Let’s now discuss how the image is formed in a photographic emulsion.
In film, the most vital components are the silver halide crystals, typically silver bromoiodide in camera films.
These are bromoiodide T-GRAIN Crystals from a fast negative film.
All these silver halides are sensitive to light to a greater or lesser degree.
During exposure in a camera, light photons are absorbed by the silver halide crystals and form a “latent” or hidden image. The latent image consists of a cluster of a minimum of about 4 metallic silver atoms in the silver halide crystal structure. The presence of these silver atoms makes the whole crystal capable of being developed. Without them, the crystal will not develop.
Chemical development of the exposed crystals converts them to 100% silver, providing a huge amplification of the latent image.
Here is the latent image formation shown schematically. Exposure to light and absorption of photons causes some of the silver halide crystals to form latent image specks consisting of a few atoms of silver. These are shown as black dots.
Where there was insufficient light, the crystals did not form latent image specks. Another way of describing this is that those crystals were not sensitive enough to record that brightness of light.
Note that some crystals received so many photons that multiple latent image specks were formed. Actually, if you leave a piece of film out in bright light, it will eventually darken to the eye as huge numbers of silver atoms are formed.
Black and White Negative Development
We use chemical development of the exposed silver halide that contains latent image specks to amplify the few silver atoms by perhaps a million times.
In a photographic developer, a chemical called a reducing agent supplies electrons to the positively charged silver ions, converting them into electrically neutral silver atoms. One of the best-known black-and -white developing agents is called Hydroquinone often used in combination with other developing agents such as Elon (also known as Metol).
Notice that in this case the action of light caused a darkening effect on the film. In other words, a negative image has been formed.
Color Negative Development
In color films, development starts in the same way as in the black-and-white films, except that a different developing agent is used, normally CD-3 for color negative films.
The exposed silver halide crystals are chemically reduced to metallic silver, and at the same time the developing agent becomes chemically oxidized.
Notice that this oxidized developing agent is formed “imagewise” as we say, as a cloud around the developed silver image grains only. The next step is to use that imagewise oxidized developer to form our color dye image.
The cloud of oxidized color developing agent around the developed silver grains reacts with a chemical called color coupler, and they join together to form a color dye. The correct color of dye -yellow, magenta or cyan- is formed in the appropriate layer of the color film. In this case we see magenta dye.
The coupler is coated as tiny droplets mixed into the same emulsion layer as the silver halide crystals, in most types of color films.
The exception is KODACHROME Film, which contains no coupler in the emulsion layers. KODACHROME Couplers are dissolved in separate yellow, magenta and cyan color developers.
So now we have developed our required color dyes, but we still have the black metallic silver image that we do not need. It has performed its function providing light sensitivity but now we must say goodbye to it.
Bleach (Color Negative only)
The rest of the processing steps in a color process are designed to get rid of the silver.
First it goes through a bleaching bath that takes what was formed in the developer and converts the black metallic silver back to silver halide, like the original light-sensitive crystals.
By the way, this bleaching process is used in color negative processing but not black and white negative processing, because it will bleach out the required silver image in black and white negative film.
Fixer (Color & Blackand White Negative)
The last major processing step in either a color or black-and-white process is the fixer. The job of the fixer is to remove all the silver halide from the film, whether formed by the bleaching of the silver image or the remaining unexposed and undeveloped light-sensitive silver halide crystals in the emulsion.
The principal chemical in the fixer is sodium thiosulfate, commonly known as “hypo”. The hypo converts the silver halide into a soluble complex which goes into solution in the fixer, leaving the film free of silver halide with just its dye image. In a black-and-white film, only the silver image would remain.
After fixing, the film is thoroughly washed to remove all traces of processing chemicals.
Here are actual photomicrographs of the cyan layer of a color film.
After development you can see the silver grains surrounded by the cyan dye clouds.
After removal of the silver by bleaching and fixing, only the dye clouds remain.
So, to summarize this simplified look at image formation in film:
- In black-and-white film, exposure to light and processing forms a metallic silver (negative) image.
- In color film, three dye images, cyan, magenta, and yellow are formed, while the silver halide that provided the light sensitivity is removed.
As a reminder, here is the dye-forming reaction. Exposed silver halide reacts with the developer to form oxidized developer, silver and halide. The oxidized developer is formed image-wise, and by allowing it to react with a chemical called a coupler, a dye image is formed. Three different couplers are incorporated into the three light-sensitive layers of the film and form the three image dyes – cyan, magenta and yellow.
In the next post, I will talk more on the difficulties in color reproduction of film and the solutions to it.
To be continued……