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Light and Dark Adaptation: Understanding Vision in Different Light Conditions, Exercises of Law

The concept of light and dark adaptation, discussing the range of light levels in the natural world, the importance of adaptation, the role of pupil changes, and the differences between rod-based and cone-based vision. It also covers the concept of Weber's Law and the response of visual neurons.

What you will learn

  • What are the differences between rod-based and cone-based vision?
  • How does Weber's Law apply to the human visual system?
  • What is the importance of light and dark adaptation in vision?
  • How does the human eye adapt to different light conditions?
  • What role do pupil changes play in light and dark adaptation?

Typology: Exercises

2021/2022

Uploaded on 09/12/2022

robinhood05
robinhood05 🇬🇧

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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
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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

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

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.

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

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

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 log luminance

The eye’s sliding scale of

brightness

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

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

3. Spectral sensitivity curves for

rod and cone vision

Relative sensitivity Wavelength (nm)

  1. Sensitivity to light of rods & cones: Dark Adaptation Low

Log. light sensitivity

High Time in dark (min) 7 minutes

Duplex function

    1. Rods are more sensitive than cones (x50)
    1. There are more rods than cones (x10)
    1. Ganglion cells have larger RFs for rods than cones (i.e. more post-receptoral summation)

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

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