Before revision: This is traditionally done with a small ruler referred to as a "PD stick" or with a corneal reflex based instrument often, but incorrectly, placed on the nose bridge and work by sighting the corneal reflection produced by an internally-mounted coaxial light source (e.g.
After revision: This is traditionally done with a small ruler referred to as a "PD stick" , but nowadays it is more likely done using a corneal reflex based instrument placed on the nose bridge and work by sighting the corneal reflection produced by an internally-mounted coaxial light source (e.g.