Photography And Negatives
( Originally Published 1963 )
The purpose of the negative in lithographic production is to provide a suitable image which can be exposed on a plate to create a final positive printing image. If the platemaking process is for negative-type plates (usually termed surface platemaking), the image on film will be clear and the nonprinting image will be opaque. If the platemaking process is for positive-type plates (usually termed deep-etch platemaking), the image will be opaque and the nonprinting image will be clear. During the process of deep-etch platemaking, the printing image and nonprinting image areas are reversed. It is essential that the platemaker receive the negative in complete page form with all illustrations properly inserted (stripped) into the text matter without further cutting or opaquing of film required to complete a page unit. The main functions of the Negative Section are photography, opaquing, cutting, and stripping.
Purpose.—By the use of high-intensity light sources, produce properly exposed and chemically processed light-sensitive negative or positive materials, which upon opaquing and stripping will provide the platemaker with the means to produce a positive image on lithographic plates.
Various kinds of photographic equipment are required because many different kinds and types of copy are received for reproduction. The equipment must be versatile. Large cameras, copyboards, lenses, and screens are necessary. Contact frames and small cameras, as well as light integrators, temperature control devices, and other processing equipment suitable to produce line and halftone negatives from 4 by 5 inches to 40 by 48 inches are required.
The process camera.—The simplest form of the process camera is merely two parallel planes at right angles to the optical axis of the lens which lies between them. There are various types of darkroom cam-eras. They differ structurally in that some are horizontal with over-head support, horizontal with floor-type support, and vertical cameras which extend copy holders in a vertical direction. The horizontal overhead type permits easy and unobstructed access to lensboard and copyboard. The horizontal floor type is worked from the side. The vertical darkroom camera has as its main advantage the conservation of space.
The Government Printing Office uses two horizontal overhead cameras 40 by 40 inches, one horizontal overhead camera 40 by 48 inches, and one horizontal floor-type camera 16 by 20 inches.
Light and illumination.—Light is used in the camera room to enable the worker to see copy and examine materials, and also to enable the camera to "see" the object. Light is a form of radiant energy that can illuminate objects. The amount of light that strikes a surface is called illumination. Various instruments are used to measure the intensity and to control the amount of light needed for a given exposure. Some of these instruments are photometer, to determine the strength of an unknown light source; exposure meter, to determine correct exposure for negatives; and various types of integrating light meters which open and close the shutter and turn on and off lights when a set amount of illumination has been reached.
Lenses.—The lens, which is the key part of any camera, is composed of one or more pieces of optical glass, specially ground and fitted to control light rays reflected from the copy so that they fall in focus on the flat film plane. The process lenses in use are corrected for various faults such as spherical aberration, astigmatism, and chromatic aberration. The lens for use on the lithographic camera must be corrected for all aberrations and be free from any flaws which might cause distortion and produce unsharp images.
The darkroom.—To process photographic material, an area must be provided for developing, stopping, fixing, and washing light-sensitive materials. The room must be lightproof.
Sinks.—The darkroom sinks should be of heavy gage stainless steel, or a reinforced plastic material, which is not easily affected by the chemicals used. The sink must be equipped with hot and cold running water and suitable temperature controls, preferably automatic. The size of the equipment would be determined by the size of the largest material to be processed. There must be room to hold developing trays, stop solution, fixing solution, and ample washing capacity.
Safelights.—The darkroom must be equipped with lights that are "safe," that is, lights that will permit the operator to see but will not cause the sensitized material being used to be exposed or fogged during processing.
Vacuum frame.—The vacuum frame is a unit of darkroom equipment used for exposing positives from existing negatives or negatives from existing positives. The vacuum printing frame consists of two frames on a stand or support. These frames are hinged so they may be opened and closed as required. The upper frame carries a pane of clear glass and the lower one has a rubber blanket and sealing ring. The blanket is connected to a vacuum pump. There are various types of vacuum frames, some equipped with arc lights which increase the versatility. The size will vary according to needs. The Government Printing Office uses two vacuum frames, one of which is equipped with arc lights which permit the use of autopositive materials and will accommodate film up to 40 by 48 inches in size.