During the 1950s, Carlos Cruz-Diez left Venezuela to travel extensively throughout Europe, where he explored the possibilities of Constructivism while continuing to develop his interest in chromatic structures and color theory. Physichromie No. 939 (1977) is part of his most important series, which he began in 1959 and has continued to investigate for decades. The exact medium of these works has developed over time but has always consisted of alternating colored strips that trap light, producing a chromatic effect that constantly changes as the viewer moves around the piece. In this way, the viewer becomes a participant in the artwork, not merely a spectator. The innovative aspects of the these works immediately caught the attention of the international milieu, and very early in his career Cruz-Diez was included in two seminal exhibitions in Europe and the United States: Movement 2 at the famous Galerie Denise René in Paris in 1964 and The Responsive Eye at the Museum of Modern Art in New York in 1965. Cruz-Diez was also the subject of a major retrospective at the Museum of Fine Arts Houston in 2011, titled Carlos Cruz-Diez: Color in Space and Time.