Agfa Library: Technical Terms - A   A Additive color mixture Additive color mixing is based on the optical mixture of monochromatic (single-colored) light in the spectral primary colors, blue, green and red. When the spectral primary colors are projected over one another in equal parts, they produce white. If two additive primaries are combined with one another, they will produce the secondary mixtures, yellow, magenta and cyan, which are, at the same time, "complementary" to the third additive primary which is not involved (complementary colors; subtractive color mixture). The first processes of color photography were based on the additive principle of color mixture. Today, additive color mixing is used, for example, in television technology and for filtering in printers. Fig. 1: Blue, green and red primary colors are made visible through a prism (simplified representation without secondary colors) and are then mixed additively again to give white light. An additive color mixture is produced by mixing together two or more sources of colored light. The colorimetrist talks of combining different, individual color stimuli (CIE system) to produce a new color sensation which differs from the single color stimuli. The new color sensation comes about through the projection of blue, green and red light on top of one another, or through the simultaneous (or apparently simultaneous) effect of very small, colored elementary areas, which the eye is unable to distinguish because of the small angle of view. A particular color sensation can be created by additive mixing in a certain ratio (examples: colored screen printing; excitation of colored luminous dots by electron radiation in color television).In the additive photographic processes, the blue, green and red primaries are produced from white light using appropriate filters (e.g. additive color filtering in printers). The original is exposed successively behind blue, green and red color separation filters onto the photo paper. The desired color can be controlled by varying the exposure times behind the individual color separation filters.   Advanced Photo System The core of the Advanced Photo System is the film which, in comparison to conventional film, has a narrower film format, a smaller film cartridge and an extra magnetic coating. A special feature of the film, which is supplied with 15, 25 and 40 frames, is the smaller negative (60 x 30 mm). The continuous perforation on conventional 35mm film has been replaced by two registration holes per negative. Three different formats can be selected: the small format (C = Classic) with the "classical" side ratio of 2 : 3 based on 35 mm film   the full format (H = High Vision or HDTV) with the side ratio of 9 : 16   the panorama format (P) with the side ratio of 1 : 3   AHU The black AHU layer (in the black-and-white films AGFAPAN APX PROFESSIONAL and AGFAORTHO) offers particularly effective protection against haloing. The AHU layer (AHU stands for Anti-Halo Undercoat) is located between the emulsion layer and the base and absorbs the light beams after they emerge from the emulsion layer. The more intense the coloring of the AHU layer, the better the protection against light scattering. Once the film has been exposed, the AHU layer has done its job, which means that its color can be removed during wet processing of the film. The AGFAPAN 35 mm films feature an additional safe-guard against haloing, namely a grey-colored base for cutting down scattered light (not the AGFAORTHO). With the AGFAPAN and AGFAPAN APX roll and sheet films, a dark-green anti-halo layer on the back of the base provides extra protection. The color is removed from this layer, too, during wet processing.   Analytical densities The analytical color densities are the individual emulsion densities of the developed component dyes of a color film. They provide information about the dyes that have formed in the light-sensitive emulsion layers and about the corrections (DIR couplers; masking) that are necessary to achieve the color rendition aimed at by the manufacturer. The measurements for determining the analytical densities are carried out by the manufacturer with the aid of specimens from individual layers and, in this respect, constitute a rather special case of densitometric measurement. Normally, densitometric measurement is carried out on the developed multi-layer film (spectral integral density). The result of adding together main color densities and secondary color densities is the total density for the three main spectral areas, which is also known as visual grey or mean neutral density. With suitable exposure and processing, the total density is wave-shaped over the three density curves of the spectral analytical densities in a density range of 1.0. Fig. 2: Absorption curves of the developed emulsion dyes of a reversal film with the total density curve on top. For example, the magenta dye component shows the highest color density in the green spectral range, but also reveals undesirable secondary densities in the blue and red spectral ranges.   Anti-halation The sharpness of a photographic image is negatively influenced, among other things, by two optical phenomena: diffusion halation and reflection halation. A diffusion halo is caused by light scattering on the silver bromide grains in the emulsion layer. The extent of the diffusion halation will depend on the layer thickness and on the distribution and size of the silver bromide grains. It can be reduced by special emulsion additives (screen dyes) and by the use of thinner emulsion layers. A reflection halo is caused by total reflection of directional beams of light at the interface of media with different refractive indices, e.g. at the interface between emulsion layer(s) and the film base, and by further reflection at the film base/air interface. Effective protection against reflection halation can be achieved by dark-colored anti-halation undercoats (AHU), protective coatings on the back of the film (e.g. gelatine NC coating, NC stands for non-curling) and by coloring the base grey. The dark color is removed from the anti-halation undercoat and from the back-coating during wet processing, but the grey coloring of the base remains. Fig. 3: Diffusion and reflection halation (side view and top view of the film surface).   ASA Abbreviation for American Standardization Association and one of several units of measure for the speed of films. Doubling or halving the arithmetic ASA value is equivalent to doubling or halving the film speed. Together with the logarithmic DIN standard (speed), the ASA standard is incorporated into the international ISO standard     alphabetic index next   Photohome - Agfa Library - Technical Terms - "A" <-- you're here About Agfa | Agfa Arcade | Agfahome | Photohome Search | News | Feedback Agfa Photo Gallery | Advanced Photo System | Trends & Experiences | Products ©1996 by Agfa-Gevaert AG, Copyright and Trademark Information