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- TABLE DES MATIÈRES
- RECHERCHE DANS LE DOCUMENT
- TEXTE OCÉRISÉ
[beville] SCREEN IN THE PHOTOMECHANICAL PROCESS
§1
The easiest way of making the transparency is to employ one of the enlarging and reducing cameras which have a partition in the middle, carrying the lens, and one of the ends arranged to receive the negative. The ordinary copying hoard is covered with white paper and set some distance away from the negative ; it is lighted, as usual, by electric light or otherwise. With a negative of proper density, the exposure need not he excessive, although longer than with the process as now worked.
It is possible to employ an arrangement like the enlarging lantern> provided the source of light be of the right kind. It must be a surface of uniform illumination; the lime-light or the arc-light will not do. Incandescent gas-light may answer the purpose. After focussing, and before exposing the plate, the condenser should be carefully adjusted. This may be done bj" marking, with ink or pencil, on a piece of white cardboard, the outline of the diaphragm’s aperture and inserting it in the diaphragm’s slot. The condenser and light are now moved until the image of the latter is formed on the cardboard ;. it must fully cover the diaphragm’s aperture and be perfectly uniform. If it were otherwise, the shape of the dots would be governed, not by the diaphragm, hut by the shape of the light.
IV. Copying through Vignetted Screens.
The screens which we have been investigating consist of opaque and transparent parts; a vignetted screen is semi-opaque over its whole surface, and divided into minute zones of varying degrees of opacity. This screen must he placed in contact with the photographic plate; its use is therefore restricted to dry plates.
It may he made to copy from an original or transparency, or from a negative ; from a practical point of view, the first case is the only one to be considered.
In copying from the original, the screen is placed as usual in the camera, but in contact with the plate ; the shape of the diaphragm is immaterial, and so is its size, provided it is not too large. A transparency may be copied in the camera or in a printing frame. The arrangement of the camera should be something like that described for copying from negatives, with the exception that any illuminant is suitable ; the arc light would answer the purpose admirably. For the printing frame, the transparency is stripped and placed between the jfiate and the screen. Exposure must not be given to diffused light, but to light emanating from a point, like an arc light ; the frame must be kept in the same position during the whole exposure. The theory is identical for the camera and for the printing frame ; we will investigate the latter only.
In a perfect transparency, the opacities are inversely proportional to the light intensities which they represent. Let L be the greatest light
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 98,45 %.
La langue de reconnaissance de l'OCR est l'Anglais.
§1
The easiest way of making the transparency is to employ one of the enlarging and reducing cameras which have a partition in the middle, carrying the lens, and one of the ends arranged to receive the negative. The ordinary copying hoard is covered with white paper and set some distance away from the negative ; it is lighted, as usual, by electric light or otherwise. With a negative of proper density, the exposure need not he excessive, although longer than with the process as now worked.
It is possible to employ an arrangement like the enlarging lantern> provided the source of light be of the right kind. It must be a surface of uniform illumination; the lime-light or the arc-light will not do. Incandescent gas-light may answer the purpose. After focussing, and before exposing the plate, the condenser should be carefully adjusted. This may be done bj" marking, with ink or pencil, on a piece of white cardboard, the outline of the diaphragm’s aperture and inserting it in the diaphragm’s slot. The condenser and light are now moved until the image of the latter is formed on the cardboard ;. it must fully cover the diaphragm’s aperture and be perfectly uniform. If it were otherwise, the shape of the dots would be governed, not by the diaphragm, hut by the shape of the light.
IV. Copying through Vignetted Screens.
The screens which we have been investigating consist of opaque and transparent parts; a vignetted screen is semi-opaque over its whole surface, and divided into minute zones of varying degrees of opacity. This screen must he placed in contact with the photographic plate; its use is therefore restricted to dry plates.
It may he made to copy from an original or transparency, or from a negative ; from a practical point of view, the first case is the only one to be considered.
In copying from the original, the screen is placed as usual in the camera, but in contact with the plate ; the shape of the diaphragm is immaterial, and so is its size, provided it is not too large. A transparency may be copied in the camera or in a printing frame. The arrangement of the camera should be something like that described for copying from negatives, with the exception that any illuminant is suitable ; the arc light would answer the purpose admirably. For the printing frame, the transparency is stripped and placed between the jfiate and the screen. Exposure must not be given to diffused light, but to light emanating from a point, like an arc light ; the frame must be kept in the same position during the whole exposure. The theory is identical for the camera and for the printing frame ; we will investigate the latter only.
In a perfect transparency, the opacities are inversely proportional to the light intensities which they represent. Let L be the greatest light
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 98,45 %.
La langue de reconnaissance de l'OCR est l'Anglais.