1. Terminology (COMPLETE) Flashcards
define spheres
lenses which form a point image of a point object. generally have rotationally symmetrical surfaces, of which one or both could be aspheric rather than spheric.
define astigmats
lenses which form one or two line images of a point object. these have at least one non-rotationally symmetrical surface in the form of a cylinder or toroid. possibly one rotationally symmetrical surface.
often referred as sphero-cylinders and cylinders. “cylinders” sometimes used to cover all astigmatic lenses.
define cylindrical lenses. draw associated diagram (p.6)
produced by cutting a section of a cylinder parallel to cylinder axis. the lens has one plane surface and one cylindrical surface. the cylindrical surface is a special case of toric surface (generating arc has infinite curvature).
the cylindrical surface has no curvature (power) along a meridian parallel to axis. any such meridian is called axis meridian.
any section through the surface perpendicular to the axis intersects in a power meridian. the axis and power meridians of the surface are referred to as the principal meridians of the lens.
what is an axis meridian?
the cylindrical surface has no curvature (power) along a meridian parallel to axis. any such meridian is called axis meridian.
what are the two meridians in a cylindrical lens?
axis and power meridians.
how is a cylinder produced?
rotation of a straight line about an axis parallel to it.
cylindrical lens is…
- a name used for an astigmatic lens with the focusing properties of a cylindrical lens.
- a name sometimes used for any astigmatic lens. it is probably an abbreviation of sphero-cylindrical lens.
define crossed cylinders
two cylindrical lenses with their axis at right angles to each other
a form of representing an astigmatic prescription
define obliquely crossed cylinders
Two cylindrical lenses with their axes inclined at an angle other than 0 or 90 degrees
define sphero-cylindrical lens
- An outdated lens form, consisting of one spherical surface and one cylindrical surface
- A name used for astigmatic lenses of all forms. This is because the focal properties of astigmatic lenses can be expressed as those of a sphero-cylindrical lens.
define toric surface
A toric surface is generated when a curve is rotated about an axis which lies in the same plane as the curve, but which does not pass through its centre of curvature.
what are the three types of toric surfaces?
The three types of toric surface are the tyre, barrel and capstan formations. Only the tyre formation is generally used for spectacle lenses. For this, the axis of rotation and the vertex of the generating arc lie on opposite sides of the vertex centre of curvature of the generating arc.
describe axis notation. include diagram (p.11)
The axis direction of astigmatic lenses is specified in degrees commencing on the right-hand side of a circle and proceeding in an anti-clockwise direction. The notation is the same for right and left lenses as they are seen on a face by an observer. Only the upper half of the circle is used for specifying an axis
The horizontal meridian is referred to as the 180 meridian, not the 0 meridian. The degree sign should be omitted so that it is not mistaken for a zero
define base curve
- The surface power in the shallower principal meridian of a toric surface (tyre and barrel forms)
- The surface power which is common to a
series of lenses of different powers
define cross curve
The surface power in the steeper principal meridian of a toric surface (tyre and barrel forms)
Negative (minus)/positive (plus) toric
Negative (minus)/positive (plus) cylinder
