CH. 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.

General Characteristics 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.

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.

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. 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. 6.5. 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.

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).

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

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).

Neutral Characteristics: Color Balance 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.

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

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.

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

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. 6.15. The two films are identical except for their red, green, and blue spectral sensitivities.

CMY density scales for a particular photographic transparency film 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.

Chapter 6 Photographic Transparencies Photographic transparency films are designed for use with specific scene illuminants. Like reflection images, images on photographic transparency media are objects that produce color stimuli only when illuminated.   Photographic transparency media are quite sensitive to changes in viewing illuminant spectral power distribution. They generally are designed for use with one specific viewing illuminant.

Photographic transparency films must depart significantly from one-to-one colorimetic relationship with the original scene in order to compensate appropriately for viewing flare and for observer general-brightness adaptation, lateral-brightness adaptation, and incomplete chromatic adaptation.   Photographic transparency films are color balanced for one scene illuminant, yet they are often used under different illuminants. This results in color-balance shifts that may require correction during the encoding process. Photographic transparency films have very large color gamuts, in part because their 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 gamuts.