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Ch04 light colorcapture

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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












Absorption
Diffusion
Reflection
Transparency
Refraction
Fluorescence
Subsurface scattering
Phosphorescence
Interreflection

Department of Mechatronics


Chapter 4 – Light & Color

4-4

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

4-5

A photon’s life choices











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










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










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










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

4-16

What humans don’t have: tapetum lucidum

Department of Mechatronics


Chapter 4 – Light & Color

4-17

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

4-19

Electromagnetic Spectrum

Human Luminance Sensitivity Function
Department of Mechatronics


Chapter 4 – Light & Color

4-20


Measuring spectra

Spectroradiometer: separate input light into its different
wavelengths, and measure the energy at each.

Department of Mechatronics


Chapter 4 – Light & Color

4-21

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

4-22

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

4-23

Image Formation

Department of Mechatronics


Chapter 4 – Light & Color

4-24


Digital camera

A digital camera replaces film with a sensor array




Each cell in the array is light-sensitive diode that converts photons
to electrons
Two common types: Charge Coupled Device (CCD) and CMOS
/>
Department of Mechatronics

Slide by Steve
Seitz


Chapter 4 – Light & Color

4-25

Sensor Array

CMOS sensor

CCD sensor
Department of Mechatronics



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