L7 Thick Lenses contd

Question 1

Factors affecting lens thickness

Answer 1

• lens thickness can be related to centre thickness or edge thickness
• back vertex power (F’v)
• surface powers
• refractive index
• lens form
• lens shape
• decentration
• minimum size uncut
• minimum centre/ edge thickness (mechanical stability)

Question 2

Lens shape

Answer 2

Influences thickness due to relationship with lens diameter and decentration

Question 3

Lens Diameter

Answer 3

• specatacle lenses are cut from a pre made lens blank
• these lens discs (blanks/uncuts) are made in a range of diff diameters - choice of diameter depends on rx and frame choice
• generally, for a positive lens, if we can have a smaller blank size, that will affect the overall thickness without even considering changes in lens materials
• blank sizes tend to go up in steps of 5mm and range available depends on the lab you’re sourcing from and the material lens is made from
• majority range from 50mm to 70/75mm

Question 4

Lens size cut from the same size diameter

Answer 4

• for a negative lens = a larger lens has thicker edges and a smaller lens size has thinner edges
• for a positive lens = a larger lens has thinner esges and a smaller lens size has thicker edgs
• going for a smaller frame size results in naturally thinner lens size

Question 5

Decentration

Answer 5

= (distance bet lenses - px pd)/2
= relationship bet the box centre of the frame and then oc of the lens
- in a perfect dispense both of them would coincide
- if the lens does have to be decentered, it will alter the thickness of the lens
- if we’re cutting centrally through the lens, thickness is gonna be relatively equal on both sides
= the displacement, horizontal and/or vertical of the centration point from the standard oc position

Question 6

Minimum size uncut

Answer 6

=(mono decentration x2) +ED +2mm
=is the minimum size of the uncut lens that can be used to cut down to the chosen lens shape
2mm to allow the lab to make a mistake

Question 7

ED

Answer 7

= Effective diameter of the lens - measurement at its widest point
= measuring from centre to one side and multiplying by 2
= can be horizontal/vertical eg for a rectangular frame, widest part of frame will be horizontal