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

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