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