Grating visual acuity testing as a means of psychophysical assessment of progressive addition lenses

Progressive addition lenses (PAL's) are subject to objectionable astigmatism due to the aspheric nature of their anterior surfaces. Optico-physical assessment of PAL's is commonly undertaken but these measures should be related to the psychophysical responses of patients whilst wearing PAL's. A technique previously used for the psychophysical assessment of contact lenses and spectacle lenses is adapted and demonstrated for the measurement of grating visual acuity (VA) through the aspheric portion of PAL's. The apparatus encompasses an astronomical telescope with magnifying power of unity designed to stabilize accommodation. This paper considers the astigmatism present with three different PAL designs along a horizontal section cutting orthogonally through the umbilical line of each lens. VA was measured, using high contrast vertical and horizontal gratings, along the same horizontal sections of the PAL's studied. A reduction in VA was noted with an increase in the angle of eccentricity from the umbilical line.