A prism is a transparent optical component bounded by planar surfaces that causes light to refract. It is used for diagnostic and therapeutic purposes in ophthalmology. The amount of light deviation caused by a prism depends on its incident angle, refracted angle, and refractive index. Prism power can be expressed in units like prism diopters and centrads and is used to relieve diplopia or build up fusional reserves. Prisms are also used in ophthalmic instruments and their power can be neutralized using hand or lensometer techniques.

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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