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

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