The major requirement in reproducing copy containing various tones of gray is that such copy be changed photographically into a series of various size dots which, when printed with a black ink, will be interpreted by the eye to be various shades of gray. To accomplish this variety of dots, two types of halftone screens are usedthe glass screen and the magenta contact screen.
Glass screen.Theory.The first practical halftone screen was in-vented by Max Levy, and is therefore often referred to as the Levy screen. The glass screen consists of two sheets of optical glass with fine parallel lines etched on one side. These lines are filled with a black opaque pigment. The glass is sealed with the etched surface together so that the parallel lines cross each other at right angles. The formation of the dots produced by the glass screen is dependent on distance from the film to the screen, size and shape of the lens aperture, intensity of light, the length of exposure, and the speed and contrast characteristic manufactured into the film.
Rulings.Halftone screens are available in standard ruling from 50 to 300 lines per inch. The lines on the screen are generally the same width as the space between them. Therefore, a 50-line screen has 50 lines and 5o open spaces to each inch, making the openings between the lines one one-hundredth of an inch wide. The width of the opening is an important factor in determining screen distance.
Procedure.The halftone screen is placed in the screen holder parallel with the film plane. The separation is checked with a screen gage, or may be determined by watching dot formation on the ground glass as the camera is focused. The making of halftone negatives with the glass screen requires the use of three or more exposures through different lens openingsone exposure for detail, one for middletone, and one for highlight. A supplementary exposure, called the flash exposure, is also made to introduce a fine pinpoint dot in the shadow areas. The lens stops must bear a uniform relationship to the camera extension. After examining the continuous tone copy, using a gray scale, the exposure times are determined and exposures are made. The halftone negative is developed in the usual manner. However, more care must be used than in line negative development to avoid developer streaks, air bells, abrasion, and over or under development which might spoil the finished product.
Magenta screen (contact screen).The contact screen is made up of vignetted translucent dots on a film base. In the case of the magenta screen, the gray silver image is replaced by a dye-coupled magenta image. The gray contact screen similar in design to the magenta screen is used for making direct halftone color separations from copy in color.
Advantages.The magenta or gray contact screens, as the name implies, are used in direct contact with the sensitized material; therefore, the screen distance ratios and various lens openings are not required or used, as they have no effect on dot formation. The lens aperture in general use is F 16. The fact that lens apertures and screen distances are not required for formation of dots simplifies the use of the contact screen in comparison to the glass screen. Other advantages are improvement in sharpness, in reproduction of fine detail, and improved tone rendering especially in middletone and highlight areas. The principal difference between the glass screen and the contact screen is the fact that lines in the glass screen are opaque and the dots in the contact screen have a variable density. The variable density of the contact screen dot permits a more variable rendering of tonal differences in the copy and therefore provides a wider interpretation of copy values than the glass screen.
Technique.The introduction of contact halftone screens has greatly simplified the techniques of making black-and-white halftones. The controlled flash exposure method and the highlight method are most commonly used for controlling contrast. The highlight method is the one used in the Government Printing Office. In this method a flash exposure is made using a yellow flashing lamp, a main exposure to white light reflected from the copy through the contact screen, and an additional exposure from the copy without the use of the contact screen. The result is an increase in the contrast of the halftone negative, particularly in the highlights.