Visual field area on the Goldmann hemispheric perimeter surface. Correction of cartographic errors inherent in perimetry

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Abstract
An isopter of visual field charted using the Goldmann perimeter represents a conical area of visual space whose apex is at the eye of the subject and whose base intersects the hemispherical surface of the perimeter bowl. Conventional isopter plotting conceals cartographic distortion and leads to serious underestimates of visual field area when measured by planimetry. Mathematical methods are presented which allow estimation of the true visual field area represented by different isopters on the surface of the Goldmann hemispheric perimeter. A correction can be applied to planimetric measurements to reduce the error inherent in the transposition of the curved surface of the perimeter onto flat, polar azimuthal visual field charts. This study may form the basis for quantitative evaluation of pathological changes in the area of the visual field.