2. ASTIGMATIC LENS
In an astigmatic lens, all the meridians do
not have the same curvature and a point
image of a point object cannot be formed.
The lens effectively gives a minimum
power in one direction, gradually
changing to a maximum power in the
other, at right angle to the first.
4. There are two types of astigmatic
lenses, namely
-Cylindrical lenses
-Toric lenses
5. CYLINDRICAL LENSES
Cylindrical lenses have one plane
surface and the other curved surface
forming the part of a cylinder.
Thus, in one meridian the lens has
no vergence power which is known
as the axis of the cylinder.
6. The axis of a cylinder is
always at right angles to the
power meridian of the
cylinder.
7. In the meridian at right angle to the axis,
the cylinder acts as a spherical lens.
The total effect is the formation of a line
image of a point object. This is called
focal line. It is parallel to the axis of the
cylinder.
12. A lens with purely cylinder power would
be described as, say – 4.00Dcyl(diopter
cylindrical)
Angles up to 180 are used for the axis of
cylinder, the full 360° protractor is only
required for the base direction of prism.
13. When describing a horizontal
cylindrical axis, it is conventional to
use the angle 180°, rather than zero,
15. ORIENTATION OF CYLINDRICAL
AXES
A good way to recognize cylindrical
axes from dispenser’s side is to view
the back of your left hand; the thumb
will point to the zero degree.
Thumb will point to zero degrees
16. TORIC LENSES
Toric lenses, also known as sphero-
cylinder lenses can be explained as
spherical lens that has been placed in
contact with a plano-cylinder lens.
17. Since a plano-cylinder lens has no power
along its axis meridian, the power along
the axis meridian of the combination
must result from spherical element alone.
The power along the other principal
meridian of the lens, at right angles to the
axis meridian of the cylinder surface, is
the sum of the sphere and cylinder.
18. Under the rotation test, it exhibits
scissor movement in the same way as a
plano-cylindrical lens and under the
movement test ; it exhibits movement
along each of its principal meridians.
19. The power of a sphero-cylindrical
lens is expressed by stating the
power of the spherical component
first, followed by the power of the
cylindrical component, and finally
the direction of the cylindrical axis.
Thus, the specification
– 3.00/– 2.00 × 90º
21. It signifies that spherical
component of the lens is – 3.00D
The cylindrical component is –
2.00DC
The axis of the cylindrical
surface lies along the 90º
meridian.
22. On representing the power of the
sphero-cylindrical lens by means of
optical cross, the principal meridians
show – 3.00D on vertical meridian
and – 5.00D on the horizontal
meridian.
24. STURM’S CONOID
The pencil of light that results from refraction
at an astigmatic lens does not focus as a point,
the interval between two line foci is called the
‘Interval of Sturm.
The best focusing occurs somewhere inside the
interval of Sturm. This point is called the
‘Circle of least confusion.
25. The complete envelop of the light
near the circle of least confusion is
called the ‘Sturm’s Conoid.
26. DETECTION OF LENS
Trial lens hand neutralizationTrial lens hand neutralization
Lensometer / FocimeterLensometer / Focimeter
Geneva lens measure or Lens clockGeneva lens measure or Lens clock
27. DETECTION OF
CYLINDRICAL LENS
In order to determine the cylinder lens and to
detect its dioptric strength.
Hold the lens a few inches away from your
eyes.
sight a straight lined object, such as window or
the door frame.
28. Rotate the lens slowly as you would turn a
steering wheel – first to the right (clockwise),
then to the left (anti-clockwise)
If a section of the door frame appears slanted,
it establishes that the lens under examination is
a cylindrical lens. This is called “scissors like
movement.
29. PLANO-CYLINDRICAL
Now, neutralize both the meridians separately.
If the movement in one meridian is nil, it is a
plano-cylindrical lens and nil movement
meridian is taken as the axis of the lens.
30. SPHERO-CYLINDRICAL LENS
If both the meridians show movement, it is
a sphero - cylindrical lens.
The weaker movement meridian is taken
as the axis of the lens under observation.
Neutralize both the meridians as before to
determine exact spherical and cylindrical
elements.
31. TRIAL LENS HANDTRIAL LENS HAND
NEUTRALIZATIONNEUTRALIZATION
For a toric lens, rotational movementFor a toric lens, rotational movement
is used to find the axisis used to find the axis
If observe “against movement” -If observe “against movement” -
use minus cylinder axis.use minus cylinder axis.
If observe “with movement” - useIf observe “with movement” - use
plus cylinder axis.plus cylinder axis.
32. TRIAL LENS HAND NEUTRALIZATIONTRIAL LENS HAND NEUTRALIZATION
For a toric lens, rotational movement isFor a toric lens, rotational movement is
used to find the axisused to find the axis
34. TRIAL LENS HAND NEUTRALIZATIONTRIAL LENS HAND NEUTRALIZATION
Toric lensToric lens