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

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