Thick Lenses

Question 1

Back Vertex Power

Answer 1

• BS states that lenses are numbered in terms of their back surface power, F’v
• isnt necessarily the same as the back surface power
• takes into account the thickness along the axial part of the lens and also takes into account the refractive index
  F’v = power of the lens at a2

Question 2

Thickness

Answer 2

When light travels through a medium other than air, the real thickness of the material (t) can be replaced by an equivalent thickness of air (d)
d=t/n

Question 3

Accurate Transposition

Answer 3

• We can compensate a surface for the change in rx due to thickness of lens
• if the thickness and one surface power are known, then to make a lens of given back vertex power, the other surface power needs to be compensated for the thickness and worked with a diff tool

Question 4

Factors affecting lens thickness

Answer 4

• 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 5

Lens shape

Answer 5

Influences thickness due to relationship with lens diameter and decentration

Question 6

Lens Diameter

Answer 6

• 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 7

Lens size cut from the same size diameter

Answer 7

• 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 8

Decentration

Answer 8

= (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 9

Minimum size uncut

Answer 9

=(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 10

ED

Answer 10

= 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