1. Prism
Dr. Meenank. B
M.S. Ophthalmology (post-graduate )
ASRAM medical college

2. Refractive surface – The two refractive
surfaces of a refractive prism inclined
at the apical angle
Axis – line bisecting the refractive angle
Reflecting surface – In some prism, internal
ray hitting the 2nd surface is subjected to
total internal reflection (reflecting prisms)
Apex – Tip of the prism where
the two refractive surfaces meet
Base – bottom of the prism / side opposite to apex or
optical angle. Helps in orientation of ophthalmic prisms
Prism is a portion of a refractive medium, bordered by 2 plane surfaces which are inclined at a finite angle

3. Prism – Refraction
Angle of deviation – Angle
between the incident and the
emergent rays
D = (n-1)α
Refractive angle / apical angle - angle
between two refractive surfaces ‘α’
 Obeying Snell's Law of refraction light passing through the
prism is deviated to the base
 Angle of deviation - D = (n-1)α ;
where n- Refractive index, α – refractive angle
Thus, D = α/2
 RI of glass – 1.5
 Factors responsible for ‘D’
 Wavelength - The angle of minimum deviation is smaller
for longer wavelengths , so red deviates less and vice-
versa
 Material of prism - directly proportional with refractive
index
 Angle of prism – directly proportional
 Angle of incidence – forma a ‘U” shaped curve

4.  Image formed – virtual, erect and displaced towards the apex
 Power of a prism – Amount of light deviation produced by the prism
Prism Dioptres ‘∆’
1 ∆ = displacement of image towards the by 1cm kept at 1m distance
Centrad ‘▼’
1 ▼ = displacement of image towards the by 1cm kept at 1m distance along an arc

5. Prism Position’s
 Ophthalmic prism's – plastic / glass and amount of deviation
depends on position in which they are held
 3 common position –
1. Prentice Position
2. Minimal Angle Deviation
3. Frontal Plane Position
 Prentice Position – glass prisms . Most common
 Requires the patients line of sight to strike the rear end of the prism @ 90˚
 Small error – large deviation
 Eg : 40∆ Prentice if held at Frontal plane - 32∆
Prentice
Position

6.  Minimal Angle Deviation – Plastic, more common
 Line of sight makes equal angle with both surface
 Difficult to maintain in clinical practice
 Frontal Plane Position – Prism is held in parallel to the frontal plane
 Holding a prism in frontal plane – very nearly produces MAD
 Error’s
 Small error b/w – F.P and MAD
 Large error b/w – P.P and F.P
 Plastic > glass
Frontal Plane Position
Minimal Deviation

7. Stacking of prism’s
 Glass prism – max – 40 ∆
 Plastic prism – max – 50 ∆
 For more than this ‘stacking’ – combining 2 prisms is done – but
never combine linearly cause they give more effect
 Eg – 50∆ + 3∆= 58∆ ( additive error)
 So, to dec. this error prism is held before both eyes
 For a V and H dev. Prism can be combined

8. Measuring strabismus with glasses
 Maximum additive error occur even when prism is held correct,
krimsky test normal, and other subjective test
 Both the line’s of sight cannot pass through the optical center of the
spectacle – glasses producing prismatic effect cause deviation
which is measured from infront of glasses, due to peripheral
prismatic effect
 More common with +/- 5∆

9. Prismatic Effect
 Spherical lens
 Spherical lens behave like prisms in all quadrants except the center
 The refractive angle between the lens and the surface inc. at the edges
viz inc. prismatic effect
 Cylinder lens
 Cylindrical lenses have no power along the axis meridian, hence the
cylinder can exert no prismatic effect along its axis.
 The power of a cylinder lies at right angles to its axis, i.e. along its power
meridian, So a cylinder exerts prismatic effect only at right angle to its
axis
 Most commonly if correction is not equal in both eyes
 High myopes
 Aphakia

10. Vector Addition
 The correction in both vertical and horizontal
planes can be achieved by one strong oblique
prism summed by vector addition or by
calculating using Pythagoras's theorem (𝑎2 +
𝑏2 = c²)
 While prescribing always mention the base and
the meridian

11. Types of Prisms
 Ground prism’s – Permanently incorporating the prism into glasses by decentring
the present spherical lens or by mounting on them
 Fresnel prism (or) Wafer prisms – Series of plastic Prisms of 1mm
 Originally developed by French physicist Augustin-Jean Fresnel for lighthouses.
 Small prisms stacked to give an effect of a large prism to over come wt.,
aberration, and achieve higher power
 Apical angle determines the strength

12. Viewing through Prism
 Prisms – Dec. image quality (or) distort
 Field of Vn – elongated at apex
 compressed at base
 Components of prismatic distortion
1. Horizontal magnification
2. Curvature of vertical line
3. Asymmetrical horizontal lines
4. Vertical magnification
5. Change in vertical magnification with horizontal
angulation

13. Viewing through Fresnel
 Poor optical quality, low resolution and contrast due to use of plastic
 Prismatic Distortion – low H and V magnification but more curvature on V and H in
Fresnel 5∆, 10∆, 15 ∆ prisms
 Fitted by cutting the membrane to the shape of the lens and placing ‘em on the
inner surface of the lens under water
 High myopes – outer side
 Commercially available in plastic of different powers from 1 – 20 giving a PD of 40
 Adv – cost effective, easy to use, light wt., flexible
 Dis-adv - glare, chromatic aberration, vision decline
 Press on prism (3M) – in changing strabismus in thyroid disease

14. Types of Prisms in Clinical Practice
1. Loose Prisms
2. Prisms Bars
3. Trial Prisms
4. Fresnel Prisms
5. Rotating Prisms – Introduced into instruments
6. Risley Double Prisms – 2 rotating Prisms of same strength on a
rotating frame
7. Vari-prisms – Single hand-held instrument where prisms power
can be changed by rotating the two glasses –
H -90 ∆, V-15 ∆
Clinicalclassificationof
Prism’s
Relieving
Inverse
Yoked
Sector (or)
Regional
Corrective
Over corrective
Rotating
Slab off

15. Uses of Prism
 Magnitude of prisms are used in orthoptic settings, mainly for
diagnosis and management
 Diagnosis
 Conformation of BSV
 Measurement of angle of deviation
 To correct angle of deviation
 Management
 Eso/ exo deviation of concomitant / incomitant
 Amblyopia, Nystagmus, Malingering, Visual Field defects

16. Diagnosis
Investigations of
BSV
Prism reflex test
Prism 4∆ test
Prism Fusional
Vergence
Amplitude
Vergence facility
Measurement of
Angle of
Deviation
Prism alt. Cover
test
Measurement of 9
positional gazes
Simultaneous
Prism Cover Test
Maddox Rods
Prism Reflection
Test / Krimsky Test
Assessment of
Torsion
Maddox double
prism
Double Maddox
rod
Correction of
Deviation
Test for ocular
symptoms
Assessment of
potential BSV
Prism adaptation
Test
Progressive prism
compensation
Pre-Op Prism
Adaptation
Test for Retinal
Correspondence
Vertical Prism Red
Filter Test
Investigation of
suppression
Post-Op Diplopia
Test (PODT)

17. Binocular Single Vision
Binocular Single Vision
Basis for testing and investigating for BSV is checking for immediate response induced by
prisms
In presence of normal BSV – adaptive response occurs on cont. viewing
Vergence adaptation in normal BSV
Normal BSV + V/H Prism in front of one/ both eyes – displaces image away from fovea
causing deviation
Normal BSV – latent deviation gradually reduced proving BSV
more faster for BO than in BI
Adaption occurs at 1sec in BO and break up on removal of stimulus

18. Investigations of BSV
• Prism reflex test
• Prism 4∆ test
• Prism Fusional Vergence
Amplitude
• Vergence facility
Investigations of BSV with Prism
 Prism Reflex Test
 Can be used to test BSV infants (4 – 6 months)
 BO infront of one eye while other eye is fixating (33cm), observe eye
movement
 Most commonly done with 10∆, 20∆
1. Normal BSV with motor fusion – eye with BO displaces image temporally – diplopia
– refixation with fellow eye – Herrings law
2. Prism over suppressed eye – initial shift will not be appreciated / no movement
3. Prism over non-suppressed eye – versional movement but, no fusinoal movement
4. Exophoria one/both eyes becoming divergent on prism intro – alt. fixation, unable
to fuse- insufficient fusinoal Vergence

19.  Prism 4∆ test
 Most commonly used for Bifoveal BSV
 By displacing the image small amount a central suppression scotoma
extending to this amount can be detected
 But, if the degree of suppression scotoma is smaller than the degree of
movement then we get a normal response
 Test performed ( 33cm / 6m) – BO prism intro and observe for patterned
movement, check for both eyes
 Micro-exo – BI

20.  Fusional Vergence Amplitude
 Vergence movement compensated for phoria and keep the eyes aligned on
target
 Exotropia – uses convergence ( strongest Vergence Improves with exercise)
 Esotropia – uses divergence (weakest Vergence, no sig imp. With exercise )
 Strength of verg. measured in P.D. – Fusional Vergence Amplitude
 Test – induce small dev. That can be fused – inc. dev until blur point – then inc.
until break point (Using – Risley prisms / Prism Bars )
 Near convergence amplitude – 40cm, start with 4 PD and inc.
 The greatest prism where patient can fuse is Fusional Vergence Amplitude
Normal Fusinoal Vergence Amplitude's
Vergence Distance (6m) Distance (40cm)
Convergence 20 – 25 PD 30 – 35 PD
Divergence 6 – 8 PD 8 – 10 PD
Vertical Vergence 2 -3 PD 2 – 3 PD

21.  Vergence facility
 Prisms used to induce convergence and divergence alt. to asses
the ability of fusional Vergence system to cope with change in
demand
 1 cpm – SV-BO(12∆)-SV-BI(3∆)-SV @ 40 cm
 Failure – less than 15 cpm

22. Measurement of Angle of Deviation
• Prism alt. Cover test
• Measurement of 9 positional gazes
• Simultaneous Prism Cover Test
• Maddox Rods
• Prism Reflection Test / Krimsky Test
Measurement of Angle of
Deviation
 Prism Alternate Cover Test
 Amount of prism needed to neutralize the full deviation tropia and any latent
phoria
 Used to measure deviation in anticipation of strab. Surgery
 Can be done using – prism bars/ lose prism
 Fixation ( 33cm/ 6m) cover test performed to detect the direction of
deviation and suitable prisms are placed
 In manifest deviation prisms can be placed over normal / deviating eye
 Eye without prism – fixating eye ( primary position )

23.  Test – 1st perform alt cover test to know the size of deviation – prism over one
eye to neutralize the dev. – alt cover test with prism – any residual movement
↑∕↓ prism with one eye covered
 In incomitant – angle may be measured by fixating one eye and performing alt.
cover test with prisms until reversal of deviation is seen ( ensuring total angle
deviation ) and dec. until no movement during alt. cover test
 Should never stack prisms for higher powers
 In V and H deviations – two prisms held with higher power closer to eye

24. Direction of prism base for correction of deviation
Deviation Prism in front of Right eye Prism in front of Left eye
Esophoria / tropia BO BO
Exophoria / tropia BI BI
Rt hyperphoria / tropia BD BU
Rt hypophoria / tropia BU BD
Lt hyperphoria / tropia BU BD
Lt hypophoria / tropia BD BU
Alt. hyperphoria / tropia BD BD
Alt. hypophoria / tropia BU BU

25.  Variables in measurement's
1. Poor control of accommodation – use targets at visual thresh-hold
2. Variable working distance – most common @ 40 cm (or) 1/3 m
3. Tonic fusion not suspended – seen in intermittent exo / accommodative eso
– dissociate BSV by prolong occlusion on prism alt. cover test
4. Physiological Redress fixation movement's – in large deviations – deviation
corrected causes over-shooting of the fixated eye
 Solution – allow peripheral vision of the occluded eye
 Take point of neutralization as a point where redress = refixation movements,
and dec. until best neutralization occurs
5. Incomitant deviation ( A,V patterns, lateral gaze, face turn, head tilt, chin
elevation/ depression) – will change measurements if incomitant
 Solution – control head position for 1 position and cardinal gazes
6. Poor vision – always conduct under full correction
 If with sensory starb/ 20/400 – use krimsky method

26.  Measurement of 9 positional gazes
 The positions of gaze are usually measured with the patient fixing on a
distance target
 Nv(33cm) / Dv( 6m) allowing only head movement (no tilt)
 With refraction corrected and prism on frontal plane position all 9
position are checked
 In deviation with head tilt – base should be parallel with lateral wall for
horizontal and floor / roof for vertical deviation

27.  Simultaneous Prism Cover Test
 Objective method which is used to measure the tropia component of the
monofixation syndrome with superadded heterophoria ( angle inc. on
disassociation ) seen in small angle deviation
 Test – Performed by first estimating the size of the tropia with corneal light reflex
testing.
 Appropriate prism on deviating eye; cover – non-deviating eye
 Remove prism and cover simultaneously – note difference – inc. prism if
needed until there's no movement of eye behind the prism eye ( prism
strength = manifestation)

28.  Maddox Rods
 Subjective method to asses the angle of deviation by prisms
 Used to detect horizontal, vertical and torsional deviation
 Most dissociating test cause both eyes see totally different image
 Has a wash-board appearance which are stacks of multiple high power plus
cylinder lens ( m.c. red colored)
 Spotlight viewed thgh Maddox as line @ 90˚ to groves
 Single Maddox test – for H and V deviation
 Test - When placed over the deviating eye the resultant displaced line
relative to light are seen by fixating eye,- can be neutralized by prism
 Vertical lines for H deviation
 Horizontal lines for V deviation
 Light pass thgh lines – orthophoric / harmonious ARC
 Doesn't distinguish b/w tropia and phoria – has to be aligned first

29.  Prism Reflection Test / Krimsky Test
 Objective method of measuring the angle of manifest deviation
 Aim – equalize corneal reflex so they appear symmetrical
 Indication – To estimate deviation in uncooperative and sensory / poor Vn (
20/400 ↑ ) patients
 Hirschberg corneal light reflex mixed with prisms for measuring strabismus
 Test – Neutralize one eye with appropriate prism – elicitate Hirschberg's on an
accommodative target with pentorch – ↑∕↓ prism until reflex is symmetrical
 Prism on fixating eye with tropia – version movement of both eyes to the apex causing
deviation of light
 Prism on non-fixating eye with tropia - eye directly moves the light reflex to the centre
of the pupil without a version shift
 Any eye can be used except in restriction and paresis
 Here, measure 1˚ deviation – prism on limited rotation
 2˚ deviation – prism on eye with duction
 Alt. prism may be place over the other eye ( deviating) until the image moves in

30. Assessment of Torsion
 Maddox double prism
 Only a qualitative test and cannot differentiate between phoria
and tropia
 Test - Two 3 0r 4 PD base to base prism mounted on a trial frame
bisecting the pupillary axis horizontally causing monocular
diplopia
 Prism eye – parallel lines with vertical spacing
 Binocular view – parallel lines if, no torsion
 Intermediate oblique line appear in presence of torsion
.
• Maddox double prism
• Double Maddox rod

31.  Double Maddox rod
 Measure the angle of torsion and more accurately but, only in
primary position
 Test – two different colored Maddox rods are place in trial frame
in horizontal
 Small vertical prism of 4PD is placed to separate the lines
 Patients is asked to rotate until the lines are parallel
 Result – measuring the angle will give the torsion angle
 In large vertical deviation – prisms can be used to bring the
image near

32. Correction of Deviation
 In heterophoria – Aid to detect if symptoms are ocular or not
 In manifest deviations –
 If normal / abr. BSV is present
 Type of retinal correspondence
 Degree of surgery required
 Out come of Sx if BSV not present
 Correction may be in the form of test / temporary wear using Fresnel
for a short interval
Correction of Deviation
• Test for ocular symptoms
• Assessment of potential BSV
• Prism adaptation Test
• Progressive prism compensation
• Pre-Op Prism Adaptation

33.  Test for ocular symptoms
 To determine that weather the symptoms experienced by the patients are in
consistence with the findings
 Monocular occlusion by removing the effort of controlling the heterophoria
relieve the symptoms
 But – field of Vn is dec. and loss of stereopsis
 So, Fresnel prisms are used to correct the angle and relieving of symptoms if
this effort was the cause
 Assessment of potential BSV
 Correction of angle will enable to regain BSV if present
 After correction of angle and BSV obtained a cover test is performed which
suggest
 If no manifest deviation – NRC and BSV
 Manifest deviation – abnormal BSV

34.  Prism adaptation Test
 Method for determining the amount of surgical correction in patients with
partially accommodative esotropia
 Involves prescribing BO prisms for residual esotropia post full hypermetropic
correction
 Patient is reviewed after 2-3 weeks and evaluated for any reminder deviation
viz over correct if needed
 Fresnel prism placed over the normal eye
1. No manifest deviation with BSV – normal BSV
2. Angle remains same and no BSV – suppression and no BSV
3. Small manifestation with prism and anomalous BSV on testing – some
form of BSV with manifest deviation
4. Similar size of manifest deviation with prism and test – ARC , patient has a
desire to maintain to angle and any inc. in prism inc. deviation

35.  Progressive prism compensation
 In manifest strabismus when prism is placed over one eye fusinoal movement
still occur which are comparatively slower than the normal response –
anomalous movement induced by disparity
 Effect the Sx outcome if strong
 So, repeating test @ 2hrs is advice to check for anomalous movements

36.  Pre-Op Prism Adaptation
 Aim –to obtain max angle of deviation pre-op and then aiming Sx correction at
this angle to dec. under/over correction
 Rx amblyopia (6/12) + alt. prism cover test + Fresnel / split prism for angle
correction
 R/a- 2wks or short duration of few hrs
 Test for sensory fusion and PCT on review
 More than 8∆ – no sensory fusion
 Prism adaptation responder – deviation stabilized at 8∆ and peripheral fusion
 Prism adaptation non-responder – exo deviation / stable angle with no
evidence of sensory fusion / angle built up more than 60∆

37. Test for Retinal Correspondence
 Vertical Prism Red Filter Test
 Used to detect ARC from NRC in patients with suppression by placing 15∆ red
vertical prism over the deviated eye
 ARC – Two vertically displaced images with red over white
 The lights are vertically aligned cause the light in the deviated eye is over
the pseudo-fovea to the true fovea of the normal eye
 NRC with central suppression scotoma – Two lights with V+H displacement
 Cause there is no pseudo-fovea and the center of reference is true fovea of
each eye
.
• Vertical Prism Red Filter Test
• Investigation of suppression

38.  Investigation of suppression
 Prisms can measure the areas of suppression by moving the
image nasally / temporally / vertically / horizontally
 Diplopia will be appreciated if the image is out side the
suppression area
 With prism –
 redirect the image into suppression areas when BSV is absent
 investigate possibility of post-Op diplopia in cases where
there's no potential BSV

39.  Post-Op Diplopia Test (PODT)
 Used to an attempt to predict the intractable diplopia of post-Op, in cases of
where BSV is potentially absent
 Angle of deviation is corrected gradually with prisms (Nv/Dv) and check for
diplopia if present
 Which may occur with an under/full/ over correction

41. Management
Prisms may be used to restore BSV, weather or not diplopia is
present (or) occasionally, to redirect the image on to a suppression
area if potential BSV is not present
Occasionally prisms are also used to separate diplopic image in
absence on fusion potential or a separation area

42.  Relieving Prisms
 Aim – stabilize sensory motor fusion
 Action – optically reduce demand on controlling fusional Vergence
 By moving light closer to fovea – moving it into foveal range – patient vergs –
fusion obtained
 Rx – less than the angle of deviation
 Base – opposite to deviation
 Uses - intermittent strabismus, phorias

43.  Inverse Prisms for training / disruptive
 Aim – To increase fusional Vergence ability
 Action – Optically increases the demand for controlling fusional
Vergence
 Base – Same direction as deviation
 Uses
 Training – used in phorias
 Disruptive – to eliminate ARC
 Inverse Prisms for cosmetic
 Indications – poor prognosis for functional care and doesn't want / not
suitable for Sx
 Aim – make eye look better
 Base – same as the deviation

44.  Yoked Prisms
 Aim – stabilizing binocular vision in non-concomitancy or dampen
nystagmus
 Action – directs the eyes in specific gaze direction.
 Optically moves the retinal images of a fixed target in a parallel towards the
base and moves the light towards the base and shows the target towards
the apex - both eyes move in same direction
 Uses – gaze palsy , Duane’s Syndrome , nystagmus

45.  Sector Prisms
 Aim – stabilize BSV in one / more gazes or distance
 Action – Reduce demand for controlling fusional Vergence in more then
one gaze or distance
 Eg – 20∆ ET @ Dv
10∆ ET @ Nv
 Rotating Prisms
 A method to change sensory input for constant strab. To precipitate a
change from ACR to NRC
 Fresnel prisms – 1 week BO then rotate BU , BI , BD
 Uses – disrupt ARC

46.  Corrective Prisms
 Aim – to stabilize normal sensory fusion
 Action – Optically neutralize the demand for controlling fusional
Vergence by elimination the oculo-motor deviation
 Rx – prism = magnitude of deviation
 Residual Vergence demand = 0
 Base – Opposite to the deviation
 Over- corrective Prisms
 Aim - To disrupt ARC
 Action – Reverse the demand for controlling fusional Vergence and
optically changing the direction of deviation
 Rx – Prism power > magnitude of deviation
 A deviation reverse is seen on cover test and ACT in these cases i.e. and
eso becomes an optical exo

47.  Slab-off Prisms
 An anisometeopic patient may experience – diplopia / asthenopia if
the line of sight doesn’t pass thgh optical center of spectacle
 this is due to displacement induced by net prismatic effect
 Eg - +1.00 OS and – 3.00 OD, will have difficulty even at 1cm below from the
optical center
 Solution
 slab-off Prisms (or) Bicentric Grinding
 Myopes- back ; hypermetropic – front
 Other – C.L , separate glasses for Nv and Dv, lowering optical center to an
intermediate

48.  Aphakia and Pseudophakia following Cataract Surgery
 Diplopia post cataract Sx can been seen as a late complication which
can be treated by prisms
 Most commonly seen with traumatic cataract due to torsion
 Malingering
 Prism Dissociation Test
 For malingering in monocular blindness
 Subjective correction with a 4∆ vertical prism will cause diplopia
 BO prism for the ill eye and when focused to eliminate diplopia malingering is
ruled out


49.  Amblyopia
 Can be use in diagnosis and treatment
 Diagnosis
 10 Prism test ( vertical Prism test / induced tropia test )
 Preverbal with straight eyes / small angle deviation – for accurate diag.
 Test – 10 to 15 BU/BD in front of one eye – induces vert. starb. And fixating
presence can be known
 Inference
1. Spontaneous alteration
2. Hold well – Smooth / blink fixation by other eye by movements for atleast 5 sec.
3. Holds briefly – refixation delayed by 3 sec.
4. Hold momentarily – fixation maintained for 1 -2 sec.
5. Will not hold – refixation as soon as prism is remover
 Treatment – Rarely done when therapy fails
 Most commonly for amblyopia eccentric fixation by passive stimulation of
amblyopic eye with full prism correction + atropinization + Nv correction with + 3 DS

50.  Field Defects
 Prisms can be used in the management of visual field defects - m.c
hemianopia's
 They expands the field of view in the direction of hemianopia's
 Object that falls on the edge of the scotoma of one eye is seen by the
other eye
 15 prism is placed over the effected eye with the base towards the
defect, trimmed to be 2mm away from the mid-pupillary line, avoiding
interference with central vision
 Can be used in stroke, field defects and visual neglect patients

51.  Other uses
 Incorporated into C.L. for vertical diplopia
 For exercise – ti increase fusional convergence
 ARMD – to relocate retinal image to an area of preserved retinal
function
 Bed ridden patients – in ankylosing spondylitis and other postural defect

52. Prisms in ophthalmic instruments
 Reflecting and dove prism are used in almost all of the of the
ophthalmic instrument and operating micro-scopes
 Using the property of total internal reflection prisms have basically
replaced mirrors in SLB, microscopes, ect

53. Thank you