POSITION OF WEAR
Darryl Meister of Carl Zeiss Vision wrote a detailed technical memo to the Technical Committee of the Vision Council. To deliver the best
optical performance in the eyewear that you dispense, this is worth a read. Thanks to Zeiss for providing excerpts and illustrations.
POW or “position of wear” is a term that is used to describe the way an eyeglass frame positions a lens in front of the eye when worn. When the lens or position of the lens changes in front of the eye, the effective power of the lens also changes, resulting in power errors from the specified prescription during both straight-ahead and peripheral viewing. Position of wear is described by three fitting parameters associated with the tilt of the frame and the position of the lens. Lens designers may use definitions for these parameters that are consistent with the ISO 13666 standard, a glossary of terminology for spectacle lenses:
Pantoscopic tilt represents the vertical angle between the plane of the frame front and a vertical plane perpendicular to the line of sight in primary (straight-ahead) gaze, which results from a rotation of the plane of the frame front around the horizontal (X) axis.
Face-form tilt represents the horizontal angle between the horizontal midline of the lens aperture and the plane of the frame front, which results from a rotation of the lens aperture around a vertical (Y) axis in the plane of the frame front.
Back vertex distance represents the distance measured along the line of sight from the apex of the cornea to the back surface of the lens with the line of sight perpendicular to the place of the frame front or, alternatively, in primary gaze.
Position of wear parameters can vary significantly from frame to frame and from wearer to wearer. Consequently, in order to provide each wearer with the most accurate prescription possible, POW should be considered. When POW values are considered during the surfacing process, lens powers can be compensated in order to deliver the correct prescription powers in the fitted frame during straight-ahead viewing. It is also possible to optically optimize the entire lens design for the position of wear, prior to free-form lens manufacturing, in order to provide the correct prescription powers over the entire lens, during both straight-ahead and peripheral viewing.
Progressive lenses are particularly sensitive to the optical effects associated with the position of wear due to the presence of the surface astigmatism that is used to “blend” the distance and near zones of the progressive surface together. Oblique astigmatism exacerbates the surface astigmatism of the progressive design, which reduces the width of the progressive viewing zones, further restricting the fields of clear vision. Furthermore, optical interactions between the oblique astigmatism and the surface astigmatism can cause the viewing zones of the lens to shift or become distorted in shape, disrupting comfortable binocular fusion and reducing the binocular fields of clear vision for the wearer.
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