Accurate measurements require knowledge of why the measurement is important, how to acquire it and the negative effects of being incorrect.
Mono PD: Think horizontal. The monocular PD is the distance from the center of the pupil to the center of the eyeglass bridge in millimeters. It enables the prescription to be centered width-wise inside the frame so the most optically precise portion of the script is placed in front of the patient's visual axis. Use a pupilometer or better yet, a digital measuring system to acquire this measurement. If you use a penlight, carefully hand-dot the corneal reflex near pupil center. If PD measurements are incorrect, it can induce prism causing dizziness, distortion, blurred vision and double vision in the worst cases.
Mono Height: Think vertical. The monocular height is the distance from the center of the pupil to the bottom-most edge of the lens. For progressives, this ensures correct reading height and that the fitting cross is positioned in front of the patient's pupil. If a digital measuring system is not available, carefully hand-dot the center of the pupil and measure to the bottom of the lens. Like mono PD, incorrect heights can result in induced prism and blurred vision. For visible multifocals (bifocals and trifocals), segment height is measured binocularly using the lower of the two eyes' heights.
Vertex: Think eyeball to lens. Vertex distance is the distance from the front of the patient's cornea to the back of the eyeglass lens. Lens design software in the lab uses this to calculate an accurate prescription by considering differences in strong prescriptions between the refracted distance and the position of the lenses. It also is used to adjust lens power over the entire lens when digital lenses are ordered. Using a digital measuring system is the easiest and most accurate way to obtain this measurement. You can also manually measure from the patient's side the distance from the front of their eye to an estimate or the position of the back of the lens. Refractions are typically performed at about a 14mm vertex. In rare occasions where the actual fitting vertex is significantly different from that of the refracted vertex, or in high power Rxs (>7D), this affects the strength of the prescription in that specific pair of eyeglasses. Lenses worn further from the eye (a great vertex distance) produce the effect of more plus power (less minus) whereas lenses closer are less plus. See the Opticians Handbook for more details.
Tilt: Think vertical lens tilt like Ray-Ban Wayfarer. Lens or pantoscopic tilt is the angle the frame positions the lens vertically in front of the face. Most frames naturally have a slight pantoscopic tilt, meaning the frame front is closer to the face on the bottom than on the top. A retroscopic tilt is the reverse; the frame is closer to the face at the top than at the bottom. Adjusting tilt can help to adjust height (segment or ocular center) issues and, because of the way the tilt affects vision, changing it can cause a patient to see more or less clearly. Lenses not ordered with actual tilt measurements are designed based on average tilt values. Adjusting a lens to the average value will usually make it "see" better.
Wrap: Think frame curvature and horizontal lens tilt. Frame wrap or faceform is how the lens is positioned horizontally in the frame in front of the face. Frames with more face wrap curve in toward the patient's temple where one with less wrap is flatter across the front. Adjustments in this area can help with side distortion issues because, like tilt, wrap also affects how the patient sees based on the average faceform angle or wrap used in the lens' design. Increasing the wrap angle can also help move areas of the lens periphery out of a wearer's field of view, adding to their comfort.
Part Three of this series provides more valuable tips for reducing remakes through Frame Fit and Adjusting.
Bethany DeWolfe, ABOC is an optician in Greensburg, Pa. She joined the optical industry 10 years ago, training under Pittsburgh's optical elite. She is currently managing an independent optical store specializing in unique frame selection and quality on-site lab work.