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- TABLE DES MATIÈRES
- RECHERCHE DANS LE DOCUMENT
- TEXTE OCÉRISÉ
[deville] SCREEN IN THE PHOTO-MECHANICAL PROCESS
49
paper may not show. Each of these exposures can be timed very accurately with trial plates—a most important consideration.
We will see later on that the second exposure alone produces a vignetted screen. The mode of action in the process last described, therefore, consists in impressing, before development, a vignetted screen upon the negative.
Unfortunately the exposure on white paper obliterates, to some extent, the image in the shadows, and increases the difficulty of obtaining sharp dots with clean edges. In that respect the process is much inferior to those which we will now investigate. It has the advantage of requiring a very short exposure.
III. Copying prom Negatives.
Exposure through a negative gives a transparency or positive ; the interposition of the screen does not modify this relation. What is to be done with the dotted transparency does not come within the scope of this paper. Whether a negative is to be made from it, or whether, by some peculiar process, the engraved block is to be made directly from the transparency, I will leave for others to decide.
To understand what is to follow, I must give briefly Messrs. Hurter and Driffield’s definitions of a perfect negative ; for further explanations I refer to their original paper.1
The opacity of a film is the reciprocal of the number expressing the fraction of the incident light which emerges from the film. An opacity of ten lets one-tenth of the light pass, stopping nine-tenths of it. An opacity of two lets one-half of the light pass. An opacity of one does not stop any light; it is perfect transparency. The density is the logarithm of the opacity.
Two opacities, O' and 0". put-together form an opacity, 0, equal to the product of O' and 0". Two densities, D' and D", produce a density, jD, equal to the sum of D’ and D".
In a perfect negative the opacities are proportional to the light intensities by which they were produced. These opacities or densities are reached at a certain stage of development; with a shorter or longer development, a density, D, becomes aD, a being the “development
factor.”
Let L be the highest light intensity of the original, and ML the corresponding opacity of the negative. Another tone pL. of the original, is represented in the negative by the opacity MpL. and the light trans-
1
mitted is
We now place this negative in front of the camera and
1 “ Photo-chemical Investigations, and a New Method of Determination of the Sensitiveness of Photographic Plates,” by Ferdinand Hurter, Ph.D., and N. C Driffield-Journal of the Societv of Chemical Industry, May 31, 1890.
Sec. III., 1895. 4.
Le texte affiché peut comporter un certain nombre d'erreurs. En effet, le mode texte de ce document a été généré de façon automatique par un programme de reconnaissance optique de caractères (OCR). Le taux de reconnaissance estimé pour cette page est de 98,39 %.
La langue de reconnaissance de l'OCR est l'Anglais.
49
paper may not show. Each of these exposures can be timed very accurately with trial plates—a most important consideration.
We will see later on that the second exposure alone produces a vignetted screen. The mode of action in the process last described, therefore, consists in impressing, before development, a vignetted screen upon the negative.
Unfortunately the exposure on white paper obliterates, to some extent, the image in the shadows, and increases the difficulty of obtaining sharp dots with clean edges. In that respect the process is much inferior to those which we will now investigate. It has the advantage of requiring a very short exposure.
III. Copying prom Negatives.
Exposure through a negative gives a transparency or positive ; the interposition of the screen does not modify this relation. What is to be done with the dotted transparency does not come within the scope of this paper. Whether a negative is to be made from it, or whether, by some peculiar process, the engraved block is to be made directly from the transparency, I will leave for others to decide.
To understand what is to follow, I must give briefly Messrs. Hurter and Driffield’s definitions of a perfect negative ; for further explanations I refer to their original paper.1
The opacity of a film is the reciprocal of the number expressing the fraction of the incident light which emerges from the film. An opacity of ten lets one-tenth of the light pass, stopping nine-tenths of it. An opacity of two lets one-half of the light pass. An opacity of one does not stop any light; it is perfect transparency. The density is the logarithm of the opacity.
Two opacities, O' and 0". put-together form an opacity, 0, equal to the product of O' and 0". Two densities, D' and D", produce a density, jD, equal to the sum of D’ and D".
In a perfect negative the opacities are proportional to the light intensities by which they were produced. These opacities or densities are reached at a certain stage of development; with a shorter or longer development, a density, D, becomes aD, a being the “development
factor.”
Let L be the highest light intensity of the original, and ML the corresponding opacity of the negative. Another tone pL. of the original, is represented in the negative by the opacity MpL. and the light trans-
1
mitted is
We now place this negative in front of the camera and
1 “ Photo-chemical Investigations, and a New Method of Determination of the Sensitiveness of Photographic Plates,” by Ferdinand Hurter, Ph.D., and N. C Driffield-Journal of the Societv of Chemical Industry, May 31, 1890.
Sec. III., 1895. 4.
Le texte affiché peut comporter un certain nombre d'erreurs. En effet, le mode texte de ce document a été généré de façon automatique par un programme de reconnaissance optique de caractères (OCR). Le taux de reconnaissance estimé pour cette page est de 98,39 %.
La langue de reconnaissance de l'OCR est l'Anglais.