A prism is a refracting medium with two plane surfaces that are inclined at an apical angle. The greater the apical angle, the stronger the prismatic effect. Prisms are used diagnostically to objectively measure deviations and fusional reserves, and therapeutically to relieve diplopia by decompensating phorias and small tropias. They produce virtual, erect images that are displaced towards the apex away from the base. Prism power is measured in diopters, with one diopter producing one centimeter of displacement per meter.

1. Prism
BY - RAJU RATHORE
B.OPTOMETRY 2nd year 3 SEM
ITM UNIVERSITY RAIPUR

2. Introduction
• A prism is a refracting medium, having
two plane surfaces, inclined at an angle
■Apical angle (refracting angle) of a prism
is the angle between two surfaces . The
greater the angle formed by two surfaces at
the apex, the stronger is the prismatic effect.
■Axis of the prism refers to the line
bisecting the optical angle.

4. ■Base of prism refers to the surface opposite
to
the apical angle. When prescribing a prism, the
orientation is indicated by the position of the
base,
e.g., ‘base-in’ or ‘base-out’ or ‘base-up’.
■Refractionthroughtheprism produces
displacement of the objects seen through it
towards apex (away from the base)

5. ■Power of a prism is measured in prism
dioptres. One prism dioptre (D) produces
displacement of an object by one cm when
kept at a distance of one metre. Two prism
dioptres of displacement is approximately
equal to one degree of arc
■Image formed by a prism is virtual, erect
and
displaced towards its apex

6. Uses in ophthalmology
1. Diagnostic uses include:
• Objective measurement of angle of
deviation
(Prism bar cover test, Krimsky test).
• Measurement of fusional reserve.
• Diagnosis of microtropia.2. Prisms are also used in many ophthalmic
equipments
such as gonioscope, keratometer and
applanation tonometer

7. 3. Therapeutic uses of prisms are:
• To relieve diplopia in patients with
decompensated phorias and small tropias
• For exercises to develop fusional
reserve

8. Lll
Thank You
By- Raju RATHORE
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