Ophthalmic Prisms: Prismatic Effects and Decentration here we discuss about the ophthalmic prisms, the prismatic effects as caused by the decentration( moving the optical center away from the visual axis)

1. Ophthalmic prisms,
prismatic effects &
decentration
Rabindra Adhikary
ravinems@iom.edu.np

2. Prisms
 Prism is simply a wedge
of refracting material
 with triangular cross-
section
 having an apex & a base.

3.  A prism is a portion of
a transparent
substance included
between two polished
non-parallel surfaces
called the refracting
surfaces.
 Aptical or refracting
angle
 Angle between two
refracting surfaces

4.  Prism has property of changing the
direction of a beam of incident light
without changing its vergence.
 Ophthalmic prisms are usually curved
rather than flat.

5. Uses of prisms
 Binocular vision disorders
 Phorias & tropias can be compensated
 In low vision
 Field enhancing channel lens
 Fresnel prisms
 In various ophthalmic instruments like
keratometer, Goldmann Applanation
Tonometer etc.

6.  When light ray traverses a plane parallel
plate (apex angle 0°)
 Equal bending occurs at two surfaces
 No deviation
 Lateral dispersion – yes

7.  When light ray
traverses through
non parallel
refracting surface
 Unequal bending
occurs
 There is net
deviation.

8. Refracting action of prisms
 Rays of light are deviated towards the base
of the prism.
 For an observer, the object appears to be
displaced towards the apex of the prism.

9. Types of prisms
 Dispersive prisms
 Polarizing prisms
 Reflecting prisms

10. Dispersive prisms
 Disperse light.
 Examples
 Triangular prism
 Abbe prism
 Amici prism

11. Polarizing prisms
 Polarize light
 Nicol prism
 Wollaston prism
 Glan-Foucault prism
 Rochon prism

12. Reflecting prisms
 Light is internally
reflected before
emerging.
 Examples
 Penta prism
 Porro prism
 Dove prism
 Dichroic prism
 Porro and Dove prisms
erect the image
 Erecting prisms

13. Ophthalmic prism power
specification
 Ophthalmic prism power usually specified in Prism
Diopters (Δ).
 A prism diopter is defined as a deviation of 1 unit
at a distance of 100 units.
 1 prism diopter
 Deviation of 1 cm at a distance of 100 cm.

14.  Prism power is related to refracting angle by
Δ = 100 tan βº (n’-1)
 For small angled prisms
1º = 1.75 Δ
& 1 Δ = 0.57º
 Ophthalmic prisms are almost never prescribed
in power greater than 5 Δ for each eye.
 Measurement of phorias and tropias involve high
power prisms.

15. Orientation of ophthalmic
prisms
 Orthoptic prisms made of glass
 Generally calibrated according to prentice
position
 Should be placed with rear surface
perpendicular to the deviated eye’s visual axis.
 40 Δ glass prism erroneously held in frontal
plane -- 32 Δ of effect achieved.

16.  American made plastic prisms & prism
bars
 Calibrated according to the angle of minimum
deviation
 Should be placed with rear surface in the
frontal plane to achieve the calibrated effect.

17. Effect of prisms on movement
of eyes
 Monocular prismatic effects
 Image of object is displaced towards the apex.
 Eyes move towards the apex through an angle
equal to the angle of deviation of the prism
 Eg :
BO eye moves inwards
BI eye moves outwards

18. Effect of prisms on movement
of eyes
 Binocular effects
 When bases of prisms are in the same direction
 Both eyes move in same direction (versions or
conjugate movements)
 Eg :
Base in OD
Base out OS
 Eyes move in same direction ie right
(dextroversion).

19.  Binocular prismatic effects
 When bases of prisms are in opposite direction
 Eyes move in opposite direction (vergence or
disjunctive movements)
 Eg :
Base in OU
 Eyes move outwards (divergence)
Base out OU
 Eyes move inwards (convergence)

20. Resultant horizontal prismatic
effects
 When prisms make eye move in same direction,
 the net effect is subtractive.
Eg :
 Base in in one eye & Base out in other eye.
 When prisms make eye move in opposite direction
 The net effect is additive.
Eg :
 Base in in both eyes.

21. Problem 1
 Given the prisms placed before the eyes
 OD 3 prism dioptres, base out
 OS 3 prism dioptres, base out
(Move eye in opposite direction)
 Resultant prismatic effect is found by adding
the prism power, ie 6 prism dioptres base out.

22. Problem 2
 Given prisms placed before the eyes
OD 6 prism dioptres, base in
OS 3 prism dioptres, base out
(Move eye in same direction)
net effect is subtractive ie resultant prismatic
effect is 3 prism dioptres, base in.

23. Resultant vertical prismatic
effects
 When bases of prism are in same direction
(both base up or base down),
 net effect is subtractive.
 When bases are in opposite directions (one
base up & other base down),
 net effect is additive.

24. Problem 3
 Given the prisms placed before the eyes
OD 4 prism dioptres base up
OS 2 prism dioptres base up
 Resultant prismatic effect subtractive
ie 2 prism dioptres base up.

25. Problem 4
 Given the prisms placed before eyes
OD 2 prism dioptres base down
OS 2 prism dioptres base up
Resultant prismatic effect is additive ie 4
prism dioptres base down OD or base up
OS.

26. Prismatic effects of lenses
 Spherical lenses – considered to
be made of infinite number of
prisms.
 Plus lens – base to base
 Minus lens – apex to apex
 For both lenses power of prism
increases from the pole of the
lens towards the periphery.

27.  Prismatic effect of lens is the function of the
 distance from the pole of lens.
 power of the lens.

28. Prismatic effect of the lens:
Prentice Rule
 Prentice Rule
 The prismatic effect at any point on the spherical
lens is equal to the distance of point from the pole
of the lens, in centimeters, multiplied by the power
of the lens.
 P = d F
 Where P = prismatic effect at a given point
d = distance from pole of lens
F = power of lens.

29. Problem 5
 A pair of spectacles having the prescription of +3.00 DS each
eye fits in such a way that the line of sight for eye passes 5 mm
nasal to the optical center of the lens. What is the prismatic
effect encountered by each lens.
 Solution
P = d F
P = 0.5 (+3.00)
= 1.5 pd base out ???

30. Base in prismatic effect
 Induced by
 Convex lens decentered
nasally.
 Concave lens decentered
temporally.

31. Base out prismatic effect
 Induced by
 A convex lens
decentered temporally.
 A concave lens
decentered nasally.

32.  When a spectacle wearer converses to read while
wearing lenses that are centered for the
interpupillary distance
 Plus lens wearer experiences a base out prismatic effect.
 Minus lens wearer experiences a base in prismatic effect.
 Clinical Significance
 Incorporation of base in prisms in spectacle magnifiers.

33. Problem 6
 Prescription
OD +3.00 DS
OS +3.50 DS
Distance PD 64 mm, Near PD 60 mm
What is the prismatic effect experienced by each eyes on reading ?
Solution
OD P = 0.2 (3) = 0.6 prism dioptres base out.
OS P = 0.2 (3.5) = 0.7 prism dioptres, base out.
net prismatic effect = 0.6 + 0.7 = 1.3 prism dioptres, BO.

34. Problem 7
 Prescription
OD -3.00 DS
OS -3.50 DS
Distance PD 64 mm, Near PD 60 mm
What is the prismatic effect experienced by each eyes on reading ?
Solution
OD P = 0.2 (3) = 0.6 prism dioptres base in.
OS P = 0.2 (3.5) = 0.7 prism dioptres, base in.
net prismatic effect = 0.6 + 0.7 = 1.3 prism dioptres, BI.

35. Problem 8
 Prescription
OD +2.00 DS
OS +2.00 DS
Lenses are centered for near PD. Patient’s line of sight is 5 mm below
the pole of the lens.what is the vertical prismatic effect ?
Solution
OD P = 0.5 (2) = 1 pd, BU
OS P = 0.5 (2) = 1pd, BU
Net prismatic effect is subtractive ie 0 prismatic
effect with both eyes.

36. Prismatic effects of cylindrical
lenses
 Prentice rule can be applied to cylindrical
lenses whose axis is either horizontal or
vertical.
 Points to note :
 Power of the cylinder is 90 degrees to the axis.
 Prismatic effect will only be in the power meridian.

37. Decentration
 An ophthalmic lens is
said to be centered when
its optic axis is aligned
with the visual axis.
 When not aligned to the
visual, then the lens is
said to be decentered.
 Decentration of the prism
induces prism.
PD
PD

38.  Prism can be induced when looking through a
lens away from the optical center.
 Prism can be induced intentionally or
unintentionally.
 Prescribing prism – intentionally induce

39. Prism reference point (PRP)
 American National Standards Institute (ANSI)
 The point on the lens, as specified by manufacturer at
which the prism value of the finished lens is to be
measured.
 also known as Major Reference Point (MRP).
 The point on the lens that provides the necessary
amount of prism.
 When no prism is prescribed, PRP is the optical
center.

40. Specification of the lens &
frame size
 Boxing system
 The size of the frame and the size of the lens
within the frame is specified by giving the
horizontal & vertical dimensions of a rectangle
that circumscribes the lens.
 The horizontal measurement of the frame is
usually greater than that of the vertical
measurement
 This difference is known as the difference.

41.  The horizontal
measurement is known as
eye size of the frame.
 Distance between lenses
(DBL)
 The horizontal distance
between the tangents at the
level of the lens bevel at the
nasal side of each lens
 Geometic Center of frame
 The geometric center of
circumscribing rectangle.

42.  Distance between Centers
(DBC)
 Distance between the
geometric centers of two
apertures of the frame.
 Also known as frame PD
 Effective Diameter
 Twice the distance from the
geometric center to the
peak of the lens bevel that
is farthest from the lens
bevel.
 Measurement useful in
determining the minimum
blank size.

43.  Minimum blank size
 Smallest possible finished uncut lens size from
which a finished lens having a particular
prescription can be cut.
Minimum blank size = ED + 2

44. Rule for decentration
 For a lens of plus power
 Decentration is in the same direction as that of
the base of the prism.
 For a lens of minus power
 Direction is in the direction opposite to the base of
the prism.

45. Problem 9
OD –4.00 DS
OS –4.00DS
PD = 64mm, eye size = 48 mm, DBL =20mm,
1) find PRP in relation to geometric center ?
2) the amount & direction of decentration ?
Solution
frame PD = eye size + DBL = (48+20) = 68mm.
but patient’s PD = 64 mm , hence PRP is 2mm
in.
Decentration in (nasal decentration)

46. Problem 10
Given a right lens of +3.00 DC axis 180º. In what direction &
how much must the lens be decentered to produce at
PRP, 1) 1 pd base down 2) 2 pd base up ?
 Solution
since prismatic effect of plano cylindrical lens is always
perpendicular to the axis. Only vertical prismatic effects can be
achieved in this lens.
1) To induce 1 pd base up
d = P / F = 1/3 = 0.33 cm downwards.
2) To induce 2 pd base up
d = P / F = 2/3 = 0.67 cm upwards.

47. Prism and prosthetic eyes
 Prism prescription
before prosthetic
eye
 Aim is to give more
natural & cosmetic
appearance.
Source : Ophthalmology Times
December 15, 2002.

48. Measuring horizontal prisms
correction
 Lensometer
 mires cannot be
centered in the
lensometer target
 the lens has to be
shifted away from the
normal viewing area in
order to center the
mires in the target.

49.  direction of the
displacement of
the intersection
from the center of
the target
-orientation of the
base of the
prism.
 Base in displacement

50. Base out displacement

51. Problem 11
 What is the correct
reading on the lens
of right eye shown in
the picture ???

52. What if the image in the
eyepiece looks something like
this ???

53. Measuring vertical prisms

54. Measuring mixed prisms

55. Special thanks
Jeewananda Bist
Nabin Paudel
Ajit Thakur
Sanjay Marasini
Rajendra Gyawali
Resource Person:
Prakash Adhikari