الطبقة الحساسة .. كتاب أسس التصوير الضوئي

SENSITIVE EMULSION

The sensitive layer is divided into two types, the first is intended for obtaining negative images, and the second is for obtaining positive images.

negative sensitive layer:
The negative sensitive layer is placed on a support that may be glass or cellulide.

It is made of the following materials:

(1) Gelatin

(2) Silver Halides that are sensitive to light. (3) Some compounds whose function is to influence the speed of sensitivity or the type of color sensitivity.

First - Gelatin: It is the medium in which light-sensitive silver salts and other accompanying materials are deposited. Gelatin is able to absorb water, and this ability increases with the increase in temperature, as it swells and swells and is in the state of a semi-solid substance, i.e. 049 degrees Celsius.
It dissolves in water at 100 degrees Fahrenheit, but it can bear a higher temperature if it is strengthened with special materials, then it dissolves at 12 degrees Fahrenheit of silver salts. It has been observed that silver salts increase their sensitivity to light by dissolving them in gelatin. It was also noted that dissolving silver salts in gelatin helps to limit the activity of the acid appearance, as it affects the granules that are located on the colored surface of the gelatin layer to a large extent, and its activity decreases as the granules are farther from the surface.

Second - Silver Halides:

It appeared that if the elements united: iodine. Chlorine. Bromine with silver results in salts that have the property of forming latent images if exposed to light. These salts are:

​​​​​​Silver Iodide Silver Chloride Silver Bromide

The sensitive layer of gelatin is prepared by dissolving potassium bromide or potassium chloride in a warm solution of gelatin, then silver nitrate is added gradually in complete darkness, and they interact according to the following equation:
Potassium Bromide + Silver Nitrate = Silver Bromide + Potassium Nitrate or Potassium Chloride + Silver Nitrate = " + Silver Nitrate = Silver Chloride + Potassium Nitrate. After cooling, it is washed with water, so the potassium nitrate dissolves, and the silver bromide or silver chloride remains stuck in the gelatin layer until it is then settled on the support (glass or celluloid).
​​​​​​(Silver iodide is not used alone), and if the amount of silver iodide exceeds what is necessary, it results in. The resulting damage is called Chemical Fog

Silver bromide is used in the manufacture of sensitive, positive papers, so they become susceptible to light, and these papers are called Bromide Paper, and are suitable for enlarging images. As for the papers used in direct printing, it is better that they be slow in sensitivity, and silver chloride is involved in their manufacture, and it is called Chloride Paper

There are papers that mediate their sensitivity between these two degrees, and both silver chloride and silver bromide are used in them. They are called Chlorobromide Paper and are used either for printing or for enlarging. These silver halide granules are small in size, ranging in diameter around 0003 of an inch. The larger the size of the granules, the faster the sensitivity. From this nucleus, the reduction of the granule begins during the process of showing, so it turns from the salt state into metallic silver, and the other part separates from it, forming other compounds that can be easily disposed of.

Third: Compounds whose function is to influence sensitivity:
These compounds are of one of two types:

(a) Either their purpose is to increase the speed of photosensitivity, such as some sulfur compounds that increase the speed of photosensitivity of the nucleus, which results in a general increase in the sensitivity of the granule.

(b) Or that its purpose is to determine the color of the rays that affect the sensitive material, as it was found that by adding some dyes to the sensitive material, the colors of the spectrum rays that affect the sensitive layer are determined: for example, panchromatic films differ from orthochromatic films and from infrared films. Red differ from each other in the type of rays that affect them. We will come back to explain that. Detailed in the next chapters.

On top of the foregoing, there are some other materials added to the sensitive layers that differ according to the different factories and are considered among the secrets of the industry. Potassium bromide is often one of them because of its property in resisting damage to the sensitive layers as a result of storage.