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Luminance and retinal illumination

The range of luminances (left) and retinal illumination (right) found in the natural world

Sun

Fluorescent light/bulbs

White paper, full sunlight Candle flame

Comfortable reading

Print read with difficulty White surface, moonlight Threshold for cone vision

White surface, moonless night

Visual threshold

How can we see over 15 log units of illumination?

•  1. Duplex function •  2. Cellular adaptation

Rod and cone operating ranges

cone vision

day

LUMINANCE RANGE

rod vision

Duplex function

•  1. Rods are more sensitive than cones (x50)

•  2. There are more rods than cones (x10)

•  3. Ganglion cells have larger RFs for rods than cones (i.e. more post-receptoral summation)

2. Distribution of rods and cones visual eccentricity (deg)

spatial density (cells/square mm)

macula lutea

cones

rods

retinal eccentricity (mm)

Convergence

receptors130 million

bipolars20 million

ganglion cells1 million

optic nerve fibres

3. Spectral sensitivity curves for rod and cone vision

Rela

tive

sen

siti

vity

Wavelength (nm)

Purkinje effect

•  A shift in the colour appearance at dusk. •  Reds look darker, blues look brighter

1. Contrast sensitivity functions at three different light levels

Spatial Frequency (cycles/mm on retina)

Spatial frequency (cycles/degree) Se

nsit

ivit

y (1

/thr

esho

ld c

ontr

ast)

Contrast sensitivity and operating range

Large operating range but poor contrast sensitivity

Relative light intensity Intensity

Brig

htne

ss

Brig

htne

ss

Good contrast sensitivity but small operating range

Receptoral adaptation

Ganglion cell adaptation

I1 I2 I3 Re

spon

se (i

ps)

1 1.5 10 15 100 150 Flash intensity log

Mean light levels

I I

I = 0.5 I = 5 I = 50

A single neuron can shift its operating range according to the mean light level. The light increment (delta I) required to obtain a criterion response is scaled up or down, according to the mean light level. This is known as GAIN CONTROL.

Increment threshold curve

WEBER’S LAW

Log background intensity (I)

Log

incr

emen

t th

resh

old

(del

ta I

)

Weber’s Law

∆ I/I = constant

•  Our sensation is determined by the percentage difference in the luminance of a surface relative to its background

•  This holds over a wide range of background (ambient) luminances

4. Sensitivity to light of rods & cones: Dark Adaptation

Low Lo

g. l

ight

sen

siti

vity

High Time in dark (min)

7 minutes

Dark adaptation curves Low Lo

g. l

ight

sen

siti

vity

High Time in dark (min)

Cones Rods

Rods & cones: 4 key differences between scotopic and photopic vision •  Contrast sensitivity

•  Distribution of rods and cones

•  Spectral sensitivity of rods and cones

•  Sensitivity to light of rods and cones.