Première page
Page précédente
Page suivante
Dernière page
Réduire l’image
100%
Agrandir l’image
Revenir à la taille normale de l’image
Adapte la taille de l’image à la fenêtre
Rotation antihoraire 90°
Rotation antihoraire 90°
Imprimer la page
- TABLE DES MATIÈRES
- RECHERCHE DANS LE DOCUMENT
- TEXTE OCÉRISÉ
38
ROYAL SOCIETY OF CANADA
As already explained, any shape of diaphragm suitable for a chessboard screen is adapted to the cross-lined screen by using pairs of apertures, the distance between the components of a pair being given by (5), but it is necessary to have a screen in which the opaque and transparent lines are of equal width. Correct gradation cannot be produced with a cross-lined screen by means of a diaphragm having a single aperture.
II. Copying prom Positives.
The object of the photo-mechanical process is to produce a print which will be an exact copy of the original. Before proceeding further we must define what an exact copy is in this particular case.
A print is an exact copy of the original when any tone of the print and the tone of the original which it rejiresents, send out or reflect light of equal intensity.
In an original on white paper, a tone is produced by a semi-opaque film. The incident light has to pass through this film a first time before reaching the underlying surface of the paper, and is partly absorbed on the way. The remainder is reflected and diffused by the paper, after which it has to pass a second time through the film, where it is again partly absorbed. What is left is the light sent out by the tone in question ; it varies, according to a certain law, with the opacity of the film.
In a half-tone print made with black ink on white paper, the incident light falling on the ink is. we will assume, entirety absorbed ; that falling on the white surface is reflected and diffused, the intense, y of the light sent out being proportional to the percentage of white paper in the tone—that is, proportional to the area of the white dots.
Let A be the intensity of the light sent out by the pure white paper : the intensity for the middle tone of the print is 0-5 A, because one-half of the surface is covered by ink. Reducing the white dots to one-half of the middle tone squares, the intensity becomes 0-25 A, because three-quarters of the surface are covered with ink. With black dots of this size, the intensity is 0'75 A, because three-quarters of the surface are bare.
I assume that black ink does not reflect any light ; this is not quite true, but is sufficient for our purpose. I find from photometric measurements on a good half-tone print, that the intensity of the light coming from the ink is only of the intensity for the white paper; this quantity is so small that we may neglect it.
Screens and diaphragms adjusted as we have seen would give very bad results in copying from positives. An investigation of the matter shows that the illumination is too small in the central parts of the dots, black and white, while it is excessive on the edges of the squares of the middle tone. With the cross-lined screen, for instance, it would be necessary to give a supplementary illumination under the central part of the trans-
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 98,83 %.
La langue de reconnaissance de l'OCR est l'Anglais.
ROYAL SOCIETY OF CANADA
As already explained, any shape of diaphragm suitable for a chessboard screen is adapted to the cross-lined screen by using pairs of apertures, the distance between the components of a pair being given by (5), but it is necessary to have a screen in which the opaque and transparent lines are of equal width. Correct gradation cannot be produced with a cross-lined screen by means of a diaphragm having a single aperture.
II. Copying prom Positives.
The object of the photo-mechanical process is to produce a print which will be an exact copy of the original. Before proceeding further we must define what an exact copy is in this particular case.
A print is an exact copy of the original when any tone of the print and the tone of the original which it rejiresents, send out or reflect light of equal intensity.
In an original on white paper, a tone is produced by a semi-opaque film. The incident light has to pass through this film a first time before reaching the underlying surface of the paper, and is partly absorbed on the way. The remainder is reflected and diffused by the paper, after which it has to pass a second time through the film, where it is again partly absorbed. What is left is the light sent out by the tone in question ; it varies, according to a certain law, with the opacity of the film.
In a half-tone print made with black ink on white paper, the incident light falling on the ink is. we will assume, entirety absorbed ; that falling on the white surface is reflected and diffused, the intense, y of the light sent out being proportional to the percentage of white paper in the tone—that is, proportional to the area of the white dots.
Let A be the intensity of the light sent out by the pure white paper : the intensity for the middle tone of the print is 0-5 A, because one-half of the surface is covered by ink. Reducing the white dots to one-half of the middle tone squares, the intensity becomes 0-25 A, because three-quarters of the surface are covered with ink. With black dots of this size, the intensity is 0'75 A, because three-quarters of the surface are bare.
I assume that black ink does not reflect any light ; this is not quite true, but is sufficient for our purpose. I find from photometric measurements on a good half-tone print, that the intensity of the light coming from the ink is only of the intensity for the white paper; this quantity is so small that we may neglect it.
Screens and diaphragms adjusted as we have seen would give very bad results in copying from positives. An investigation of the matter shows that the illumination is too small in the central parts of the dots, black and white, while it is excessive on the edges of the squares of the middle tone. With the cross-lined screen, for instance, it would be necessary to give a supplementary illumination under the central part of the trans-
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 98,83 %.
La langue de reconnaissance de l'OCR est l'Anglais.