Index type CRT having a photosensor with a function of amplifier

A display device comprising:

• a beam index type cathode ray tube having a fluorescent screen with an effective screen area and an ineffective screen area, with red, green and blue fluorescent materials formed in strip configurations on the effective screen area and with index fluorescent materials formed in strip configurations with a predetermined pitch on both the effective screen area and the ineffective screen area;
• a photomultiplier receiving a driving voltage for converting light generated from the index fluorescent materials into an electric signal in accordance with a multiplication ratio when an electron beam of the cathode ray tube horizontally scans the fluorescent materials and the electron beam impinges on the index fluorescent materials, and for producing an output signal having a level responsive to said driving voltage;
• an index signal processing circuit for switching red, green and blue signals in with said output signal from said photomultiplier, for adding to one of said red, green and blue signals a detection signal for detecting the index fluorescent materials in the ineffective screen area of said cathode ray tube, and for controlling luminance of said cathode ray tube in accordance with a luminance level control signal fed thereto;
• a driving voltage generating circuit for controlling a driving voltage applied to said photomultiplier in an inverse relationship relative to said luminance level control signal to thereby control said multiplication ratio of said photomultiplier;
• and
• a variable circuit for changing a level of said detection signal in accordance with said luminance level control signal also fed thereto.