3rd Year Student Of Optometry at ISRA School Of Optometry

2.  Prism is transparent refracting body bounded
by two triangular and three rectangular
surfaces .
OR
 Portion of refracting body/medium bordered
by two plane surfaces which are inclined at a
finite angle.

4.  How much light deviated after
passes through prism.
 It depends upon these factors
i. Incident angle
ii. Refracted angle
iii. Refracting index of prism
material

5.  Power of prism can be expressed in various
units:
 PRISM DIOPTER:
 A prism of one prism diopter power produces
a linear apparent displacement of 1cm of an
object situated at 1m(100cm).

6.  CENTRAD:
 This unit differs from prism diopter only in that
the image displacement is measured along an arc
1m from prism.
 Refracting angle:
 A prism may also be described by its refracting
angle but refracting index of prism must be
known.

7.  GRINDING:
 This is a manufacturing technique.
 Usually applied when large amount of prism is needed.
 DECENTRATION:
 A prism is added to spectacle lens by decentering the lens
in frame so that patient visual axis in primary position
passes through an off center portion of the lens.
 Usually applied when small amount of prism is needed.

8.  Prentice rule
Prismatic power of a lens in prism diopter at any
point on its surface is equal to the distance from
optical center(in cm) times the power of the lens
(in diopter).
Uses:
It is used to determine amount of prescribed
prism in lenses by decentration.
It is used to determined either unprescribed
prism present in lenses by decentration.

9. PRENTICE’S RULE
p=d*F
p=Prism power(in prism diopter)
D=Decentration(in cm)
F=Refracting power of lens(in diopter)

10.  Prismatic power is neutralized by two ways:
HAND NEUTRALIZATION :
Two prisms of equal strength placed base to apex.
LENSOMETER:
 Two types of lensometer available
1. Automatic lensmeter
2. Manual lensmeter

11.  Diagnostic uses
 Objective measurement of angle of deviation (PCT, krimsky
test).
 Subjective measurement of angle of deviation with Maddox
Rod.
 Measurement of fusional reserve.
 Diagnosis of microtropia (4Dprism test).
 To asses diplopia after squint surgery in adults.
 After subjective refraction in binocular balancing.
PRISM USED FOR DIAGNOSTIC PURPOSE :
 Risley prism.
 Mounted prism from trial lens set.
 Prism bar(horizontal and vertical).

12. Therapeutic uses:
 To build up fusional reserve in patient with
convergence insufficiency (BO).
 To relieve diplopia .
 Prism ballast
Prism used for therapeutic purpose:
 Clip on spectacle prisms
 Fresnel prism
 Permanent wear(incorporate prism in lens by
decentration)

13.  Prism are commonly used as reflectors of
light in ophthalmic instruments such as:
 Slit lamp bimicroscope
 Keratometer
 Applanation tonometer
 Gonioscope

14. REFERENCE
 A.K Khorana
 Duke elder
 Clinical optics
 American academy
 Internet
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