Prior to the development of photography in the early 19th century, the ability to capture or depict the reality human vision perceives was limited by the skills, methods, and techniques of artists/artisans engaged in the image-making process. The introduction of oil painting in the 15th century greatly expanded the tonal range an artist could depict, but the process was laborious, time consuming, and could not be readily reproduced. Photography made it possible, albeit at first only in grayscale (black & white), to capture images of remarkable clarity and fidelity to human perception. As printing press technology advanced during the 19th century, printers were eager to develop a means by which photographic images could be reproduced en masse via a printing press. The challenge was to find a way to reproduce the continuous tones a photographic captured which, prior to the invention and development of the halftone process in the mid 19th century, had not been possible in the printing process.
But first, what is Continuous Tone?
When humans gaze upon nature, we perceive a nearly infinite range of colors and gradations of value (the lightness or darkness of color). For example, if you glance upward at the sky on clear day, the deep azure directly overhead gradually lightens as your eye meets the horizon. Even at sunrise or sunset, there are no discrete bands of color or changes in value as your eye sweeps downward — the colors and values blend together. This smooth change in values and color is known as continuous tone in the realm of photography (color or grayscale) and is what the halftone process makes possible to reproduce in printed pieces.
What is a Halftone?
Initially conceived by William Fox Talbot in 1852, a halftone converts a continuous tone image into dots of varying size. When the dots are made sufficiently small (i.e., below the threshold of normal human perception) a continuous tone image can be printed. In essence, a halftone image is an optical illusion: what appears to the naked eye as a smooth transition between values and colors is the result of tiny dots of printed ink. In the case of color printing, the illusion is further enabled by the use of semi-transparent inks that allow colors to blend as dots of the different CMYK (cyan, magenta, yellow, black) inks are overlayed.
Creating a halftone was traditionally done via photographic process, today halftones are created via computer software when a file is output directly to a digital press or to plates for offset printing. Halftones, also called halftone screens in the industry, are defined by their resolution, measured in lines per inch (lpi). The higher the lpi, the smaller the dots and the greater the quality of the printed image. However, the use of higher lpi screens is constricted by the nature of the paper or substrate being printed. Newspapers commonly use 85-line screens as newsprint is highly absorbent and the dots spread as the ink is absorbed into the printing stock. Most offset and digital color presses use 105 or 133-line screens on coated (matte or gloss finish) stock. 150 to 185 line screens are reserved for the highest, most costly printed pieces.
If you have any questions about continuous tone or halftones, give us a call. We’re happy to help you get the most out of your printing project.
