Photoreceptors

The Retina

The retina is a layer of photoreceptors (light-detecting cells) that convert light into neural signals.

Types of Photoreceptors

Type	Function	Location
Cones	Daylight vision, colour	Concentrated in fovea
Rods	Night vision, motion	Peripheral retina

Cone Cells: Colour Vision

Cones are responsible for colour perception. Three types exist, each sensitive to different wavelengths:

Cone Type	Peak Sensitivity	Colour
L-cones	Long wavelength (~560 nm)	Red
M-cones	Medium wavelength (~530 nm)	Green
S-cones	Short wavelength (~420 nm)	Blue

Cone Response Formula

Each cone type outputs a single value—the total light absorbed:

$L_{\text{response}} = \int L(\lambda) \cdot S_L(\lambda) \, d\lambda$

where $L(\lambda)$ is the light spectrum and $S_L(\lambda)$ is the L-cone sensitivity function.

Limitations of Cone Vision

We cannot distinguish between:

• Different spectra that produce the same L, M, S responses (metamers)
• A single wavelength from a mixture that produces the same response

Rod Cells: Night Vision

Rods are specialised for low-light conditions:

• More sensitive than cones (single photon detection possible)
• Single type—no colour discrimination in dim light
• Concentrated in peripheral retina
• Responsible for motion detection in peripheral vision

Distribution Across Retina

Region	Cone Density	Rod Density
Fovea centre	Maximum	Zero
Fovea periphery	High	Low
Peripheral retina	Low	High

Achromatic vs Chromatic Mechanisms

Achromatic (Luminance)

Combines L and M cone signals (L+M):

• Provides luminance perception
• High spatial resolution
• Motion detection

Chromatic (Colour)

Compares cone outputs:

• L−M: Red-green opponent
• S−(L+M): Blue-yellow opponent

Light Adaptation

The eye adapts to different light levels:

Adaptation State	Dominant Receptor	Characteristics
Scotopic	Rods	Dim light, no colour, high sensitivity
Mesopic	Mixed	Twilight, limited colour
Photopic	Cones	Daylight, full colour, lower sensitivity

Adaptation Time

• Dark adaptation: ~30 minutes to full sensitivity
• Light adaptation: Seconds

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Summary

• Cones provide colour vision; rods provide night vision
• Three cone types (L, M, S) with different spectral sensitivities
• Rods are more sensitive but do not detect colour
• The fovea has maximum cone density; periphery has more rods

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Past Paper Questions

2024 Paper 3 Question 3: Explain the role of cones and rods in human vision. How do their distributions differ across the retina?

2023 Paper 3 Question 3: Why do we lose colour perception in dim light?

2022 Paper 3 Question 3: What are the three types of cone cells? What is their peak sensitivity?