10/4/2013
Timothy A. Wingert, OD, FAAO
Prisms Terminology
 Consist of two plane surfaces joined at an angle
 Intersection of the two surfaces is called the apex (or
edge)
 The thick side is called the base
 Base is used to specify the orientation of the prism
(base-apex line)
 Section through prism perpendicular to edge –
principal section
Prisms Terminology
 Angle between the two surfaces is called
 Apical angle
 Prism angle
 Refracting angle
1
10/4/2013
Prisms
Terminology
 Angle between the two surfaces is called
 Apical angle
edge
 Prism angle
apex
 Refracting angle

 Often designated by  (beta)
base
Prism Optical Properties
A prism changes the direction of light without changing
its vergence


 (refracting angle, beta)
 (angle of deviation, epsilon)
PrismHowever,
Optical
Properties
the deviation of light causes the
image to be relocated
Rays of light - deviated toward base
Observer viewing object through prism – object appears to be displaced
toward the apex
2
10/4/2013
Prism Refracting Power
Angle of deviation is usually called the prism power.
The unit is the prism diopter (∆).
The power in prism diopters is ∆ = x (cm)
X (M)
x
∆
X
1 ∆ causes deviation of 1 unit at a distance of 100 units
Prism
 What is a 1∆ prism?
Prism
1∆
1cm
1m
1D of prism will bend light
1cm at a distance of
1 meter
3
10/4/2013
Diopter
Unit of measure for lenses
1∆
1cm
1m
1.00D lens will focus light at 1 meter
2.00D lens will focus light at 1/2 meter
3.00D lens will focus light at 1/3 meter
Prism
Apex
Image
Light ray
Base
Light is redirected toward the base
The object appears closer to the apex
Prism
Prism #1
BASE
Prism #2
• Plus lenses are base to base
• Thicker in middle, thinner on edges
• Used to correct hyperopia
4
10/4/2013
Prism
Prism #1
Apex
Prism #2
• Minus lenses are apex to apex
• Thinner in middle, thicker on edges
• Used to correct myopia
Prisms - Effects on Eyes
 Depends on:
 Visual Status

Monocular versus binocular
 Prism Orientation

Described by location of the base
Prisms - Effects on Eyes
 Prism Orientation
120
090
060
150
030
120
090
060
150
030
180
000
180
000
210
330
210
330
240
270
300
240
270
300
Note: 270°≠ 090°
5
10/4/2013
Prisms - Effects on Eyes
 Location relative to eyes
Base Up
Base Out
Base Up
Base In
Base In
Base Down
Base Out
Base Down
Prisms - Effects on Eyes
 Location relative to eyes
 Horizontal Prism


Base In
Base Out
 Vertical Prism


Base Up
Base Down
Prisms - Effects on Eyes
 Monocular prismatic effects
 One eye occluded
 Image is projected toward the apex
 Eye must rotate an amount equal to the angle of
deviation

Duction movement
 Base down-eye moves up
 Base out- eye moves in
6
10/4/2013
Prism
 Are BU and BD in the same direction?
 Are BO and BI in the same direction?
Prisms - Effects on Eyes
 Binocular Prismatic Effects
 Both eyes open
 Place BI in front of OD, BO in front of OS
 Both eyes move right
 Conjugate eye movement or Version
OS
OD
Prisms and Decentration
 Binocular Prismatic Effects
 Both eyes open
 Place BO in front of both eyes
 Both eyes move inward
 Disjunctive eye movement or Vergence
7
10/4/2013
Prisms - Effects on Eyes
 Binocular Prismatic Effects
 Bases in opposite direction (one R one L)
 Base Out OU

Vergence movement
Prisms - Effects on Eyes
 Binocular Prismatic Effects
 Bases in same direction (both to R or L)

Version movement
 Both eyes shift in the same direction
Prisms - Effects on Eyes
 Binocular Prismatic Effects
 Bases in same direction (both to R or L)

Version movement
 Both eyes shift in the same direction
 Bases in opposite direction (one R one L)


Vergence movement
Same effect if placed only in front of one eye while binocular
 Base in oblique direction

Nausea and dizzy
8
10/4/2013
Prisms - Effects on Eyes
 Resultant Prismatic Effect
 Defining as: Extent of the disjunctive movement
 If both eyes shift in same direction



Version movement
No prismatic effect if same amount in each eye
No difference between lines of sight
Prisms - Effects on Eyes
 Resultant Prismatic Effect
 Extent of the disjunctive movement
 Horizontal Prism

If both BI or both BO
 Net binocular effect is SUM of each prism

Same effect if total prism was all in one lens

i.e. 2.5∆ BO OD and 2.5∆ BO OS = 5 ∆ BO OD
Prisms - Effects on Eyes
 Resultant Prismatic Effect
 Vertical Prism
 If both BU or both BD



Version movement
No prismatic effect
No difference between lines of sight
 If One BU and other BD



Net binocular effect is SUM of each prism
Same effect if total prism was all in one lens
i.e. 2.5∆ BU OD and 2.5∆ BD OS = 5 ∆ BU OD
9
10/4/2013
Prisms - Effects on Eyes
 Problems in Note Set
 Resultant Prism

Horizontal
 4∆BO OD, 2 ∆BI OS → 2∆ BO (disjunctive movement)
 4∆BI OD, 2 ∆BI OS → 6 ∆ BI (OD)

Vertical
 3∆BU OD, 1 ∆BU OS → 2 ∆ BU OD (BUOS)
 3∆BD OD, 1 ∆BU OS → 4 ∆ BD OD (BUOS)
Prism
 What is the prismatic effect for the following?
 2 BU OD
 4 BO OD
 4 BO OD
 2 BU OD
2 BD OS
4 BO OS
4 BI OS
2 BU OS
Prismatic Effect of Lenses
 Spherical Lenses
 Infinite number of prisms stacked
 Plus

Base to Base
Minus
Apex to Apex
 Prism power increases
from pole to periphery
10
10/4/2013
Prism
Sphere Cylinder
Axis
-1.50
Base
1.00 BD
 May be prescribed if the eyes don’t align
properly
 Oriented in any direction, referenced by
the direction of the base
Prism
Sphere Cylinder
Axis
+1.50
Base
1.00 BD
Prism
Sphere Cylinder
+1.50
+1.50
Axis
Base
5.00 BI
5.00 BI
11
10/4/2013
Prism
Sphere Cylinder
Axis
+1.50
+1.50
Base
5.00 BD
5.00 BU
 When calculating the total prism in a pair
of glasses the horizontal prism is added
together and the vertical prism is the
difference between the two eyes.
Alternating Cover Test
 Occluder placed in front of
one eye
 Occluder is moved to the
other eye after 2 to 3
seconds
 Repeat the pattern
 Observe movement of the
eyes
Alternating Cover Test
 Exophoria
 Eyes move from out to in
 Esophoria
 Eyes move from in to out
 Hyperphoria
 Eyes move from up to down
 Hypophoria
 Eyes move from down to up
 Orthophoria
 No movement of the eyes
12
10/4/2013
Cover-Uncover Test
 Used to detect the presence of
a tropia
 One eye is covered and the
patient views your finger
 Move finger around to break
the normal reflex
 Uncover eye and see if there is
movement back to the correct
position
Cover-Uncover
Test
 Used to detect the presence of
a tropia
 One eye is covered and
uncovered at a time
 The eye not being covered is
watched to determine if it
moves as the first eye is
occluded
Hyper
Eso
Exo
Hypo
13
10/4/2013
Fresnel Prism
 Press- on prisms
 Constructed from stacking series of identical small
plastic prisms, parallel and adjacent to each other
Fresnel Prism
 Press- on prisms
 Constructed from stacking series of identical small
plastic prisms, parallel and adjacent to each other
 1mm thick, flexible
 Plasticized PVC, n = 1.525
 One surfaced grooved, other is flat
Fresnel Prism
 Advantages
 Thin and lightweight
 Flexible
 Can apply to existing spectacles, in-house
 Can be cut into any shape
 Reduce magnification differences from prism
14
10/4/2013
Fresnel Prism
 Disadvantages
 Look different
 Harder to clean – lots of small edges
 Slight loss of acuity and contrast due to reflections,
distortions, and chromatic aberrations.
Fresnel Prism
 Clinical Applications
 High Amount of Prism
 Use and Reuse


Can see which works best,
On and off for vision therapy
 Sectorial Application

Trial before actually ordered in specs
 Visual Field Defects
 Nystagmus – direct eye into null position
15