High rx dispensing

Question 1

What issues can a px face when they have a high rx?

Answer 1

Thicker/heavier lenses
Magnification or minification of face through lenses

Question 2

Define refractive index

Answer 2

The ratio of velocity of light in a vacuum to the velocity of light in the medium

Question 3

What happens to TCA with an increase in RI?

Answer 3

Increases

Question 4

How is TCA measured?

Answer 4

V-value

Question 5

Define density

Answer 5

mass per unit volume
i.e. how much of the material is within each unit of volume

Question 6

Define reflectance

Answer 6

% of reflection at each surface

Question 7

Define specific gravity

Answer 7

density of a material relative to the density of water

Question 8

What does the weight of a lens depend on?

Answer 8

Density and specific gravity

Question 9

What is the n of crown glass?

Answer 9

1.523

Question 10

What is the n of CR39?

Answer 10

1.498

Question 11

What is the n of Polycarb?

Answer 11

1.586

Question 12

What is the n of trivex?

Answer 12

1.532

Question 13

What is the density of crown glass?

Answer 13

2.5

Question 14

What is the density of CR39?

Answer 14

1.3

Question 15

What is the density of polycarb?

Answer 15

1.2

Question 16

What is the density of trivex?

Answer 16

1.1

Question 17

What is the v-value of crown glass?

Answer 17

59

Question 18

What is the v-value of CR39?

Answer 18

58

Question 19

What is the v-value of polycarb?

Answer 19

30

Question 20

What is the v-value of trivex?

Answer 20

46

Question 21

What is the reflectance of crown glass?

Answer 21

4.3

Question 22

What is the reflectance of CR39?

Answer 22

4.0

Question 23

What is the reflectance of polycarb?

Answer 23

5.2

Question 24

What is the reflectance of trivex?

Answer 24

4.4

Question 25

What measurements do you need to take for SV high index dispenses?

Answer 25

Mono PD Heights to pupil centres with dispenser's rule applied BVD

Question 26

What measurements do you need to take for BIF high index dispenses?

Answer 26

Binoc PD Heights to lash line Inset BVD

Question 27

What measurements do you need to take for Vari high index dispenses?

Answer 27

Mono PD Heights to pupil centre BVD

Question 28

At what rx should BVD be taken?

Answer 28

+/- 5D

Question 29

If you move a lens away from the eye it becomes more:

Answer 29

Plus

Question 30

If you move a lens towards the eye it becomes more:

Answer 30

Minus

Question 31

Why should BVD be kept to a minimum?

Answer 31

Maximise FOV Reduce distortion Reduce chromatic abberation Reduce mag/minification

Question 32

Why is the correct centration important?

Answer 32

Rx most effective when looking through centre of lens Reduces unwanted induced prism Reduces possibility of ghost images Reduces one sided thickness

Question 33

What do you need to consider in terms of the lens for high myopic dispenses?

Answer 33

Lens material/form High index lenses High v-value AR coat aspheric/lenticulars

Question 34

What do you need to consider in terms of the frame for high myopic dispenses?

Answer 34

Small frame (to cut off edge thickness) Centration as central as poss Thick rims to hide thickness of lenses (e.g. plastic) If nasal thickness will affect positioning of nosepads

Question 35

What benefits could lenticular lenses have to a high minus px?

Answer 35

Reduced aperture reduces edge thickness Good VA and FOV Controlled abberations Can choose from wider range of frames Reduces face minification Ok cosmetically Cheaper than high index

Question 36

Who are lenticulars normally dispensed to?

Answer 36

>-12.00D (more common if >-20.00D) Special assessment px's

Question 37

What do you need to consider in terms of the lens for high hyperopic dispenses?

Answer 37

Lens material/form High index lenses High v-value AR coat Aspheric/lenticulars Small blank size

Question 38

What do you need to consider in terms of the frame for high hyperopic dispenses?

Answer 38

Centration as central as poss Small eye size Good fit to avoid slipping Lightweight frame material Regular/Fixed/strap bridge

Question 39

What benefits could lenticular lenses have to a high plus px?

Answer 39

Reduced aperture reduces centre thickness Good VA and FOV Controlled peripheral abberations Can choose from wider range of frames Reduces face magnification Ok cosmetically Cheaper than high index Reduces jack in the box effect

Question 40

Which px's are lenticular bifocals useful for?

Answer 40

High powered presbyopes

Question 41

What measurements are needed for lenticular bifocals?

Answer 41

Mono PD Heights of seg top Inset BVD