1. Light and Dark Adaptation
Mohammad Arman Bin Aziz
Faculty, ICO

2. Light and Dark adaptation
• The range of light levels over which we can see
• Why do we need adaptation?
• The design problem
• Role of pupil changes
• The duplex retina: four comparisons of rod-
based and cone-based vision
• Dark adaptation and pigment bleaching
• Light adaptation:
Weber’s Law
The response of visual neurons

3. 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

4. Rod and cone operating ranges
cone vision
day
LUMINANCE RANGE
rod vision

5. Light reflected from a surface under
low and high illumination
The ratio of light intensities reflected from the white
surround and the black letter is 9:1 under both low and
high illuminations.

6. Consider and comment:
A white page inside a room reflects less
light than a black stone on a sunny
beach, yet the page looks white and the
stone looks black

7. The sensations of blackness and
lightness depend on the contrast
of the stimulus, not on the
absolute amount of light
reflected off any one part of it.

8. Luminance difference between ‘L’ and background =
80 units 8000 units
Background =
90 units 9000 units
Response  Difference/Background
80/90 (89%) 8000/9000 (89%)

9. A piece of white paper that is dimly lit (A) looks white because its luminance
lies at the top of its local scale, even though this luminance may be less than
that of a piece of black paper that is brightly lit (B).
log illuminance
logluminance
The eye’s sliding scale of brightness

10. Mechanisms that enable us to see over
a wide range of light intensities:
• Pupil changes
• Duplex retina: rods & cones 
• Dark adaptation & pigment bleaching
• Light adaptation of the visual system
and individual neurons (Webers Law)

11. 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.

12. 1. Contrast sensitivity functions at
three different light levels
Spatial Frequency (cycles/mm on retina)
Spatial frequency (cycles/degree)Sensitivity(1/thresholdcontrast)

13. 2. Distribution of rods and cones
visual eccentricity (deg)
spatial density
(cells/square mm) macula lutea
cones
rods
retinal eccentricity (mm)

14. Convergence
receptors
130 million
bipolars
20 million
ganglion cells
1 million
optic nerve fibres

15. 3. Spectral sensitivity curves for rod
and cone vision
Relativesensitivity
Wavelength (nm)

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

17. 4. Sensitivity to light of rods &
cones: Dark Adaptation
Low
Log.lightsensitivity
High
Time in dark (min)
7 minutes

18. 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)

19. Dark adaptation curves
LowLog.lightsensitivity
High
Time in dark (min)
Cones
Rods

20. The “design” problem
• To detect differences in luminances across
the visual scene
• Scale the response to these differences
according to the ambient light level
In solving the problem, we must:
1. Have good sensitivity to luminance
differences
2. Be able to operate across a wide range of
ambient light levels
3. Cope with a limited neural response range

21. Contrast sensitivity and operating range
Large operating range but
poor contrast sensitivity
Relative light intensity Intensity
Brightness
Brightness
Good contrast sensitivity but
small operating range

22. Increment threshold curve
WEBER’S
LAW
Log background intensity (I)
Logincrementthreshold(deltaI)

23. 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

24. I1 I2 I3
Response(ips)
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.

25. Ganglion cell adaptation

26. Receptoral adaptation