RX and Bifocal Lens Producer

RX and Bifocal Lens

The term RX refers to the prescription parameters that define the corrective power needed for an individual's vision. When combined with a bifocal design, the lens must accommodate multiple focal lengths within a single optical device.

How does the prescription influence bifocal design?

The distance portion of the lens occupies the upper section and is ground to the patient's distance prescription. The near segment, typically located in the lower portion, contains an additional plus power—referred to as the "add"—that enables clear vision at reading distances. The total power in the near zone is the sum of the distance prescription and the add power.

What measurements are required beyond the basic prescription?

Fitting bifocals requires additional clinical measurements. The segment height determines where the near portion is positioned relative to the pupil. This measurement ensures the wearer looks through the near zone when reading. Monocular pupillary distances, measured separately for each eye, are also necessary for accurate optical center placement.

Can any prescription be made as a bifocal?

Many prescriptions can be produced in bifocal form, though certain parameters may affect availability. Very high add powers, unusual prism requirements, or cylinder values may require special ordering or custom surfacing. The lens material also influences feasibility, as some materials are better suited to bifocal production than others.

Bifocal lenses are categorized by the method used to create the near segment and the visibility of the segment line. The following table outlines common bifocal types and their characteristics.

A bifocal stock lens is a finished lens held in inventory, requiring only edging to fit a selected frame. Understanding the characteristics of stock bifocals helps in setting appropriate expectations for availability and performance.

Prescription range limitations: Stock bifocal lenses are typically available for distance powers within a moderate range, often from -4.00 to +4.00. Cylinder corrections usually do not exceed -2.00. Add powers are commonly available from +1.00 to +3.00 in 0.25 diopter steps. Prescriptions outside these parameters generally require custom surfacing.

Segment height fixation: The near segment on a stock bifocal is positioned at a predetermined height, typically around 5mm below the lens center. This fixed position means the lens cannot accommodate unusual segment height requirements. Wearers needing higher or lower segment placement must consider custom lenses.

Material options: Common stock bifocal materials include CR-39 plastic, which accepts tinting readily, and polycarbonate, valued for impact resistance. Glass bifocal stock lenses are less common but available through some suppliers. Material choice affects weight, thickness, and tintability.

Coating availability: Some stock bifocals come with factory-applied hard coatings. Anti-reflective coatings may be applied after edging, depending on laboratory capabilities. Uncoated stock lenses offer flexibility for later coating selection but require additional processing time.