As a photographic emulsion, a thin photosensitive layer is referred to with the various support materials such as glass and films of cellulose or polyester coated. The light-sensitive photographic emulsion forms the basis for all photographic plates, films and papers.
Structure and composition
In the " photographic emulsion " it is actually not an emulsion. The photosensitive layer is rather a suspension, because it is a slurry of finely divided crystals in gelatin, so a gel. Nevertheless, the term has naturalized photo emulsion.
Photo emulsions consisting of gelatin with the embedded halide of silver chloride, silver bromide or silver iodide. Halides arising from reactions 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 present loaded (Ag , Cl- ) = ionic lattice. Theoretically, each silver six chloride ions is (analogous to bromide and iodide surrounded ) and each chloride of six silver ions. In practice, however, it is so that 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 history of a latent image.
Impurity silver halides are sensitive to light, and the sensitivity of iodide decreases over the bromide to chloride. However, they are only sensitive to blue and UV light. At longer wavelengths, they do not respond. Therefore, they must be spectrally sensitized. They are coated with dye molecules, which capture the light quanta and deliver electrons in the direction of the crystal.
Upon impact of such an electron ( photoelectron ) to an interstitial silver ion, the two unite to silver atom, but separate immediately. In the chemical ripening of the emulsion, however, arise Ag2 - ripening bacteria. Exceeds the electron in the vicinity of such a maturation germ a interstitial silver ion, the separation does not occur. The resulting silver atom attaches to the Ag2 - seed, which a Ag3 cluster is created. This process is repeated a second time, creating a long-term stable and developable Ag4 cluster. Thus, a latent image is formed.
Any sufficiently exposed crystals are thus to metallic silver. The unexposed crystals are removed later in the fixing of the layer which is thus stabilized. This results in the negative.
For color - reversal films and a more complex sequence of layers or development for the desired effects makes.
Silver halide emulsion
An orthochromatic emulsion is sensitive to visible light plus ultraviolet, but without the color red
In the early days of photography thus coated was recorded, the rule, which is why in particular seem human skin tones in old photographs often quite strange. To mitigate this effect, to portraying people sometimes had to go through an extensive make-up procedure.
Silver halide or non-sensitized films and papers are usually used in applications where it is already used uniformly black (eg black and white enlargement of the black - and-white negative ) or the conversion of the colors in harmonious shades of gray is not critical, such as in many areas of reprophotography.
A panchromatic emulsion is sensitive to light of each color or each wavelength of the visible electromagnetic spectrum.
The spectral sensitivity ranges over the entire visible range from 400 to 700 nm, this is in the production of emulsion by mixing special light-absorbing substances reached ( sensitization).
Panchromatic films are characterized by a tone quality of the colors: the perception of the gray scale corresponds to the brightness impression of the eye. Black -and-white films for use in the pictorial photography are therefore nowadays almost exclusively sensitized panchromatic.
To produce color worth real black-and- white prints from color negatives a few photo papers are with panchromatic emulsion available (eg Kodak Panalure ). This can, of course - just like a normal Pan. negative film - not be used with the usual red darkroom illumination. However, there are the dark material with a reduced sensitivity in the sodium lines (about 589 nm ), so that with a limited exposure period, a weak sodium vapor lamp can be used for illumination.