Ophthalmic lenses theory

• 3 different waysvwe can produce prism when dispensing


  • Surfacing into the lens

  • decentration of the lens

  • Fresnel prism
• Write short notes explaining why prism may be dispensed

  • Aid convergence for near vision

  • Help to allign visual and optical axis

  • Restore binocular vision
• Explain how the uncut diameter impacts the centre thickness of a plus lens

  As we increase the blank size, the centre thickness increases
• State 3 advantages of freeform surfacing

  • Reduces aberrations

  • Better optical properties

  • Reduced number of surfacing tools
• What symptoms is the patient likely to experience without prisms

  Headaches

  Double vision

  Visual discomfort
• Define prismatic effect


  The deviating power of a lens at any given point is its prismatic effect at that point
• 2 advantages and 2 disadvantages of using polycarbonate as a lens material

  + impact resistant

  + lightweight

  × Scratches easily

  × Low v value
• Explain how different prismatic effects may impact on the patients vision

  Horizontal diplopia or vertical diplopia
• Describe the shape of each surfacing tool

  Plus = hollow tool

  Minus = mushroom tool
• Comment on how a glazing error causing vertical and horizontal differential prismatic effects may affect vision

  Double vision

  Headaches

  Visual discomfort
• Write short notes on what steps could be taken to improve cosmetic appearance of specs with high minus

  High index material

  Plastic frame

  Aspheric lenses

  Round frame style
• 2 advantage and 2 disadvantages of plastic lenses

  Safer than glass/ tougher

  Lightweight

  Fewer values of n
• What is meant by meniscus and toric lenses

  Meniscus is a curved lens whose both surfaces are spherical

  A toric lens is used to correct astigmatism, a curved lens whose surfaces: one will be spherical and other toroidal
• Define refractive index

  Ratio between speed of light in a vacuum and speed of light in particular medium
• Describe how, in practice, you could distinguish between an astigmatic and spherical lens without a focimeter

  Lens measure

  Solid movement on rotation
• State 3 limitations of the lens measure

  High degree of accuracy not to be expected

  All 3 points must be in contact with lens

  Cannot provide exact axis of cyl
• Describe base setting, apical angle and angle of deviation

  Base setting refers to direction of the prism base

  Apical angle is the angle of the prism apex: the larger the angle, the greater the deviation

  Angle of deviation refers to displacement of an image: Ray of light will be diverted towards base

  d=(n-1)a
• Define the prism dioptre

  Unit specifying amount of light deviation by an ophthalmic prism

  1 prism dioptre represents a deviation of 1 cm on a flat surface 1m away from prism
• Describe the optical principles of the lens measure and its use in practice

  Measures approximate power of lens surface

  3 pointed legs in same plane

  Outer 2 legs are fixed, centre leg is spring loaded

  Its position in relation to fixed legs is sag

  The mechanics and calibration of the instrument convert linear movement of leg into circular movement of dial which records surface power
• Write short notes on what steps can be taken to improve the cosmetic factors when dispensing a high plus lens

  Minimal decentration

  High index

  Smaller blank size

  Rounder style frames
• Explain why the uncut diameter of a lens will impact the centre thickness of a plus lens compared to a minus lens

  Minus lens = centre thickness remains the same

  Increasing uncut diameter increases centre thickness in plus lens
• 2 types of single vision lenses that cannot be decentred

  Aspherics

  Plano
• What are lenticular lenses for

  High prescription

  With plus usually aphakic
• How do lenticulars allow for larger eye size frames

  Reduce centre thickness for plus

  Reduce edge thickness for minus
• Solid (conventional) lenticulars

  Wider FoV

  Aperture/margin boundary cosmetically noticeable

  Ring scotoma
• Blended Lenticulars

  Cosmetically more acceptable

  More expensive

  No ring scotoma but peripheral distortion at boundary