Radiation Safety Glasses Model 400

How To Read Your Prescription

BiFocal ADD

Bifocal lenses are designed to correct vision for two different distances, typically near and far, in a single lens. The "ADD" in "Bifocal ADD" stands for "additional" or "additive." It represents the additional power needed to correct near vision in bifocal lenses.

Pupil Measurements

The pupil distance is the distance (measured in mm) between the center of your pupils when looking far away in the distance. All prescriptions require a pupil distance (PD) measurement. Your PD remains constant (does not change) and is usually on record at your eye doctor. The average PD for an adult male is 65mm and female is 63mm. The PD may be noted as a (two-number) monocular measurement somewhere between 26mm – 36mm; the first number denotes the right eye. You may also have a distance and near PD, as your pupils are slightly closer when looking at a book or something else near your eyes. See our video on how to measure your pupil distance.

Segment Height

The segment height is the distance (in millimeters) from the bottom of your lens to the top (or beginning) of your bifocal. For lined (Flat-Top) bifocals, your segment height is generally measured from the bottom of the lens to the top of your bottom eyelid. For progressive (no-line) bifocals, your segment height is measured from the bottom of the lens to the center of your pupil. We recommend leaving segment height blank for lined bifocals as the standard height works for most people. For progressives, it’s a good idea to order the frame first to get the measurement just right. If you prefer to leave the information blank, we will make the segment height standard for the specific frame that you ordered. Our lab has over 50 years of optical experiencing with creating progressive lenses. We use a very accurate progressive lens formula that is precisely calculated to the frames you order. Our formula is accurate in most cases.