discussion of different types of prism in Optometry practice

1. Prisms in Optometry practice
Hira Nath Dahal
10/07/17 1

2. Introduction
Prism is a portion of a refracting medium
bordered by two plane surfaces which are
inclined at a finite angle.
10/07/17
2
AxisAxisββ
2

3. Prism Terminologies
Apex
Base
Refracting angle
Angle of deviation
10/07/17
33

4. Properties of prism
Prisms deviates light without
changing its vergence.
Obeys Huygen’s principle
It obeys Snellen’s law i.e. it
displaces the image towards its
base
It also causes dispersion.
10/07/17
44

5. Image formation of
prisms:
Erect, virtual and displaced towards the apex of
the prism.
10/07/17
55

6. Dispersive prisms
Polarizing prisms
Reflecting prisms
10/07/17
66

7. Dispersive prism
Dispersive prisms are used to break up light into its
constituent spectral colors because the refractive index
depends on frequency;
The white light entering the prism is a mixture
of different frequencies, each of which gets
bent slightly differently.
Blue light is slowed down more than red light
and will therefore be bent more than red light.
Examples:
- Triangular prism
- Abbe prism
- Pellin-Broca prism
- Amici prism 10/07/17
77

8. Polarizing prism
can split a beam of light into components of varying polarization.
These are typically made of a
birefringent crystalline material.
Examples:
- Nicol prism
- Wollaston prism
- Rochon prism
- Glan-Foucault prism
- Glan-Taylor prism
- Glan-Thompson prism
10/07/17
88

9. Nicol prism
Made from a piece of calcite crystal cut
diagonally in half with the two halves
cemented together with Canada balsam or a
low refractive index optical cement.
Incident light is split into ordinary and
extra-ordinary linearly polarized rays in the
prism:
- the ordinary rays reaches the interface
and is totally reflected,
- the extra-ordinary ray is transmitted.
10/07/17
99

10. Wollaston prism
2 right-angled prisms of equal angle made of a double
refracting surface such as quartz or calcite cemented
together by their hypotenuse faces to form a
rectangular unit.
A beam of unpolarized light incident on it will emerge as
two diverging beams which are oppositely polarized and
almost free of dispersion.
10/07/17
1010

11. Glan Foucault prism
Constructed only from calcite
As it is transparent from
230 nm to about 5000nm,
it can be used over a wide
range of wavelengths.
10/07/17
1111

12. Reflecting prisms
A prism in which light is internally reflected at one or more of the plane
surfaces before emerging.
This happens due to total internal reflection.
Example:
-Penta prism
-Porro prism
-Porro-Abbe prism
-Abbe-Koenig prism
-Schmidt-Pechan prism
-Dove prism
-Dichroic prism
-Amici roof prism
10/07/17
1212

13. Erecting prism
A prism system placed in an optical
system for the purpose of forming an
erect image .
Examples:
-Porro prism
-Dove prism
10/07/17
1313

14. 10/07/17 14

15. Types of prism of clinical use
1. Relieving prisms
2. Corrective prisms
3. Over-corrective prisms
4. Inverse prisms
5. Yoked prisms
6. Rotating prisms
7. Regional prisms
10/07/17
15

16. Decentration
Prisms can be prescribed in the spectacles just
by decentrating the optical center of the lens.
This applies only to low power prism and
moderate power prism in high power lens.
Its effects is identical to grinding.
10/07/17 16

17. USES OF PRISM
Optical
Diagnostic
Therapeutic
10/07/17
1717

18. In optical devices
Risley prisms:- In phoropters
10/07/17
1818

19. Risley Prism
Consists of 2 prisms of equal power
Are mounted in such a way that by turning a
knob, both prisms are rotated equally but in
opposite directions.
The resultant prismatic power in any given
direction varies with the degree of alignment of
two prisms.
10/07/17
19

20. Fresnel prisms
10/07/17
2020

21. Fresnel Membrane Prism (Fresnel press-on
prism)
Thin disc of transparent plastic
Consists of one flat surface which
can adhere to a clean lens surface
when pressed in place; and another
on which are incorporated small
prismatic elements laid parallel to
each other.
10/07/17
21

22. Keratometers
10/07/17
2222

23. Applanation Tonometer
Internal doubling prism
10/07/17
23

24. 10/07/17 24
Telescopes
24

25. Magnifying aid
Binocular loupe available in
powers up to about(10.0 or
12.0 Dioptre), usually
incorporate some prism or
decentration to facilitate
convergence.
10/07/17
25

26. Ophthalmoscope
May prism
Made up of solid glass
Consisted of a convex lower end, acting as a
condenser and an angled upper portion serving
as the reflector.
10/07/17
26

27. 10/07/17 27
Diagnostic and therapeutic uses
• Measurement of deviations
• Hirscberg test
• Krimsky test
• Krimsky test
• Modified Krimsky test
• Prism Cover test
• Simultaneous cover test
• Maddox rod
27

28. Hirshberg test
• 1mm = 22 PD
10/07/17 28

29. Krimsky test
10/07/17 29

30. Maddox rod
10/07/17 30

31. Distance and Near Fusional
reserves
• Equipment:
• Use of prism bars
• Measurement of Horizontal Fusional Vergence Reserve
• Base in prism– measuring divergence
• Base out prism- measuring convergence
• Measure: Blur /Break/Recovery
• Expected results:
• Base in …/10 / 5
• Base out 10/20/10
10/07/17 31

32. Microtropia – Find out
• 4 PD Base out.
10/07/17 32

33. Fresnel prism (Press on Prism)
10/07/17 33
-Series of prisms of equal power on a plastic
sheet (PVC)
-Total power = ADD of total Units
-Press on glasses under water – Trial to see
the prism tolerance for therapeutic purpose

34. 10/07/17
34

35. To detect Intermittent Deviations
The difference between blur point and recovery point should be (2-4) prism
dioptre.
If recovery point is much less than 4 dioptre. it is intermittent deviation.
10/07/17
35

36. Convergence
To measure convergence
As a therapy to convergence
Divergence insufficiency
10/07/17
3636

37. Accommodation
To measure accommodative spasm
The need to accommodation to aid convergence is
alleviated by the introduction of base in prism
The plus sphere power is added so long as the
desired level of Visual acuity at distance is achieved.
The original spherical end-points found monocularly
are often accepted binocularly.
10/07/17
3737

38. To measure AC/A ratio
By fixation disparity method
Changes in fixation disparity induced by prisms and that
induced spherical lenses is noted and graphically represented to
calculate AC/A ratio.
This method has great advantage because fusion is maintained
throughout the test.
Complicated and time consuming.
10/07/17
38

39. To check binocular balance after
subjective refraction
Steps:
Keep the 3dioptre base up prism in one eye and 3 dioptre
base down prism in the other eye.
Tell the patient to focus on (20/40) line.
The patient now sees two line.
Place 0.25 sphere in one eye and ask which one is clear
and in next time keep that power in next eye.
If the patient reports the eye without the sphere power
is clear in both instances, then the eyes are balanced.10/07/17
3939

40. To measure Visual
acuity in infants
Base down prism
Ten diopter base
down prism test
10/07/17
4040

41. For neural cases
3rd
nerve palsy
Do not response well with prisms due to gross
incomitance
Prism is useful as recovery is occurring.
6th
nerve palsy
Adjust extremely well to prism
Placed over the involved eye if primary and secondary
deviations are present
10/07/17
4141

42. 4th
nerve palsy
Unilateral palsy
Respond well to prism therapy
At times, may need 2 prisms for near and distance fixation
Bilateral palsy
Do not respond well to prism therapy
May be helpful for distance fixation
But not for near due to torsion at reading position
10/07/17
42

43. Infra-nuclear eye muscle palsies
Treatment is towards eliminating diplopia.
Minimum prism correction that allows Binocular Single Vision in
primary position.
10/07/17
43

44. Supranuclear dysfunction
Skew deviation (vertical deviation caused by a supranuclear
dysfunction)
May be comitant or incomitant.
The more comitant the deviation, the more likely the prism will be
helpful.
10/07/17
4444

45. Neuro-muscular defect
oOcular myasthenia: usually a variable deviation
oWhen deviation is sufficiently stable for a long period of time,
prisms can be used to eliminate diplopia.
10/07/17
4545

46. Graves ophthalmopathy
Prisms in spectacles are effected in reducing
double vision when the restriction is mild to
moderate.
10/07/17
4646

47. Degenerative disease (i.e.
multiple sclerosis)
Only some conditions can be helped with prisms.
Those conditions are:
Bilateral internuclear ophthalmoplegia
Cerebellar eye signs
Sixth nerve paresis
Pendular nystagmus
10/07/17
4747

48. Retinal wrinkling
Such patients do not respond to prism
therapy.
Prisms can be given for symptoms like
diplopia or ghostlike images.
But when complain is of dips or curves in
the images, they cannot be helped with
prisms.
10/07/17
4848

49. Nystagmus
Yoke prisms (Base out) can be prescribed
to stimulate fusional convergence and
decrease the nystagmus.
A trial with prisms prior to considering
surgery to improve head turn.
10/07/17
4949

50. Contact lens
As a ballast lens
To stabilize the lens. i.e. stop it rotating on the eye
To correct a vertical extra-ocular muscle imbalance
Between 1 and 3 Dioptre usually 1.5 Base-down at 270
degrees
First choice in condition where good distance vision is
needed and near vision can be intermediate like flying,
driving.
10/07/17
5050

51. Prosthesis
Prism prescription before prosthetic eye
Aim is to give more natural &
cosmetic appearance.
10/07/17
51

52. Visual field defects
Hemianopia
Constricted visual field
Prism relocation
10/07/17
5252

53. 10/07/17 53

54. Prismo-therapy in eccentric fixation
Can be approached in two ways:
Direct method
Indirect or reverse method
10/07/17
5454

55. Prismotherapy in Anomalous correspondence
Aim is to secure a state of sensory ortho-tropia
of the eyes
so that simultaneous stimulation of the fovea and other
corresponding points of the 2 retina is attained in natural
condition of seeing.
Prolonged form of treatment extending between
2 to 8 or even 18 months
Sometimes, involves frequent change of glasses.
10/07/17
5555

56. For Developmental oculomotor dysfunction
For such patients, yoke prisms introduce a new ocular
input to change the individual’s orientation in visual
space.
Patient has to walk with yoked prisms (5 minutes each,
bases left, right, up and down).
The clinician, by changing the amount and orientation of
the yoked prisms periodically, rehabilites the
orientation reflex, requiring the individual to become
aware of his spatial orientation. 10/07/17
5656

57. In ocular misalignment and oculomotor
dysfunction due to closed head trauma
Prisms can be given
As an adjunct to active vision therapy
To temporarily restore Binocular single vision
Yoked prism is given to compensate for visual
field defects, oculomotor range of muscle
restrictions.
10/07/17
5757

58. For sports vision therapy
Can be used to load visual performance.
Applications of base up prism
More convergence
Central processing
Application of base down prism
Less convergence
Peripheral processing
10/07/17
5858

59. For Relief
Orthoscopic spectacles
Are prismo-spheres where equal spherical and
prismatic power are combined.
Worn with advantage in performing extremely fine
tasks.
This helps to attain the necessary magnification
And in such an attitude strain is avoided since the
spheres relieve the accommodation and the prisms,
the convergence.
10/07/17
5959

60. Prismatic spectacles
Enables a patient to read in supine position.
So designed that the incident light is reflected twice & emerges
from the third surface free from chromatism and without
inversion of the image.
With this, the patient can read with both the eyes and hold the
book in a normal and comfortable position.
10/07/17
60

61. Recumbent Spectacle 10/07/17 61