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- TABLE DES MATIÈRES
- RECHERCHE DANS LE DOCUMENT
- TEXTE OCÉRISÉ
Section III., 1895.
[29]
Teaks. R. S. C.
1Y.—The Theory of the Screen in the Photo-mechanical Process.
Bv E. Detille.
(Read May 17, 1895.)
I. The Shadow oe the Screen.
Although comparatively new. photo-engraving, or the " half-tone process:’ as it is popularly called, has grown so rapidly that it has now become an important branch of the printing trade. An investigation of its theory may therefore prove of interest.
The object of the process is to break the continuous tones of an original into equivalent tones consisting of white and black dots suitable for printing in the typographic press. Dor this purpose, the original is copied in the camera, but a short distance in front of the sensitive plate a screen is inserted consisting of minute opaque and transparent figures ; it is adjusted to project a diffused shadow over the plate, the light being strongest under the transparent parts and weakest under the opaque parts, with varying degrees of intensity between. "Whatever may be the subsequent operations, the result, if they have been properly performed, is that all parts of the print corresponding to those parts of the photographic plate which have received less than a certain amount of illumination are covered with ink. while for all parts which have received more than the said amount of illumination the surface of the paper is left bare. Illumination means here the product of the intensity of light by the time of exposure.
The first question that arises is this : How does the illumination vary within the shadow of the screen ?
In copying a subject in the camera, the aperture of the lens' diaphragm may be taken as the source of illumination ; seen from a point of the photographic plate, its whole surface appears evenly illuminated, if the subject is properly focussed, the illumination being proportional to the intensity of the light sent by the corresponding point of the subject.
Let ABGD, Fig. 1, be the diaphragm, and MM the plane of the sensitive plate. For a screen, let us take a single opaque figure, LL. There is, on the plate, a certain space, MM, outside of which the whole of the diaphragm is visible and the illumination uniform. There is another space, AAV. inside of which the diaphragm is invisible and where the illumination is zero : this is the shadow proper. In the space inside of MM and outside of ATV, a portion only of the diaphragm is visible, and the illumination varies according as the diaphragm is more or less covered
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 98,27 %.
La langue de reconnaissance de l'OCR est l'Anglais.
[29]
Teaks. R. S. C.
1Y.—The Theory of the Screen in the Photo-mechanical Process.
Bv E. Detille.
(Read May 17, 1895.)
I. The Shadow oe the Screen.
Although comparatively new. photo-engraving, or the " half-tone process:’ as it is popularly called, has grown so rapidly that it has now become an important branch of the printing trade. An investigation of its theory may therefore prove of interest.
The object of the process is to break the continuous tones of an original into equivalent tones consisting of white and black dots suitable for printing in the typographic press. Dor this purpose, the original is copied in the camera, but a short distance in front of the sensitive plate a screen is inserted consisting of minute opaque and transparent figures ; it is adjusted to project a diffused shadow over the plate, the light being strongest under the transparent parts and weakest under the opaque parts, with varying degrees of intensity between. "Whatever may be the subsequent operations, the result, if they have been properly performed, is that all parts of the print corresponding to those parts of the photographic plate which have received less than a certain amount of illumination are covered with ink. while for all parts which have received more than the said amount of illumination the surface of the paper is left bare. Illumination means here the product of the intensity of light by the time of exposure.
The first question that arises is this : How does the illumination vary within the shadow of the screen ?
In copying a subject in the camera, the aperture of the lens' diaphragm may be taken as the source of illumination ; seen from a point of the photographic plate, its whole surface appears evenly illuminated, if the subject is properly focussed, the illumination being proportional to the intensity of the light sent by the corresponding point of the subject.
Let ABGD, Fig. 1, be the diaphragm, and MM the plane of the sensitive plate. For a screen, let us take a single opaque figure, LL. There is, on the plate, a certain space, MM, outside of which the whole of the diaphragm is visible and the illumination uniform. There is another space, AAV. inside of which the diaphragm is invisible and where the illumination is zero : this is the shadow proper. In the space inside of MM and outside of ATV, a portion only of the diaphragm is visible, and the illumination varies according as the diaphragm is more or less covered
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 98,27 %.
La langue de reconnaissance de l'OCR est l'Anglais.