Round-bottom picture tube
The round-bottom picture tube is the earliest type of Braun tube , which has a round picture tube and a round screen. However, since a television picture can only have a rectangular format because of the line-by-line transmission of the picture content, compromises have to be made between picture detail and screen in the round-bottom picture tube.
The simplest option is to use the full, round screen as a viewing area, whereby the parts of the image that lie outside the round screen remain invisible to the viewer. Televisions with such screens were mainly manufactured in the USA by the companies Zenith and Raytheon and were known as porthole (German: porthole).
The much more widespread way to use the round screen was to crop the round screen with a header at the top and bottom. This means that more image content is visible to the viewer, but usable screen space is lost. This technique was used in black and white television in the United States and Great Britain in the 1940s and 1950s , and also used in American color television from the mid-1950s to the mid-1960s , before it was replaced by the rectangular picture tube.