Photographic Chemistry : The Process of Producing Photographs
The word photography comes from two Greek words, which mean “drawing by light”.
There is at present no field of human activity that is not directly or indirectly touched upon by photography.
Photographic process was not discovered overnight.
It took many years and a lot of effort from several scientists to discover the process.
Let us have a historical insight.
The early alchemists used to know about the effect of light on silver salts.
At the beginning of the 19th century, Wedgewood, successfully reproduced images as negatives on paper impregnated with silver salts.
A few years later, in 1819, Herschel discovered a unique property of thiosulphate, which we call is the fixing property.
Thus he set the doors open for the first pictorial reproduction and in 1839 for the first exhibit, called photographs.
The same year, Daguerre, a french scientist published his formula for manufacturing of the familiar daguerreotype.
A few more years later, Maddox discovered the gelatin dry plate.
Vogel, in 1873, found that the light sensitivity of silver halide crystals is greatly increased by the dye molecules adsorbed by them. Transparent roll film was introduced by George Eastman in 1889 and the famous Wratten panchromatic plates were introduced in 1906.
Now it is time we looked at the photographic process.
Silver halides are the most widely used sensitive substances in photographic processes.
These light-sensitive silver salts are prepared in the dark as an aqueous gelatin emulsion.
Certain dyes, various classes of sulfur and gold compounds, anti-fogging agents, hardening agents, stabilizers, coating aids, etc are added with the emulsion.
After some additional treatments, this emulsion is coated on film support of cellulose acetate, polyester, or glass.
Exposure of the emulsion in a camera results in a photochemical reaction on the surface of the silver salt crystal.
In silver halide crystals, internal dislocations act as electron traps. When some energy in the form of light is absorbed by the silver halide, a free atom of silver is liberated.
This silver atom acts as a center which invites more photolytically generated silver atoms until a mono positive tetra silver ion is formed. Thus photolytic silver constitutes an image and forms effectively in proportion to the incident light of the color to which the crystal is sensitive.
Thus an object is produced in the form of a dark image.
The result of this process described here is negative. The density of the image depends upon the number of crystals exposed.
Positives are generally produced by first developing the negative silver image and then dissolving the silver image in an oxidizing solution or bleach.
The residual complement of silver halide, which has the configuration of the positive image, is uniformly fogged either physically or chemically and then developed to give a positive silver image.
Silver halides of the emulsion supply the film with sufficient metallic silver when the developer is employed. As a result, we obtain a visible image from the negative.
Now let’s see how black and white materials are processed.
Modern developing solutions contain mainly four functional constituents. They are- an organic reducing agent, a preservative, an accelerator, and a restrainer.
The reducing agent chemically reduces the silver halide to metallic silver. Chemically, they are substances like polyhydroxide, amino hydroxide, polyamine derivatives, etc.
Preservatives guard the developer against air oxidation. The most common preservative used is sodium sulfite.
Accelerators increase the alkalinity of the developing solution. Eventually, the activity of the reducing agent is increased.
To maintain the speed of the developer, a restrainer, for example, potassium bromide or benzotriazole is employed.
The processing of black and white photographic materials is ended by a wash. After washing, the materials are dried with heat.
This is how we get what are called photographs.
Now we will see how colored photographs are produced.
Color photography is based upon the principle that the colors of nature can be represented to human eyes and brains by mixtures of blue, green, and red light.
Mixtures of these kinds have been produced by projecting colored beams of light in ‘register’.
These beams of light emanate from properly prepared transparent positive images.
They can also be produced by modulating or by silver images, microscopic blue, green, and red filters which are juxtaposed on support.
In the method, the eye receives the required amounts of green blue and red light to reproduce the intended color. An ideal electronic analog of this system is a color television picture tube.
The use of green, blue and red beams or filters is difficult in practice and requires a lot of light energy.
Alternatively, most methods of color photography are based on the use of the complements of blue, green, and red, which are yellow, magenta, and cyan respectively.
Yellow results when blue is absent from white light.
Similarly, magenta is absent when the green light is not present, and cyan, when the red light is absent.
Thus à yellow filter can prevent the blue component of white light and permit only green and red; magenta prevents the green light from passing and allows only blue and red light to pass, and cyan controls red while permitting blue and green light to go through. Combinations of different proportions and densities of yellow and magenta produce a variety of colors, generally including orange and red.
Similarly, yellow and cyan produce a variety of colors including green and magenta and cyan produce a group of colors including blue. There is a way these combinations can be affected.
The way is to superimpose layers of dyes on a single support.
Originally pigment layers were applied on one upon another and that is how color prints were prepared.
But, as there are much better dye-transfer systems available now, this process has almost become obsolete.
However, there is still some highlight on the pigment process as prints thus produced possess more permanent colors than those done with the help of dyes.
Dyes for color photography are produced according to the following set of reactions:
Exposed silver salt + developer → oxidized developer + silver
Oxidized developer + coupler → dye
This process requires that only one layer develop at a time and also that all reactants
be washed out of the photographic material before the next dye is produced.
Preparation of each layer for development requires careful control.
The coupler is introduced to the emulsion by the manufacturer.
Then the exposed silver salts can comfortably be developed in all layers.
The oxidized developer reacts with the coupler immediately and thus a proper dye is formed.
Two types of color photographic processes are available.
They are- negative and reversal.
Negative processing is the one where an incorporated coupler is used.
It results in color development in the very region of the exposed silver salts.
In the regions that are exposed to blue, a yellow dye is formed.
Similarly, a magenta dye is formed in regions exposed to green and a cyan dye is formed in regions exposed to red.
The negative can be used to produce a positive.
In making the print, the dyes in the negative are responsible for controlling the amounts of blue, green, and red light reaching the layers of emulsion on the print material.
The emulsions also contain incorporated couplers.
The reversal process involves the development of exposed silver salts to a silver negative.
The remaining silver salts are not exposed universally but selectively and color developed to yield dye layer by layer.
New processes of color photography are being developed.
We hope that what is done in a somewhat difficult way nowadays can be done with much ease tomorrow.
We are awaiting that tomorrow eagerly.
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