Field restriction and vignetting in contact lenses with opaque peripheries.
BACKGROUND: Some previous researchers have shown empirically that visual fields can be restricted by contact lenses with opaque and semi-opaque peripheral zones. However, there has been no formal statement of the optical theory behind such restriction. METHODS: Calculations are made of fields of view with opaque periphery contact lenses, based on paraxial theory and meridional finite ray tracing in the Gullstrand-Emsley schematic eye. RESULTS: This analysis shows that paraxial theory is adequate for predicting 'half fields of full illumination' for ocular pupil diameters smaller than six millimetres, although paraxial estimates of 'half total field' can substantially underestimate those obtained by finite ray tracing for a wide range of ocular pupil sizes. Therefore finite ray tracing may be a more appropriate method of calculating half total field. 'Total field' will be smaller with smaller ocular pupils and smaller contact lens apertures. A clinical method is suggested for assessing total field with such contact lenses in situ. The analysis applied to opaque periphery contact lenses is extended to the semi-opaque periphery contact lenses used for cosmetic alteration of eye appearance. CONCLUSION: Semi-opaque periphery contact lenses will decrease retinal illuminance most at regions outside those tested by previous empirical static perimetry studies.
Carkeet
Andrew
A
Department of Optometry and Vision Science, The University of Auckland, New Zealand or Optometry Centre CPB, Singapore Polytechnic, 500 Dover Rd, Singapore 139651.