Fig. 1. Diagram of an ideal refracting superposition eye in the dark-adapted state. Light entering an eye first passes the corneal facet lenses (c) and subsequently the crystalline cones (cc). Proximal to the array of crystalline cones is the clear zone (cz) and the layer of rhabdoms (rh). A beam of light parallel to the optical axis of an ommatidium is focused on the rhabdom of the central ommatidium. Sheets of screening pigment surround the crystalline cones. In many species, tracheolar tapeta and/or screening pigment isolate the rhabdoms from each other. The set of facets that contributes to the superposition image of a distant point source is called the superposition aperture.