OL 5

Question 1

Which kind of lens gives better image quality particularly in the lens periphery?

Answer 1

a meniscus lens

Question 2

what does a toroidal surface represent?

Answer 2

a bent cylinder

Question 3

In a toroidal surface, The lowest power (flattest curve) is the called the what?

Answer 3

base curve

Question 4

in a toroidal surface, The highest power (steepest curve) is the called the what?

Answer 4

cross curve

Question 5

how is a toriodal surface formed?

Answer 5

by rotating an arc of a circle in the plane of the circle but lying outside it.

Question 6

what are the three forms of toroidal surface?

Answer 6

capstan
barrel
tyre

Question 7

describe capstan formation?

Answer 7

• Concave in one principal meridian
• Convex in second principal meridian
• Axis of rotation and centre of curvature of the generating arc lie on opposite sides of the vertex

Question 8

describe barrel formation?

Answer 8

• Axis of rotation lies between the vertex of the generating arc and its centre of curvature

Question 9

describe the tyre formation?

Answer 9

• Axis of rotation and the vertex of the generating arc lie on opposite sides of the centre of curvature of the generating arc

Question 10

which toroidal surface is barely used in ophthalmic lenses?

Answer 10

capstan (sometimes used for bifocals)

Question 11

which toroidal surface gives good off axis performance but not suited for mass production?

Answer 11

barrel formation

Question 12

which toroidal surface is used for mass production and the most common surface found on ophthalmic lenses

Answer 12

tyre formation

Question 13

in a toric lens, where is the base curve, cross curve and sphere curve?

Answer 13

sphere curve is the power of the sphere, base curve is lowest power and cross curve is the highest power, usually with the sphere curve is specified.