مدة تعريض الورق الحساس للضوء لتكبير الصورة .. كتاب التحميض والطبع والتكبير

Exposure time of photosensitive paper to enlarge the image:

Estimating the exposure time of photosensitive paper when enlarging the image depends on several factors, which we mention as follows:

The power of the light source.

2- The color of the rays emitted by the light source, and the extent of their effect on sensitive paper.

3- The distance between the lens and the photosensitive paper, noting that the illumination on the photosensitive paper is inversely proportional to the square of the distance between it and the lens.

4- The diameter of the diaphragm aperture of the magnifier lens.

e- The density of the negative image (ie the extent of its blackness).

6- The speed of the sensitivity of the paper used for the magnification, noting that the speed of the sensitivity of the bromide paper is much higher than that of the chloride paper, and between them the speed of the sensitivity is that of the carobromide paper.

7- The degree of contrast required.

It is easy to estimate the correct exposure time if it is possible to measure the intensity of luminance on the surface of the paper and to know the speed of its sensitivity. However, this may not be achieved in practice for two reasons:

(a) Many photographers do not have Enlarging Exposure Meters due to their high price and the possibility of replacing them with simple practical methods (we will mention them later).

(b) Many of the sensitive paper factories deliberately neglected to set the sensitivity of the sensitive paper, as they follow with regard to the sensitive films prepared to obtain a negative image, and therefore, it is not expected that the photographer will find on the envelope of the sensitive papers only brief words to indicate the The paper is included in the family of bromide leaves or chloride leaves, without indicating its sensitivity.

For the aforementioned reasons, it is necessary to conduct initial experiments to estimate the appropriate exposure time for each image. There are two lame ways to do this. . They are both described in
(Figures 86 on page 326).

In order to make it easier to accurately adjust the exposure time, an automatic electric clock is usually used, which is connected to the electrical circuit in the amplifier. And after setting it for the required period and then pressing a special button for it, the electric current is conducted and then it is cut off by itself after the prescribed period has passed. This hour is considered one of the important auxiliary supplies for the work, especially if we want to obtain many copies of one negative image, or the negative images are of low density and require only an exposure of no more than a part of a second, which is a very short period of time that cannot be estimated by the person who does the work without Help this hour. The automatic watch is considered an essential tool when enlarging a color image, as it is necessary to increase or decrease the exposure for fleeting moments that may reach parts of a second, according to the intensity of the colored optical filters that are used in this case.