Next we have segment heights, commonly referred to as seg height. This refers specifically to multifocal lens designs.
First, we'll talk about a bifocal lens which is the lens that we can clearly see one segment placed at the bottom of the lens which is primarily for mirror viewing, while the rest of the lenses for distance viewing. To properly measure a bifocal lens for the segment height, we would measure with our PD ruler from the bottom of the lens to the lower lid line of the patient to give us our proper segment height.
Next we have the trifocal lens. The trifocal lens again has a visible segment in the bottom for near vision but also has a rectangular segment in the middle of the lens for intermediate viewing. To properly measure this seg height, you will again measure from the bottom of the lens but this time we want to go to the center of the patient's pupil and drop down approximately 2 millimeters and that would represent our segment height for a trifocal.
For a progressive lens, we measure the same that we do for a trifocal. So again, we would measure from the bottom of the lens to the center of the pupil and drop approximately 1-2 millimeters based on whatever the standard is in your own practice. It's important that all of the opticians in the same practice discuss this technique so that everyone uses the same drop from the center of the pupil in order to ensure uniformity among all of the prescriptions that are fit in your practice.
The other need for seg height would be for anti-fatigue lenses. These are the lenses that are primarily meant for distant viewing but have a slight bump in the reading vision in order to aid non-presbyopic patients that do up close work for the majority of their day. For proper measurement on this you would measure the same as you would a trifocal lens, from the bottom of the lens to the center of the pupil, dropping down approximately 2 millimeters.
Next, we have the vertex. Vertex is defined as the distance from the front of the patient's cornea or eye, to the back side of their prescription lens. The default vertex for all glasses starts at 13 millimeters, which is also the distance that each phoropter in your practice is set up for. If you don't specify a different vertex, your lab will default to 13 millimeters. Which means for this patient, if the glasses were closer than 13 millimeters from the front of their eye, which is the case one, we will actually add minus power to their prescription and their new lenses.
Conversely, if we move the glasses further away, which is typical for people with longer eye lashes that don’t want them hitting the back of the lenses, the longer vertex will actually add plus power to the prescription. So, it’s important to first get the proper fit for your patient and then measure the vertex accordingly.