topic 5B physical characteristics of ophthalmic lenses (cyl lens forms)

Question 1

what is a astigmatic lens?

Answer 1

These lenses produce focal lines instead of single focal point.

Vision of the wearer
will be affected by lens rotation

No focal point image, Line image is formed with the same orientation of the axis

Question 2

what are the 4 Types of Astigmatic Lenses

Answer 2

1. Plano-cylindrical lens
2. Bi-cylindrical lens (crossed cylinder form)
3. Sphero-cylindrical lens
4. Toric lens (one sph, one toroidal)

Question 3

what is a plano-cyl lens?

Answer 3

The surface of the lens is part of a cylinder.
The posterior surface is flat hence has zero power.

Eg. Plano X 180 & -1.50 X 90

Question 4

what is a bi-cyl lens?

Answer 4

The lens has two cylindrical surfaces, one at the front surface and the other at the back surface.

Usually the curves (power meridians) are at right angles to one another.
one convex the other concave, of equal strength, placed with their axes at right angles to each other

Eg -0.50 X 180 & +0.50 X 90
AKA. JCC (Jackson crossed cylinder)

Question 5

what is a sphero-cylindrical lens?

Answer 5

There is a flat lens with one spherical surface and one cylindrical surface.

Eg +3.00DS & pl/-0.75 X 180 or - 4.50DS & pl/+1.25 X 90

Question 6

what is a toric lens?

Answer 6

Bent lens having one spherical and one toroidal surface

Eg -2.50 / -2.50 X 180 & +1.00 DS or -4.50DS & -3.00 / -1.50 X 90 (back surface toric)

**None of the front or back surfaces has a zero power meridian

Currently, all astigmatic lenses are made in back toric form