vertex power #01130OIML <p><span class="" dir="ltr" style="left: 775.366px; top: 633.061px; font-size: 15.1511px; font-family: serif; transform: scaleX(1.00256);" role="presentation">(1) The unit for expressing vertex power is the </span><span dir="ltr" style="left: 775.366px; top: 650.484px; font-size: 15.1511px; font-family: serif; transform: scaleX(1.00285);" role="presentation">reciprocal metre (m</span><span dir="ltr" style="left: 894.96px; top: 647.719px; font-size: 9.68645px; font-family: serif; transform: scaleX(1.00607);" role="presentation">-1</span><span dir="ltr" style="left: 903.056px; top: 650.485px; font-size: 15.1511px; font-family: serif; transform: scaleX(1.00211);" role="presentation">). The name for this unit </span><span dir="ltr" style="left: 775.369px; top: 667.996px; font-size: 15.1511px; font-family: serif; transform: scaleX(1.00222);" role="presentation">is the “dioptre”, for which the symbol is D.</span><br role="presentation" /><span class="" dir="ltr" style="left: 775.369px; top: 685.332px; font-size: 15.1511px; font-family: serif; transform: scaleX(1.00427);" role="presentation">(2) Conventionally the back vertex power, in </span><span dir="ltr" style="left: 775.369px; top: 702.844px; font-size: 15.1511px; font-family: serif; transform: scaleX(1.00176);" role="presentation">dioptres, is specified as the “power” of a </span><span class="" dir="ltr" style="left: 775.369px; top: 720.268px; font-size: 15.1511px; font-family: serif; transform: scaleX(1.00205);" role="presentation">spectacle lens, although the front vertex power </span><span dir="ltr" style="left: 775.369px; top: 737.779px; font-size: 15.1511px; font-family: serif; transform: scaleX(1.00217);" role="presentation">is required for certain purposes (for example </span><span dir="ltr" style="left: 775.369px; top: 755.203px; font-size: 15.1511px; font-family: serif; transform: scaleX(1.0024);" role="presentation">in the measurement of some multifocal </span><span dir="ltr" style="left: 775.369px; top: 772.627px; font-size: 15.1511px; font-family: serif; transform: scaleX(1.00192);" role="presentation">lenses)</span>.</p>