A bifocal lens is made by integrating an additional lens segment, usually for near vision, into the main lens. In the case of glass lenses, this is done by fusing two pieces of glass together; for plastic lenses, different methods are used.
Here's a breakdown of the process, primarily focusing on older glass lens manufacturing, as it explains the core concept best:
Glass Bifocal Lens Production
- Creating the Near Vision Segment (Addition): A small, higher-powered lens segment, the "addition," is crucial for near vision correction. The back surface of this additional lens is ground and polished to precise specifications.
- Fusing the Segment: This segment is then fused into the main lens blank. This traditionally involved using different types of glass with different refractive indices to achieve the desired power correction for reading or close-up work. The main lens blank (the semi-finished lens) usually has the upper portion with the distance correction, and the lower part is where the near-vision segment will be fused.
- Grinding and Polishing the Whole Lens: After the segment is fused into the main lens, the entire lens is then ground and polished to the final prescription of the patient. This process shapes the lens to the correct curve for distance vision, while the fused segment provides the additional power for near vision.
Plastic Bifocal Lens Production
While glass bifocals were the traditional method, plastic bifocals are more common today. The techniques vary:
- Fused Plastic: Similar in concept to glass, but uses different types of plastic with differing refractive indices, fused together. This process is less common than other plastic methods.
- Molded Bifocals: The bifocal segment is molded directly into the main lens during the lens casting process.
- Cemented Bifocals: In some cases, the bifocal segment is cemented onto the main lens. This is less common due to aesthetic and durability concerns.
General Steps for All Bifocal Lenses (After Segment Creation)
Regardless of material, the following steps generally apply after the bifocal segment is created:
- Lens Surfacing: The front and back curves of the lens are generated to meet the patient's prescription for distance correction, taking into account the bifocal segment.
- Lens Polishing: The lens surfaces are polished to ensure clarity and optical quality.
- Edge Finishing: The lens is edged to fit the chosen frame.
- Coating (Optional): Anti-reflective or scratch-resistant coatings can be applied.
Key Considerations
- Segment Shape and Placement: The shape (e.g., D-segment, round segment) and placement of the bifocal segment are important for visual comfort and functionality. The segment is typically placed in the lower portion of the lens, allowing the wearer to look down to engage the near-vision correction.
- Power of Addition: The power of the near vision segment (the "add" power) is customized based on the individual's reading needs and age-related vision changes (presbyopia).
- Material Properties: The choice of lens material (glass or plastic) affects weight, impact resistance, and optical properties.
