Cathode ray tube and method of displaying picture

A cathode ray tube displaying a main picture in a single color comprising:

• an electron gun for emitting electron beams;
• a first picture display unit, including a phosphor which emits a single color upon incidence of electron beams while displaying a main picture by emission of the single color emission phosphor;
• and
• a second picture display unit including another phosphor which emits another color different from the single color emission phosphor upon incidence of electron beams in a region different from the region where the single color emission phosphor is provided for displaying another picture in a color different from that of the display of the main picture by emission of the another phosphor,
• wherein, in a system comprising a front-door intercom which comprises a subsidiary apparatus, including an image pickup apparatus and a master apparatus including a display unit which can display the picture picked up by the image pickup apparatus, and a peripheral apparatus for safety surveillance connected to the front-door intercom, the cathode ray tube is applied to the display unit of the master apparatus of the front-door intercom, and the picture picked up by the image pickup apparatus is displayed in the first display unit as the main picture, while displaying another picture different from the main picture in the second picture display unit according to surveillance signals from the peripheral apparatus.