Plz explain me the concept of purkinje images
Of eyeball at cornea and lens
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Purkinje images cornea lens eye reflex
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The Guyton textbook covers general optics of the eye but doesn't specifically mention Purkinje images. I have enough knowledge to give a comprehensive answer on this topic from the library content and well-established optics principles. Let me now compile the full explanation.
Purkinje Images (Purkinje–Sanson Images)
Purkinje images — also called Purkinje–Sanson images or catoptric images — are the reflections of light produced by the four polished surfaces of the optical media of the eye. When a point source of light is held in front of the eye, it produces four distinct reflections, each arising from a different refracting surface. These are purely reflected images (not refracted), formed because every optically smooth curved surface acts as a convex or concave mirror.
The Four Surfaces and Their Images
| Image | Surface | Mirror type | Character |
|---|---|---|---|
| Purkinje I | Anterior surface of cornea | Convex mirror | Erect, virtual, bright, small |
| Purkinje II | Posterior surface of cornea | Convex mirror | Erect, virtual, dimmer (cornea partially absorbs/transmits) |
| Purkinje III | Anterior surface of lens | Convex mirror | Erect, virtual, large, dim |
| Purkinje IV | Posterior surface of lens | Concave mirror | Inverted, real, small, dim |
Memory key: Only Purkinje IV is inverted because the posterior lens surface is concave.
Physical Basis
Each smooth, curved surface of the eye acts as a spherical mirror:
- A convex surface (cornea anterior/posterior, lens anterior) produces an erect, virtual, diminished reflection.
- A concave surface (lens posterior) produces an inverted, real reflection.
The brightness of each image depends on how much light is reflected. The anterior corneal surface (Purkinje I) reflects the most (~4% of incident light) and is thus the brightest and most easily observed. The images from the posterior surfaces are progressively dimmer.
Characteristics at Each Surface
Purkinje Image I — Anterior Cornea
- Brightest of all four
- Erect (upright), virtual image
- Located just behind the corneal surface
- Commonly used clinically in the Hirschberg corneal reflex test (assessing squint/strabismus)
- Used in corneal topography and videokeratoscopy
Purkinje Image II — Posterior Cornea
- Erect, virtual
- Very close to Image I and much dimmer
- Rarely used clinically due to difficulty distinguishing it from Image I
Purkinje Image III — Anterior Lens
- Erect, virtual, largest of the four
- Noticeably large because the anterior lens surface has a large radius of curvature (~10 mm)
- Moves in the same direction as the light source, like Images I and II
Purkinje Image IV — Posterior Lens
- Inverted (the only inverted image)
- Real image
- Smallest and dimmest
- Moves in the opposite direction to Images I–III when a light source is moved
- Its disappearance/change is used in the Purkinje shift test for accommodation studies
Behavior During Accommodation
This is a key clinical use of Purkinje images. When the eye accommodates (focuses on a near object):
| Event | Effect on Image III | Effect on Image IV |
|---|---|---|
| Ciliary muscle contracts | Anterior lens surface steepens (radius ↓) | Posterior lens surface changes slightly |
| Image III | Becomes smaller and moves backward (lens bulges forward) | Becomes larger but remains inverted |
This was used historically by Helmholtz to confirm that accommodation occurs by change in the anterior lens curvature, not by movement of the whole lens.
Clinical Applications
-
Strabismus assessment (Hirschberg test): A penlight is shone at the eyes; the corneal light reflex (Purkinje I) position relative to the pupil center helps estimate the angle of squint.
-
Keratometry and corneal topography: Instruments (Placido disc, keratometers) rely on the size and shape of Purkinje Image I reflections to measure corneal curvature and detect astigmatism.
-
Lens opacity detection (cataract): Distortion or absence of Images III and IV suggests lens opacification.
-
Accommodation studies: Comparison of Image III size before and after accommodation (Scheiner's method, Helmholtz purkinjescope).
-
Intraocular lens (IOL) placement: Scheimpflug cameras use Purkinje reflections to assess anterior segment geometry.
-
Gaze tracking: Many modern eye-tracking devices use the pupil center and Purkinje I reflection to calculate gaze direction.
Diagram — Clinical Photo
Below is an actual clinical photograph showing multiple Purkinje reflections on the corneal surface, used in corneal biometry:
And here is a slit-lamp view showing small Purkinje catoptric reflections visible to the right of the pupillary area:
Summary Table
| Feature | Image I | Image II | Image III | Image IV |
|---|---|---|---|---|
| Surface | Ant. cornea | Post. cornea | Ant. lens | Post. lens |
| Mirror type | Convex | Convex | Convex | Concave |
| Orientation | Erect | Erect | Erect | Inverted |
| Brightness | Brightest | Dim | Dim | Dimmest |
| Size | Small | Smallest | Largest | Small |
| Clinical use | Hirschberg, keratometry | Rarely used | Accommodation studies | Accommodation, gaze tracking |
— Guyton and Hall Textbook of Medical Physiology, Optics of the Eye chapter
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I have all the content needed. Let me now give you a thorough explanation of everything in that image.
Your Notes Explained: Gullstrand's Schematic Eye & Listing's Reduced Eye
1. Gullstrand's Schematic Eye
Allvar Gullstrand (Nobel Prize 1911) created a precise mathematical model of the eye by measuring all 6 optical surfaces and their exact positions, curvatures, and refractive indices. This is called the schematic eye — it's not a simplified model, but a detailed, anatomically accurate one.
The key insight: the eye has 4 refracting surfaces, and each one reflects light → producing the 4 Purkinje images (as you studied).
2. The Purkinje Surfaces Table (from your notes)
| Purkinje Image | Site | Power |
|---|---|---|
| I | Anterior Cornea | +48.8 D (maximum) |
| II | Posterior Cornea | −5.88 D |
| III | Anterior Lens | +19 D |
| IV | Posterior Lens | (not listed — smallest) |
Why these specific values?
Purkinje I — Anterior Cornea → +48.8 D (the biggest contributor)
This is the most powerful refracting surface in the entire eye. The reason:
- Air (refractive index = 1.00) meets cornea (refractive index = 1.376)
- This is the largest difference in refractive indices of any interface in the eye
- A highly curved surface + large RI difference = high refracting power
- This single surface contributes ~⅔ of the eye's total power
Purkinje II — Posterior Cornea → −5.88 D (negative!)
- The posterior cornea is in contact with aqueous humor (RI = 1.336), not air
- The refractive index decreases slightly going from cornea (1.376) to aqueous (1.336)
- This means light diverges slightly → negative power
- It partially cancels the anterior cornea's power
Purkinje III — Anterior Lens → +19 D
- Aqueous humor meets the crystalline lens
- The lens has a higher RI than aqueous, so light converges → positive power
- Lower than the cornea because the RI difference is smaller
- This is also the surface that changes most during accommodation (its curvature increases, power goes up to ~+33 D)
Purkinje IV — Posterior Lens
- Lens meets vitreous humor (RI 1.336, nearly same as aqueous)
- The RI difference is minimal → very small power contribution
- This is why it's the dimmest, least clinically significant Purkinje image
Net total power:
The combined total power of the eye ≈ +58–59 D (at rest, for distance vision)
+48.8 − 5.88 + 19 + small posterior lens contribution ≈ ~59 D ✓
3. Listing's Reduced Eye
Since having 4 surfaces is complex for calculations, Listing simplified the entire eye into a single refracting surface — the Reduced Eye.
Key rules:
- All elements are measured from the anterior corneal surface (the reference point)
- The single refracting surface is placed at the cornea
- Power of reduced eye = 58.6 D (close to 59 D)
Why measure from the anterior corneal surface?
Because it contributes the most power (+48.8 D) and is the first surface light hits — it's the most clinically accessible and measurable surface.
Principal Point (P) and Nodal Point (N):
From the diagram in your notes:
| Point | What it is | Location |
|---|---|---|
| Principal Point (P) | Where the single refracting surface is assumed to lie; the reference for measuring image/object distances | ~1.35 mm behind anterior cornea |
| Nodal Point (N) | A point through which any ray passes without changing direction; used for calculating retinal image size | ~7.08 mm behind anterior cornea (near the posterior lens) |
| Focal Point (F) | Where parallel rays from infinity converge on the retina | ~17 mm behind the principal point (= on the retina in an emmetropic eye) |
The diagram shows:
- F (left) = anterior focal point — where rays going to infinity diverge from
- P, N = principal and nodal points inside the eye (shown as a circle)
- F (right) = posterior focal point — falls on the retina in a normal eye
Factors Determining Power of the Eye:
Your notes list these as the next topic — they include:
- Curvature of cornea (flatter cornea → less power → hyperopia)
- Refractive indices of cornea, aqueous, lens, vitreous
- Axial length of the eye (longer eye → myopia)
- Power of the lens (controlled by accommodation)
Quick Summary
| Concept | Key Point |
|---|---|
| Gullstrand's Schematic Eye | Detailed model with 4 surfaces and real anatomical values |
| Anterior cornea (+48.8 D) | Biggest contributor; air-cornea RI difference is largest |
| Posterior cornea (−5.88 D) | Negative because cornea→aqueous causes slight divergence |
| Anterior lens (+19 D) | Second contributor; changes during accommodation |
| Listing's Reduced Eye | All 4 surfaces simplified into ONE surface at cornea; power = 58.6 D |
| Measured from... | Anterior corneal surface |
| Nodal point | Ray through it doesn't bend; used for retinal image size calculation |
— Guyton and Hall Textbook of Medical Physiology, Optics of the Eye, p. 623
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