the first steps in vision: seeing stars - biomotion lab · the first steps in vision: seeing stars...
TRANSCRIPT
1/17/2011
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The First Steps in Vision: Seeing Stars
2Outline
What is light?
• Types of waves
• Dual nature of light: waves and particles
• Spectrum of electromagnetic radiation
• Intensity, wavelength, polarization, direction
• Range of light intensities
• Interactions between light and matter
Anatomy and function of the eye
• univariance principle
• eye cups
• pinhole eye
• lens eye
Optics of the eye
• accommodation
• refractive errors
• role of the pupil
To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree.
Charles Darwin
Additional reading
Richard Dawkins (1996). Climbing Mount Improbable. W.W. Norton & Company. A wonderful text about how evolution works. Chapter 5 describes the 40 different ways of designing light sensing organs that nature came up with.
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Longitudinal
Transverse
Types of waves
wavelength λ [m]
frequency f [Hz] (1/s, number of waves per second)
speed c [m/s]
λ
λ ∗ f = c
speed of light: 300,000,000 m/s
Wavelength, frequency, speed
speed of sound: 340 m/s
-meter 100
kilo- 103
mega- 106
giga- 109
tera- 1012
Big numbers and small numbers
pico- 10-12
nano- 10-9
micro- 10-6
milli- 10-3
distance earth - sun 150 x 109 m
distance earth - moon 380 x 106 m
wavelength of green light: 500 x 10-9 m
The spectrum of electromagnetic radiation
Light: A wave; a stream of photons, tiny particles that each consist of one quantum of energy
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Light intensities
Luminance[ cd m-2 ]
photonsm-2 sr-1 s-1
photons per receptor
paper in starlight 0.001 1013 0.01
paper in moon light 0.2 1015 1
computer monitor 65 1017 100
room light 350 1018 1,000
blue sky 2,500 1019 10,000
paper in sun light 40,000 1020 100,000
• our visual system has to cope with a HUGE range of intensities
• bright sunlight is about 10,000,000 times more intense than starlight
absorption &reflection
refraction
scattering(defraction)
Intensity
Wavelength
absorption &transmission
Direction
Interactions between light and matter
Polarisation
Direction
Sens
itiv
ity
Number of photons absorbed
Cel
l res
pons
e Principle ofUnivariance
(William Rushton, 1972)
A photoreceptor'sresponse corresponds to
just a single variable:the number of photons
absorbed
Sens
itiv
ity
Direction
Seeing direction
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Sens
itiv
ity
Direction
Seeing direction
The eye cup makes an array of photoreceptors sensitive to direction
Eye cup
flatworm
bivalve mollusc
polychaet worm
limpet
Eye cups An eye cup cannot generate an image
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The pinhole eye can generate an image on the retina
Pinhole eye
marine snail bivalve mollusc
abalone
Nautilus
Pinhole eyes
Nautilus
The pinhole eye can generate an image on the retina
... however, lots of valuable light is wasted
Pinhole eye
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The solution:a collecting lens
Lens eye Compound eye
Cross section of the vertebrate eye: the lens
Zonules
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Analogies between eye and camera
� Aperture: Iris/pupil. Regulates the amount of light coming into the eye and affects depth of field
� Focus: Lens changes shape to adjust focus
� Film: Retina records the image
Accommodation in eye and camera
The pinhole eye can generate an image on the retina
... however, lots of valuable light is wasted
Pin hole eye
The solution:a collecting lens
Lens eye
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Accommodation
focal length: the distance between lens and image plane for a distant stimulus
f
Accommodation
f
focal length: the distance between lens and image plane for a distant stimulus
Cross section of the vertebrate eye: the zonules
Zonules
Lens, zonules and ciliary muscle
Distantfocus
Closefocus
Ciliary muscle
relaxed contracted
Zonules tense relaxed
Lens flat spherical
Accommodation
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Accommodation Accommodation
Accommodation
Near-point: the closest distance at which accommodation is still possible
Presbyopia: far-sightedness in elderly peopledue to reduced flexibility of the lens
Refractive errors of the eye
Emmetropia
Myopia
“Short-sightedness”
Hyperopia
“Far-sightedness”
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Refractive errors of the eye
Smaller aperture sharpens the imageand thus increases depth of field
Role of the pupil
Role of the pupil
Smaller aperture sharpens the imageand thus increases depth of field
The pinhole eye can generate an image on the retina
... however, lots of valuable light is wasted
Pin hole eye
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� Reduce amount of incident light(but only by a factor 10)
� Reduces refractional problems
� Increases depth of field
What is the pupil for?
10 1000
1
2
1
3
Fechner‘s law: S = c * log I
Intensity
log(Int)0 1 2
Brig
htne
ss
Brightness: Perceived intensity (sensation magnitude)
10 10000
1
2
1
3
Fechner‘s law: S = c * log I
Intensity
log(Int)0 1 3
Brig
htne
ss
Brightness: Perceived intensity (sensation magnitude)
2100
Log (Number of photons absorbed)
Cel
l res
pons
e
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Outline
What is light?
• Types of waves
• Dual nature of light: waves and particles
• Spectrum of electromagnetic radiation
• Intensity, wavelength, polarization, direction
• Range of light intensities
• Interactions between light and matter
Anatomy and function of the eye
• univariance principle
• eye cups
• pinhole eye
• lens eye
Optics of the eye
• accommodation
• refractive errors
• role of the pupil