Astigmatic Lenses Flashcards
How are cylindrical lenses made
one plane surface and the other forms part of a cylinder. Thus, in one meridian the lens has no vergence power and this is called the axis of the cylinder
In which meridian of a cylindrical axis does the cylindrical lens act as a spherical lens
In the meridian at right angles to the axis
The total effect is the formation of a line image of a point object. This is called the focal line.
What is the focal line parallel to
It is parallel to the axis of the cylinder
What is the Maddox rod used for
diagnosis of extraocular muscle imbalance, consists of a series of powerful convex cylindrical lenses mounted side by side in a trial lens
How is the maddox rod test performed
The patient views a distant white point source of light through the Maddox rod, which is placed close to the eye (in the trial frame). The spotlight must be far enough away for its rays to be parallel on reaching the patient (at least 6 m). Light in the meridian parallel to the axis of each cylinder passes through undeviated and is brought to a focus by the eye
What is the location of the line seen by the patient using the maddox rod
The line seen by the patient lies at 90° to the axis of the Maddox rod and is formed by the focusing mechanism of the eye. It is not the real line image of the Maddox rod
How is the Maddox rod used to test muscle imbalance
Maddox rod is placed close in front of the right eye (in the trial frame) and the distant white spotlight is viewed with both eyes. The right eye therefore sees a red line, at 90° to the axis of the Maddox rod, while the left eye sees the white spotlight.
How to test for a horizontal muscle imbalance using Maddox rod
To test for horizontal imbalance, the rod must be horizontal to give a vertical line and vice versa
Remember that the eye behind the Maddox rod (conventionally the right) is deviating in the opposite direction to that indicated by the red line
Describe Maddox rod findings in horizontal orthophoria
Vertical line and rod intersect in the middle of patient view
Describe Maddox rod findings in right eye Exophoria
Crossed diplopia
Vertical line formed to left of circle
Describe Maddox rod findings in right eye Esophoria
Vertical line to right of circle
Describe Maddox rod findings in vertical orthophoria
Horizontal line and circle intersect in the middle of patient view
Describe Maddox rod findings in Right hyperphoria
Circle above horizontal line
Describe Maddox rod findings in Right hypophoria (left hyperphoria)
Horizontal line above circle
What is a toric surface
Imagine that the cylindrical lens is picked up by its ends and bent so that the axis XY becomes an arc of a circle. The previously cylindrical surface is now curved in both its vertical and horizontal meridians, but not to the same extent. It is now called a toric surface
In a toric lens, what is the principal meridian of minimum curvature and therefore minimum power called
base curve
What is a lens with one toric surface called
toric lens or sphero-cylindrical lens. For example a toric lens with a power of +2 D in one principal meridian and +4 D in the other principal meridian can be regarded as a +2 D sphere with a +2 D cylinder superimposed. This is therefore written as +2.0 DS/+2.0 DC
How to calculate the spherical equivalent of a toric lens
algebraic addition of the spherical power and half the cylindrical power, e.g. the spherical equivalent of +2.00 DS/+2.00 DC is +3.00 DS, while that of +2.00 DS/–2.00 DC is +1.00 DS
The focal point of the spherical equivalent would coincide with the circle of least confusion of the toric lens’s Sturm’s conoid
What is the jackson cross cylinder
a sphero-cylindrical lens in which the power of the cylinder is twice the power of the sphere and of the opposite sign
The lens is mounted on a handle which is placed at 45° to the axes of the cylinders.
What is the +0.50D cross cylinder used for
he 0.50 D cross-cylinder is used to check the power of the trial cylinder where the patient has good vision
What is the +1.00D cross cylinder used for
The 1.00 D cross-cylinder is used to check the axis of the trial cylinder, and the power in patients with poor visual acuity
Clinical use of the cross cylinder
check the axis of the cylinder prescribed and then its power. It can also be used to verify that no cylindrical correction is necessary for the patient if no cylinder was detected on retinoscopy.
Which side of the JCC is plus axis and which is minus
White axis of plus cylinder
Red axis of minus cylinder
