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Interferometry and Optical Coherence
Optics & Refraction
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31. Introduction - The Physics Beneath the Clinic
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- Clinical Analogy: Think of two stone-drops in a still pond. Where the ripples overlap, they either amplify (constructive interference) or cancel (destructive interference). Light behaves identically - and ophthalmology has weaponised this principle into instruments that measure the eye with nanometre precision.
- Interferometry is the science of using the interference of light waves to make ultra-precise measurements.
- In ophthalmology, it underpins:
- Optical Coherence Tomography (OCT) - cross-sectional retinal imaging
- Laser Interferometry / Coherence Interferometry - preoperative visual potential assessment
- Partial Coherence Interferometry (PCI) - biometry (IOLMaster, Lenstar)
- Specular Microscopy - corneal endothelial imaging
- Wavefront Sensing (Hartmann-Shack) - aberrometry
2. Fundamental Wave Physics
Wave Properties of Light
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1- Light behaves as a transverse electromagnetic wave characterised by: wavelength (λ), frequency (f), amplitude (A), and phase (φ).
- Two waves interact to produce interference.
Interference - Constructive vs Destructive
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1| Parameter | Constructive Interference | Destructive Interference |
|---|---|---|
| Phase difference | • 0° • 360° • 720° • (multiples of 2π) | • 180° • 540° • (odd multiples of π) |
| Path difference | nλ (n = 0,1,2,3…) | (n + ½)λ |
| Resultant amplitude | A1 + A2 (maximum) | |A1 − A2| (minimum/zero if equal) |
| Resultant intensity | Maximum - bright fringe | Minimum - dark fringe |
| Clinical use | Signal peaks in optical coherence tomography (OCT) | Null zones used in interferometry |
| Parameter | Constructive Interference | Destructive Interference |
|---|---|---|
| Phase difference | • 0° • 360° • 720° • (multiples of 2π) | • 180° • 540° • (odd multiples of π) |
| Path difference | nλ (n = 0,1,2,3…) | (n + ½)λ |
| Resultant amplitude | A1 + A2 (maximum) | |A1 − A2| (minimum/zero if equal) |
| Resultant intensity | Maximum - bright fringe | Minimum - dark fringe |
| Clinical use | Signal peaks in optical coherence tomography (OCT) | Null zones used in interferometry |
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- Intensity ∝ Amplitude². Equal amplitudes in destructive interference → complete cancellation. This is exploited in anti-reflection coatings on spectacle lenses.
Coherence - The Critical Concept
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1Exam pearl
- Clinical Analogy: Coherence is like two singers staying perfectly in tune.
- Temporal coherence = the electric field at one point maintains a stable phase relationship over time.
- Spatial coherence = electric fields at different positions across the beam maintain a stable phase relationship.
- A sufficiently coherent source, with spatial and temporal overlap and suitable polarization, produces stable, measurable interference fringes; reduced coherence lowers fringe visibility and may wash out the pattern.
Coherence has two complementary aspects:
- Temporal Coherence: Phase relationship maintained over time → characterised by coherence length (Lc)
- – Coherence length (Lc) ≈ λ²/∆λ (where ∆λ = spectral bandwidth; the numerical factor depends on the spectral shape and definition used)
- – Narrow bandwidth (e.g., single-frequency laser) → long coherence length (Lc) → high temporal coherence
- – Broad bandwidth (superluminescent diode (SLD)/white light) → short coherence length (Lc) → reduced temporal coherence - used in optical coherence tomography (OCT)!
- Spatial Coherence: Phase relationship maintained across the wavefront width
- – Point source → high spatial coherence
- – Extended incoherent source → low spatial coherence
| Property | Laser (helium–neon, diode) | Superluminescent diode | White light |
|---|---|---|---|
| Bandwidth | • Narrow (source dependent) [14] | • Moderate (20–100 nm) [14] | • Very broad (source dependent) [14] |
| Coherence length | • Long (source dependent) [15] | • Short (source dependent) [15] | • Very short (~1 µm) |
| Temporal coherence | • Very high [15] | • Low [15] | • Very low [16] |
| Spatial coherence | • Very high [16] | • High | • Low (source dependent) [16] |
| Ophthalmic use | • Laser interferometry [17] | • Optical coherence tomography (time-domain and spectral-domain) [18] | • Specular microscopy |
| Axial resolution | • Poor depth discrimination | • High axial (depth) resolution in optical coherence tomography [19] | • Poor depth discrimination |
| Property | Laser (helium–neon, diode) | Superluminescent diode | White light |
|---|---|---|---|
| Bandwidth | • Narrow (source dependent) [14] | • Moderate (20–100 nm) [14] | • Very broad (source dependent) [14] |
| Coherence length | • Long (source dependent) [15] | • Short (source dependent) [15] | • Very short (~1 µm) |
| Temporal coherence | • Very high [15] | • Low [15] | • Very low [16] |
| Spatial coherence | • Very high [16] | • High | • Low (source dependent) [16] |
| Ophthalmic use | • Laser interferometry [17] | • Optical coherence tomography (time-domain and spectral-domain) [18] | • Specular microscopy |
| Axial resolution | • Poor depth discrimination | • High axial (depth) resolution in optical coherence tomography [19] | • Poor depth discrimination |
MCQ Trap
- Optical coherence tomography (OCT) uses a SHORT coherence length source (broad bandwidth superluminescent diode (SLD)), NOT a laser.
- The short coherence length is what gives optical coherence tomography (OCT) its axial resolution.
- A laser's long coherence length would make depth discrimination impossible.
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Keep reading Interferometry and Optical Coherence
12 more sections are included with the Clinical plan.
- The Michelson Interferometer - Foundation Principle
- Laser Interferometry - Potential Acuity Assessment
- Partial Coherence Interferometry (PCI) - Optical Biometry
- OCT - Interferometric Cross-Sectional Imaging
- Interference in Clinical Optics - Everyday Applications
- Wavefront Sensing and Aberrometry