Measuring the ring scotoma

The aim of this study was to analyse how to reduce the ring scotoma (RS) induced by a +10.00D ophthalmic lens in a patient who had previously worn only soft contact lenses and how to minimise the disturbing effects due to this scotoma. The patient wearing a +10.00D spherical lens complained of serious mobility problems since he began to use spectacles. Previously, he always wore soft contact lenses. He used a plastic spectacle frame with the lens before his right eye and a balance lens on the left due to significant amblyopia. After analysing the clinical history we suspected that the origin of his problem was the ring scotoma. In order to find the best solution for this patient, we assessed the properties of different types of +10.00D ophthalmic lenses by using different techniques in RS determination. We designed a real ray tracing program to calculate its size and correlated these calculations with objective measurements performed with an optical bench apparatus, and we used a modified tangent screen to measure the subjective RS. Moreover these last measurements were performed in two different illumination conditions, 0.3cd/m2 and 400cd/m2, in order to determine the influence of pupil size. We found that the measured subjective scotoma is greater than the objective one because it includes the influence of the spectacle frame, although greater pupil size does tend to reduce it. The objective method is the most reliable method of assessing RS one. As a result we observed the temporal RS reached its lowest values if a lenticular ('fried-egg') design was employed in low illumination conditions and with a small diameter ophthalmic lens. So, as a final recommendation to our patient, we suggested him a fried-egg design ophthalmic lens mounted in a thin rim or rimless spectacles.