Interesting ‘surface’ research of circular (halo) motifs in Christian Iconography, and its perpetual decline in Art History. This ties in well to my research development ~ looking at circles first, before advancing into the sphere.
The halo was incorporated into Early Christian art sometime in the 4th century with the earliest iconic images of Christ, initially the only figure shown with one (together with his symbol, the Lamb of God). Initially the halo was regarded by many as a representation of the Logos of Christ, his divine nature, and therefore in very early (before 500) depictions of Christ before his Baptism by John he tends not to be shown with a halo, it being a matter of debate whether his Logos was innate from conception (the Orthodox view), or acquired at Baptism (the Nestorian view). At this period he is also shown as a child or youth in Baptisms, though this may be a hieratic rather than an age-related representation … With increasing realism in painting, the halo came to be a problem for artists. So long as they continued to use the old compositional formulae which had been worked out to accommodate haloes, the problems were manageable, but as Western artists sought more flexibility in composition, this ceased to be the case. In free-standing medieval sculpture, the halo was already shown as a flat disk above or behind the head. When perspective came to be considered essential, painters also changed the halo from an aura surrounding the head, always depicted as though seen full-on, to a flat golden disk or ring that appeared in perspective, floating above the heads of the saints, or vertically behind, sometimes transparent. “ (Wikipedia)