Reflection from Spectacle Lens Surfaces: Ghost Images & Bifocal Reflections
1) Introduction
Every lens surface reflects a portion of incident light. For typical crown glass (n≈1.523), reflection loss at each surface is ~4%. Thus, a simple pair of uncoated spectacles can lose ~8% of light by reflection alone. These reflections can also produce ghost images, glare, and reduced contrast, affecting both vision and cosmesis.
2) Basic Optics of Reflection
- At any interface between two media of different refractive indices, a portion of light is reflected according to Fresnel’s equations.
- Reflectance (R) ≈ ((n1 - n2)/(n1 + n2))² for normal incidence.
- For air (n=1.0) to glass (n=1.5): R ≈ (0.5/2.5)² ≈ 4% per surface.
- Higher index materials reflect more light (e.g., n=1.7 lens has ~6% reflection per surface).
3) Ghost Images
Ghost images are secondary, faint images caused by reflections from lens surfaces (usually the back surface).
- When bright light sources are present (street lamps, headlights), reflections from the concave back surface can project a secondary out-of-focus image onto the retina.
- These ghost images are often displaced and blurred, reducing visual comfort, especially at night.
- Factors influencing ghost images:
- Lens curvature and base curve—steeper curves increase risk.
- High-index lenses—greater reflectance increases ghosting.
- Lack of anti-reflection (AR) coatings.
- Cosmetic issue: Ghost reflections on lens front surface make wearer's eyes less visible to others.
4) Reflections in Bifocal Lenses
In bifocals, the dividing line between distance and near segments introduces additional reflections due to changes in refractive index and surface discontinuity.
- Executive (Franklin) bifocals: Junction line often produces a sharp reflected image, visible to others and sometimes disturbing to wearer.
- Round and D-segment bifocals: The boundary between segments may reflect light, producing noticeable glints.
- These reflections are generally more of a cosmetic problem than a visual one, but can distract wearers sensitive to glare.
5) Consequences of Lens Reflections
- Reduced Transmission: Each reflection reduces light reaching retina → decreased contrast sensitivity.
- Glare & Halos: Strong reflections cause glare, troublesome in night driving.
- Ghost Images: Faint double images impair comfort.
- Cosmetic Effects: Reflections obscure wearer’s eyes, reducing eye contact and appearance.
- Patient dissatisfaction: Especially in high-index lenses without coatings.
6) Methods of Reducing Reflections
- Anti-Reflection (AR) Coatings:
- Thin-film interference layers reduce surface reflectance to < 1%.
- Improve light transmission, reduce glare, eliminate ghost images.
- Modern AR stacks also include scratch-resistant and hydrophobic topcoats.
- Lens Material Selection: Lower index materials reflect less light.
- Lens Curvature: Proper base curve selection minimizes ghosting.
- Tinted/Polarized Lenses: Reduce intensity of reflections but do not eliminate them.
7) Clinical & Dispensing Relevance
- Patients with high prescriptions or high-index lenses should always be advised AR coatings.
- Night drivers, computer users, and students particularly benefit from AR coatings to reduce glare and ghosting.
- Bifocal wearers should be informed about possible reflections at the dividing line; PALs avoid this problem.
- AR-coated lenses improve cosmesis, allowing clear view of wearer’s eyes.
Lens Coatings: Anti-Reflection, Mirror, Hard Multi-Coating (HMC) & Hydrophobic Coatings
1) Introduction
Modern spectacle lenses are almost never left uncoated. Coatings enhance optical performance, durability, and cosmetic appeal. Among these, anti-reflection, mirror, HMC, and hydrophobic coatings play a central role in ensuring patient satisfaction and long-term lens performance.
2) Anti-Reflection (AR) Coating
Principle: Uses thin-film interference to reduce surface reflections. A thin layer (e.g., MgF₂, n≈1.38) of ~¼ wavelength thickness causes destructive interference of reflected light waves.
- Without AR: Crown glass reflects ~4% per surface, high-index ~6–8%.
- With AR: Reflections reduced to <1 increases="" to="" transmission="">99%.
- Benefits:
- Reduces glare and ghost images.
- Improves contrast sensitivity, especially at night.
- Cosmetically superior—eyes more visible to others.
- Indications: High-index lenses, night drivers, computer users, students, photographers.
- Modern AR stacks: Multiple layers tuned for broad-spectrum reduction, often with added blue-light filtering and UV protection.
3) Mirror Coating
Principle: Thin-film coatings that reflect a portion of visible light, reducing transmission. Achieved with alternating dielectric layers deposited by vacuum evaporation.
- Types: Silver, blue, green, red, gold mirror coatings.
- Applications:
- Snow fields, deserts, high-altitude glare environments.
- Sports eyewear, fashion sunglasses.
- Benefits: Significantly reduces brightness, cosmetic appeal.
- Limitations: Show fingerprints and scratches more easily; usually combined with hydrophobic topcoat.
4) Hard Multi-Coating (HMC)
Definition: A multilayer coating system applied to plastic lenses to improve durability and performance.
- Hard Coat: Base layer (siloxane or UV-cured resin) that resists scratches; essential for soft materials like CR-39 and polycarbonate.
- AR Layer: Applied over hard coat to reduce reflections.
- Top Layer: Hydrophobic/oleophobic to resist water and smudges.
- Importance: Provides a package solution for modern lenses—scratch resistance, glare reduction, and easy cleaning.
5) Hydrophobic / Oleophobic Coatings
Principle: Surface layer with very low surface energy (e.g., fluoropolymers) causes water and oil to bead up rather than spread.
- Hydrophobic: Repels water, prevents fogging, easier cleaning.
- Oleophobic: Resists fingerprints, smudges, and skin oils.
- Application: Always applied as the topmost layer of the AR/HMC stack.
- Maintenance: Reduces need for frequent cleaning, prolongs lens life.
6) Coating Stacks & Manufacturing
- Vacuum deposition: Coatings applied in ultra-clean vacuum chambers.
- Multilayer stacks: Typical AR stack has 5–7 layers of alternating high- and low-index films.
- Combined packages: Modern lenses sold as “Super HMC” or “Premium AR” with all-in-one protection (scratch + AR + hydrophobic).
7) Advantages of Coated Lenses
- Improved light transmission and visual comfort.
- Reduced ghost images and glare.
- Better night driving and computer use.
- Scratch resistance prolongs life of lenses.
- Hydrophobic surfaces stay cleaner for longer.
- Cosmetically superior appearance.
8) Limitations & Care
- AR lenses may show smudges more readily without hydrophobic topcoat.
- Coatings can peel or craze if exposed to high heat (e.g., car dashboard).
- Require proper cleaning (microfiber cloth, mild soap)—avoid strong chemicals.
- Mirror coatings may scratch more easily without protective layers.
9) Dispensing Relevance
- Always recommend AR/HMC coatings for high-index lenses and for patients sensitive to glare.
- Night drivers: Emphasize AR for safety and comfort.
- Children: Scratch-resistant hard coats essential.
- Sports eyewear: Mirror coatings + hydrophobic layers improve performance.
- Computer users: Blue-light AR coatings may reduce digital eye strain complaints.
