A photo emulsion is a thin, light-sensitive layer with which various carrier materials such as glass and foils made of cellulose or polyesters are coated. The light-sensitive photo emulsion forms the basis for all photographic plates , films and papers .
Structure and composition
The "photo emulsion " is actually not an emulsion . Rather, the light-sensitive layer is a suspension , because it is a slurry of finely divided crystals in gelatine , i.e. a gel . Nevertheless, the term photo emulsion has become common.
Photo emulsions consist of gelatine with the embedded halides silver chloride , silver bromide or silver iodide . The halides arise from the reaction of common salt ( sodium chloride (NaCl)), potassium bromide (KBr) and potassium iodide (KI) with silver nitrate .
Silver halides form crystal structures in which the elements are charged (Ag + , Cl - ) = ion lattices . Theoretically, each silver ion is surrounded by six chloride ions (analogous to bromide and iodide) and each chloride ion is surrounded by six silver ions. In practice, however, at least some of the silver ions, so-called interstitial silver ions, leave their place and move in the crystal. These ions play an important role in the further development of a latent image.
Impurity silver halides are photosensitive, with the sensitivity decreasing from iodide to bromide to chloride. However, they are only sensitive to blue and UV light . They do not respond to longer wavelengths . Therefore they have to be spectrally sensitized . They are coated with dye molecules, which capture the light quanta and deliver electrons to the crystal .
When such an electron ( photoelectron ) hits an interstitial silver ion, the two combine to form the silver atom, but immediately separate again. During the chemical ripening of the emulsion, however, Ag2 ripening nuclei arise . If the electron meets an interstitial silver ion in the vicinity of such a ripening nucleus, the separation does not take place. The resulting silver atom attaches to the Ag2 nucleus , creating an Ag3 cluster . If this process is repeated one more time, a long-term stable and developable Ag4 cluster is created. A latent image was thus created.
All sufficiently exposed crystals thus turn into metallic silver. The unexposed crystals are later released from the layer during fixing , which is thus stabilized. This creates the negative.
In the early days of photography, recording material coated with this was the rule, which is why the human skin tones in particular often appear strange in old photographs. In order to mitigate this effect, the people to be portrayed sometimes had to endure an extensive make-up procedure.
Orthochromatic or non-sensitized films and papers are mostly used where black and white is already used (e.g. black-and-white enlargement of black-and-white negative) or the conversion of color tones into coherent gray levels is not critical, for example in many areas of repro photography .
The spectral sensitivity extends over the entire visible range from 400 to 700 nm. This is achieved during the production of the emulsion by adding special light-absorbing substances ( sensitization ).
Panchromatic films are characterized by a correct tonal reproduction of the colors: The perception of the gray levels corresponds to the impression of brightness of the eye. Black and white films for use in pictorial photography are therefore almost always panchromatically sensitized nowadays.
A few photo papers with a panchromatic emulsion are available (e.g. Kodak Panalure) to produce black and white prints of color negatives with the correct color value. Of course, like a normal Pan. Negative film, these cannot be used with the usual red darkroom lighting. However, there is material for the darkroom with reduced sensitivity in the range of the sodium lines (approx. 589 nm), so that a weak sodium vapor lamp can be used for lighting if the exposure time is limited .