High rx dispensing Flashcards
What issues can a px face when they have a high rx?
Thicker/heavier lenses
Magnification or minification of face through lenses
Define refractive index
The ratio of velocity of light in a vacuum to the velocity of light in the medium
What happens to TCA with an increase in RI?
Increases
How is TCA measured?
V-value
Define density
mass per unit volume
i.e. how much of the material is within each unit of volume
Define reflectance
% of reflection at each surface
Define specific gravity
density of a material relative to the density of water
What does the weight of a lens depend on?
Density and specific gravity
What is the n of crown glass?
1.523
What is the n of CR39?
1.498
What is the n of Polycarb?
1.586
What is the n of trivex?
1.532
What is the density of crown glass?
2.5
What is the density of CR39?
1.3
What is the density of polycarb?
1.2
What is the density of trivex?
1.1
What is the v-value of crown glass?
59
What is the v-value of CR39?
58
What is the v-value of polycarb?
30
What is the v-value of trivex?
46
What is the reflectance of crown glass?
4.3
What is the reflectance of CR39?
4.0
What is the reflectance of polycarb?
5.2
What is the reflectance of trivex?
4.4
What measurements do you need to take for SV high index dispenses?
Mono PD
Heights to pupil centres with dispenser’s rule applied
BVD
What measurements do you need to take for BIF high index dispenses?
Binoc PD
Heights to lash line
Inset
BVD
What measurements do you need to take for Vari high index dispenses?
Mono PD
Heights to pupil centre
BVD
At what rx should BVD be taken?
+/- 5D
If you move a lens away from the eye it becomes more:
Plus
If you move a lens towards the eye it becomes more:
Minus
Why should BVD be kept to a minimum?
Maximise FOV
Reduce distortion
Reduce chromatic abberation
Reduce mag/minification
Why is the correct centration important?
Rx most effective when looking through centre of lens
Reduces unwanted induced prism
Reduces possibility of ghost images
Reduces one sided thickness
What do you need to consider in terms of the lens for high myopic dispenses?
Lens material/form
High index lenses
High v-value
AR coat
aspheric/lenticulars
What do you need to consider in terms of the frame for high myopic dispenses?
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
What benefits could lenticular lenses have to a high minus px?
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
Who are lenticulars normally dispensed to?
> -12.00D (more common if >-20.00D)
Special assessment px’s
What do you need to consider in terms of the lens for high hyperopic dispenses?
Lens material/form
High index lenses
High v-value
AR coat
Aspheric/lenticulars
Small blank size
What do you need to consider in terms of the frame for high hyperopic dispenses?
Centration as central as poss
Small eye size
Good fit to avoid slipping
Lightweight frame material
Regular/Fixed/strap bridge
What benefits could lenticular lenses have to a high plus px?
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
Which px’s are lenticular bifocals useful for?
High powered presbyopes
What measurements are needed for lenticular bifocals?
Mono PD
Heights of seg top
Inset
BVD
