is not possible to produce a photographic element having spectral sensitivities whose response is negative.
To date, no available photographic system has been developed which has spectral sensitivities which approximate a set of color-matching functions or a linearly combination thereof. Numerous ranges of spectral sensitization have been claimed for specific color reproduction advantage, but none approximate color-matching functions as spectral sensitivities and therefore do not have colorimetrically accurate color recording or reproduction.
A photographic element could be built using all-positive color-matching functions as spectral sensitivities, but these color-matching functions would not correspond to the primaries of the photographic system. Those skilled in the art will recognize that linear exposure-space signal processing (matrixing) would be required to transform the linear exposures recorded by all-positive color-matching-function spectral sensitivities to the linear exposures corresponding to the display primaries of the system. The signal processing available in photographic elements, however, is inherently non-linear in nature, i.e. it operates in what is effectively a log-exposure space, rather than a linear-exposure space. For example, the amount of chemical signal processing (hereafter referred to as interlayer interimage) produced by a dye-forming layer of a photographic element is essentially proportional to the amount of silver developed and/or the amount of image dye formed in that layer; and both silver development and dye formation are in turn essentially proportional to the logarithm of the exposure of that layer, rather than to the exposure. Color correction may also be produced by other methods. For example, colored dye-forming couplers can be used (in negative working and other intermediary photographic elements), and the hues of the image-forming dyes themselves can be adjusted. The color correction produced by these methods, however, is also logarithmic in nature and not of the linear type required in order to use color-matching-function spectral sensitivities.
If a conventional photographic element were to be built with all-positive color-matching functions, the preferred choice of spectral sensitivities would be an all-positive set with minimum overlap. David L. MacAdam derived a set of single-peaked all-positive functions with minimum overlap which very closely approximate color-matching functions. By minimizing the overlap of the spectral sensitivities, competition for light between image recording units during imagewise exposure and the amount of interimage required is minimized. Use of the MacAdam sensitivities reduces the problems encountered with spectral sensitivities which are color-matching functions but not sufficiently to make the use of such sensitivities practical in a conventional photographic element.
Further, the inter-record chemical interactions available in photographic chemistry are limited in their ability to address individual records. For example, it is difficult to affect a chemical interaction from layer A to layer C, if layer B is located between them, without affecting layer B. Inter-record chemical interactions are useful in correcting for the effects of unwanted absorptions of the imaging dyes and optical crosstalk, but the control of their magnitude and specificity is limited.
For these reasons, conventional photographic elements require spectral sensitivities which differ significantly from color-matching functions. The spectral sensitivities used in conventional photographic systems are designed to minimize the need for linear-space signal processing (color correction) because such color correction is not available from chemical color-correction mechanisms. Conventional photographic elements are therefore not well suited for applications in which the photographic elements of the present invention are intended.
References can also be found in the prior art suggesting the use of spectral sensitivities for various purposes which differ from conventional sensitivities but which do not reasonably approximate color-matching functions. For example, U.S. Pat. No. 3,672,898 entitled MULTICOLOR SILVER HALIDE PHOTOGRAPHIC MATERIAL AND PROCESSES by J. Schwan and J. Graham describes photographic elements incorporating red, green, and blue spectral sensitivities of specified peak wavelengths and specified ranges of spectral widths which provide good color rendition and acceptable neutrals under a variety of illuminants such as sunlight, tungsten or fluorescent.
U.S. Pat. No. 5,180,657 entitled COLOR PHOTOGRAPHIC LIGHT-SENSITIVE MATERIAL OFFERING EXCELLENT HUE REPRODUCTION by F. Fukazawa et al describes photographic elements incorporating red, green, and blue spectral sensitivities with specified ranges of peak wavelengths and increased levels of interlayer interimage for improved color reproduction, particularly of colors of certain difficult-to-reproduce hues.
In each of these and other related patents and applications, the photographic element spectral sensitivities, described by various ranges of peak locations and widths, do not reasonably approximate sets of color-matching functions. In order to achieve acceptable color reproduction, either directly or from subsequent imaging processes, the spectral sensitivities of the photographic elements described in these patents represent compromises constrained by the type and amount of color correction available within the conventional photographic system. These compromises result in a colorimetrically inaccurate recording of original scene colors, in the form of an exposed latent image.
Further, much of the prior an for the spectral sensitivity ranges of photographic elements specifies the response of the respective image recording units independently and a selection of any set of three in no way assures that the resultant photographic element's sensitivity will yield colorimetrically accurate recording or be satisfactory for a given set of imaging chemistry. The specification of a test method for evaluating color recording is necessary to ensure that the set of spectral sensitivities chosen will deliver the required performance.
It is well known and typical in the photographic an to judge the color reproduction of films and film-based systems using human judgments of a limited number of colors (whether in patch form or contained in an image). The selection of colors used, images selected for judgment, and individual preferences play a role in the judgment of color reproduction and therefore cannot lead to a definitive measure of film's or imaging system's colorimetric capabilities. To definitively differentiate between the color reproduction capabilities of various spectral sensitivities, a quantitative measure is required.
Quantitative measures based on correlation of spectral sensitivities to a set of color-matching functions have been proposed. The ability to predict color recording capabilities of a photographic element based on the correlation of its spectral sensitivities to color-matching functions is limited, as discussed by F. R. Clapper in The Theory of the Photographic Process, T. H. James, 4th Ed., Macmillan, N.Y., 1977, Chapter 19, Section D, pp. 566-571. Clapper points out that such a correlation is unable to differentiate the colorimetric accuracy of sets of spectral sensitivities which have equal correlation to color-matching functions but significantly different color recording properties. Therefore, a quantitative measure which will more effectively differentiate the colorimetric recording capabilities of various sets of spectral sensitivities in commonly encountered imaging situations is required. Such a quantitative measure requires the specification of the illumination source, test colors, and the metric to be calculated. The distribution of test colors are selected such that they are evenly distributed in color space, and have spectral reflectance representative of the colors typically encountered in imaging.
The following is a color test which meets all the aforementioned criteria, quantifies the colorimetric accuracy of a photographic element (or system), differentiates between the colorimetric capabilities of various photographic element spectral sensitivities, and simulates typical imaging conditions with colors which are distributed in color space and whose spectral reflectance is representative of real-world surface colors. For the test, color accuracy is judged according to the value of .DELTA.E*.sub.ab. .DELTA.E*.sub.ab is the average CIE 1976 (L*a*b*) color difference, .DELTA.E*.sub.ab, between the CIE 1976 (L*a*b*)-space (CIELAB space) coordinates of the test colors and the CIE 1976 (L*a*b*)-space coordinates corresponding to a specific transformation of the exposure signals recorded by the photographic element. .DELTA.E*.sub.ab computed for a specified set of colors of known spectral reflectance using a D.sub.65 illuminant. D.sub.65 is a CIE standard illuminant which is specified to be representative of a daylight source with a correlated color temperature of 6500.degree. K. The exposure signals are calculated using the measured spectral sensitivity of the photographic element. The exposure signals are transformed using a 3.times.3 matrix, Matrix M (applied in (linear) exposure space). The 3.times.3 exposure matrix is derived to minimize ##EQU2## using standard regression techniques. The test colors consist of 190 entires of known spectral reflectance specified at 10 nm increments (see Appendix A).
The foregoing discussion is mathematically described as follows: The red, green, and blue record relative exposures captured by the photographic element for the i.sup.th color (H.sub.red.sbsb.i, H.sub.grn.sbsb.i, H.sub.blu.sbsb.i, respectively) are calculated as: ##EQU3## where red, grn, blu designate the records of the photographic element, S.sub..lambda. is the spectral power output of the illuminant, D.sub.65
R.sub..lambda. is the spectral reflectance of the i.sup.th test color PA1 I.sub..lambda. is the measured spectral sensitivity of the photographic element,
and ##EQU4## where E.sub..lambda. is the narrow bandwidth exposure of peak wavelength .lambda., required to achieve a defined density in the photographically processed photographic element, and values of n.sub.red, n.sub.grn, and n.sub.blu are determined such that ##EQU5##
From the CIE 1931 system, the aim tristimulus values for the i.sup.th color patch, X.sub.aim.sbsb.i, Y.sub.aim.sbsb.i, and Z.sub.aim.sbsb.i, are computed: ##EQU6## where: ##EQU7## are the CIE 1931 color-matching functions.
All mathematical integrations are performed over the range from to 730 nm as discussed by R. W. G. Hunt in Measuring Color, John Wiley and Sons, New York, Chapter 2, pg. 50.
The aim CIELAB values (L*.sub.aim.sbsb.i, a*.sub.aim.sbsb.i, b*.sub.aim.sbsb.i) of the i.sup.th -color patch are computed: ##EQU8## X.sub.n, Y.sub.n, Z.sub.n are the tristimulus values (95.04, 100.00, 108.89, respectively) which describe a specified white achromatic stimulus (D.sub.65 illuminant).
The tristimulus values (X.sub.PE.sbsb.i, Y.sub.PE.sbsb.i, Z.sub.PE.sbsb.i) of the i.sup.th color patch for the photographic element are calculated as follows: ##EQU9## where: ##EQU10##
Matrix P is the phosphor matrix for a video monitor having primaries defined by CCIR Recommendation 709, Basic Parameter Values for the HDTV Standard for the Studio and for International Programme Exchange, published 24 May 1990. The chromaticity coordinates (CIE 1931) of the primaries are red (x=0.640, y=0.330), green (x=0.300, y=0.600), and blue (x=0.150, y=0.060). The assumed chromaticity for equal primary signals, i.e. the reference white, is (x=0.3127, y=0.3290), corresponding to D.sub.65. Matrix P in no way influences the magnitude of .DELTA.E*.sub.ab, it is included so that the magnitude of the terms in matrix M are relevant in the noise test described below. The signals resulting after application of matrix M are suitable to drive a video monitor with phosphors having the specified chromaticities. Matrix M is derived using standard regression techniques and is calculated so as to minimize the quantity, ##EQU11## where .DELTA.E*.sub.ab is determined for each test color as defined below. The transformed exposure signals of the photographic element are used to calculate CIELAB coordinates as follows: ##EQU12##
The average CIELAB color difference, .DELTA.E*.sub.ab, is defined as: ##EQU13## where ##EQU14##
Although the color recording and/or reproduction of an imaging system is an important characteristic to be considered in its design, it is not the only factor. Preferred embodiments of the invention have, as one of their features. excellent signal-to-noise properties for use in hybrid imaging systems. Image quality aspects of photographic elements used in hybrid systems must therefore be considered. R. W. G. Hunt in The Reproduction of Colour in Photography, Printing, and Television, 4th Ed., Fountain Press, England, 1987, Chapter 20, Section 20.10, pp. 414-416 points out "The practical choice of spectral sensitivities is usually based on a compromise aimed at achieving a balance between several conflicting requirements. Thus if the coefficients of the matrix are too high, the signal-to-noise may be adversely affected." The matrix coefficients to which Hunt refers are those used to transform from the spectral sensitivities of a video camera to the color-matching functions which correspond to the primaries of the output device or medium, which in Hunt's discussion are the phosphors of a video system. It is therefore important to also consider the signal-to-noise implications of a particular selection of spectral sensitivities. As in the case of assessing the color recording capabilities of a set of spectral sensitivities, it is useful to have a quantitative measure of the signal-to-noise implications of a particular choice of spectral sensitivities.
The measure used to quantify the noise implications is ".PSI.", or noise-gain factor. As alluded to in Hunt's reference, the noise-gain factor, .PSI., is computed from the matrix used to transform the photographic element's exposures to a specified set of color-matching functions. The color-matching functions chosen for reporting the noise results correspond to the primaries outlined in the CCIR Recommendation 709, Basic Parameter Values for the HDTV Standard for the Studio and for International Programme Exchange, published 24 May 1990. The chromaticity coordinates (CIE 1931) of the primaries are red (x=0.640, y=0.330), green (x=0.300, y=0.600), blue (x=0.150, y=0.060), and the assumed chromaticity for equal primary signals, i.e. the reference white, is (x=0.3127, y=0.3290), corresponding to D.sub.65. .PSI. is the sum of the square roots of the sum of the squares of the elements of each row in the matrix M which transforms the exposure signals. Mathematically this is expressed as: ##EQU15## where i and j represent the row and column number, respectively.
The tests described are useful measures to predict the capabilities of a photographic element and to differentiate between the capabilities of photographic elements. The color test is designed specifically to measure the colorimetric accuracy of the spectral sensitivities of the photographic element and does not indicate the colorimetric accuracy of the reproduced image; it is a measure of the colorimetric accuracy of the recorded image only.
With the emergence of computer-controlled data processing capabilities, interest has developed in extracting the information contained in an imagewise exposed photographic element instead of proceeding directly to a viewable image. It is now common practice to scan both black-and-white and color images. The most common approach to scanning a black-and-white negative is to record point-by-point or line-by-line the transmission of a light beam, relying on developed silver to modulate the beam. In color photography blue, green and red scanning beams are modulated by the yellow, magenta and cyan image dyes. In a variant color scanning approach, the blue, green and red scanning beams are combined into a single white scanning beam modulated by the image dyes that is read through red, green and blue filters to create three separate records. The records produced by image dye modulation can then be read into any convenient memory medium (e.g., an optical disk). Systems in which the image passes through an intermediary, such as a scanner or computer, are often referred to as "hybrid" imaging systems.
A hybrid imaging system must include a method for scanning or for otherwise measuring the individual picture elements of the photographic media, which serve as input to the system, to produce image-bearing signals. In addition, the system must provide a means for transforming the image-bearing signals to an image representation or encoding that is appropriate for the particular applications of the system.
Hybrid imaging systems have numerous advantages because they are free of many of the classical constraints of photographic embodiments. For example, systematic manipulation (e.g., image reversal, hue and tone alteration, etc.) of the image information that would be cumbersome or impossible to accomplish in a controlled manner in a photographic element are readily achieved. The stored information can be retrieved from memory to modulate light exposures necessary to recreate the image as a photographic negative, slide or print at will. Alternatively, the image can be viewed on a video display or printed by a variety of techniques beyond the bounds of classical photography--e.g., xerography, ink jet printing, dye-diffusion printing, etc.
For example, U.S. Pat. No. 4,500,919 entitled "COLOR REPRODUCTION SYSTEM" by W. F. Schreiber, discloses an image reproduction system of one type in which an electronic reader scans an original color image and converts it to electronic image-bearing signals. A computer workstation and an interactive operator interface, including a video monitor, permit an operator to edit or alter the image-bearing signals by means of displaying the image on the monitor. When the operator has composed a desired image on the monitor, the workstation causes the output device to produce an inked output corresponding to the displayed image. In that invention, the image representation or encoding is meant to represent the colorimetry of the image being scanned. Calibration procedures are described for transforming the image-bearing signals to an image representation or encoding so as to reproduce the colorimetry of a scanned image on the monitor and to subsequently reproduce the colorimetry of the monitor image on the inked output.
U.S. patent application Ser. No. 059,060 entitled METHODS AND ASSOCIATED APPARATUS WHICH ACHIEVE IMAGING DEVICE/MEDIA COMPATIBILITY AND COLOR APPEARANCE MATCHING by E. Giorgianni and T. Madden describes an imaging system in which image-bearing signals are converted to a different form of image representation or encoding, representing the corresponding colorimetric values that would be required to match, in the viewing conditions of a uniquely defined reference viewing environment, the appearance of the rendered input image as that image would appear, if viewed in a specified input viewing environment. The described system allows for input from disparate types of imaging media, such as photographic negatives as well as transmission and reflection positives. The image representation or encoding of that system is meant to represent the color appearance of the image being scanned (or the rendered color appearance computed from a negative being scanned), and calibration procedures are described so as to reproduce that appearance on the monitor and on the final output device or medium.
Each of these forms of image representation or encoding, produced by transformations of image-bearing-signals, is appropriate and desirable for applications where the intent is to represent the colors of the image reproduced directly on, or to be subsequently produced from, the color-imaging element being scanned into the system. For other applications, however, it would be more desirable to produce an image representation or encoding that is a colorimetrically accurate representation of original scene colors, rather than reproduced colors.
An improved photographic element for use in applications requiring coIorimetrically accurate representations of captured scenes would provide the capability to produce image representations or encoding that accurately represent original scene colorimetric information. The improved photographic element could be used to form and store a colorimetrically accurate record of the original scene and/or used to produce colorimetrically accurate or otherwise appropriately rendered color images on output devices/media calibrated by techniques known to those skilled in the art.
One requirement for the use of photographic elements capable of colorimetrically accurate recording is the ability to remove color alterations produced by the color reproduction properties of the imaging element. U.S. Pat. No. 5,267,030 entitled METHODS AND ASSOCIATED APPARATUS FOR FORMING IMAGE DATA METRICS WHICH ACHIEVE MEDIA COMPATIBILITY FOR SUBSEQUENT IMAGING APPLICATIONS, filed in the names of E. Giorgianni and T. Madden, provides a method for deriving, from a scanned image, recorded color information which is substantially free of color alterations produced by the color reproduction properties of the imaging element. In that patent, a system is described in which the effects of media-specific signal processing are computationally removed, as far as possible, from each input element used by the system. In addition, the chromatic interdependencies introduced by the secondary absorptions of the image-forming dyes, as measured by the responsivities of the scanning device, are also computationally removed. Use of the methods and means of the invention transform the signals measured from the imaging element to the exposures recorded from the original scene.
The extraction of recorded exposure information from each input element allows for input from disparate types of imaging media, such as conventional photographic negatives and transmission and reflection positives. For the purposes of the present invention, that same process of extracting recorded exposure information can be used to effectively eliminate any contribution to color inaccuracy caused by chemical signal processing and by the image-forming dyes. However, the recorded exposure information so extracted will, in general, still not be an accurate record of the colorimetric values of colors in the actual original scene that was recorded photographically using the element, as described previously. The reason for this inaccurate recording is the selection of spectral sensitivities in conventional photographic products.
Values of .DELTA.E*.sub.ab and .PSI. were calculated as previously described for a variety of commercially available photographic elements. Table I contains representative photographic elements from that survey. Spectral sensitivity was measured for negative-working photographic elements by determining the exposures required to achieve a density of 0.2 above the minimum density formed in the absence of exposure. Spectral sensitivity for positive-working photographic elements was measured by determining the exposures required to achieve a density of 1.0. Included for reference are the MacAdam spectral sensitivities. The entry "J. Schwan and J. Graham" refers to spectral sensitivities selected from the ranges cited in U.S. Pat. No. 3,672,898 entitled MULTICOLOR SILVER HALIDE PHOTOGRAPHIC MATERIAL AND PROCESSES by J. Schwan and J. Graham. The entry "F. Fukazawa" refers to spectral sensitivities selected from ranges cited in U.S. Pat. No. 5,180,657 entitled COLOR PHOTOGRAPHIC LIGHT-SENSITIVE MATERIAL OFFERING EXCELLENT HUE REPRODUCTION by F. Fukazawa et al.
TABLE I ______________________________________ Entry Identification .DELTA.E* ab .PSI. FIG. ______________________________________ 1 Color Reversal Film #1 7.0 3.4 1 2 Color Reversal Film #2 5.4 3.6 2 3 Color Negative Film #1 5.0 3.7 3 4 Color Negative Film #2 5.6 3.5 4 5 Color Negative Film #3 3.9 3.8 5 6 Color Negative Film #4 3.4 4.0 6 7 MacAdam 0.1 7.3 7 8 J. Schwan/J. Graham 3.8 4.4 8 9 F. Fukazawa 3.9 3.8 9 ______________________________________
The following discussion relates to the data presented in Table I. Entries 1-6 are representative of the normal range of colorimetric accuracy for photographic elements currently available based on measurements of their spectral sensitivities. Entry 6 marks the lower limit of .DELTA.E*.sub.ab of the photographic elements surveyed. Entry 7 establishes the value of .DELTA.E*.sub.ab for the MacAdam spectral sensitivities, the residual error is caused by the truncation of small negative responses present in the color-matching functions on which the MacAdam spectral sensitivities are based. The spectral sensitivities of the photographic elements listed in Table I are shown in FIGS. 1-9. The area under each spectral sensitivity response is normalized to unity for convenience.
From the data in Table I, it is clear that conventional photographic elements are not sensitized to achieve colorimetric accuracy. Subsequent stages in the color reproduction of these photographic elements will alter the colorimetric performance but can not improve the colorimetric accuracy. The colorimetric accuracy is fundamentally limited by the spectral sensitivity of the photographic element.
The data in Table I also illustrates that the prior art as manifest in the patents of J. Schwan and J. Graham and F. Fukazawa is insufficient in its specification of spectral sensitivities to produce colorimetrically accurate data. Because of the inter-related nature of the choice of spectral sensitivities, it is not possible to select, for example, the green spectral sensitivity independently of the red spectral sensitivity. The specification of spectral sensitivity must therefore be in terms of the colorimetric capability of the photographic element if it is to achieve a specified level of colorimetric accuracy.