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Optics > 6. Astigmatic Lenses > Flashcards

6. Astigmatic Lenses Flashcards

1
Q

In astigmatic lenses all meridans are

A

do NOT have the same curvature

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2
Q

what two types of astimagtic lenses are there

A

Cylindrial
toric lenses

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3
Q

how are cyclindical lenses made up

A

one plane surface and the other forms part of a sphere

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4
Q

what is the axis of the cylinder

A

one meridian of the lens that has no virgence power –> plane surface

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5
Q

when does the cylinder act like a spherical lens

A

in the meridian at right angles to the axis

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6
Q

what is the total effect of a cylinder

A

formaiton of a line image of a point object –> called the focal line

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7
Q

what is the focal line parellel to

A

the axis of the sphere

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8
Q

what is a maddox rod

A

convex cylindrical lens mounted side by side

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9
Q

if the maddox rod is held vertically (so that the cylinders are orientated vertically) what orientation will be the light

A

horizontal
i.e. light is a 90 degress to the principal axis

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10
Q

how to use the maddox rod

A
  1. pt views a distant white point source of light (at least 6m)
  2. maddox rod is placed over the right eye (by convention) - this dissociates the eyes
  3. 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
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11
Q

why do you only get one line using a maddox rod

A

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

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12
Q

to test for horizontal imbalance what orientation must the maddox rod be in

A

horizontal to get a vertical line

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13
Q

Maddox rod results - a

A

horizontal orthophoria (normal)

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14
Q

Maddox rod results - b

A

exophoria

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15
Q

Maddox rod results - c

A

esophoria

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16
Q

Maddox rod results - d

A

vertical orthophoria (normal)

17
Q

Maddox rod results - e

A

right hyperphoria

18
Q

Maddox rod results - f

A

right hypophoria (or left hyperphoria)

19
Q

describe a toric lens

A

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

20
Q

with toric lenses, what are the meridians of maximum and minimum curvature called

A

principal meridians

21
Q

the maximum and minimum curvatures are at

A

90 degrees to each other

22
Q

the principal meidian of minimum curvature is called the

A

base curvature

23
Q

the base curvature is the area of

A

minimum power

24
Q

why don’t toric lenses form one image

A

because the principal meridians form separate line foci at right angles to each other

25
Q

how is Sturm’s conoid formed

A

between the two line foci of rays of light from horizontal and vertical meridians

26
Q

Distance between two lines foci is called

A

interval of Sturm

27
Q

Circle of least confusion

A

is where the two meridians intersect

28
Q

Blur circle

A

images are formed at all other planes lying between the two foci
if there difference is small, this can allow for vision

29
Q
A

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

30
Q

how are toric lenses defined

A

as a fraction
spherical power / cyclindrical power

(as it can be thought of as a spehereical lens with a cylindical lens superimporased)

31
Q

Lens units

A

Dioptres sphere = DS
Dioptres cyclinder = DC

32
Q

+2D spehere with a +2D cylinder superimposed

A

+2.0DS / +2.0 DC

33
Q

Sphereical equivalent

A

Cylinder / 2 + sphere

34
Q

+2.00DS / +2.00DC

A

+2.00/2 + 2
1+2
= +3.00 DS

35
Q

+2.00DS / -2.00 DC

A

-2.00 / 2 + 2.00
-1 +2
= +1.00 DS

36
Q

the focal point of the speherical equivalent

A

would concide with the ircle of least confusion

Optics (18 decks)

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