عملية الاظهار الرفرسال .. كتاب التحميض والطبع والتكبير

Chapter Thirteen

Referral display process
REVERSAL DEVELOPMENT

We knew before that the process of showing the sensitive film or board is a means of converting the latent image into a transparent negative image, while the reverse display process (or reversal display) is a means of converting the latent image into a positive transparent image directly, or to obtain - immediately after showing a negative image Transparent from printing, enlarging, or transferring another negative image with negative transparency, or to obtain a positive transparent image by means of Photographic Reproduction, or printing or enlarging another positive image without the need to go through the stage of preparing the negative image first:

Advantages of the referral display method over the negative-positive method:

Remember, in the following, what distinguishes the transparent positive images obtained by the “reversal” method from those obtained by the “positive negative” method.
(Whether the image is on transparent film or on sensitive paper).

(a) It provides a negative image preparation stage, and is therefore a popular method among amateur cinematographers with small films 16mm, 8mm

(B) The image is sharper Image due to what we know as cutting the sharpness of the image during the printing or enlarging operations.

(D) Traces of dust (which may fall on the film) appear as black spots or lists in the image if it is treated in the “reversal” method, while it appears as white spots or dots if the “negative-positive method” is followed. It is well known that the black spots on the film do not attract attention as the transparent white spots that appear clear and clear when viewing the image.

(D) It is characterized by its superior ability to transmit Tonal Gradation colors of the photographed subjects with accuracy (3) and with much greater fidelity than the ability of the positive images printed or enlarged on paper, in which we see the image by means of the rays reflected from it.

The basis of the reversal display theory:

The theory of “reversal display” is based on the fact that when a film is exposed to light (while it is in a camera, for example), two potential images are formed on it, one negative and the other positive. If we suppose that the highly luminous parts (in the original subject that we photographed) affected the sensitive film sufficiently to make 90% of the silver salts reducible to metallic silver, it will follow that after the exposure process other quantities of silver salts remain that have not been reduced. In the first manifestation stage, and these salts are capable of becoming another positive form, if they were treated according to the steps that we will mention after this table, Figure (112). We summarize in the following table what we have mentioned.

Referral films:

Although it is possible to display some negative films using the reversal method, it is more appropriate to use films that are specially made for this purpose.

Characteristics of reversal films:

Reversal films have the following characteristics:

(1) The sensitive layer is thin coated emulsion, and a quantity of silver salts is usually deposited in it when it is made, less than what the usual negative films carry, and the amount of silver salts in reversal films does not exceed twice the amount that is used to form the negative image.

As long as the amount of silver salts in the sensitive layer did not decrease (in the aforementioned manner), it would be necessary to increase the exposure during imaging by a large increase so that only the amount of silver salts that suffices to form the final positive image remains.

(2) The silver salts granules (small and large) are spread and distributed evenly throughout the sensitive layer area.

(3) It is stipulated that these films do not lead to any haze, as haze badly affects the degree of transparency of high light areas. in the positive form.

(4) Its contrast is slightly higher than the usual limit in other negative films. The acceptable positive transparent image is the one that combines the existing blackness that represents the areas of shadows and the full transparency that represents the areas of high light or the white areas in the photographed subject.

Sensitivity speed:

Although it is usual to show the sensitivity of reversal films in units similar to those we are used to with regard to negative films (such as DIN - H & D - Scheiner), in reality they are not based on the same principles as sensitometry in negative films, rather they are not numbered according to Film sensitivity units can be a means to assist the photographer in estimating the duration of exposure in the reverse film. In this regard, we mention that mirror films do not have the same degree of latitude as that of positive films.

Spectral sensitivity:

These films may be of the panchromatic family (which is the majority) or of the orthochromatic family of films.

Granules:

Due to the frequent use of reverse films in cinematography (under the standard, that is, the size of 16 millimeters or 8 millimeters), where it is necessary to enlarge them a lot when displaying on the screen, therefore, it is imperative that they be of the category of films with fine grains.

Anti-glare layer:

These films carry an anti-halo layer, which is usually found between the sensitive layer and the support, and it is preferable to place them in this way than to make a colored support due to the importance of the color of the transparent positive image in assessing its artistic value.