L.4- Varible Focus Lenses
Chapter 1: Introduction
Introduction to Variable Focus Lenses
Reference to an expert from Berkeley explaining variable focused lenses (VFLs)
Importance of terms:
Name change from "progressive addition lens" (PAL) to "variable focus lenses" due to the focus changing throughout the lens.
The term "progressive" derives from the gradual change in power from distance through intermediate to near.
Distinction from traditional lenses
Other types of lenses exist, not necessarily designed for distance, intermediate, or near focus (e.g., lenses designed specifically for specific near tasks).
Overview of upcoming lecture regarding various designs of variable focus lenses and the interchanging terminology used in both the community and public.
Terminology Confusion
PAL refers to Progressive Addition Lenses but often confused with blended bifocals.
Blended bifocal: Typically has the line ground out, lacks intermediate vision, and misnomer for PALs.
No line bifocal/Invisible bifocal: Other terms used by the public that might confuse understanding of lens functionality.
Trifocals and other multifocal lenses are also discussed with advantages and disadvantages noted:
Disadvantages of bifocals: fixed working distance and no intermediate zone.
Image jump: Transition between different lens segments causing image displacement perceived as a jump.
Cosmetic issues: Visibility of the line and lens fit can affect the aesthetic appeal for the wearer.
Historical Context of Lenses
Origins of progressive lenses and variable focus lenses back to the 1950s, developed by Essilor in France.
Resurgence in popularity in the 1980s with significant market share held by Essilor.
Mention of technological advancements in lenses leading to wider acceptance, noting the adaptation challenges faced by many new users.
Visual Mechanics and Lens Technology Development
Base curve blending was highlighted, discussing earlier generation lenses and their limitations compared to modern lenses.
A mention of the optical center and functional zones in lenses.
Chapter 2: Surface Of Lens
Overview of Lens Surface Design
Conventional progressive lenses only incorporated on the front surface, leading to limitations including visual distortion.
Shift to modern technology allowing designs on either surface or both surfaces, creating new optical possibilities for patients.
Digital Surfacing Technology
Progression from conventional lens surfacing to digital.
Precision of prescriptions increased, allowing for lenses to be made with tolerances of of a diopter compared to of a diopter for traditional methods, thus enhancing visual clarity.
Incorporation of progressive surfaces on both lens sides leads to better design options, wider fields of view, and improved overall vision.
Transitioning Lens Powers and Astigmatism Effects
Blending of base curves promotes gradual power transitions rather than abrupt changes, as seen with bifocal lenses.
Issues related to surface design, such as cylindrical surfaces that cause distortion referred to as surface astigmatism.
Mention of astigmatism effects from looking through lenses not at the optical center leading to distorted visuals, especially with older lenses.
Chapter 3: Progressive Addition Lens
Features of Progressive Lenses
Progressive lenses have inherent surface astigmatism, varying with design and technology used for production.
Diagram mapping lens power helps understand change rates across the lens.
Advantages and Disadvantages of Progressive Lenses
Advantages:
Single lens for multiple distance viewing, avoiding need for multiple pairs of glasses.
Improved cosmetic appeal via seamless design, avoiding bifocal lines and thus providing natural vision flow.
Mimics natural vision by smooth eye movement rather than jumping between lens zones.
Disadvantages:
Limited field of view, variable restriction per design.
Persistent surface astigmatism affecting patient adaptation, especially for high myopes or those with substantial astigmatism making adaptation challenging.
Selection of appropriate lens based on specific use-cases (e.g., driving, desk work) becomes critical for patient satisfaction.
Importance of Frame and Measurement Accuracy
Highlighting the importance of proper fitting and frame selection to maximize effectiveness and support effective prescription functionality.
Emphasizes on frame structure (like sag height, monocular PD) impacting overall effectiveness of progressive lenses.
Chapter 4: Lens Cost and Adaptation Challenges
Cost Considerations for Progressive Lenses
Notable instances of high prices, highlighting the financial implications on doctors and cost of lenses for patients; some pairs costing upwards of $1300.
Encouragement for early adaptation to progressive lenses for patients showing signs of presbyopia rather than delaying the need for vision correction.
Discussions on the myth of dependency on glasses and the resistance faced from patients adjusting to progressive lens recommendations.
Chapter 5: Detailed Lens Calculations
Knowledge of Equations
Introduction of Minkwitz's theorem discussing surface astigmatism correlation to increasing add power.
Key learning focus on understanding how increased add power results in higher surface astigmatism levels, establishing a clear relationship with lens prescription effects.
Chapter 6: Innovations in Lens Design
Differences in Lens Quality and Performance
Explanation of variable focus lens performances across types.
Understanding how modern “digital” lenses can better handle surface astigmatism and improve design performance through computational optimization.
Utilization of spherical lens performance and how it varies for patients regardless of astigmatism levels represented in their prescriptions.
Chapter 7: Conclusion
Emphasis on the correlation between add power, surface astigmatism, and vision clarity.
Closing remarks highlight the importance of proper frame selection and lens fitting and the understanding of how specific add powers impact progressive lens effectiveness in varying daily activities.