Describir: Temporal variation in the luminance level of stimuli displayed on a cathode ray tube monitor :

__    __    _____     _____     _____    __   __    _____     ______  
\ \\ / //  |  ___||  /  ___||  |  ___||  \ \\/ //  |  ___||  /_   _// 
 \ \/ //   | ||__   | // __    | ||__     \   //   | ||__    `-| |,-  
  \  //    | ||__   | \\_\ ||  | ||__     / . \\   | ||__      | ||   
   \//     |_____||  \____//   |_____||  /_//\_\\  |_____||    |_||   
    `      `-----`    `---`    `-----`   `-`  --`  `-----`     `-`'   
                                                                      
   ___    
  / _ \\  
 / //\ \\ 
|  ___  ||
|_||  |_||
`-`   `-`