Question: Frans Luycx became the leading portrait painter to which Emperor?
Related documents: [0]  [DOC] Ferdinand III (13 July 1608 – 2 April 1657) was Holy Roman Emperor from 15 February 1637 until his death, as well as King of Hungary and Croatia, King of Bohemia and Archduke of Austria. He was the last emperor to have real power over the Holy Roman Empire. [END] 
[1]  [DOC] Frans Luycx or Frans Luyckx (before 17 April 1604 – 1 May 1668) was a Flemish painter who became the leading portrait painter at the imperial court of Emperor Ferdinand III in Vienna. He is best known for his portraits of the Emperor's family and various members of the Habsburgers, including its Austrian and Spanish branches. [END] 
Answer: 
Ferdinand III
Ferdinand III