Chapter 6: Astygmatic Lens Flashcards
What are the 2 types of astigmatic lenses
Toric and cylindrical
Describe how a cylindrical lens works
These lenses have one plane surface and the other forms part of a cylinder. In one meridian (plane surface) the lens has no vergence power and this is called the axis of the cylinder. In the meridian at right angles to the axis, the cylinder acts as a spherical lens. This produces a focal line that is parallel to the axis of the cylinder. Cylinders act like lots of semi circles stacked on each other
Describe how a Maddox rod works
Instrument used to measure extro-ocular ima balance.
1. White light is placed 6m away from patient
2. Patient wears a trial frame that’s contains a red tinted cylindical lens on one eye and nothing on the other eye.
3. If lens is put infront of the right eye, it will see a red line, perpendicular to the axis of the lens. The left eye will see a white dot.
4. To test horizontal imbalance, the line must be seen vertically, therefore, the Maddox rod must be horizontal.
5. Prism of varying power is then placed on uncovered eye until line and dot overlap to measure the deviation
What is a toric surface?
In a toric lens, the meridian with the least power is also known as
The base curve
What is Strums conoid?
On a Toric surface (sphero-cylindrical lens), each meridian has a different focal length. So light on the toric surface is focused at 2 different points.
The plane where the two pencils of light intersect is called the circle of least confusion or the circle of least diffusion.
How are toric lenses named mathematically
Can be considered as a spherical lens with a cylinder placed on top
Numerator = spherical lens
Denominator = 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.
In a Jackson cross:
How to calculate spherical equivalent of a toric lens?
Spherical power + 0.5*cylindrical power
eg +2.00 DS/–2.00 DC is +1.00 DS
It’s focal point coincides with strums conoid of the original toric lens
On a Jackson cross, what do the annotations on the side mean
When do we use a 1 diopter cross cylinder and when do we use 0.5
1 = check axis in poor visual acuity patient’s lenses
0.5 = check axis in good visual acuity patient’s lenses
What is the spherical equivalent value of a Jackson cross cylinder
How to refine axis of a trial lens using a JCC
To check the axis, the cross-cylinder is held before the eye with its handle in line with the axis of the trial cylinder (axis of lens and jcc are 45 degrees to each other). The cross-cylinder is turned over and the patient asked which position gives a better visual result. The cross-cylinder is held in the preferred position and the axis of the trial cylinder rotated slightly towards the axis of the same sign on the cross-cylinder.
IE: The axis of the trial lens is moved towards the axis of the same sign on the cross-cylinder.
How to refine power of a trial lens using a JCC
To check the power of the trial cylinder the cross-cylinder is held with its axis In line with the axis of the trial cylinder. The JCC is then flipped (which basically add or minuses 0.5 dioptres). This has the effect of increasing and then decreasing the power of the trial cylinder. You can then adjust the trial lens accordingly
How to use JCC to check if a cylinder is needed for correction
To confirm the absence of a cylinder, the cross-cylinder is offered as an addition to the trial sphere in four different orientations, with its + axis at 90°, 180°, 45°, and 135°. If the patient prefers one of these options to the sphere alone, a cylindrical correction is necessary. The exact axis and power can then be determined by the methods described above.
