At the heart of obtaining the photographic image physical properties and chemical action of light lie. Physical properties of light allow to receive by means of the camera on a plane surface the optical (luminous) pattern of subjects surrounding us, and chemical action — to imprint this image on photographic materials (photoplates, a film, photographic paper).
The surface layer of photographic materials consists of the substances changing under the influence of light. Such substances are called light sensitive.
Thus, we receive that light sensitivity of photographic paper expresses property of its light sensitive layer to change under the influence of light, therefore in it the latent image which after development is formed turns into the visible one.
Light sensitivity of photographic materials is measured in the relative units: DIN, ISO, GOST, etc.
Now ISO units are the international standard which are using for designation of light sensitivity.
A photographic paper light sensitivity is much lower than a film one. Too high light sensitivity of photographic papers would create only difficulties in operation. Therefore for photographic papers is been prepared a less-sensitive emulsion, hundreds times less sensitive than for photoplates and films.
Bromosilver photographic papers — Unibrom and Photobrom are most sensitive to light. Chlorobromosilver photographic papers — Bromportrait are less sensitive. The composition of an emulsion of these papers includes a mixture of silver bromide and silver chloride. Chlorosilver photographic papers of Photocont and also Chlorobromoiodosilver photographic paper of Iodocont are even less sensitive to light.
To foreknow a light sensitivity level of photographic paper is not necessarily. It is always possible to define light sensitivity of photographic paper having made on it one or two trial prints. For this reason light sensitivity of photographic papers cannot be designated on labels.
Property of photographic paper to increase light sensitivity often use in practice when while printing the small underexposure has been done. In this case by increasing duration of development it is possible to «hold out» the image to normal density.
Photographic papers with light sensitivity from 3 to 15 units are used for a contact and projection printing. Photographic papers with lower light sensitivity — only for the contact printing.
The contact printing gives bigger resolution of details than the projection one on condition of the equal scale of the image (1: 1) and the printing on the same kind of photographic paper.
In the first method of printing the resolution value depends on a contact rate between a negative and a light sensitive layer of photographic paper: the better it has been done the higher detail resolution of a negative will be got on a positive.
In the second method it depends on the resolving power of a photographic enlarger lens and focusing accuracy: the more its resolving power and the more exact focusing in case of a projection the bigger number of details of a negative will be reproduced on a positive.