During his years in Antwerp, Claus mainly painted portraits and realistic, anecdotal genre pieces.Claus is considered to be the pioneer of Belgian luminism. In 1904, he founded the society Vie et Lumière ('life and light') and became known as the 'sun painter' and the 'painter of the Lys'. A magnificent example is his painting Cows crossing the Lys (1899), which shows a group of motley cows being herded across a small river, with sunlights reflecting off the moving water. The painting hangs in the Royal Museum of Fine Arts in Brussels (Belgium).During the First World War, while in exile in London, he painted a series of views on the river Thames, known as "reflections on the Thames", in the style of Monet. They are his most traditional impressionist works. On 14 June 1924, Claus died in Astene. His last words were: “Bloemen, bloemen, bloemen …” (‘Flowers, flowers, flowers’). The day before his death, he had painted a pastel of a bouquet of flowers, sent to him by Queen Elisabeth of Belgium. Claus is buried in his own garden in Astene and a street is named after him in Brussels.