زيادة سرعة الحساسية في الطبقات الحساسة السالبة .. كتاب الإضاءة والفلم

Increased sensitivity speed in negative sensitive layers

It may be necessary to photograph in inappropriate light conditions (due to the lack of light), or that the light is sufficient, but the photographer made a mistake and did not open the lens diaphragm properly, or a high shutter speed was estimated when a lower speed should have been estimated.

Such pictures may have great value, and then the photographer should not refrain from taking pictures, otherwise he will miss important opportunities. In such abnormal cases, it is very sufficient to obtain any result that serves the required purpose. It is not necessary to obtain excellent results from a technical point of view as long as the circumstances did not allow it.

The methods that we will all mention are summed up in benefiting as much as possible from the sensitivity of films, and these methods are:

First - Increasing the sensitivity before imaging if we know the imaging conditions in advance. This method is called Hypersensitizing

Second - Increasing the sensitivity after imaging and before the rendering process: It is called the process of strengthening the latent image, i.e. I atent Image Intensification.

Third - Using a High Energy Developer solution

Fourth - Increasing the density of the negative image after Intensification. Since we have already discussed the last two methods in detail in the book Acidification, Printing and Enlargement (1), therefore it is sufficient here to discuss the first and second methods.

Hypersensitising before imaging

A questioner may ask: As long as it is possible to increase the sensitivity of films for required purposes, what prevents sensitive film factories from preparing and offering them for sale after increasing their sensitivity? In response to that, we say: These methods of increasing the sensitivity that we mention now are temporary, not permanent, and the film whose sensitivity has been increased should be used as soon as possible, provided that the period between the completion of the process of increasing the sensitivity and between filming does not exceed a few days, and this is the reason why it is not shown. Films that were treated with these methods commercially because they could not be stored with the knowledge of the manufacturer.

Methods of increasing the sensitivity are suitable to a large extent if the films were originally slow in sensitivity, and their effect is less in the case of fast films, and their effect is greater in fauchromatic films than in other types of films. Increasing the sensitivity is of great importance for Infra Red Emulsions sensitive films.

The sensitivity is increased prior to imaging in one of the following ways:

(a) Use of solutions: Bathing
We have already indicated in the matter of making the sensitive layer that potassium bromide salt is included in it, either intentionally in the composition because of its property of resisting damage resulting from storage and resisting fog, or because there is like the rest that silver nitrate did not interact with, which we add when preparing the sensitive layer to form and despite the importance of bromide salt Potassium in resisting the fogging that results from storing the sensitive layer, it has another effect, as it reduces the speed of sensitivity. Accordingly, our method to increase the sensitivity is to get rid of potassium bromide, and this is done in one of the following ways:

1- Put the sensitive film or plate in distilled water to dissolve the potassium bromide, then dry it.

2- Washing the sensitive film or plate in a dilute alkaline solution such as borax or an ammonia solution of the following composition:

30 parts water
Noshader is 88% concentrated. 1 part

The sensitive film or plate is treated for two minutes in this solution at a temperature of .. Fahrenheit (12 Celsius).

3- The sensitive film or plate should be washed in a solution of 5% triethanolamine for two minutes at a temperature of 550 degrees Fahrenheit.

4- It is washed in a diluted solution. Silver nitrate . Noting that it may result in an increase in fog as a result of dissolving potassium bromide.

comments :

(A) The aforementioned operations take place in complete darkness, away from its covers

(b) Liquid droplets shall be lifted as soon as possible from the surface of the sensitive layer and dried as soon as possible without being re-wrapped in their covers after drying is complete.

(H) The low temperature (55 Fahrenheit) does not result in any damage (such as swelling of the sensitive layer, for example), so this temperature is preferred when performing the operations referred to.

(D) When displaying sensitive films and plates that were treated in the previous way, a diluted solution of 1:30,000: benzotriazol must be added to the appearing solution to prevent fogging, which is the function that was performed by potassium bromide (and this solution can be obtained prepared for use from the production of factories Kodak and its trade name is 1. Kodak Anti Fog No.).

(B) Increased sensitivity to vapors: Vapor Treatment:

This process is carried out by placing the sensitive film or plate in a tightly closed non-metallic bell that does not allow light or air to pass through, and is placed with the sensitive plate or flag in a small bag containing a quantity of the substance in whose vapor the process will be completed (such as mercury for example). We leave the crucible and the film as well for 4824 hours. Bonnet sealing The mercury and its vapor foci evaporates in the film after removing the spool from it if it is metal. This method is distinguished from the previous one by not having the need to remove the film from the paper cover, meaning that the process is done in a dry way without the need to wash the flag in liquid, and therefore there is no need to dry it. This method does not affect the sensitivity of the film to colors (ie, it does not affect the spectral sensitivity). Despite the low temperature in which this process takes place, the amount of simple steam rising is sufficient to affect the sensitive layer during the process period (24-48 hours) once the crucible is sealed, especially if the air pressure inside the bell can be reduced from the normal pressure.

(c) Increasing sensitivity by exposure to light:

This method may seem strange, but experiments have proven that if we show the film or photosensitive plate for a very short period before imaging, we gain an exponential increase. Photography, then the film is exposed to light by placing a white surface like white paper without lines in front of the lens, provided that this paper is very close to the lens out of focus in order to prevent the appearance of the surface of the paper (texture) in the image.

The natural result of the previous process is that we have formed a light haze on the surface of the negative image as a result of exposing it to light, which results in a decrease in the degree of contrast of the negative image. The degree of clarity of the negative image, which if we show it and prolong the period of showing it a little, it will result in a high-density image that clarifies the required details.

We explain the following steps that took place in this process according to the well-known current facts:

1- By exposing the film to light, we obtained a light haze, which always results in a decrease in the degree of contrast.

2- By increasing the exposure period by a reasonable percentage, the degree of contrast increases to an extent sufficient to compensate for its deficiency in the previous stage, and the intensity also increases.

3- And it is established for us in this imaging process (for which we want to increase the sensitivity) that the lighting is not sufficient for the image to obtain its rightful amount of light, which results in a blurring of the details of the shadow areas, which is a defect that is compensated by the increase in the over development period, which increases the density in the image. shadows area.

As a result of the previous operations, we will finally find that we have obtained a negative image with a normal degree of contrast, and clear details in both the areas of light and shadows.

But how long does it take for this optical haze to work on the sensitive layer?
And what is the period required to increase the manifestation?

In response to that, we say: It is necessary to do several experiments to estimate these periods. It is difficult to estimate it here due to the different sensitivity of the used sensitive films and plates, and due to the different strengths of the many highlighting solutions.

We may notice in the result that we obtained that the density has increased too much, and here we can follow one of the methods that we have already talked about on the issue of reducing the density of the negative image Reduction Process (1)