Diazo refers to a group of chemical compounds, specifically diazonium compounds, that are used in certain photographic and printing processes. When exposed to intense blue or ultraviolet light, these compounds decompose and react to form an image composed of azo dyes, producing a visible and stable colour or pattern.
Diazo materials were historically used in blueprinting, architectural reproductions, and some photographic processes because they could create clear, high-contrast images quickly and reliably. Today, diazo techniques are less common but remain an important part of the history of chemical imaging and light-sensitive materials.