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Technical Documents - Documentos Técnicos: Exposing and processing the Photographic Paper.
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Exposing the Photographic Paper (Printing) Several concepts about colour : It is important first to consider that the "colour" is not an absolute characteristic of an object, but a human perception. The colour stimuli registered by the retina of the eye are made up of the energy distribution and the spectral properties of the visible light passing through or reflected by an object. The sensation of "colour" only comes about after a complex operation, in which the brain processes the information relating to the incoming stimuli. According to the laws of "additive colour mixing" all the colours that we can discern with our bare eyes are composed "additively" of blue, green and red light. Even non-colourful colours - grey and white - consist of additive colours (i. e. blue, green and red) of visible light. In a color negative, the special colour developer simultaneously produces three part images (colour dye negatives) in the three emulsions: The first layer responds only to blue light (i. e. it is blue-sensitive) and therefore produces a yellow negative, the second one is green-sensitive and produces a magenta negative, and the third one is red-sensitive so that the the negative is cyan. The photo-receptors of the retina (rods and cones) convert light into nerve impulses. Three kinds of cones with different sensitivities for different wavelengths allow our eye to see the visible radiation in the range from approx. 380 to 750 nanometres (nm). There are three types of cones with different spectral sensitivity which are responsible for colour vision, i. e. responding to stimuli in the visible spectrum. As the basis for colorimetry, it is therefore adequate to have three primary colours, each of which can, in turn, be fully defined by three values (hue, saturation and brightness) . One such colorimetric system is the international CIE system (CIE: Commission Internationale de l'Eclairage). The representation of colours in CIELAB diagrams largely corresponds to the physiological perception of colour. Since all impressions of colour in the human eye are generated with just the three colours of light, blue, green and red (three-range division of the visible spectrum), the CIE system is based on the three primary colours, blue, green and red (as monochromatic, i. e. not additively mixed primary colours) with the wavelengths 435.8 nm (blue), 546.1 nm (green) and 700.0 nm (red). The ability to see colours is, over a broad range, independent of the brightness of the visible light (range of photopic vision). In the case of radiation-sensitive systems, e. g. measuring devices, photographic films, printing materials and the human eye, we use the term "spectral sensitivity", not "colour sensitivity". Complementary colours
Colours which together make up white (additive colour mixture) or black (subtractive colour mixture) are complementary . (The term "complementary" is used in the colloquial sense. What is meant is the "opposite colour", the colours that are opposite each other in the colour circle. Strictly speaking, pairs of colours which produce white as an additive mixture are complementary ) . Additive colour mixture (e. g. with coloured light) Subtractive colour mixture (e. g. with coloured filters) The negative : The light and dark areas of the negative are the reverse of those in the original scene. The colored dyes contained in the emulsion layers of the negative are also the opposite (complementary) colors of the original scene. You can follow the formation of the complementary coloured negative or in a more detailed chart, the formation of a colour negative in the following figure, where the various stages are illustrated:
Figure : Chromogenic principle: Formation of the colour negative (II). This process requires certain substances (so-called "colour couplers") which are incorporated in each emulsion, in addition to the silver halide. The colour couplers are colourless substances, situated above the silver halide. During colour development, whenever exposed silver halide is converted to a silver image, an additional colour-dye image is also formed. In conjunction with each colour coupler contained in the three emulsions, the oxidized developer substance forms a certain yellow, magenta or cyan dye, depending on the layer. The formation of colour dyes is known as a "chromogeneous process". It involves a chemical reaction between the oxidation of the colour developer and the colour couplers in the emulsion. All colour materials produced by Once this important stage of forming colour dyes in the colour developer has been concluded, the colour negative looks opaque, almost black, rather than coloured. This is due to the metallic silver overlaying and hiding the three colour negatives. The next stage is therefore to "bleach" and then "fix" away the metallic silver in a series of processing solutions. What's left is the colour negative, consisting of yellow, magenta and cyan dyes. "Can the production of a colour negative be illustrated with an example?" Let us assume you're taking a photo of a blue sky and a flower with green leaves and a red blossom. As soon as you press the shutter release, you expose all three emulsions of your colour negative film, thus producing three latent images - one in the blue-sensitive layer (the sky), another in the green-sensitive emulsion (the leaves) and the third one in the red-sensitive layer (the blossom). And so the finished colour negative has a yellow sky, purple (magenta) leaves and a bluish green (cyan) blossom! So how do we get a positive print, with the colours the right way round, so that the sky is blue again. The answer is, by enlarging the colour negative with its unusal complementary colours onto a colour negative paper. This paper works follows the same principle as our colour negative film. How exactly this process works will be described in the following lines. How does colour copying work ? Let's just remember our complementary colour negative with its yellow sky, magenta leaves and cyan blossom: To copy the negative, we are now going to "project" it with an enlarger onto colour negative paper in the darkroom, i. e. we produce a latent image in each of the light-sensitive emulsions of the colour paper, using the copying light that has been coloured by our colour negative. You must reverse the colors again in order to produce a final print that looks like the scene originally photographed. You accomplish this by using the negative to expose the photographic paper on the printer. The photographic paper contains light sensitive dye forming layers. This paper is exposed by projecting light through the negative image. When this has been done, a latent image is present on the paper, although nothing is yet visible. In order to see the image, you must process the exposed paper in chemicals to produce the final print. In principle, the colour negative paper will respond to coloured light in the same way as it did in the case of the colour negative, i. e. the blue, green and red-sensitive emulsions will absorb either blue, green or red light. Unfortunately, however, these colours are not contained in our negative! The reason why a correct positive picture is produced anyway is due to the laws of additive colour mixing: 1. The copying light, coloured yellow by the yellow sky of the negative, is composed of green and red light. The green and red light produces latent images in the layers that are sensitive to green and red copying light. Now compare this with the illustration below :
Fig. How colour printing works: The formation of a colour print (colour negative process).
If you take a closer look, you will notice that, under the colour developer, the green and red-sensitive emulsion layers of the colour paper form magenta and cyan colour dyes (i. e. the colour developer - in conjunction with the colour coupler in the red-sensitive layer - reacts to a cyan colour dye, while the coupler in the green-sensitive layer reacts to a magenta dye.) The combination of both colours - magenta and cyan - produces blue. Again, we have come full circle, and the coloured print now shows the blue sky of the original subject. The same principle is followed for restoring the leaves to green and the blossom to red: 2. The copying light consists of blue and red light. These colours are due to the purple (= magenta) leaves. As a result, the copying light affects the blue and red-sensitive emulsions of the colour paper. In conjunction with the colour developer, the colour couplers contained in these layers respond to yellow and cyan colour dyes. Yellow and cyan produce green. 3. The copying light consists of blue and green light. These colours are due to the bluish green (= cyan) blossom. As a result, the copying light affects the green and blue-sensitive emulsion layers of the paper. In conjunction with the colour developer, the colour couplers contained in these layers respond to magenta and yellow dyes. Magenta and yellow produce red. Processing the Paper (Making a Final Print) .
How are photographic materials processed ? Photographic materials are treated in chemical processing solutions under narrowly defined conditions (e. g. processing time and temperature). To process B/ W negative materials you need a developer and a fixer. In addition, it is advisable to use a stop bath after the development, so that the development can be halted quickly after the prescribed period. The fixer is then followed by a thorough final wash. Colour negative materials go through a colour developer, a bleach and a fixer. However, instead of separate bleaches and fixers, colour papers are often processed in a combined solution, a so-called bleach-fix bath. Again, the final stage is a wash or a stabilization bath. During paper processing, a sequence of chemical solutions (similar to those used to process the film) make the latent image on the paper both visible and permanent. Once the paper is processed, you have a print that gives the same visual impression as the original scene. You have now completed the steps from camera to finished print. Now, let's go on and look at each of these steps in more detail as they occur in your minilab.
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