Ophthalmic prisms

1. Ophthalmic Prisms
OPTOM FASLU MUHAMMED

2. Ophthalmic Prisms
 A prism is defined as a portion of a refracting
medium bordered by two plane surfaces which
are inclined at a finite angle.
 The angle between the two surfaces is called
the refracting angle or apical angle
of the prism.

3.  A line bisecting the angle is called the axis of the
prism.
 The opposite surface is called the base of the
prism.
 When prescribing prisms ,the orientation is
indicated by the position of the base , ‘base in’,
base up’, base down, base out.

4.  A prism consists of two angled refracting
surfaces.
 The simplest form of a prism is two flat
surfaces that come together at an angle at the
top. The point is called the apex of the prism;
the wider bottom of the prism is called the
base.

5. Ophthalmic Prisms

6. REFRACTION THROUGH PRISM
 Light passing through a prism obey snell’s law
at each surface.
 The ray is deviated towards the base of the
prism.
 This causes objects to be displaced away from
the base of the prism towards its apex.

7. The net change in direction of the ray , angle D is
called the angle of deviation.

8.  All varieties of spectacle lens has the effect of
a prism when viewed through a point away
from the optical center.
 The further the away from the optical center,
the greater is the prismatic effect

9.  For a prism in air ,the angle of deviation is
determined by three factors
1. The refractive index of the material of which
the prism is made.
2. The refracting angle of the prism
3. The angle of incidence of the ray considered.

10.  Light rays entering and leaving a prism are
bent towards the base of the prism.
 This causes objects to be displaced away from the
base of the prism towards its apex.
Base down prism - upward.
Base up prism - downward
 To the right or left when seen through a base-out or
base-in prism

12. CHARACTERISTICS OF PRISM
 A prism has a thickest edge, the base and a
thinnest edge, the apex.
 A prism displaces the incident rays towards the
base of the prism.
 A prism displaces the image towards the apex
of the prism.

13. CHARACTERISTICS OF PRISM
 A prism does not change the vergence of the
rays.
 A prism does not magnify or minify the image.
 A prism also disperses incident pencil rays into
its component colours.

15. Angle of Minimum Deviation
 For any particular prism, the angle of
deviation D is least when the angle of
incidence equals the angle of emergence.
Refraction is then said to be symmetrical and
the angle is called the Angle of minimum
deviation.

16. Image Formation
 The image formed by a prism is
Erect
Virtual
Displaced towards the apex of the prism.

17.  There are two primary positions in which the
power of a prism may be specified
The position of minimum deviation
The prentice position.

18. Units of prisms
 The power of a prism can be expressed in
various unit.
1. The prism dioptre
2. The centrad

19. The prism dioptre (Δ)
 A prism of one prism dioptre power produces
a linear apparent displacement of 1 cm ,of an
object O, situated at 1 m.
 Denoted by the symbol Δ.

20. The centrad
 This unit differs from the prism dioptre only in
that the image displacement is measured along
an arc 1m from the prism.
 The centrad produces a very slightly greater
angle of deviation than the prism dioptre, but
the difference ,in practice is negligible.

21. Refracting angle
 A prism may also be described by its refracting
angle.

22. DEVIATION PRODUCED BY PRISM
 The deviation so produced can be measured by
the Prentice’s Rule:
 Δ = cF
is commonly known as Prentice’s
c = image displacement in cm
F =lens power

23. PRISM ORIENTATION
 Prism can be oriented in front of the eye using
notations
Base-in
Base out
Base up
Base down
 All other base directions require 360° notation

24. PRISM ORIENTATION

25. PRISM ORIENTATION

26. Use of prisms
 Diagnostic prisms
 Therapeutic prisms

27. Diagnostic prisms
 1. Assessment of squint and heterophoria
(a) Measurement of angle objectively by prism cover test.
(b) Measurement of angle subjectively by maddox rod.
(c) To assess likelihood of diplopia after proposed squint
surgery in adults.
(d) Measurement of fusional reserve.
(e) Four dioptre prism test. (microtropia)

28.  2. Assessment of simulated blindness
If a prism is placed in front of a seeing eye, the
eye will move to regain fixation.

29. Forms of prism used in diagnosis
 Single unmounted prisms
 The prisms from the trial lens set
 Prism bars
These are bars composed of adjacent prisms of
increasing power.

30. Therapeutic prism
1. Convergence insufficiency
 commonest therapeutic use of prisms in
orthoptics,for building up the fusional reserve of patients
with C.I.
 Base out prisms are using in exercise periods.
 They are not wore constantly.

31. 2 .To relieve diplopia in certain cases of
squint
Include decompensated heterophorias,
small vertical squint and some paralytic squint with
diplopia in the primary position.
 Prisms are reserved for those patients for whom surgery is
not indicated.

32. Forms of therapeutic prism
 Temporary wear
prisms used in treatment include clip-
on spectacle prisms for trial wear.
Eg:-Fresnel prism (pronounced fre-nell') prisms,)

33. Fresnel prism
 To understand how a Fresnel prism works,
imagine cutting off the tops of a large number
of equally powered prisms and gluing them,
one above the other, onto a thin piece of
plastic.
 A Fresnel prism is only 1 mm thick.

34. Fresnel prism
 It consists of plastic sheet of parallel tiny
prisms of identical refracting angle.
 The overall prismatic effect is the same as that
of a single large prism.
 The sheets are lighter than a glass prism and
can be stuck on to the patient’s glasses.

35. Fresnel prism

36. Advantages of a Fresnel Prism
 It is very thin and extremely lightweight.
 It is flexible and can be applied to an existing
spectacle lens.
 It can be cut to any shape with scissors or a
razor blade.
 Reduce magnification differences considerably.

37. Disadvantages
 They are harder to clean than conventional
lenses.
 Fresnel prisms also cause a slight loss of visual
acuity caused by reflections for prisms greater
than 10Δ.

38. WHEN ARE FRESNEL PRISMS USED?
 High Amounts of Prism
 Use and Reuse
 Sectorial Application
 Slowing of Nystagmus

39. Permanent wear
 Prism can be mounted in spectacles
permanently .

40. Vector addition of prisms
 Sometime a patient requires a prismatic correction in
both the horizontal and vertical directions.
 This can conveniently be achieved by using one
stronger prism mounted at an oblique angle. This is
calculated by vector addition, either graphically or
mathematically.

41. Risley prism
 There is an application of obliquely crossed
prisms that is used on a regular basis in
ophthalmic practice.
 That application is called a rotary or Risley’s
prism

42.  A rotary prism is a combination of two prisms.
 These prisms are placed one on top of the
other. Initially, their base directions are exactly
identical, but as the prisms are rotated, their
bases move by equal extents in opposite
directions.

43.  For example, suppose two prisms of 10Δ each
are placed on top of one another base-to-base.
The total prismatic effect is 20Δ .
 But if they are placed base-to-apex, then their
total prismatic effect is zero

44. Risley prism

45. Interpretation of orthoptic reports
 1 prism dioptre = ½ degree
 eg: 20 degree = 40 prism diopter

46. Prescription of prisms
 Generally when prescribing prisms, the
correction is split between the two eyes.
 The apex of the prism must always be
placed towards the direction of deviation of
the eye.(apex towards deviation )

47.  Convergence or eso deviation
Base out prism ,e.g. 8 prism diopter base out R and
L.
 Divergence or exo deviation
prisms must be base -in , e.g. 6 prism diopter
base in R and L.

48. Vertical deviation
 To correct vertical deviation the orientation of
the prism is opposite for the two eyes.
 2 base - down RE
2 base – up for R hypertropia

50. Prisms in optical instruments
 Slit lamp bio microscope
 Applanation tonometer
 keratometer

51.  Right angle prism
 Porro prism
 Dove prism

52. DETECTION OF PRISM IN AN OPTICAL LENS
 Hold the glass up between the eye and an object
which forms a straight line.
 If the continuity of the line in broken as seen in
Figure ,it implies that the prism is present and
since the line appears to be deviated towards the
apex, we know the direction of apex of the prism.
 The amount of displacement produced is the
strength of the prism, and can be measured by
neutralising.

53. •To do so put the prism lens of known strength in
contact with the lens under checking with its apex
in opposite direction.
•The prism lens with which the continuity of the
lens is again established, represents the strength of
the prism.

54. Lensometer
 Lensometer can also be used to detect the prism
dioptre. To check this get the best focus point of the
target image and read the prismatic power.
 An additional accessory called ‘prism compensator’ is
to be attached to the lensometer for higher dioptre of
prism.

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