الخصائص الضوئية للأوراق الحساسة الموجبة .. كتاب التحميض والطبع والتكبير

Light properties of positive photosensitive leaves

First) the speed of sensitivity to light:

It is not usual for sensitive papers prepared to produce positive images to show any units to indicate the speed of their sensitivity, similar to what we know about the speed of negative film sensitivity units (eg: ASA. - DIN - 11 & D - Scheiner).

Which calls for following experimental methods when exposing those papers. Print or enlarge. (The way to conduct these experiments will be explained when talking about the zoom process). However, the sensitive paper factories used to publish data on the relative sensitivity of the sensitive papers they produce, for example what Kodak factories published about the sensitivity ratio of each type of sensitive paper they produce to the other type, as shown in the following table:

The rate of sensitivity of the positive leaves varies according to the type of salt that is included in the composition of the sensitive layer. Whether it is silver chloride or silver bromide, and according to some other chemical compounds that are added to the sensitive layer during its manufacture, the types of sensitive papers are divided into the following factions:

(a) Chloride Paper:

(B) Bromide Paper:

(C) Chlorobromide Papers:

Chloride leaves:
Sometimes known as Gas light Paper, it is a very slow sensitive paper that can be handled in weak artificial light or in a yellow safety light. The images that are printed on chloride papers are blue-black in color if they appear in a solution of palmitol and hydroquinone, and they require only a relatively short period of time (about 45 seconds at a temperature of 18 °C) to appear.

Due to the slow sensitivity of these papers, they are suitable for contact printing by means of printing boxes whose lamps emit very large light energy.

Bromide papers:

It is the one in which the silver bromide salt is the main sensitive salt, and sometimes very little silver iodide is added to it. They are very fast sensitive papers, and their sensitivity speed exceeds chloride papers by an amount that may range from 100 to 1000 times approximately, and therefore they are very suitable for image enlargement operations, (as the amount of light emitted by the lamps of enlargers is relatively less than that available in contact printing boxes. Printers).

In view of the speed of the sensitivity of these papers, they should not be handled in the next dark room except in a standing safe light, olive green, orange, or sometimes red. The positive image, which is enlarged on this type of paper, will have a dark black color if it is shown in a solution of palmitol and hydroquinone. Showing it requires a period of usually between one and a half minutes and two and a half minutes at a temperature of 20 C.

Chlorobromide leaves:

The positive images that are printed or enlarged on this type of sensitive paper are characterized by their warm colors. This type of leaves is divided into two factions:

(a) Slow chlorobromide leaves.

The percentage of silver chloride is much higher than that of silver bromide. The speed here and the sensitivity of these papers increase by ten times the speed of the sensitivity of the chloride papers. They want from me, and therefore they are used for printing sometimes and at other times for enlarging in small proportions.

(b) Chlorobromide rapid leaves: in which the percentage of silver bromide is much higher than the amount of silver chloride, and sometimes another amount (very small) of silver iodide salt is added to it. These papers are mainly used in enlargement operations. It usually requires an exposure (during enlargement) that ranges from 50 to 4 times more than what is appropriate for bromide leaves, and since green rays may somewhat affect some fast chlorotromid leaves (in addition to their sensitivity to blue and violet rays), it is therefore more correct not to handle them. In the dark room, in a safety light of green color, but it is better if it is orange or red.

These papers lead to warm black colors in positive images if they appear in a solution of palmitol and hydroquinone, and require a period of approximately 3 to 3 minutes to appear.

It is possible that these papers lead to warm brown colors if the duration of exposure is increased during overexposed and then floated in diluted or exhausted exposure solutions. Slow chlorobromide papers also have this feature, but it is easy to manipulate the colors of the image by increasing their orientation towards warm brown colors if the chlorobromide paper is fast.

Second) the degree of variation in positive sensitive papers:
The following factors affect the degree of contrast you get in an image:

(a) The type of sensitive paper: some of them are designed to produce images with high contrast, while others lead to low contrast according to the way they are made.

(b) Duration of exposure: during printing or enlargement and its relationship to display, as the contrast decreases if the duration of exposure of the paper is increased with a decrease in the exposure period.
(H) The degree of glossiness of the paper: It is one of the natural factors that affect the degree of variation. It can be said in general that the degree of contrast increases to some extent if the paper is not together.

(E) The composition of the appearing solution also affects the degree of contrast in the image. There are solutions that increase the contrast and others that reduce it. It is clear that by studying and taking into account the previous factors, we can control to a large extent the correction of errors in negative images. The negative image that lacks contrast can be corrected by printing or enlarging it on sensitive paper with high contrast, and the image with high contrast can of course be corrected or enlarged on low contrast paper. Factories that produce delicate papers manufacture their products with different degrees of variation: it may be two, three, five, or sometimes six degrees, and they are symbolized by symbols that differ from each type to another.

The following is a statement of the varying degrees of sensitive papers, and the conditions for benefiting from each type of them.

1- Paper with a very high contrast, and it is symbolized by .. sometimes, i.e. Ultra Vigorous, Ultra Contrast, or Ultra Hard, and it is used in cases where the negative image has very little contrast (such as underexposing or underexposing, or if it is the opposite of That is, if the definition period is greatly increased, then the contrast decreases and the overall density of the negative image increases).

This type is also used if I want to obtain images of subjects characterized by very high contrast, such as photographing papers written in black ink on white paper, for example.

2- Contrast paper, symbolized by (Vigorous) and sometimes by (Hard). It is also used in printing or enlarging a soft negative image. .

3- Paper with medium contrast, Normal and sometimes symbolized by N: When the negative image is correct, it is used to insert and display a pen, the duration of either of which is more than or less than necessary (i.e. the image has medium contrast).

4- Paper with less contrast than the average Soft, sometimes symbolized by the letter 2, and used if the negative image has very high contrast, which calls for work to reduce it by using suitable sensitive paper.

Although some factories producing sensitive papers may replace the previous symbols with numbers such as (1, 2, 3, 2, 3, 4, 5) instead of letters, and they mean that these numbers indicate the degree of discrepancy, meaning that the number 3 indicates On paper with medium contrast and that the number E indicates paper with very high contrast while the number 1 indicates paper with little contrast, but care should always be taken when considering these numbers, as the degree of contrast in paper with a specific manufacturer code may be equal to the degree of contrast in Paper produced by another factory and marked with the number 3. Accordingly, we conclude that these numbers do not indicate anything but a relative indication of the degree of discrepancy between a group of sensitive papers produced by only one factory.

III) Spectral sensitivity:

Sensitive papers are generally affected only by blue and violet rays, as their spectral sensitivity is very limited, with the exception of very few types of sensitive papers. Variable Contrast Papers * whose spectral sensitivity is greater than that (and will be discussed below):

And sensitive papers of another type prepared for printing or enlarging transparent negative color images to obtain positive “black and white” images, and this is a case that requires that the papers have a sensitive layer of the “panchromatic” species on them. We have previously talked about this type of sensitive papers in the book Colored Photography, and these papers are known as Panalure paper

Variable Contrast Papers:

Some bromide papers are made so that the degree of contrast can be controlled. These papers bear two sensitive layers, one on top of the other, and the first is very bright and the contrast is very high, and the second is very low contrast, as the first is sensitive to green and yellow rays, while the second is sensitive to blue and violet rays only in the bare form, and the degree of contrast in the positive image depends on the color of the rays Which is used in the process of magnification, so if the magnifier lamp is lit, for example, with a blue filter placed on its lens, the low-contrast sensitive layer (which is sensitive to blue rays) will be mainly affected, and the positive image will thus be low-contrast. Therefore, this method is suitable when the negative image is of high contrast. Therefore, this method is suitable when the negativity is of little contrast.

But if the image is to be of medium contrast, it is easy to achieve this by making the exposure through the blue filter for a specific period of time, and then through the yellow filter for another period. It is also possible to control the degree of variation as required by adjusting the ratio between the nursing period during the first filter and the nursing period during the second filter.