Multifocal Lenses: Introduction, History & Development, Types of Bifocal, Trifocal & Progressive Addition Lenses
1) Introduction
Multifocal lenses are ophthalmic lenses designed to provide clear vision at more than one focal distance—usually distance, intermediate, and near. They are prescribed primarily for presbyopes, who lose the natural ability of the crystalline lens to accommodate with age.
Initially, presbyopes managed by using two separate pairs of glasses—one for distance and one for near. The inconvenience of constantly switching glasses led to the development of bifocals, followed by trifocals, and eventually progressive addition lenses (PALs) that provide a continuous power gradient.
2) Need for Multifocal Lenses
- Presbyopia: Natural decline in accommodation with age (~40+ years).
- Convenience: Reduces need for multiple pairs of spectacles.
- Occupational needs: Office workers, professionals, and artisans require vision at multiple distances.
- Binocular balance: Maintaining single pair avoids adaptation issues of switching between glasses.
3) History & Development
- Benjamin Franklin (1784): Credited with inventing the first bifocals—two lens halves (distance + near) joined in one frame, known as Franklin bifocals.
- 19th century: Introduction of cemented bifocals where a near segment was glued onto a distance lens.
- Late 19th – early 20th century: Development of fused bifocals, where the near segment was fused within a glass distance lens, offering better cosmetics and durability.
- 1930s: First trifocals designed to provide intermediate vision in addition to distance and near.
- 1959 (Varilux, Essilor): Introduction of the first commercial progressive addition lens (PAL), eliminating segment lines and offering a continuous power change.
- Modern era: Freeform digital surfacing allows highly customized PALs with wider fields and reduced distortions.
4) Bifocal Lenses
Definition: A lens with two distinct optical powers—one for distance and one for near. The powers are separated by a visible dividing line.
Types of Bifocals
- Franklin (Executive) Bifocal:
- Two halves of lenses joined horizontally; full width near segment at bottom.
- Advantages: Wide near field, simple design.
- Disadvantages: Visible dividing line, heavy, prone to dirt at junction.
- Cemented (Glued) Bifocal:
- A small near segment cemented onto distance lens.
- Advantages: Easy to make.
- Disadvantages: Poor cosmetics, segment can detach.
- Fused Bifocal:
- Near segment made of high-index glass fused into distance lens cavity.
- Advantages: Good cosmetic appearance, durable.
- Disadvantages: Limited to glass; heavier than plastics.
- Solid Bifocal:
- Distance and near segments ground in a single piece of material.
- Common in CR-39 and plastic bifocals.
Segment Shapes
- Round segment (Kryptok): Circular near portion; cosmetically neat but small near area.
- D-segment (Flat-top): Most common design; wide reading area with flat top.
- Executive (Franklin): Full-width near field; large reading area but thick and heavy.
5) Trifocal Lenses
Definition: A trifocal lens provides three focal zones—distance, intermediate, and near—using two visible segment lines.
- Intermediate segment: Usually occupies the area between distance and near segments, about 7–14 mm deep.
- Uses:
- Beneficial for office workers and professions requiring intermediate vision (e.g., computer use, music reading).
- Disadvantages:
- Two visible lines—cosmetically less appealing.
- Limited intermediate area compared to PALs.
6) Progressive Addition Lenses (PALs)
Definition: PALs provide a continuous gradient of power from distance through intermediate to near, with no visible dividing line.
- Design: Progressive corridor (~14–20 mm) where power increases gradually from distance Rx to near ADD.
- Advantages:
- Cosmetically appealing—no segment line.
- All distances covered (distance, intermediate, near).
- Smoother adaptation compared to abrupt bifocal jumps.
- Limitations:
- Unwanted astigmatism at lens periphery → “swim effect.”
- Smaller near zone compared to bifocals.
- Requires careful fitting (monocular PDs, accurate fitting height).
- Modern Advances:
- Freeform PALs: Customized to wearer’s prescription, frame, and posture; provide wider fields.
- Occupational PALs: Special designs for computer use, office, or sports.
7) Design Variations & Optical Principles
- Bifocal image jump: Caused by sudden prismatic change at segment top; minimized in round-seg designs but still present.
- Trifocal zones: Intermediate addition typically 50% of near add.
- Progressive unwanted astigmatism: Mathematical consequence of surface geometry; balanced by lens design trade-offs.
8) Clinical Applications
- Bifocals: Patients needing clear distance and near without concern for intermediate (e.g., elderly, readers).
- Trifocals: Patients with significant intermediate demands (computer users, machine operators, musicians).
- PALs: Patients desiring cosmesis, multiple focal distances, and convenience in a single lens.
9) Dispensing Considerations
- Correct monocular pupillary distance (PD) measurement is critical, especially for PALs.
- Bifocal segment height usually 2 mm below lower lid margin; PAL fitting cross at pupil center.
- Frame choice: Sufficient vertical depth required (≥28 mm for PALs).
- Discuss adaptation time, especially for PAL wearers.
10) Limitations & Challenges
- Bifocals: Poor cosmesis, lack of intermediate vision, image jump.
- Trifocals: Two lines, limited intermediate width, adaptation issues.
- PALs: Peripheral blur, adaptation period, higher cost.
Progressive Lenses: Design and Optics
1) Introduction
Progressive addition lenses (PALs) are ophthalmic lenses that provide a continuous change in power from the distance portion to the near portion without any visible dividing line. They are designed to eliminate the cosmetic drawbacks and abrupt image jumps of bifocals and trifocals, offering presbyopic patients a more natural visual experience.
The first commercial PAL was introduced in 1959 by Essilor (Varilux 1). Since then, progressive lens technology has advanced rapidly, incorporating digital freeform surfacing, individualized designs, and specialized occupational variants.
2) Concept of Progression of Power
- Power gradually increases from the upper (distance) portion to the lower (near) portion through a vertical corridor of progression.
- The corridor length is the vertical distance over which the add power builds up (commonly 14–20 mm).
- Intermediate powers are distributed along this corridor, allowing comfortable vision at all distances.
- Unlike bifocals, there is no abrupt change—making transitions smoother.
3) Optical Principles of PALs
The power progression in PALs is achieved by continuously varying surface curvature across the lens. This design inherently introduces areas of unwanted astigmatism due to surface geometry limitations.
- Fundamental principle: According to Minkwitz theorem, as power changes vertically, unwanted astigmatism is produced laterally in proportion to the rate of power change.
- Result: Clear central corridor but blurred/distorted peripheral regions, known as “swim effect.”
- Design challenge: Balancing corridor width, peripheral blur, and ease of adaptation.
4) Zones of a Progressive Lens
- Distance Zone: Upper portion designed for distance vision. Typically wide and clear.
- Progressive Corridor: Vertical strip where power gradually increases; narrow, with peripheral astigmatism.
- Intermediate Zone: Middle area of lens corresponding to computer/desk work distances.
- Near Zone: Lower portion with full add power for reading.
- Unwanted Astigmatism Areas: Peripheral lateral regions of blur/distortion.
5) Design Types of PALs
- Hard Design:
- Rapid power change over short corridor.
- Wider near and distance zones.
- More peripheral blur (harsh swim effect).
- Preferred for experienced bifocal wearers.
- Soft Design:
- Gradual power change over longer corridor.
- Narrower near zone, but less peripheral blur.
- Easier adaptation, especially for first-time PAL users.
- Balanced/Modern Design:
- Compromise between hard and soft designs.
- Customized depending on patient’s needs.
6) Manufacturing Techniques
- Molded (Conventional) PALs: Front surface molded with progressive design; limited customization.
- Freeform/Digital Surfacing:
- Computer-controlled surfacing of back or both surfaces.
- Allows high customization for prescription, frame shape, and wearer posture.
- Provides wider fields and reduced peripheral astigmatism.
7) Advantages of PALs
- Cosmetically appealing—no segment line.
- Provide vision at all distances: distance, intermediate, and near.
- No image jump as in bifocals/trifocals.
- Modern designs minimize adaptation difficulties.
8) Limitations and Challenges
- Peripheral blur and distortion (“swim effect”).
- Smaller near zone compared to bifocals.
- Precise fitting required—errors in PD or fitting height cause non-tolerance.
- Higher cost than bifocals and single vision lenses.
9) Modern Digital & Freeform PALs
Freeform lens technology has revolutionized PALs:
- Individualized designs: Customized for wearer’s prescription, PDs, pantoscopic tilt, vertex distance, and frame wrap.
- Occupational PALs: Special progressives for computer work (enhanced intermediate zone), office PALs, and sports-specific designs.
- Short-corridor PALs: Designed for small fashionable frames with limited vertical depth.
- Advanced coatings: AR, blue-light filtering, hydrophobic layers enhance comfort and durability.
10) Dispensing Considerations
- Accurate monocular pupillary distance (PD) is essential.
- Fitting cross must align with pupil center in primary gaze.
- Minimum fitting height required (usually ≥ 18 mm, short-corridor PALs allow ~14 mm).
- Frame choice: Ensure adequate vertical depth and stable fit.
- Explain adaptation period—patients may initially experience peripheral swim effect.
- Occupational needs should guide design selection (standard vs office vs sports PALs).
11) Clinical Applications
- General presbyopia correction: Most popular solution for modern lifestyles.
- Office workers: Special office PALs with wider intermediate zones.
- Cosmetic-conscious patients: Desire seamless appearance with no segment lines.
- Active lifestyles: Sports-specific progressives with wide distance fields and stable peripheral optics.
