Segment height is one of the essential measurements used when creating a pair of lined, or standard, bifocal lenses.
Measured in millimeters, the segment height is the distance between the bottom of the lens and the bifocal line. This measurement establishes the height of the magnification portion of the lens, which takes up most of the lower portion.
Segment height is essential because it will determine the proportions of your lens. If the segment height is too high, the lens will consist mostly of the lower magnifying prescription and not feature enough of your long-distance prescription. Too low a segment height number and you won’t have enough of a reader lens to use without adjusting your head.
And yes, you can measure it yourself. The process is easy and requires no special equipment. Measuring segment height on your own is especially simple if you already own a pair of bifocals. Simple put on your glasses and, using a ruler that features millimeter markings, measure (or have someone measure for you) the distance between the bottom of one of the lens and the top of your bottom eyelid. This measurement is the segment height and you can now enter it in the appropriate spot when ordering prescription bifocals online.
Don’t let the importance of the segment height measurement dissuade you from measuring it yourself. For one thing, it’s not an exact science. If you like a bigger magnifying surface at the bottom, you can bump the measurement up by a millimeter or two; if want more of your standard corrective prescription to take up the lens, you simply shave off one or two millimeters. As a rule of thumb, however, the segment height shouldn’t exceed the halfway point on your frame.
If you prefer not to measure segment height by yourself, you can leave that portion of the prescription order form blank. If you do, your new lenses will be made with the standard segment height that corresponds to the frame style you’ve selected. So if you’re comfortable with the standard height of a bifocal, you don’t have to measure segment height at all. If you’d rather adjust the location of that bifocal line, provide a segment height and the optics lab will customize your bifocals to your liking.
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