Chapter 5- Dispensing part II (1) Flashcards
What are the factors we need to consider in choosing frames and lenses?
1- Distance Rx- high or low Rx
2- The patient’s current type of specs- don’t want to change it to something too drastic that will make them uncomfortable.
3- Occupation- Safety considerations - certain people may need impact or chemical resistance in their glasses
- Visual considerations- may have anti- reflective coating which will help assist them when working at a computer screen.
What are the considerations for distance Rx (high prescription)?
1- Lens thickness
2- Lens weight
3- Magnification/ minification- this will either make the eyes look bigger or smaller through the lens
The smaller the uncut lens…
The thinner the lens will look when it’s cut into a frame.
What is an uncut lens?
The big lens that the manufacturers produce that will be cut down to produce the lens that goes in the frame.
When it comes to lenses and frames, what should you always aim for?
Aim for the smallest frame and minimum decentration of lenses in frame.
What is decentration?
The difference of the distance between the optical centre and pupil distance.
What is the optical centre?
The point lying on the lens where light passes through without undergoing any deviation.
What do you want the boxed centre distance to be equal to?
The patient’s PD
What is the boxed centre distance
The distance from the centre of one lens to the centre of the other lens.
In a myopic lens, where is the thickest part?
It is thickest at the edges
In a hypermetropic lens, where is the thickest part?
It is thickest in the middle
Why does an oval type shape work well for a myopic lens?
Because this will cut out the thick bits from the top and the bottom of the lens.
Why does an oval type shape for a hypermetrope not work as well?
Because it is cutting into the thick part of the lens which means you end up with quite thick lenses at the top and the bottom.
What does an oval shape frame look better on aesthetically?
A myopic lens
What is the formula for refractive index?
n = Velocity of light in air/ Velocity of light in material
What does it mean if the n value for the refractive index is high?
It means that the material slows light down a lot as it passes through a material, which means it bends light more.
So a higher n value/ refractive index means that the material is better at refracting light
What can you do to lens curvature and thickness if you have a high refractive index?
It can be reduced
What are the advantages of a high refractive index on lenses?
1- Thinner lens
2- Lighter lens
What are the disadvantages of a high refractive index?
1- The high refractive index materials are more expensive than the standard ones
2- More reflections from lens surfaces so an antireflective coating is necessary
What do aspheric lenses do?
1- Creates a thinner, flatter lens profile
2- Reduces spectacle magnification- reduces the size of images seen
3- Good for high powered Rx (prescription)
What are power rings used for?
For patients with a high myopic (minus prescription).
What does Px stand for?
Patient
How do myopic lenses make the world look?
Makes the world look smaller to px, and px’s eyes look smaller to everyone else (minification)
How do hypermetropic lenses make the world look?
It magnifies everything
What type of spectacle lenses might be appropriate for the following presbyopic patients?
1- 78 year old avid reader with osteoarthritis in the neck and upper spine
2- 68 year old office worker who drives and likes fiddling with her ipad at home
3- A 60 year old libarian who is struggling to see the titles of books when putting them away on higher shelves
1- Single vision lens, readers and distance
2- Progressive addition lenses
3- A trifocal lens with the reading lens on top.
