Analog image carriers – negative film

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In essence, we have learned up to this point that a section of three different wavelength ranges of the spectrum is sufficient for our visual apparatus to construct all other colors from it. – The photoreceptors solely determine this section. Since we intend to view color photographs, they don’t need to display the entire spectrum’s intensities; instead, they can focus on recording three carefully selected spectral ranges. Since three widely separated wavelength ranges are most suitable, the system of color photography developed along the cornerstones of blue, green and red. The first to practically drive in these pegs was the Scottish physicist James Clerk Maxwell. In 1861, he had photographer Thomas Sutton take three black-and-white photographs of a check-patterned piece of fabric, each with a differently colored filter in front of the lens. He used a blue, green, and red filter, adhering to the Young-Helmholz three-color theory of vision that was prevalent at the time. He then projected the three resulting positives onto a screen, each with its own projector, and each projection apparatus was equipped with the filter used in the photograph. The result was a clearly visible color image on the projection screen. The imaging techniques used today still work according to this principle of three color separations.

In the case of color negative film, we are dealing with three light-sensitive layers, as stated at the beginning. Separated by filter layers (also called intermediate layers) for the more blue (380-500 nm), more green (500-600 nm) and more red (600-700 nm) part of the spectrum, their sequence in the emulsion is staggered according to the energy density and penetration depth of the light. This at least partially prevents light of one wavelength from penetrating the wrong layer and causing a color cast there.

In addition to the silver halide crystals, the layers of the emulsion also contain so-called color couplers. These react with the developing liquid oxidized on the developing nuclei to form complementary (visually opposite) insoluble dyes. In this process, a dye image of the same density forms alongside the silver image. After development, a bleach bath removes the useless silver image. What remains are three superimposed monochrome images, which together form the complete color image. Thus, when one speaks of the grain in color film, one is referring to the combined dyes.

The principle of subtractive color mixing creates the colors, as the dyes extract the corresponding portion of their complementary color from the light. The layer under the blue filter forms the yellow dye. The green-sensitive layer prepares the magenta dye, and the third, red-sensitive layer contains color couplers for cyan.

Let’s run through it once for all three layers. Blue light exposes the blue-sensitive layer in the negative, where yellow dye is formed. When we copy the negative, we send white light through the film piece, which is yellow after the pass. On the photographic paper, this yellow light exposes all color layers except the yellow one, which is yellow because it reflects the yellow color components and does not absorb them. The magenta and cyan layers of the paper are therefore exposed. In subtractive color mixing, magenta and cyan produce blue. When viewed under white light, the magenta layer absorbs the green color components, and the cyan layer absorbs the red color components. Ergo, the blue light components remain. Green light exposes the green-sensitive layer in the negative, which forms the magenta dye. During copying, the photographic paper is therefore illuminated magenta and the yellow and blue-green layers are exposed. Both of these processes reduce the incident white light to a green hue. Red light exposes the red-sensitive layer of the negative, in which cyan dye is formed. When enlarged, blue-green light passes through and exposes the yellow and magenta layers. In subtractive color mixing, the combination of these two colors yields red once more. The reproduction of dark and light areas is analogous to the process described above, except that black and white are produced according to the rules of subtractive color mixing by mixing all three primary colors evenly. The paper print is thus a negative of the negative because it has the opposite color and brightness values as the latter. However, this reversal means that it shows the subject again in the correct colors and brightnesses.

Three complementary color layers thus add up to the total spectrum – the process sounds simple. But the technology sets limits to the colorful hustle and bustle because it is not possible to produce truly pure dyes. Instead, another hue always contaminates the dominant color to a greater or lesser extent, and mixing these dyes only amplifies the error. The industry tries to compensate for this with chemical tricks in the form of corrective masking layers, but it is still impossible to accurately reproduce all the colors of the spectrum at the same time. If you manage to match one color, all the others will be slightly „off“. This technical limitation accordingly leaves the photographer with only one desirable goal: to create an acceptable balance between hues within each image carrier’s own reality.

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Main Lightness and Color

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