topic 5A physical characteristics of ophthalmic lenses (sph lens forms and factors affecting lens thickness) Flashcards
what defines a lens form?
The front and back surface (curvatures) powers, together with the central thickness define the lens form
what are the 6 different spherical lens forms?
- biconcave
- biconvex
- plano-concave
- plano-convex
- positive meniscus
- negative meniscus
what is a spherical lens?
All meridians will have same curvature and therefore same power
Vision of the wearer will not be affected by lens rotation.
These lenses produce sharp focal points when parallel light from infinity enters the lenses.
The front and back surfaces are generated by spherical surfaces, hence each surface will have only one surface power.
how to reduce lens thickness by adjusting RI?
For Negative lenses, higher the power, greater the edge thickness. So giving a high-index lens can help reduce the edge thickness.
For Positive lenses, higher the power, greater is the center thickness. So, giving a high-index lens can help
reduce the center thickness.
***Higher the Refractive index, Thinner the lens
What is precal and how does it help to reduce thickness in plus lens
precal= pre calculated custom made + lenses where diameter is reduced and base curve altered to get thinner + lens
***central thickness can decrease by decreasing the diameter of lens
how does diameter affect thickness of minus lens?
edge thickness of minus lens can decrease by decreasing the diameter of lens
**note that If edge thickness increase, centre thickness will increase as well.
Same will apply to both plus & minus lenses.
how does base curve affect plus lens thickness?
So increasing the base curve of a plus lens, increases the lens centre thickness.
**note that If the frame chosen is curved, a steeper base curve is needed
What is an ophthalmic lens?
Lenses used in optical trade AKA eye-glasses
