4.1.5 Dispenses a range of lens forms to include complex lenses, multifocals and high corrections, and advises on their application to specific patients’ needs.

Question 1

Bifocals

Answer 1

 Contain more than 1 focal power, employed to correct presbyopia
 Solid bifocals; ridge at edge of segment
o R = PLUS
o D = Minus; NVP is better controlled to avoid jump

Question 2

Varifocals (variable focus)

Answer 2

• Varifocal design can be determined based on the isocylinder plots
• Soft designs = long progression lengths, more gradual astigmatic gradient in lens periphery, but have smaller stable reading area and distortions in the distance
  o Easier to get used to – less of a change per 1mm when looking down
  o More intermediate area
  o E.g., Admin worker
• Hard = wider stable reading area, wider progression and distortion free distance area; however, the distortions are more severe as they are crammed into smaller area, E.g., driver
• Prism thinning = vertical prism to reduce thickness/weight, it is equal in both ye so prismatic effect (BD prism reduces centre thickness)
• Mono PDs, heights from HCL are required
• Varis are always aspheric – not spherical as not every point is coming from same radius of curvature
• Gradual change in curvature = distortion at edges / astigmatic periphery
• Free form lenses – require several measurements however specsavers don’t supply true free form lenses
  o Basic free forms – back surface varifocal and wider FoV
• Higher add = narrower corridor & more distortion
• Copy of electronic progressive identifier catalogue

Question 3

Reflections & anti-reflection coating

Answer 3

Theory of Anti-Reflection (AR) Coatings

• Based on Interference Principles
  
  • Light travels as a wave and reflects in-phase upon a surface, creating a high amplitude bright reflection.
  • To reduce reflections, a single-layer AR coating is applied.
  • The coating causes destructive interference, making reflected waves out-of-phase with zero amplitude.
  • Result: No reflection occurs, and more light is transmitted to the next surface.
• Key Concepts
  
  • Constructive interference → occurs on uncoated lenses (in-phase reflections).
  • Destructive interference → occurs on AR-coated lenses (out-of-phase reflections).
  • Coating thickness should be ¼ of the wavelength of light.
• Limitations of Single-Layer AR Coatings
  
  • Poor refractive index compatibility between the coating and base lens reduces efficiency.
  • Colour effects can appear at oblique angles.
• Multi-Layer AR (MAR) Coatings
  
  • Used to enhance performance by stacking alternating layers of low and high index materials.

Advantages of AR Coatings
- Improved visual performance → reduces veiling glare, increases contrast.
- Better cosmesis → reduces reflections on lenses.
- Essential for high-index lenses → increases light transmission.

Disadvantages of AR Coatings
- Smear easily → require hydrophobic coating.
- Expensive → require vacuum coating.
- Very thin (hard MAR) → prone to chemical damage (e.g., hairspray).

Question 4

High index lenses

Answer 4

Refractive Index & British Standards

• Definition: Ratio of the velocity of light in a vacuum to its velocity in a specific medium.
• Refractive Index Categories (British Standards)
  • Normal Index → 1.48 – 1.54 (Abbe ~58)
  • Mid Index → 1.54 – 1.64 (Abbe ~37)
  • High Index → 1.64 – 1.74 (Abbe ~32)
  • Very High Index → >1.74 (Abbe ~33)

Effects of Increasing Refractive Index
- Higher index = thinner lens → bends light more efficiently.
- Thinner due to increased index & flatter radii of curvature.
- Higher index = lower Abbe number → increased chromatic aberration.
- Abbe number measures light dispersion.
- Chromatic aberration occurs when white light splits into its component colors, preventing all wavelengths from focusing at the same point.
- High Abbe = desirable → means low dispersion.

Other Considerations
- MAR Coatings Required
- Higher index lenses cause more reflections due to their denser material.
- CR39 vs. High Index Lenses
- Some patients prefer CR39 over high index due to fewer off-axis aberrations.
- Density & Weight
- As refractive index increases:
- Density & aberrations increase
- Weight decreases (plastic) or increases (glass)

Question 5

features Aspheric lenses

Answer 5

Conventional vs. Aspheric Lenses

• Conventional Lenses → Spherical front surface.
• Aspheric Lenses → Front curvature gradually changes from center to periphery.

Key Features of Aspheric Lenses
- Flatter base curve → typically 2–3D less than standard lenses.
- Reduced spectacle magnification → due to thinner center thickness, improving cosmetic appearance.
- Minus Lenses → Back surface aspheric design allows for closer fit to the eye, providing a wider field of view (FoV).
- Curvature variation across the surface results in:
- Lighter lens.
- Better optical quality with reduced off-axis aberrations.

Additional Considerations
- Heights required for accurate fitting.
- Each point on the lens has a different curvature, meaning different effective optical centers (OC).

Question 6

Dispensing high index

Answer 6

• Match HCD (Horizontal Centre Distance) to PD → reduces the need for decentration.
• Use a smaller lens → helps reduce thickness & weight.
• Ensure frame PD matches patient’s PD → avoids excessive decentration, which can increase thickness.
• Use a smaller blank size for plus lenses → minimizes edge thickness.

Question 7

High minus vs high plus lenses

Answer 7

• Ring diplopia (high minus) - overlap of fields at & around the lens causing diplopia towards very periphery but hardly noticed by patients
• Ring scotoma (high plus) - as image is more magnified & converged, potential for scotoma in far periphery so that if a random object suddenly comes in from periphery after being in that blind spot, it can shock the patient. Not likely but using aspheric lens helps avoid this or even a polynomial surface