Now let's review the basic lens measurements that we need to be aware of in order to give your patients the best possible vision with their prescription in their new glasses
First let's discuss PD, which stands for Pupillary Distance. Pupillary distance can be measured two different ways. The first is called binocular PD. Binocular PD refers to measuring from the center of one pupil to the center of the other pupil. To place the center of the Prescription at that measurement.However, that also assumes that your patient is perfectly symmetrical, which is not normally the case. So, what We prefer to use is what we call monocular PD. Monocular PD measures from the center of the nose or bridge of the glasses to the center of the patient’s pupil each eye individually. This makes sure we center the prescription on each eye to give your patient the best possible vision.
Next we have optical center or OC. OC is the opposite which means it means the height of the center of the pupil in the lens. So, for that, we would measure with our PD ruler or digital measuring device from the bottom of the lens to the center of the pupil in millimeters. Each eye individually again. So that we can center the prescription on patients eyes individually rather than together. This is exceptionally important for stronger single vision prescriptions because the patients are more susceptible to changes in the prescription within the lens. If you do not specify optical center in your patients prescription your lab will default to putting the prescription at the geometric center of the lens which would be here which is just slightly lower than what it should be which means for the patient to see clearly they would normally have to just tilt their head up in order to see that center vision.