Photocopying – Projection printing and copying cameras – Methods
Utility Patent
1993-09-01
2001-01-02
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Methods
C355S038000
Utility Patent
active
06169599
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for determining the individual amounts of light for each primary color in a color copying process.
In a known method of determining the amount of light in each primary color to which a colored original (i.e., a negative or transparent positive such as a slide) is to be exposed during copying, each of a series of regions or points of the original is scanned in the three primary colors, e.g. to measure the transparency of the original to light of each color. The measurements obtained from each region are compared with reference values in order to determine whether the respective regions contain color dominants. The amount of light in each primary color to which the original is to be exposed during copying is then calculated disregarding the regions with color dominants altogether or giving less weight to such regions.
U.S. Pat. No. 4,279,502 of Thurm et al. describes how to determine color density difference curves for a film from the color density values and average densities of individual color originals, e,g. color negatives. The regions of the original are checked first to see which are relatively neutral in color and which belong to a dominant color region. Also, the regions are analyzed to see whether there was atypical photographic illumination, which would also result in a clear color shift. Image containing regions with a photographic object of a dominant color or with a color tinge due to the illumination cannot be used for the formation of color density difference curves.
Techniques for reliably identifying color-dominant regions are described in U.S. Pat. No. 4,561,768 of Fursich et al. The method employed in the '768 patent searches for the thinnest or most transparent regions on the film. Locating an impermissibly large color deviation from a neutral value is made by comparison with the mask value on the assumption that the thinnest point also at the same time represents the mask value. It is also possible to start from absolute values, determined for example from an average of the mask values of a large number of films. Finally, a color difference curve, obtained from a first passage of the measured values through a computer, can itself serve as a comparison scale for a second, improved passage.
The foregoing methods provide good values for certain negatives and values that are not as good for other negatives. Since the nature of the negatives is difficult to classify, the identification of dominant colors is difficult, particularly in films, the negatives of which reproduce the same color-dominant motif in large numbers. Moreover, the identification of dominants is difficult in very short films, particularly in individual negatives, if there is no probability of a statistical equalization of the motifs over the whole of the film. In the case of professional films, which are used as portrait films in a studio under constant conditions of illumination and for which the requirements of color fidelity are particularly high, a sufficiently accurate recognition of dominants is difficult.
OBJECT OF THE INVENTION
It is an object of the invention to provide an improved method for identifying dominants and color tinges in a color copying system of the type disclosed in U.S. Pat. Nos. 4,297,502 and 4,561,768.
SUMMARY OF THE INVENTION
Standard color density difference curves provide a reliable indication of whether the scanned regions of the color original are suitable for establishing a color density difference curve specific to the film containing the original. According to one embodiment of the invention, the color density difference values for a particular film type are stored in memory and the required value determined by optically reading a DX code, for example, on one edge of the film. Alternatively, the film type identification can be entered manually and the color density difference curves taken from memory to calculate the copying light amounts.
The standard color density difference data for each film type can be obtained in various ways. For example, gray fields or gray steps can be exposed on test films. These films may be subsequently developed and measured. From the point of view of exposure, this technique provides reliable measurement values. However, the method does not take into consideration the fact that the properties of photographic films of one type fluctuate to some extent. Very accurate information concerning the standard curves would therefore be possible only on the basis of a large number of test films. However, the cost when a large number of film types is used is not justifiable.
According to a further feature of the invention, the standard color density difference curves for a film type are formed by averaging color density difference curves obtained from a large number of films of one type. Such averaging is based on actual experience and also takes into consideration certain trends in the properties of photographic film types and the film development in the laboratory, particularly, if over longer periods of time the older measured values are no longer taken into consideration or if less weight is attached to such values. One method of discounting the effect of older data is to assign backwards directed ordinal numbers to each new set of data.
Furthermore, the standard color density difference curves can be used as a color standard for identifying special color surfaces in the originals, such as skin shades or white textiles. The correct reproduction of the colors of such characteristic surfaces is, to a high degree, an indication of the quality or correctness of a copy, so that various methods are directed particularly to the correct reproduction of such surface areas. However, such special colored surfaces can be identified correctly only if it is possible to accurately define the colored space in which they are located. Such a definition requires a standard, for which the film type-specific standard color density difference curves are particularly well suited. The film typespecific standard color density difference curves can be used as a gray standard for locating such specially colored surfaces.
According to a further feature of the invention, the standard color density difference curves can also be averaged with color density difference curves determined for the color film on the basis of its image containing regions and the image values can be specified for calculating the copying light quantities. These film-specific values are less susceptible to the accumulation of particular motifs within a film and are good for equalizing color tinges of the films, which can arise due to lengthy storage, wrong development, etc. Finally, the standard color density difference curves are helpful for color originals in which the film-specific values can be based only on a very small number of values that are not dominant, particularly in the case of copying individual negatives.
Further details and advantages of the invention arise out of the description of the embodiments, illustrated in the drawings. The drawings, however, are illustrative only. The scope of the invention should be ascertained from the claims.
REFERENCES:
patent: 4279502 (1981-07-01), Thurm et al.
patent: 4561768 (1985-12-01), F{umlaut over (u)}rsich et al.
patent: 4566786 (1986-01-01), F{umlaut over (u)}rsich et al.
patent: 5148213 (1992-09-01), Terashita
patent: 5555073 (1996-09-01), Gossman et al.
patent: 5561494 (1996-10-01), Terashita
Fuersich Manfred
Treiber Helmut
Zahn Wolfgang
Adams Russell
Agfa-Gevaert Aktiengesellschaft
Darby & Darby
Mahoney Christopher
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