Optics and Human Vision

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Optics and Human Vision
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Light

• Light
• Particles known as photons
• Act as waves
• Two fundamental properties
• Amplitude
• Wavelength
• Frequency is the inverse of wavelength
• Relationship between wavelength (lambda) and
  frequency (f)

Where c speed of light 299,792,458 m / s
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Light
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Light
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Simple Optics
1/S1 1/S2 1/f
M image size / object size S1 / S2
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Optics and f-stop

• Lens aperture is given by f-stop
• Ratio of focal length to the diameter of the area
  of opening
• Successive halving of the area

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Anatomy of the Human Eye

• Light processing
• Cornea acts as a protective lens that roughly
  focuses incoming light
• Iris controls the amount of light that enters the
  eye
• The lens sharply focuses incoming light onto the
  retina
• Absorbs both infra-red and ultra-violet light
  which can damage the lens
• The retina is covered by photoreceptors (light
  sensors) which measure light

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Anatomy of the Human Eye
Source http//webvision.med.utah.edu/
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Human Eye
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Two Kinds of Photoreceptors

• Rods
• Approximately 100-150 million rods
• Non-uniform distribution across the retina
• Sensitive to low-light levels (scotopic vision)
• Lower resolution
• Cones
• Approximately 6-7 million cones
• Sensitive to higher-light levels (photopic
  vision)
• High resolution
• Detect color by the use of 3 different kinds of
  cones each of which is sensitive to red, green,
  or blue frequencies

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Photoreceptor density
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Density of Photoreceptors inthe Human Eye
Source http//webvision.med.utah.edu/
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Comparision
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Illumination and Reflectance
f(x,y) i(x,y)r(x,y) r reflectance i
illumation
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Color and Human Perception

• Chromatic light
• has a color component
• Achromatic light
• has no color component
• has only one property intensity
• Three Related Terms
• radiance total amount of energy emitted from a
  source
• luminance actual amount of light coming from a
  source
• brightness perceived amount of light coming
  from a source

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Human Visual Perception

• Range of light intensities that can be seen is on
  the order of 1010 ranging from the darkest
  scotopic threshold to the brightest glare limit
• Cant discriminate all these intensities at the
  same time! We adjust to an average value and
  then discriminate around the average.
• Subjective Brightness is a logarithmic function
  of the actual light intensity

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Brightness Adaptation
The eye can see over the full range of
intensities between scotopic threshold and the
glare limit, but not at the same time!
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Brightness Adaptation and Mach Banding
Exaggerates intensity changes across edges.
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Brightness Adaptation Revisited
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(No Transcript)
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Simultaneous Contrast
The intensity of the interior rectangle appears
to change as the intensity of the background
changes. Perceived intensity depends on
background information.
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Chromatic Adaptation
The color above is actually GREEN! In the image
to the right, the yellow region from the trick
image was cut and pasted onto the original.
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Stroop Effect(context matters)
ZEWIAAAKS KAKBKEALK IEIEAEPMX KQPPENAEP KEANEEAKT
QKEELKKSS KEJAMEMMT
BLUE RED YELLOW BLACK GRAY GREEN PURPLE
A white box with colored lines of text will
appear. Name the color of each line of
text. See how fast you can identify the colors.
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Shape Contrast
Structural and spatial context influence
perception. Which of the two center circles is
larger?
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PsychoVisual Information
Relative Size As an object shrinks while moving
we interpret it to be further away
Interposition An object that overlaps another is
interpreted as closer to the observer
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PsychoVisual Information
Linear Perspective Parallel lines converge with
distance from the observer
Shading Light is assumed to come from above
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PsychoVisual Information
Binocular Cues Disparity between left-right eyes
gives depth cues.
http//www.purveslab.net/seeforyourself/