ASTIGMATIC LENSESOPTOM FASLU MUHAMMED

ASTIGMATIC LENSIn 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.

There are two types of astigmatic lenses, namely

-Cylindrical lenses

-Toric lenses

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.

The axis of a cylinder is always at right angles to the power meridian of the cylinder.

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.

NOTATION FOR CYLINDER LENSES AND ORIENTATION OF AXIS

ORIENTATION OF AXIS

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.

When describing a horizontal cylindrical axis, it is conventional to use the angle 180°, rather than zero,

ORIENTATION OF CYLINDRICAL AXES ON A PROTRACTOR

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

TORIC LENSESToric lenses, also known as sphero-

cylinder lenses can be explained as spherical lens that has been placed in contact with a plano-cylinder lens.

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.

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.

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

OPTICAL CROSS REPRESENTATION OF – 3.00/– 2.00 × 90°

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.

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.

STURM’S CONOID

STURM’S CONOIDThe 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.

The complete envelop of the light near the circle of least confusion is called the ‘Sturm’s Conoid.

DETECTION OF LENS

Trial lens hand neutralizationTrial lens hand neutralization

Lensometer / FocimeterLensometer / Focimeter

Geneva lens measure or Lens Geneva lens measure or Lens clock clock

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.

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.

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.

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.

TRIAL LENS HAND NEUTRALIZATIONTRIAL LENS HAND NEUTRALIZATION For a toric lens, rotational For a toric lens, rotational

movement is used to find the axismovement is 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” - If observe “with movement” - use plus cylinder axis.use plus cylinder axis.

TRIAL LENS HAND NEUTRALIZATIONTRIAL LENS HAND NEUTRALIZATION

For a toric lens, rotational movement For a toric lens, rotational movement is used to find the axisis used to find the axis

TRIAL LENS HAND NEUTRALIZATIONTRIAL LENS HAND NEUTRALIZATION

Toric lensToric lens

LENSOMETER

Lensometer can also be used to determine the lens power of an astigmatic lens.

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