Magnification and Related Factors in the Alleviation of Impaired Vision

Abstract

The technology of magnification has been developed to the level at which. modern optical design could permit most visually-handicapped persons a degree of functional independence, particularly in near vision tasks. The more severely disabled patients, however, have been aided only with the higher magnification and supplementary advantages of closed-circuit television (CCTV). Experimental CCTV magnifiers were constructed for studying the effects of magnification on performance. Reading rate and ability in a hand-eye co-ordination task were quantitatively assessed, and the most severely handicapped patients were found to benefit the most fron CCTV. Further, the pattern of improvement in reading performance with practice was determined over several days, using partially-sighted schoolchildren. In addition, the relatioship of reading rate with high magnification led to the formulation of a mathematical model expressing the upper limits of performance. Magnification, display aperture width, and the number of alphanumeric characters displayed influenced the reading rates of visually-handicapped subjects. Recommendations were given for prescribing appropriate angular magnification and display field size. When the field size is pathologically constricted as occurs in retinitis pigmentosa, inverse magnification creates an expansion of the visible field, allowing detection while attenuating the visual acuity. An experimental binocular search task revealed that extensive adaptation would be essential before selected patients could expect to gain increased detection skill under these unusual conditions of magnification. Since magnification changes the angular spatial frequency, and little is known about contrast thresholds in low vision, the contrast sensitivity function (C.S.F.) was investigated. In a pilot study, a procedure was devised for relating specific C.S.F. parameters to the optimal magnification required in CCIV reading. Finally, the experimental deductions are brought together in the conclusion, and suggestions are given for further research.