Progressive lenses provide clear sight at all distances without the visible lines of bifocals or trifocals. They achieve this smooth, continuous power transition through a precisely calculated gradient of increasing magnification from top to bottom. This gradient corrects presbyopia, the age-related condition where the eye struggles to focus on nearby objects. Producing this complex optical surface requires specialized, advanced manufacturing techniques that ensure the lens is optically correct and customized for the wearer.
The Progressive Lens Blueprint
The design of a progressive lens relies on a sophisticated mathematical blueprint that dictates the power variation across the lens surface. This map divides the lens into four functional zones to accommodate different viewing distances. The top section is the distance vision zone, used for tasks like driving. Beneath this is the intermediate vision zone, which corrects for mid-range tasks, such as viewing a computer screen.
The smooth connection between these zones is the progressive corridor, where the power gradually increases. This corridor eliminates the abrupt “image jump” experienced with bifocals. The bottom portion contains the near vision zone, offering the highest magnification for reading and close-up work.
The fourth component is the peripheral distortion zone, which occupies the edges of the lens. Because power changes continuously along the corridor, the sides contain non-prescribed cylinder power that appears blurred. Modern designs minimize this effect by pushing the distortion further out of the wearer’s typical line of sight.
Surfacing the Lens with Digital Technology
The complex blueprint is physically realized through Digital Freeform Surfacing. The process starts with a semi-finished lens blank, which has a molded front surface. The progressive design is then sculpted onto the back surface of this blank, allowing for maximum customization.
The core technology is the Freeform generator, a multi-axis Computer Numerically Controlled (CNC) machine. This generator uses a diamond-tipped cutting tool, guided by a digital file, to physically remove material from the lens surface. The tool sculpts the precise curves that form the progressive corridor and the customized prescription.
This direct-to-lens surfacing allows for customization down to the micrometer level. Unlike older methods using fixed molds, Freeform technology allows every point on the lens surface to be individually adjusted. This precision tailors the lens not only to the exact prescription but also to the wearer’s unique frame choice and fitting parameters, such as the frame’s tilt.
The digital approach compensates the prescription across the entire lens surface, resulting in a wider field of clear vision. Advanced computer-aided design and manufacturing (CAD/CAM) technologies ensure the resulting optical surface is sophisticated, incorporating complex aspheric and atoric shapes. This capability to create an individualized lens makes Digital Freeform Surfacing the standard for modern progressive lens production.
Polishing, Coating, and Inspection
After the complex curves are generated, the lens requires further treatment to ensure it is optically clear and durable. The first step is polishing, which smooths the surface after cutting. This process uses flexible tools and precise computer control to smooth the material without altering the intricate Freeform curves.
Next, several functional layers are applied to enhance performance and longevity. A hard coating is applied first for scratch resistance, protecting the softer lens material. This is followed by the anti-reflective (AR) coating, composed of multiple microscopic layers that reduce glare and reflections. The AR coating is especially important for high-index lenses, which reflect more light.
Optional layers, such as UV protection or tinting, are also applied, often using vacuum deposition chambers. The final stage is a thorough quality inspection using advanced instruments. These devices verify the lens power point-by-point to confirm the accuracy of the prescription and the integrity of the progressive corridor. This inspection ensures the finished lens matches the original digital blueprint before shipping.