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
40.
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
