Bài giảng Chapter 4: Light and color capture
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Chapter 4 – Light & Color
4-1
Chapter 4
Light and Color Capture
James Hays, Brown University
Department of Mechatronics
Chapter 4 – Light & Color
4-2
Contents
• Review of lighting
– Color, Reflection, and absorption
• What is a pixel? How is an image represented?
– Color spaces
Department of Mechatronics
Chapter 4 – Light & Color
4-3
A photon’s life choices
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Absorption
Diffusion
Reflection
Transparency
Refraction
Fluorescence
Subsurface scattering
Phosphorescence
Interreflection
Department of Mechatronics
Chapter 4 – Light & Color
4-4
A photon’s life choices
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Absorption
Diffusion
Reflection
Transparency
Refraction
Fluorescence
Subsurface scattering
Phosphorescence
Interreflection
Department of Mechatronics
light
source
λ
Chapter 4 – Light & Color
4-5
A photon’s life choices
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Absorption
Diffuse Reflection
Reflection
Transparency
Refraction
Fluorescence
Subsurface scattering
Phosphorescence
Interreflection
Department of Mechatronics
light
source
λ
Chapter 4 – Light & Color
4-6
A photon’s life choices
•
•
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Absorption
Diffusion
Specular Reflection
Transparency
Refraction
Fluorescence
Subsurface scattering
Phosphorescence
Interreflection
Department of Mechatronics
light
source
λ
Chapter 4 – Light & Color
4-7
A photon’s life choices
•
•
•
•
•
•
•
•
•
Absorption
Diffusion
Reflection
Transparency
Refraction
Fluorescence
Subsurface scattering
Phosphorescence
Interreflection
Department of Mechatronics
light
source
λ
Chapter 4 – Light & Color
4-8
A photon’s life choices
•
•
•
•
•
•
•
•
•
Absorption
Diffusion
Reflection
Transparency
Refraction
Fluorescence
Subsurface scattering
Phosphorescence
Interreflection
Department of Mechatronics
light
source
λ
Chapter 4 – Light & Color
4-9
A photon’s life choices
•
•
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Absorption
Diffusion
light source
Reflection
λ1
Transparency
Refraction
λ2
Fluorescence
Subsurface scattering
Phosphorescence
Fluorescence occurs when a substance absorbs
Interreflection
radiation of one wavelength, and immediately
Fluorescent scorpion. Image courtesy of The Firefly Forest.
emits radiation of a different wavelength.
Department of Mechatronics
Chapter 4 – Light & Color
4-10
A photon’s life choices
•
•
•
•
•
•
•
•
•
Absorption
Diffusion
Reflection
Transparency
Refraction
Fluorescence
Subsurface scattering
Phosphorescence
Interreflection
Department of Mechatronics
light source
λ
Chapter 4 – Light & Color
4-11
A photon’s life choices
•
•
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•
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Absorption
light source
Diffusion
t=1
Reflection
Transparency
t=n
Refraction
Fluorescence
Subsurface scattering
Phosphorescence Phosphorescence is a related type of photoluminescence
in which absorbed radiation is re-emitted more slowly, so
phosphorescent
objects can still glow for periods up to
Interreflection
several hours after the source of incident radiation is
removed.
Department of Mechatronics
Chapter 4 – Light & Color
4-12
A photon’s life choices
•
•
•
•
•
•
•
Absorption
Diffusion
Reflection
Transparency
Refraction
Fluorescence
Subsurface
scattering
• Phosphorescence
• Interreflection
Department of Mechatronics
light source
λ
(Specular Interreflection)
Chapter 4 – Light & Color
4-13
The Eye
• Your eyes work a lot like a camera. The lens of a camera
focuses light onto the film inside. The cornea and lens in the
front of the eye focus light onto the back, where lightsensitive tissue called the retina is located. When the retina
receives an image, it sends a signal through the optic nerve
to the brain for the image to be developed.
• />
Department of Mechatronics
Chapter 4 – Light & Color
4-14
The Eye
The human eye is a camera!
• Iris - colored annulus with radial muscles
• Pupil - the hole (aperture) whose size is controlled by
the iris
• What’s the “film”?
– photoreceptor cells (rods and cones) in the retina
Slide by Steve Seitz
Department of Mechatronics
Chapter 4 – Light & Color
4-15
The Retina
Cross-section of eye
Ganglion axons
Ganglion cell layer
Bipolar cell layer
Receptor layer
Department of Mechatronics
Cross section of retina
Pigmented
epithelium
Chapter 4 – Light & Color
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What humans don’t have: tapetum lucidum
Department of Mechatronics
Chapter 4 – Light & Color
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Two types of light-sensitive receptors
Cones
cone-shaped
less sensitive
operate in high light
color vision
Rods
rod-shaped highly
sensitive operate
at night gray-scale
vision
cone
rod
Department of Mechatronics
Chapter 4 – Light & Color
4-18
Rod / Cone sensitivity
The famous sock-matching problem…
Department of Mechatronics
Chapter 4 – Light & Color
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Electromagnetic Spectrum
Human Luminance Sensitivity Function
Department of Mechatronics
Chapter 4 – Light & Color
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Measuring spectra
Spectroradiometer: separate input light into its different
wavelengths, and measure the energy at each.
Department of Mechatronics
Chapter 4 – Light & Color
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The Physics of Light
Any patch of light can be completely described
physically by its spectrum: the number of photons (per
time unit) at each wavelength 400 - 700 nm.
Department of Mechatronics
Chapter 4 – Light & Color
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The Physics of Light
% Photons Reflected
Some examples of the reflectance spectra of surfaces
400
700 400
700 400
700 400
700
Wavelength (nm)
Department of Mechatronics
© Stephen E. Palmer,
2002
Chapter 4 – Light & Color
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Image Formation
Department of Mechatronics
Chapter 4 – Light & Color
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Digital camera
A digital camera replaces film with a sensor array
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Each cell in the array is light-sensitive diode that converts photons
to electrons
Two common types: Charge Coupled Device (CCD) and CMOS
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Department of Mechatronics
Chapter 4 – Light & Color
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Sensor Array
CMOS sensor
CCD sensor
Department of Mechatronics
