العوامل المؤثرة في زيادة أضرار الهالة الضوئية .. كتاب الإضاءة والفلم

Factors affecting the increase in corona damage:

The extent of damage to the image as a result of the occurrence of the optical corona depends on the following factors:

1- The thickness of the sensitive layer: the distance (BD in Figure 38) decreases as the thickness of the sensitive gelatin layer decreases.

2- The thickness of the pillar: as the distance (BK in Figure 38) decreases whenever the thickness of this pillar decreases.

The ratio between the thickness of the sensitive layer to the thickness of the support varies greatly, as it may not exceed the ratio of 1:10 in the thin films of the support, and may increase to the ratio of 1:200 sometimes in some sensitive glass panels.

3 - Intensity of incident light: It is obvious that the different reflections increase as the amount of incident light increases on the sensitive layer, so in addition to the light beam (WTCL) reflected from the point (F) other successive reflections occur such as (CT ) and then (mn) the amount of light.

Factors that adversely affect the sensitive layer.

(a) Humidity: Gelatin has the ability to absorb water vapor, and this absorption may be inconsistent, so spots appear in the negative after displaying it. Some manufacturers of photographic materials enclose their products with moisture-resistant casings designed for hot regions. If these casings are not completely sealed, these products are damaged by moisture. As an extra measure of caution, it is better to put some water vapor absorbing materials in places where sensitive materials are stored, such as calcium chloride or dried rice if the storage is in a wide place.

(b) The length of the storage period: If the sensitive material is stored for a long time after its manufacture, then the silver salts granules become capable of being pinched by the apparent solution despite not being exposed to light, so the clarity of the negative image becomes less clear and it becomes covered with a gray color. It is possible to add some chemical compounds to the highlighting solution to fix the defect resulting from the long storage. Perhaps the best place to store for a suitable time is a cool, dry place.

(H) Chemical fumes: Sensitive layers are affected by fumes and gases, which leads to the formation of what is called chemical fog. Examples of these gases are: carbonic gases resulting from the combustion of turpentine or oils, gases resulting from chemical reactions, and vapors of some materials.

(d) Light fog: This defect results from the arrival of any amount of light to sensitive materials, either directly or as a result of not wrapping them tightly after use, or as a result of placing them in intense light while they are wrapped.

The thickness of the photosensitive layer and its light diffusion:

The thickness of the sensitive layer differs according to the purpose for which it was prepared. Films with a thick sensitive layer increase the amount of silver salts, so their density increases as a result of the developing process to a degree more than that of a thin layer, but the latter is characterized in turn by an increase in image sharpness in it compared to what is seen in the same thick layer. Irrediation decreases as the sensitive layer thins, which increases the sharpness of the image.

And in an explanation of what we mean by “photoscattering”, we mention that in the sensitive layer there is an infinite number of granules of grains of silver salts that are sensitive to light, and they are very small in size and are not visible to the naked eye. More or less at night, and despite its infinitely small size, it has an important effect in scattering the light that falls on the sensitive layer, so the rays reach parts of the flag that did not receive direct rays. (Fig. 41) This negatively affects the sharpness of the image after showing it, as the light point emanating from the object becomes, accordingly, an indefinite spot of light (Fig. 42). The original size of each granule (Fig. 43) is because the separate, very small granules have gathered during the process of showing, so that each group of them is one large granule (Fig. 40, p. 114). It is possible to overcome this problem to some extent by using developer solutions that lead to super fine grain developpers. This type of developer solution has been discussed in detail in the book on acidification, printing and enlargement.

The amplitude of the light diffusion increases with the increase in the thickness of the sensitive layer (Fig. 44), and it also increases with the increase in the exposure time beyond the appropriate limit, and this is self-evident, as the increase in light energy results in an increase in its ability to reach deeper parts of the sensitive layer, so that silver salts become crystals. Existing in areas that were not exposed to light directly can be reduced during the process of showing (developing).