Successfully reported this slideshow.
We use your LinkedIn profile and activity data to personalize ads and to show you more relevant ads. You can change your ad preferences anytime.
Light and Dark Adaptation
Mohammad Arman Bin Aziz
Faculty, ICO
Light and Dark adaptation
• The range of light levels over which we can see
• Why do we need adaptation?
• The design prob...
Luminance and retinal illumination
The range of luminances (left) and retinal illumination (right)
found in the natural wo...
Rod and cone operating ranges
cone vision
day
LUMINANCE RANGE
rod vision
Light reflected from a surface under
low and high illumination
The ratio of light intensities reflected from the white
sur...
Consider and comment:
A white page inside a room reflects less
light than a black stone on a sunny
beach, yet the page loo...
The sensations of blackness and
lightness depend on the contrast
of the stimulus, not on the
absolute amount of light
refl...
Luminance difference between ‘L’ and background =
80 units 8000 units
Background =
90 units 9000 units
Response  Differen...
A piece of white paper that is dimly lit (A) looks white because its luminance
lies at the top of its local scale, even th...
Mechanisms that enable us to see over
a wide range of light intensities:
• Pupil changes
• Duplex retina: rods & cones 
•...
Rods & cones: 4 key differences
between scotopic and photopic vision
• Contrast sensitivity
• Distribution of rods and con...
1. Contrast sensitivity functions at
three different light levels
Spatial Frequency (cycles/mm on retina)
Spatial frequenc...
2. Distribution of rods and cones
visual eccentricity (deg)
spatial density
(cells/square mm) macula lutea
cones
rods
reti...
Convergence
receptors
130 million
bipolars
20 million
ganglion cells
1 million
optic nerve fibres
3. Spectral sensitivity curves for rod
and cone vision
Relativesensitivity
Wavelength (nm)
Purkinje effect
• A shift in the colour appearance at dusk.
• Reds look darker, blues look brighter
4. Sensitivity to light of rods &
cones: Dark Adaptation
Low
Log.lightsensitivity
High
Time in dark (min)
7 minutes
Duplex function
• 1. Rods are more sensitive than cones (x50)
• 2. There are more rods than cones (x10)
• 3. Ganglion cell...
Dark adaptation curves
LowLog.lightsensitivity
High
Time in dark (min)
Cones
Rods
The “design” problem
• To detect differences in luminances across
the visual scene
• Scale the response to these differenc...
Contrast sensitivity and operating range
Large operating range but
poor contrast sensitivity
Relative light intensity Inte...
Increment threshold curve
WEBER’S
LAW
Log background intensity (I)
Logincrementthreshold(deltaI)
Weber’s Law
∆I/I = constant
• Our sensation is determined by the
percentage difference in the luminance
of a surface relat...
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 ...
Ganglion cell adaptation
Receptoral adaptation
Upcoming SlideShare
Loading in …5
×

Light and Dark Adaptation

3,384 views

Published on

Published in: Education

Light and Dark Adaptation

  1. 1. Light and Dark Adaptation Mohammad Arman Bin Aziz Faculty, ICO
  2. 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. 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. 4. Rod and cone operating ranges cone vision day LUMINANCE RANGE rod vision
  5. 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. 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. 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. 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. 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. 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. 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. 12. 1. Contrast sensitivity functions at three different light levels Spatial Frequency (cycles/mm on retina) Spatial frequency (cycles/degree)Sensitivity(1/thresholdcontrast)
  13. 13. 2. Distribution of rods and cones visual eccentricity (deg) spatial density (cells/square mm) macula lutea cones rods retinal eccentricity (mm)
  14. 14. Convergence receptors 130 million bipolars 20 million ganglion cells 1 million optic nerve fibres
  15. 15. 3. Spectral sensitivity curves for rod and cone vision Relativesensitivity Wavelength (nm)
  16. 16. Purkinje effect • A shift in the colour appearance at dusk. • Reds look darker, blues look brighter
  17. 17. 4. Sensitivity to light of rods & cones: Dark Adaptation Low Log.lightsensitivity High Time in dark (min) 7 minutes
  18. 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. 19. Dark adaptation curves LowLog.lightsensitivity High Time in dark (min) Cones Rods
  20. 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. 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. 22. Increment threshold curve WEBER’S LAW Log background intensity (I) Logincrementthreshold(deltaI)
  23. 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. 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. 25. Ganglion cell adaptation
  26. 26. Receptoral adaptation

×