Halftone Printing

What's On
May 3, 2022

Here at Hungate, we have been working with volunteers to unwrap, clean, sort, and catalogue printing blocks from the newspaper of the Norfolk and Norwich Archaeological Society. Inspired by what we’re come across in this process, here is a run-down on how photographic images were reproduced in the 19th and 20th centuries.

A printing block from the NNAS exhibiting the halftone technique

Creating images—whether physical or digital; colour or black and white—generally requires some tricks of the eye. This may be because of limitations to the medium or to cut cost, time, or effort. A well-known example of this is how the pixels on our computer monitors or TV screens can only light up yellow, green, or blue, but are able to create all different tones and values of colour. This can be done because the pixels are so small that when a fine grid of red and blue pixels illuminates our screen, our eye blends them into violet.

From Perdue University

               Similar tricks of the eye can be found in the reproduction of photography from the 19th and 20th centuries. At Hungate, our Unwrapping Archaeology project has uncovered printing blocks which use halftone to produce convincing images of different tones of black, white, and grey. The halftone method uses dots in different concentration to achieve the illusion of continuous colour. As you can see in the image below, what appears from a distance as a dark shade of grey or black is a result of densely-packed dots. The lighter shades appear when there are fewer dots that are more spaced apart, or when there are no dots at all.

               Colour is perceived as either lighter or darker depending on how it responds to waves of light. Darker colours absorb more light, while lighter colours reflect light back to our eyes. Halftone printing works with how our eyes interpret the world around us to create convincing images. The paper, referred to as the substrate in printing, will reflect less light in areas where dots of ink are printed in higher concentration.

From Demystifying the Halftoning Process: Conventional, Stochastic, and Hybrid Halftone Dot Structures

 For the printing blocks we’ve found at Hungate, we can assume the older ones were printed using the conventional method, as it was the primary method of halftone conversion for photographs for over a century. This method, sometimes called amplitude modulated screening, was patented by English polymath William Henry Fox in 1852. To create images, this method uses varied sizes of dots in a criss-cross pattern. The thicker dots result in areas of greater light absorption (darkness), and the smaller dots leave most of the substrate exposed, resulting in more light reflection (lightness). While this method is effective, it can be vulnerable to human error. For example, if an image has a small point of light surrounded by a darker value, pressing too hard on the plate can result in the dots becoming too large, covering the area that was intended to be unprinted or only lightly printed.

If you found this topic interesting, you would be a great candidate to help us out at the church as we sort through the Norfolk and Norwich Archaeological Society’s printing blocks. These blocks were used to print their newspaper, which started publication in the mid-19th century. We work on this project Saturdays and Sundays—all are welcome!

Much of the information for this post on halftone printing came from Garth R. Olver and Jerry J. Waite’s Demystifying the Halftoning Process: Conventional, Stochastic, and Hybrid Halftone Dot Structures

Written by Johanna Boyes