The Apse Mosaic at Cefalù Cathedral: Detail – SS. Paul, John the Evangelist, and Luke
Cathedral of the Transfiguration, Cefalù, Sicily
As usual, St. Paul has a long beard and is balding from the front. The other two would not be recognized without their labels. St. John is labeled IOANNES THEOLOGOS, "John the Theologian," the title by which he is known in the Eastern Church. It is also the practice of the Eastern Church for the John of portraits to be an old man, as here.
It is unusual for Luke to be portrayed with a tonsure. I do not know of another image of him with a tonsure, and the twelve other saints in this composition do not have tonsures, though Peter is shown balding from the back in the section on the left.
This image in full resolution
View the entire apse
More of St. Paul
More of St. John the Evangelist
More of St. Luke
Photographed at the site by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.