PRISMS
Prof: Vasudev Anand Rao
INTRODUCTION
 A wedge of refracting medium
 Triangular cross-section with an
apex and a base
 The angle α between the two
surfaces is the refracting or apical
angle of the prism. A line bisecting
the angle is called the axis of the
prism. The opposite surface is
called the base.
 When prescribing prisms, the
orientation is indicated by the
position of the base ex: base-in ,
base-up
PRISM OPTICS
 Light is refracted at each
surface as it enters and exits
the prism.
 The ray is deviated towards
the base of the prism. The net
change in direction of the ray,
angle D is called the angle of
deviation
 For a prism in air, the angle of
deviation is determined by
three factors:
The refractive index
The refracting angle α
The angle of incidence
The image formed by a prism is
 erect,
virtual and
displaced towards the apex of the prism.
NOTATION OF PRISMS
The prism dioptre (Δ)
 The power of a prism is said to be one diopter when
it produces 1cm of linear apparent deviation of a light
ray from where it would have otherwise traveled,
measured 100 cm from the prism.
 A 15 Δ prism produces 15 cm of deviation if a light
ray when measured 1mt from the prism, the same
prism would produce 7.5 cm of deviation measured
50 cm from the prism, and 30 cm of deviation when
measured at 2mt from the prism.
USES: DIAGNOSTIC
EVALUATION OF SUPPRESSION
4Δ BASE-OUT PRISM TEST
 This test is used to detect suppression
in small-angle esotropia and
surgically corrected large angle
squint.
 When a 4Δ base-out prism is placed
in front of a normal eye, the sudden
displacement of the image to a
parafoveal temporal point elicits a
refixation movement.
 No movement is seen in a microtropic
4Δ base-out prism test
eye, since the image is shifted within
the central suppression scotoma.
 Movements of the other eye also do
not occur.
DIAGNOSIS AND ACCURATE ASSESSMENT OF
STRABISMIC DEVIATIONS
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Prism bar cover test
The alternate cover test is
performed first.
Prisms of increasing strength
placed in front of one eye with
the base opposite the direction of
the deviation . ie base-in in a
divergent strabismus.
Alternate cover test is performed.
End point is reached when first
neutralization and then reversal of
the corrective occurs.
The angle of deviation then
equals the strength of the prism.
KRIMSKY TEST WITH
PRISM
 Hirschberg’s test to locate the
corneal reflex.
 Placing the prism with
appropriate base and gradually
increasing the power in front
of the fixing eye unless the
corneal reflex is central – base
out for esotropia, base in for
exotropia.
 The strength at which the
corneal reflex is symmetrical
is estimated.
MEASUREMENT OF AC/A (convergence ratio)
 Denotes how many prism dioptres of convergence is
produced for each dioptre of accommodation. The
normal ratio is 3Δ-5Δ.
 Important in the classification and management of
accommodative and non-accommodative esotropia
AC/A is calculated as:
AC/A = (Δ¹- Δ²) / D , where
 Δ¹ is the original deviation in prism dioptre, Δ² is the
deviation in prism dioptre after putting the lenses and
D is the power of new lens in dioptres
COMPONENTS OF DIAGNOSTIC OPHTHALMIC
EQUIPMENT:
The diagnostic instruments that contain prisms are:
 Maddox double prism
 Synaptophore
 Direct ophthalmoscope
 Indirect ophthalmoscope
 Operating microscope
 Slit lamp
 Goldmann applanation tonometer
 Stereoscopes
 Pachymeter
 Keratometer
PORRO-ABBE PRISM
PORRO
PRISM
SLIT LAMP
OPERATING MICROSCOPE
INDIRECT OPHTHALMOSCOPE
STNAPTOPHORE
BIPRISM
USES: THERAPEUTIC
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To restore and maintain BSV in children with late onset
strabismus, usually esotropia, who are awaiting surgical
treatment.
 To obtain BSV if there is a small surgical under-correction
or overcorrection.
Prism Exercises: useful when the angle of deviation is 15° or
less, and patient can appreciate diplopia.
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prism enabling the patient to fuse two images is held in
front of one eye.
strength gradually decreased.
patient encouraged to fuse the images at whatever distance
it is possible.
the object is then moved nearer and further, while patient
tries to maintain fusion without assistance of prisms.
Prism vergence
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To fuse diplopia: starting at the distance at
which the diplopia is maximum, a prism bar is
placed in front of one eye and the prism is
gradually increased until the images join and
fusion can be maintained without undue
effort.
If this is impossible, fresnel prisms can be
fitted as bifocals
To improve the cosmesis of a blind or
disfigured eye or prosthesis. A base down
prism can make hypotropic prosthesis appear
slightly higher to the observer.
FRESNEL PRISMS:
Composed of concentric annular rings.
 Principle: the prism apex deviates light
just as much as any other part of the
lens.
 Sheet of prism apices on a thin base
sheet used to obtain a prismatic effect
across the lens without creating
additional lens thickness.
 Method of choice for temporary use.
Upto 30° can be applied to either eye
but high powered prisms may not be
tolerated.Upto 20° can be worn
comfortably
 The prism is normally fitted to the back
surface of the spectacle lens.
THANK YOU