Chemistry in Photography
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A photograph is an image made by a photo-chemical reaction which records the impression of light on a surface coated with silver atoms. The reaction is possible due to the light-sensitive properties of silver halide crystals. Equation form for silver halides: Ag + + e – Ag
Species produced include: Ag2+, Ag2o, Ag3+, Ag3o, Ag4+, Ag4o In 1556, the alchemist Fabricius was the first to discover that light can photochemically react with these crystals to change the silver ions (Ag+) to elemental silver (AgO). As the reaction proceeds, the silver atoms grow into clusters, which are large enough to scatter light and produce colors in a pattern identical to that of the original light source. Photography utilizes this chemical principle to record color or black and white images. One of the first researchers to produce photographic images using silver halide chemistry was Schultze.
As early as 1727, he formed metallic silver images by first reacting solutions of silver nitrate and white chalk and then exposing these solutions to light through stencils. Schultze’s work was improved upon through the efforts of Louis Jacques Mandé Daguerre who, in 1837, developed a process for printing images on a silver coated copper plate. This type of printed image is called a daguerreotype, and is made by polishing and cleaning a silver-coated copper plate and then reacting the silver coating with iodine vapors to form light-sensitive silver iodide. The silver iodide coated plate is then exposed to light through the optics of a camera that projects and focuses an image on the plate. In the ensuing reaction, the silver ions are reduced to silver metal. Finally, the plate is treated with mercury to produce an amalgam. In this type of print, the areas of the plate exposed to light appear white and the unexposed areas remain dark. The problem with this method was that it required long exposure times because the intensity of the image depends solely on the strength of the light forming the image.
An example of a daguerreotype photograph
In 1841, William Henry Fox Talbot overcame the problem encountered by Daguerre by developing a quicker method that did not depend entirely on reflected light to produce the image. He found that silver halide could be exposed in such a way so as to produce a preliminary latent image which required only a small amount of light. This latent image could then be subsequently reacted, without additional light, to produce a final image. Using this technique, known as calotyping, Talbot was one of the first to produce continuous tone images. Unfortunately, these early images were not stable and darkened over time. Fortunately, around the same time Talbot did his work, John Frederick William Herschel discovered a way to stabilize images. His process, known as fixation, chemically converts unexposed silver halide to silver thiosulfate, which can easily be washed off of the image. Equation Form: 2AgBr + 3 Na2S2O3 = Ag2Na4(S2O3)3 + 2 NaBr
The next major advance in photography came with the discovery that certain materials could enhance the sensitivity with which latent images are formed. This enhancement is achieved by coating the silver halide crystals with chemical agents, such as sulfur and gold, which increase the light sensitivity of crystals. Gelatin, which for years had been used as a photographic coating agent, was found to be an effective medium for these light-sensitive materials. Equation form: AgNO3 + KBr = AgBr + KNO3 in gelatin * AgBr precipitates due to light sensitivity and remain in the gelatin to form minute grains. The formed latent image can then be developed further. In 1888, George Eastman, who pioneered modern film development, coated gelatin-dispersed silver halide crystals onto celluloid sheets. By the next year, Eastman had commercially sold rolls of films prepared by dissolving nitrocellulose with camphor and amyl acetate in a solution of methanol. In the last century, both film processing and camera equipment have improved considerably but these same basic principles are still used to make photographs today. Materials
* Film – Modern film is made by coating light-sensitive ingredients onto a flexible plastic surface. * Special paper – which is coated in light sensitive materials * Enlarger – increase the size of the image and developing and toning solutions, which help control its intensity and color. The chemicals used in developing are designed to grow the microscopic silver atoms into silver centers that are larger enough to be visible to the unaided eye. These developer solutions are composed of: * Reducing agents: commonly Hydroequinone
* Restrainers: Bromide Ions: Equation form: Br – + light Br + e – * Preservatives: Sodium sulfite (Na2SO)
Equation form of the reactants of the developer: C6H4(OH)2 + Na2SO3 + 2AgBr + NaOH In addition to the materials described above, developing and printing operations require a variety of equipment such as trays, measuring glass ware, thermometers, drying screens, timers, mixing pails and stirring paddles, and paper cutters. Process
* Making of the film
The first step in the process is to grow microscopic silver halide crystals from silver nitrate and halide ions. After the crystals are grown in solution to a certain minimum size, they are separated and mixed into a gelatin base. This mixture is washed to remove sodium, potassium, and nitrate ions and the resulting silver halide/gelatin emulsion is chilled and allowed to gel. This emulsion is both light and temperature sensitive and must be carefully stored. The emulsion is later melted and the silver grains are coated with chemical agents to enhance sensitivity to certain wavelengths of light. In its molten form, the emulsion is coated onto a support structure, usually a polymeric film. One common coating method runs the film below a hopper filled with the emulsion. As the film passes under the hopper, the emulsion is dispensed onto the film. After coating, the emulsion is spread evenly on the film with rollers and is transported to a cooling chamber where the emulsion gels. Finally, the film is sent through a heated chamber which dries and hardens the emulsion. Multiple layers can be coated onto the film in this fashion and specific coatings can be added in order to control how light is reflected/absorbed. Additives used for this purpose include small carbon particles, dyes, or colloidal silver. The last layer is a gelatin overcoat, which seals the film and holds the lower layers in place. In general, the thicker the layers of the emulsion and the larger the silver crystals, the more light sensitive the image.
After the making of film, there are three key steps involved in making a photograph: exposing the film to light, developing the image, and printing the photograph.
Once the film is loaded inside the camera it is ready to be exposed. The camera optics focus an image through the lens and onto the emulsion grains. The camera controls the light through a combination of the size of the opening in the lens (the aperture) and the length of time the aperture stays open (the shutter speed). A wide variety of exposure effects can be achieved by varying these two factors. The reaction between the emulsion and the light forms a latent image on the film. The focal length of the camera lens determines the magnification of the latent image, while the penetration of light into the film depends on the combination of lens optics and the chemical properties of the film. The image formed is a negative, meaning it is opposite of how it is seen by the eye. * Developing
Film is handled in special darkrooms, which are illuminated with safe red light that does not effect the film. Once inside the darkroom, the film is removed for the development from its canister, wound onto a spool, and stored in a plastic container to protect it from light and physical damage. The film may then be submerged in a tank containing a solution of the developing chemicals described above. This solution reacts with the exposed areas of the film to amplify the light impressions of the latent image. This process produces variable results depending on the type and temperature of the developer solution used and the level of the original exposure to light. After this stage is complete, the solution is poured off and a stop bath treatment consisting of dilute acetic acid is added to the tank to prevent the film from overdeveloping. After the development is stopped, a fixative can be added to lock in the image. The finished negative then may be washed and rinsed. The reel is then removed from the tank and the fresh negatives are hung up to dry This step produces a negative image, which can then be used to print a final picture. Equation form of the development process:
C6H4(OH)2 + Na2SO3 + 2AgBr + NaOH C6H3(OH)2SO3Na + 2NaBr+ H2O + 2Ag Hydroquinone sodium sulphite silver bromide sodium hydroxide hydroquinone sulphonate sodium bromide water SILVER! * Printing
Printing is the process of producing a final image from a negative. If photography is the art of taking a picture, printing is the science of making a picture. Printing requires light, a negative, and printing paper. The light source is an enlarger, which uses a lens to focus light through the negative and project it onto light-sensitive paper. The positive image on this paper is then developed in a manner similar to that described above for developing negatives. Finally, the print may be mounted on cardboard or other backing material. Fixing the image
The biggest problem after the invention of photography in the 1830’s was the lack of permanency. You have to get rid of that remaining bromide, or eventually the photograph will go black. There are no true solvents of AgBr. When sugar is dissolved in water, and then evaporated, the sugar is recovered. This never happens with AgBr. The residue left behind is always a transformed salt. So what we need to do is make sure the transformed salt is soluble, so it can be washed away. AgBr + Na2S2O3 = AgNaS2O3 + NaBr (only slightly soluble)
But if we have a more liberal solution of sodium thiosulphate: 2AgBr + 3 Na2S2O3 = Ag2Na4(S2O3)3 + 2 NaBr (bingo!)
Color photography is photography that uses media capable of representing colors, which are traditionally produced chemically during the photographic processing phase. By contrast, black-and-white (monochrome) photography records only a single channel ofluminance (brightness) and uses media capable only of showing shades of gray. In color photography, light-sensitive chemicals or electronic sensors record color information at the time of exposure. This is usually done by analyzing the spectrum of colors into three channels of information, one dominated by red, another by green and the third by blue, in imitation of the way the normal human eye senses color. The recorded information is then used to reproduce the original colors by mixing together various proportions of red, green and blue light Monochrome images which have been “colorized” by tinting selected areas by hand or mechanically or with the aid of a computer.