A spinning disk television camera at the Jenkins Television Co. studios in 1931. Mechanical scan television technology was used in experimental TV broadcasting during the 1920s and 30s until it was superseded by electronic-scan technology in the 1940s, which was used in the first large scale TV broadcasting in the 1940s. This television camera uses a large spinning disk pierced with a spiral pattern of holes "(its housing is visible at front of camera)", called a Nipkow disk. Each hole passing across the image creates one scan line. The light passing through the hole hits a phototube which converts it into an electric current which is proportional to the brightness of the image at each particular point. The images produced by these mechanical television systems had very low resolution, usually either a 60 line image at 20 frames per second, or a 48 line image at 15 frames per second.