Excerpt from
https://en.wikipedia.org/wiki/Video_camera_tube
The size of video camera tubes is simply the overall outside diameter of the glass envelope. This differs from the size of the sensitive area of the target which is typically two thirds of the size of the overall diameter. Tube sizes are always expressed in inches for historical reasons. A one-inch camera tube has a sensitive area of approximately two thirds of an inch on the diagonal or about 16 mm.