General Characteristics Neutral Characteristics: Viewing Illuminant Sensitivity Neutral Characteristics: Color Balance Grayscale Characteristics Color Characteristics Color-Encoding Considerations 6. Photographic Transparencies

 Photographic transparencies, such as 35-mm slides and large-format sheet films, are the most commonly used form of input for high-end color imaging systems. There are good reasons for this. Photographic transparency films are capable of extraordinary image quality-high sharpness, low noise-and they can record and produce a wide range of colors. Photographic transparencies are favorite medium of most magazine and catalog editors. CH. 6 Photographic Transparencies

 Figure 6.1 shows a simplified cross-section of a photographic transparency film. Because both the green light-sensitive and red-sensitive layers also are inherently sensitive to blue light, a yellow filter layer is coated above these layers to prevent any blue light from reaching them. General Characteristics

Photographic transparency films are designed to be used with specific image-capture illuminants. For example, some films are designed for daylight-illumination photography, while others are designed for tungsten- illumination photography.

 Fig. 6.2 shows the relative spectral power distributions for CIE Standard Illuminant and a tungsten light source.

 Fig. 6.3 shows the red, green, and blue spectral sensitivities of a representative daylight transparency film, which is balanced for scene illumination. If this film were used to photograph a scene illuminated by a tungsten light source, the resulting slides would have an overall orange (red-yellow) color balance due to the relatively high red power and low blue power of the tungsten source.

 Fig. 6.3 also shows the red, green, and blue spectral sensitivities for a representative photographic transparency that is balanced specifically for tungsten- illumination photography. Note that its red sensitivity is red low, and its blue sensitivity is relatively high, compared to the respective sensitivities of the daylight film.

 When an exposed area of photographic transparency film is chemically processed, yellow, magenta, and cyan dyes are formed in the blue-light, green-light, and red-light-sensitive layers, respectively (Fig.6.4). A positive image results, i.e., the maximum amount of dyes forms at the minimum exposure; the minimum amount of dyes forms at the maximum exposure. Also during chemical processing, the yellow filter layer is made colorless.

 Fig. 6.5 shows the spectral transmission densities of the cyan, magenta, and yellow image-forming dyes of a representative photographic transparency film (solid lines), compared to those of a representative photographic paper (dotted lines). Note that these dyes seem to be purer than those of the photographic paper, which are also shown in Fig Purer means that each dye more nearly absorbs light of just one primary color, and each has less unwanted absorption of light of the other two primary colors. For example, the cyan dye of the photographic transparency film absorbs mostly red light and relatively little green or blue light.

 For example, a dye that appears to be a pure yellow on a transmissive support may appear quite orange on a reflective support because the reflection optics magnify any unwanted green- light absorption the dye might have.

 Hue change, from yellow toward orange, which results when the same yellow dye is used on a transmissive support and on a reflective support.

 Comparison of the resulting color garmuts when the same set of CMY image-forming dyes is used on a transmissive support (the gumut boundary shown by the wire frame) and a reflective support (gamut boundary shown by the solid).

 Colorimetric neutral produced by the CMY image- forming dyes of a representative photographic transparency film. The viewing illuminant is a tungsten- halogen projection lamp. Neutral Characteristics: Viewing Illuminant Sensitivity

 Spectral transmission density characteristics of two sets of cyan, magenta, and yellow image-forming dyes. The dyes of set A (solid lines) have much lesser amounts of unwanted absorptions than do the dyes of set B (dotted lines).

 Neutrals formed by dye set A are more spectrally selective, and therefore more viewing illuminant sensitive, than those formed by dye set B.

 The color gamut of dye set A (indicated by the wire frame) is greater than that of dye set B (indicated by the solid).

 A colorimetric neutral and the actual neutral produced by a representative photographic slide film. Slides made by this film are designed to be viewed using a tungsten-halogen projection lamp. Neutral Characteristics: Color Balance

 Grayscale characteristic of a high-quality transparency film. The grayscale of a high-quality reflection-print system is shown for comparison. Grayscale Characteristics

 Comparisons for absolute luminance level and viewing flare do not fully explain the grayscale characteristics of a photographic transparency film.

 Densities of the reproduction of a perfect white on the grayscales of a photographic transparency film and a photographic reflection system.

 A photographic transparency film is a complete imaging system. Color Characteristics

 Each of the basic imaging-system functions-image capture, signal processing, and image formation–is performed entirely within the transparency film itself(Fig. 6.14).

 The colorimetric characteristics of the image-capture function of a transparency film are determined by the spectral sensitivities of that film. These sensitivities can differ somewhat from product to product, as shown in Fig  Comparison of the red, green, and blue spectral sensitivities of two photographic transparency films.

 Comparison of test target colors, as reproduced by the two films of Fig The two films are identical except for their red, green, and blue spectral sensitivities.

 CMY density scales for a particular photographic transparency film. Note the overall cyan-blue color balance.

 Spectral sensitivities of a representative photographic transparency film, compared to a set of all-positive color-matching functions.  The film sensitivities are spectrally narrower and more separated than this set, or any other set, of all-positive color-matching function.

 The colorimetric characteristics of a photographic transparency film result from a complex relationship involving the characteristics of a spectral sensitivities, grayscale signal processing, color signal processing, and image–forming dyes.  One consequence of this complexity is that each transparency film tends to have its own distinctive appearance.  Sometimes the color-production characteristics that contribute to that particular appearance are created deliberately, and sometimes they are a result of various design compromises. Color-Encoding Considerations

 Photographic transparency films are designed for specific scene illumination; yet in practice they may be used under a variety of different scene illumination conditions. This results in color-balance shifts in the photographic image that may require correction during the encoding process.

 Photographic transparency films have very large color garmuts, in part because image dyes are formed on a transmissive, rather than reflective, support.  Successful color encoding of these media therefore must be capable of numerically representing large color garmuts.  In addition, the image-forming dyes are designed for a particular viewing illumination.  This is a consideration for scanning, in that the measured colorimetry will be strongly affected by the spectral properties of the scanner illumination.

 Of particular importance for color encoding is the fact that because photographic transparencies are viewed in a dark environment, the relationship between their colorimetric measurements and their visual appearance is not straightforward.  For example, standard colorimetric measurements would indicate that photographic transparencies intended for projection are too dark, too high in luminance contrast, and cyan-blue in overall color balance compared to an original scene on a reflective image.  This discrepancy between colorimetric measurement and color appearance is a very significant complication. * alternative methods for encoding color information