To what extent does Jan Van Eyck's 'Arnolfini Double Portrait' fulfill the demands of its patron?

First I would look at the general function of portraiture in the 15th century which was primarily to memorialize the features of the patron for posterity and to ensure rememberance, as well as to celebrate the earthly riches of the patron and his devotion to god. 

Then I would look at how this is fulfilled in the work itself: the specific objects that allude to the earthly riches of the couple for example, such as the imported items such as the oranges and the carpet and the expensive materials (fur, velvet, mirror). The piety of the patrons is emphasized through a number of objects such as the paternoster hanging from the wall, the scenes of the Passion around the mirror, and the shoes discarded on the floor. (NOTE that it important not only to convey general facts but also to ground them in specific visual details in the painting)

One could then talk about how a naturalistic likeness of both sitters has been achieved and how, such as through the use of oil paint and glazes. The naturalism would have allowed them to be faithfully remembered and therefore prayed for after their death. (Talk about the specific properties of oil paint such as the slow drying time and how it can therefore achieve such naturalistic likenesses)

This painting may well have been a document to provide legitimacy of the marriage between the two figures in the work. Not only does Van Eyck leave a prominent signature (almost as if he has signed a marriage document) but he also shows in the mirror at the back that a third person in present to witness the union. He uses colour symbolism to allude to certain traditional qualities in a marriage, for example Constanza's green dress which emphasizes her fertility.

The varying points of views of critics relating to the meaning of the work could then be discussed and how this is backed up with visual detail.