When Benjamin Franklin came up with the idea of making one pair of glasses out of two, he solved a huge problem for people who need glasses to read and see in the distance, too. The technology of today's eyeglasses requires accurate placement of the bifocal height, also called the segment or seg. Progressive, or no-line, bifocals and bifocals with lines are measured in a few simple steps but each type is measured in a slightly different way.
Place a piece of clear tape from the top of the frame to the bottom in the approximate centre of the lens. You will be marking a dot on the tape. If necessary, you can reposition the tape once you begin. This prevents marking on existing lenses. If the frame does not have lenses in it, be sure the tape is taut. You will use the dot to record two measurements in the following steps.
Position the person you are measuring directly across from you at the same height. Ask him to sit so he is comfortable. Make sure the frame is adjusted properly and in the right place.
Measure the right eye first. Optical measurements and prescriptions always start with the right eye, also called the O.D. (oculus dexterous). Make a dot on the piece of tape directly in front of the other person's pupil. Repeat with the left eye, or O.S.(oculus sinister).
Take off the frame and measure from the bottom edge of the frame to the dot you made on each lens. Record the number. Opticianworks.com notes that most progressive bifocals have a segment height range between 15 to 25 millimetres.
Measure the monocular PD or pupillary distance for each eye. The PD measurement is used to place the optical centre of the lens in front of the pupil. Take and record the measurement from the middle of the bridge (the centre of the frame) to the dot for each eye. Each eye will have a PD measurement when you measure for a no-line bifocal.
Place a piece of clear tape from the top of the frame to the bottom in the approximate centre of each lens. You will be marking a dot and a line on the tape or lens.
Check the fit of the frame and make sure it is comfortable and in the right place. Position the person you are measuring directly across from you at the same height. Ask him to sit so he is comfortable and in a normal sitting position.
Have the person look straight ahead and draw a line along the edge of the bottom of the eyelid on the tape. Record the number for the right eye, or O.D. Repeat with the left eye, O.S. This line will be the seg height measurement.
Place a dot on the tape in front of the pupil of each eye. Remove the frame and measure dot to dot. This is the person's binocular PD. For lined-bifocals, you use the binocular measurement.
Measure from the bottom edge of the frame to the line you drew at the edge of the lower lids. This is the lined bifocal measurement.
You can double-check your measurement by "free-hand" measuring before you begin.