These pictures are often categorized as blueprints, referring to a method of reproduction most frequently used for architectural plans. This is a contact print process on photosensitive paper; white lines on a blue background distinguish the finished product. (Other types of paper produced different background colors.)
The technique Woodman used was diazotypy: a dry photographic process on paper coated with diazonium compounds, which are sensitive to blue and UV light and developed by ammonia vapour. Woodman experimented with this technique. She created the largest of these images by hanging a long sheet of photosensitive diazo paper on the wall of a darkroom. A photographic slide was projected onto the paper from a slide projector, often for hours. The paper was then developed in a diazo processor at a company that made commercial reproductions of architectural plans. The result was a set of magnificent works in blue, purple, and sepia tones.