First implemented in the 1880s, halftone printing is the process of turning a continuously tonal image into a series of tiny dots of slightly varying sizes such that when printed with ink and seen from a normal distance, it retains the tonality of the original picture (Figure 2). A typical halftone pattern is comprised of 150 lines per inch, which equates to a dot spacing of .16mm, hardly detectable to human eye. A photograph is transferred to a halftone screen print, which is then used to recreate the image through printing. Light exposure through the original image determines the size and spacing of the dots needed for the final reproduced image. Once the dot pattern is created using the screen, it is then transferred to printing plates or cylinders. These plates then direct the placement of the ink onto the final paper. The more typical screen processes uses a contact screen and a dye depositing system in tiny beehive-type compartments. When the dye emulsion on the contact sheet is put in contact with a film emulsion and exposed to light, dots will be produced on the film. The dye lodged in the contact sheet determines the size of the dot produced by controlling light penetration. Strong light is able to penetrate all the way through the dye, while weaker light gets partially absorbed by the dye resulting in a smaller dot (Figure 3).
Herron, Stephen. “A Guide to Halftone Technologies: Strategies and Methods.” Isis Imaging Corporation, 2008.
Stulik, Dusan and Art Kaplan. “The Atlas of Analytical Signatures of Photographic Processes: Halftone.” Los Angeles: The Getty Conservation Institute, 2013.
Wright, Helena E. “Photography in the Printing Press: The Photomechanical Revolution.”In Presenting Pictures, edited by Bernard Finn. London: Science Museum, 2004.