6. Astigmatic Lenses Flashcards
In astigmatic lenses all meridans are
do NOT have the same curvature
what two types of astimagtic lenses are there
Cylindrial
toric lenses
how are cyclindical lenses made up
one plane surface and the other forms part of a sphere
what is the axis of the cylinder
one meridian of the lens that has no virgence power –> plane surface
when does the cylinder act like a spherical lens
in the meridian at right angles to the axis
what is the total effect of a cylinder
formaiton of a line image of a point object –> called the focal line
what is the focal line parellel to
the axis of the sphere
what is a maddox rod
convex cylindrical lens mounted side by side
if the maddox rod is held vertically (so that the cylinders are orientated vertically) what orientation will be the light
horizontal
i.e. light is a 90 degress to the principal axis
how to use the maddox rod
- pt views a distant white point source of light (at least 6m)
- maddox rod is placed over the right eye (by convention) - this dissociates the eyes
- Light in the meridian parallel to the axis of each cylinder passes through undeviated and is brought into focus by the eye –> a row of foci are formed
why do you only get one line using a maddox rod
Incident light on the maddox rod outwith the 90o axis is focussed too close to the eye to form a distinct image as it is scattered over the reting
to test for horizontal imbalance what orientation must the maddox rod be in
horizontal to get a vertical line
Maddox rod results - a
horizontal orthophoria (normal)
Maddox rod results - b
exophoria
Maddox rod results - c
esophoria
Maddox rod results - d
vertical orthophoria (normal)
Maddox rod results - e
right hyperphoria
Maddox rod results - f
right hypophoria (or left hyperphoria)
describe a toric lens
a cylindrical lens bent so that the XY axis meets
Meaning that the surface is now curved in both the vertical and the horizontal meridans
with toric lenses, what are the meridians of maximum and minimum curvature called
principal meridians
the maximum and minimum curvatures are at
90 degrees to each other
the principal meidian of minimum curvature is called the
base curvature
the base curvature is the area of
minimum power
why don’t toric lenses form one image
because the principal meridians form separate line foci at right angles to each other
how is Sturm’s conoid formed
between the two line foci of rays of light from horizontal and vertical meridians
Distance between two lines foci is called
interval of Sturm
Circle of least confusion
is where the two meridians intersect
Blur circle
images are formed at all other planes lying between the two foci
if there difference is small, this can allow for vision
V is vertical meridian
A is the horizonal meridian
B is the foci of the horizontal meridian
D is foci of the vertical meridian
E is the interval of Sturm
C is the circle of least confusion
how are toric lenses defined
as a fraction
spherical power / cyclindrical power
(as it can be thought of as a spehereical lens with a cylindical lens superimporased)
Lens units
Dioptres sphere = DS
Dioptres cyclinder = DC
+2D spehere with a +2D cylinder superimposed
+2.0DS / +2.0 DC
Sphereical equivalent
Cylinder / 2 + sphere
+2.00DS / +2.00DC
+2.00/2 + 2
1+2
= +3.00 DS
+2.00DS / -2.00 DC
-2.00 / 2 + 2.00
-1 +2
= +1.00 DS
the focal point of the speherical equivalent
would concide with the ircle of least confusion
