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Prisms and Optical Principles
Optics & Refraction
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91. Physics of a Prism
Exam pearl
- Clinical Analogy: Think of a prism as a 'deflection machine'.
- Just as a river bends around a rock, light bends toward the base of the prism while the image seen by the eye appears displaced toward the apex.
- This single principle underlies clinical applications including strabismus measurement and the prismatic effects of bifocal lenses.
Structure and Geometry
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1A prism is a transparent medium bounded by two non-parallel plane surfaces (the refracting surfaces) meeting at an angle called the apical angle (A). The third surface is the base.
| Component | Definition | Clinical Relevance |
|---|---|---|
| Apex | Thinnest edge; tip of the prism | Image displacement occurs toward apex |
| Base | Thickest edge; opposite the apex | Light bends toward base |
| Apical angle (A) | Angle between the two refracting faces | Determines power of the prism |
| Angle of deviation (D) | Angle between incident and emergent ray | Related to prism diopter (PD) by: • Prism diopter (PD) = 100 × tan(angle of deviation (D)) |
| Minimum deviation | Position where deviation is least; • The refracted ray inside the prism is parallel to the base | Used in spectrometry; • Not the usual dispensing position for ophthalmic glass spectacle prisms • Minimum-deviation positioning may be used with plastic prisms and is difficult to obtain clinically |
| Component | Definition | Clinical Relevance |
|---|---|---|
| Apex | Thinnest edge; tip of the prism | Image displacement occurs toward apex |
| Base | Thickest edge; opposite the apex | Light bends toward base |
| Apical angle (A) | Angle between the two refracting faces | Determines power of the prism |
| Angle of deviation (D) | Angle between incident and emergent ray | Related to prism diopter (PD) by: • Prism diopter (PD) = 100 × tan(angle of deviation (D)) |
| Minimum deviation | Position where deviation is least; • The refracted ray inside the prism is parallel to the base | Used in spectrometry; • Not the usual dispensing position for ophthalmic glass spectacle prisms • Minimum-deviation positioning may be used with plastic prisms and is difficult to obtain clinically |
Snell's Law and Prism Deviation
- For a prism of refractive index n and apical angle A, the deviation D depends on the angle of incidence. At minimum deviation:
- Dmin = 2i – A | n = sin[(A + Dmin)/2] / sin(A/2)
- For small apical angles (thin prisms used clinically): D ≈ (n – 1) × A
- For thin prisms, the net deviation is the vector sum of the individual prism deviations.
Prism Dioptre - Definition and Units
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1- A prism dioptre (∆ or PD) is defined as a deviation of 1 centimetre at 1 metre distance.
- It is the universal clinical unit of prismatic power.
- 1 prism dioptre (PD) = 1 centimetre displacement of an image at 1 metre.
- Conversion: tan(θ°) = prism dioptres/100.
- 1° ≈ 1.75 prism dioptres (PD).
- For small angles: 1° ≈ 2 prism dioptres (∆) is the common approximation.
- 10 prism dioptres (∆) ≈ 5.7°.
- 20 prism dioptres (∆) ≈ 11.3°.
- 45 prism dioptres (∆) ≈ 24.2°.
- TRAP: The relationship 1° = 2 prism dioptres (∆) is only an approximation.
- For an accurate conversion at any deviation, use the tangent formula.
- The Krimsky test uses prisms to measure the deviation directly in prism dioptres (PD).
Prentice's Rule - The Most Tested Formula
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1Prentice's Rule quantifies the prismatic effect at any point on a lens away from the optical centre: • P (prism dioptres) = c (centimetres) × F (dioptres) Where: • P = prismatic power induced • c = distance from optical centre in centimetres • F = lens power in dioptres (use absolute value; sign determines direction) Direction rule: • Convex (+) lens → base DOWN above centre, base UP below centre • Concave (–) lens → base UP above centre, base DOWN below centre
Worked Example: • A +4.00 D lens with optical centre 3 millimetres above the visual axis: • P = 0.3 centimetres × 4 D = 1.2∆ base UP (because looking through the lens below its optical centre produces a base-up effect)
| Lens Type | Looking Up (above optical centre) | Looking Down (below optical centre) | Looking In (nasal) | Looking Out (temporal) |
|---|---|---|---|---|
| +Convex | Base DOWN | Base UP | Base OUT | Base IN |
| –Concave | Base UP | Base DOWN | Base IN | Base OUT |
| Lens Type | Looking Up (above optical centre) | Looking Down (below optical centre) | Looking In (nasal) | Looking Out (temporal) |
|---|---|---|---|---|
| +Convex | Base DOWN | Base UP | Base OUT | Base IN |
| –Concave | Base UP | Base DOWN | Base IN | Base OUT |
- TRAP: In anisometropia, differential prismatic effect at near (looking down through reading area) creates vertical imbalance.
- This is the basis for slab-off prism prescription - always prescribed in the MORE MINUS (or less plus) eye.
2. Types of Prisms and Their Clinical Uses
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1Standard Ophthalmic Prisms
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1| Prism Type | Construction | Key Use | Advantage/Limitation |
|---|---|---|---|
| Loose prism bar | • Calibrated prism bar • Base-in (BI) • Base-out (BO) • Base-up (BU) • Base-down (BD) | • Cover test • Prism and alternate cover test (PACT) | • Prisms of different orientations can be combined for horizontal and vertical deviations |
| Fresnel press-on prism | • Polyvinyl chloride (PVC) membrane with microprisms • Press onto spectacle lens | • Large deviations • Postoperative use • Temporary use • Diplopia treatment | • May reduce visual acuity (VA) • May cause chromatic aberrations, particularly at higher powers • May be unsuitable for tasks requiring fine visual resolution |
| Ground-in prism | • Incorporated into spectacle lens during grinding | • Long-term correction of small deviations | • Can exceed 8 prism dioptres per lens • Higher powers make lenses thicker, heavier, and more expensive |
| Risley rotary prism | • Two prisms counter-rotated • Power varies 0–30 prism dioptres | • Binocular vision assessment • Smooth vergence testing | |
| Maddox rod with prism bar | • Used with Maddox rod | • Dissociation • Subjective deviation measurement | • Subjective • Requires patient cooperation |
| Base-out prism test | • 4 prism dioptres base-out (BO) placed before one eye | • Microtropia/monofixation detection | • Classic diagnostic use |
| Prism Type | Construction | Key Use | Advantage/Limitation |
|---|---|---|---|
| Loose prism bar | • Calibrated prism bar • Base-in (BI) • Base-out (BO) • Base-up (BU) • Base-down (BD) | • Cover test • Prism and alternate cover test (PACT) | • Prisms of different orientations can be combined for horizontal and vertical deviations |
| Fresnel press-on prism | • Polyvinyl chloride (PVC) membrane with microprisms • Press onto spectacle lens | • Large deviations • Postoperative use • Temporary use • Diplopia treatment | • May reduce visual acuity (VA) • May cause chromatic aberrations, particularly at higher powers • May be unsuitable for tasks requiring fine visual resolution |
| Ground-in prism | • Incorporated into spectacle lens during grinding | • Long-term correction of small deviations | • Can exceed 8 prism dioptres per lens • Higher powers make lenses thicker, heavier, and more expensive |
| Risley rotary prism | • Two prisms counter-rotated • Power varies 0–30 prism dioptres | • Binocular vision assessment • Smooth vergence testing | |
| Maddox rod with prism bar | • Used with Maddox rod | • Dissociation • Subjective deviation measurement | • Subjective • Requires patient cooperation |
| Base-out prism test | • 4 prism dioptres base-out (BO) placed before one eye | • Microtropia/monofixation detection | • Classic diagnostic use |
Fresnel Prisms - Expanded
Fresnel (press-on) prisms are made of flexible optical-grade polyvinyl chloride (PVC) with a series of angular grooves on one side. The smooth side is applied to the lens surface with water.
- Available powers: 1–40 prism dioptres (∆) (standard kit)
- Visual acuity (VA) and contrast sensitivity decrease as Fresnel prism power increases; higher powers may be less well tolerated.
- Chromatic aberration - polyvinyl chloride (PVC) Fresnel prisms increase chromatic dispersion.
- Reduced contrast sensitivity - contrast sensitivity decreases as Fresnel prism power increases.
- Indicated when: Fourth nerve palsy with small vertical deviation; thyroid-related orbitopathy with diplopia; neurological diplopia awaiting recovery; preoperative assessment; postoperative diplopia.
Exam pearl
- Fresnel prisms may reduce visual acuity, particularly at higher prism powers. Warn patients that they may cause blur and reduced visual acuity. Clean prism grooves of dust or debris; refit or replace the prism if it peels, curls, or no longer lies flat.
Reflecting Prisms
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1| Prism | Type of Reflection | Application |
|---|---|---|
| Porro prism | • Two total internal reflection (TIR) reflections • Image inverted (single Porro prism) | • Binoculars • Telescopes |
| Dove prism | • Single total internal reflection (TIR) • Inverts image | • Rotating image systems |
| Roof (Abbe) prism | • Roof edge • Compact image erection | • Compact binoculars |
| Pechan prism | • Multiple internal reflections • Image erect | • Endoscopes • Periscopes |
| Amici prism | • Roof + total internal reflection (TIR) • Rotates image 180° about the optical axis | • Slit-lamp design |
| Prism | Type of Reflection | Application |
|---|---|---|
| Porro prism | • Two total internal reflection (TIR) reflections • Image inverted (single Porro prism) | • Binoculars • Telescopes |
| Dove prism | • Single total internal reflection (TIR) • Inverts image | • Rotating image systems |
| Roof (Abbe) prism | • Roof edge • Compact image erection | • Compact binoculars |
| Pechan prism | • Multiple internal reflections • Image erect | • Endoscopes • Periscopes |
| Amici prism | • Roof + total internal reflection (TIR) • Rotates image 180° about the optical axis | • Slit-lamp design |
Exam pearl
- Total Internal Reflection (TIR) occurs when light hits the interface at an angle > critical angle (C), with light travelling from higher to lower refractive index.
- For a glass–air interface, sin C = 1/n; generally, sin C = n₂/n₁ (n₁ > n₂).
- For glass (n=1.5), C ≈ 41.8°.
- Prism binoculars use total internal reflection to fold the optical path, shortening instrument length.
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Keep reading Prisms and Optical Principles
10 more sections are included with the Clinical plan.
- Prisms in Strabismus - Clinical Framework
- Prism and Alternate Cover Test (PACT)
- Therapeutic Uses of Prisms in Specific Conditions
- Prisms in Spectacle Design
- Prisms in Ophthalmic Instruments
- Dispersion, Chromatic Aberration and Abbe Number