Typical color photographic negatives have three records which are sensitive to respective areas of the visible light spectrum, namely red, green and blue. Each record is usually made up of one or more layers each containing a light sensitive silver halide emulsion. These records also contain couplers which imagewise produce cyan, magenta and yellow dyes, respectively. In a color negative film, the records are usually arranged on a support in the order of red, green and blue sensitive records (that is, the blue sensitive record is furthest from the support).
Conventional silver halide emulsions usually have grains which are primarily cubic, octahedral, cubo-octahedral or polymorphic in shape. Such grains typically have an inherent sensitivity to visible light in the region of about 400-430 nm. Therefore, sensitizing dyes are used on the emulsions to sensitize them to the required red and green region of the spectrum. Optionally, a blue sensitizing dye can be used to provide sensitivity to the 450-500 nm region of the visable spectrum. Such conventional shaped grains, while not necessarily requiring sensitizing dyes to provide sensitivity in the blue region of the visible spectrum, contribute to optical degradation of the image being captured by the underlying green and red records.
Tabular grain emulsions are known for use in the blue sensitive layer of a color negative film. Tabular grains, when present in the blue sensitive layer, result in improved transmission of incident light to the underlying green and red sensitive layers. In order to provide practical photographic efficiency, such grains are typically sensitized in the 450-500 nm region by a spectral dyeing technique to yield blue sensitive emulsions as high tabularity, low bulk iodide tabular grains have relatively little inherent sensitivity in the 400-500 nm range. By spectrally sensitizing these emulsions where there exists a higher number of photons per unit energy (that is, between 450-480 nm of the blue region), the sensitivity and hence the efficiency of the blue sensitive record containing these elements is maximized. However this is not without consequence as discussed below.
Following imagewise exposure and chromogenic processing, the dye image (the negative) thus obtained is usually printed onto a receiving element (typically having a paper base although other supports or media such as those used in digital image manipulation are applicable) to yield a positive image.
Automatic printers, common to the art of modern photofinishing, have been developed to attain rapid and economical printing from color (dye image) negatives. Well designed printers have one large area sensor or any number of smaller sensors with red, green and blue sensitivities that are used by the printer algorithm to assess the red, green and blue densities, integrated over the entire negative, in effectively the same way as does a photographic paper which is used in the printer. Color negative films are designed so that, for a specific taking (exposing) illuminant (typically daylight), a specified red, green and blue density relationship is effected when a uniform neutral (typically gray) target is photographed. Automatic printers, in turn, are set up so that this red, green and blue density relationship of a standard negative (when exposed with a gray target under the design illuminant) are recognized as being a neutral exposure. Thus, for such a negative, the integrated red, green and blue density relative to a gray center, referenced as D', is given a value of D'=0. In any printer this leads to adjustment of the appropriate red, green or blue light exposures of the subject negative to the print (for example, by controlling the duration or intensity of those colors through the use of direct control of the light source(s) and/or filters), to yield a perfect gray print balance. The controlling logic, or algorithms, used by automatic printers further assumes that even though most scenes are composed of objects of many colors, most photographed scenes integrate to a near neutral gray.
However, when such an automatic printer encounters an exposed negative for which D' is not equal to zero, the printer algorithm is designed to alter (or "correct") the red, green and/or blue light exposure, in a manner which depends on the value of D'. The degree to which this correction is applied varies depending on the particular printer algorithm used. Due to the diverse causes of color bias, well designed printers do not apply 100% correction. Simple algorithms apply some smaller correction, often 50% to minimize the chances of removing all the color bias in the film which can significantly alter the appearance of captured scenes which do not integrate to gray. More complex algorithms alter the amount of correction depending on the color bias direction(hue) to make a more intelligent assessment as to how much of the bias to correct based on known hue-dependent bias causes. The operation of such algorithms is described in "Modern Exposure Determination for Customizing Photofinishing Printer Response" by E. Goll, D. Hill, and W. Severin, published in Journal of Applied Photographic Engineering, Vol 5, Number 2, pages 93-104, 1979.
By the foregoing process the automatic printer attempts to remove some or all of the color bias (that is, the degree to which D' differs from O, sometimes referenced in this application as "saturation" of a negative) recognized by the printer in the film frame. The goal of the printer is to reduce in the print as much as possible, all the color bias in the negative to be printed except that caused by the objects in the scene itself and occasionally some of the bias caused by the scene illuminant (as in pictures taken at sunset) so that the printed reproduction appears to the viewer as the original scene is remembered.
It would be desirable to provide a color negative which uses a tabular grain emulsion in the blue sensitive layer, and which can be printed in automatic printers of the above described type and produce prints which have low objectionable color bias even though the negative may have been exposed under different lighting conditions, and particularly under fluorescent lighting.