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Results found in several dictionaries: Collins, Collaborative Dictionary
fade in 
  vt sep 
 [+picture] 
faire apparaître en fondu 
 [+scene] 
ouvrir en fondu 
 [+sound] 
faire monter progressivement 
Fade in the violins ...   
  vi 
[picture] 
apparaître en fondu 
[sound] 
monter progressivement 
The opening credits fade in and fade out.   

Collaborative Dictionary     English-French
'fade in' found in translations in French-English dictionary
fondu n. 1) dissolve (m), 2) fade out (m) Reverso General Dictionary 

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