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Electromagnetic properties

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Frequency is a major factor in the primary characteristics ... Radiometer. Measurement of reflected intensity - Fiber-Optic Spectrometer. Optical. Glass Fiber ... – PowerPoint PPT presentation

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Title: Electromagnetic properties


1
Electromagnetic properties
  • Part I

2
Electrical and magnetic properties
  • Electromagnetic fields are propagated through and
    reflected by materials
  • Characterized as
  • Current flow at low frequencies
  • Magnetism in metals
  • Optical absorbance / reflectance in light
  • etc.
  • Frequency is a major factor in the primary
    characteristics
  • Low frequency electrical properties
  • High frequency optical properties

3
Fundamentals of high frequency electromagnetic
waves (Light)
  • Light Energy (radiant energy)
  • Readily converted to heat
  • Light shining on a surface heats the surface
  • Heat energy
  • Light Electro-magnetic phenomena
  • Has the characteristics of electromagnetic waves
    (eg. radio waves)
  • Also behaves like particles (e.g.. photons)

4
The electromagnetic spectrum
5
Relationship between frequency and wavelength
l
Plus
Plus
Wavelength speed of light divided by
frequency (miles between bumps miles per hour
/ bumps per hour) l Wavelength m n
Frequency Hz c 3x108 m/s in a vacuum
Minus
Minus
6
Relationship between frequency and wavelength
l

-
Antenna
Plus
Plus
Minus
Minus
l KOSU 3 x 108 / 97.1 x 106 l KOSU 3 m l red
6.40 x 10- 7 m 640 nm Bohrs Hydrogen 5 x
10 - 11 m
7
Plants light harvesting structure - model
Jungas et. al. 1999
8
Light emission / absorption governed by quantum
effects
Planck - 1900
Einstein - 1905
One photon
DE is light energy flux n is an integer
(quantum) h is Plancks constant n is frequency
9
Frequency bands and photon energy
10
Changes in energy states of matter are quantitized
Bohr - 1913
  • Where Ek, Ej are energy states (electron shell
    states etc.) and frequency, n , is proportional
    to a change of state
  • and hence color of light. Bohr explained the
    emission spectrum of hydrogen.

Hydrogen Emission Spectra (partial representation)
Wavelength
11
Measurement of reflected intensity Typical
Multi-Spectral Sensor Construction
One Spectral Channel
Photo-Diode detector / Amplifier
Analog to Digital Converter
CPU
Optical Filter
Illumination
Collimator
Radiometer
Computer
Target
12
Measurement of reflected intensity - Fiber-Optic
Spectrometer
One Spectral Channel at a time
Optical Glass Fiber
Optical Grating
Analog to Digital Converter
CPU
Element selection
Computer
Photo Diode Array
13
Visual reception of color
  • Receptors in our eyes are tuned to particular
    photon energies (hn)
  • Discrimination of color depends on a mix of
    different receptors
  • Visual sensitivity is typically from wavelengths
    of 350nm (violet) to 760nm (red)

Wavelength
700 nm
400 nm
500 nm
14
Quantification of color
  • Spectral measurements can be used to quantify
    reflected light in energy and spectral content,
    but not very useful description of what we see.
  • Tri-stimulus models represent color as
    perceived by humans
  • Tri-stimulus models
  • RGB - most digital work
  • CYM - print
  • HSI, HSB, or HSV - artists
  • CIE Lab
  • YUV and YIQ - television broadcasts

15
CIE XYZ model
Y
  • Attempts to describe perceived color with a three
    coordinate system model

X
Z luminance
16
CIE Lab model
  • An improvement of the CIE XYZ color model.
  • Three dimensional model where color differences
    correspond to distances measured colorimetrically
  • Hue and saturation (a, b)
  • a axis extends from green (-a) to red (a)
  • b axis from blue (-b) to yellow (b)
  • Luminance (L) increases from the bottom to the
    top of the three-dimensional model
  • Colors are represented by numerical values
  • Hue can be changed without changing the image or
    its luminance.
  • Can be converted to or from RGB or other
    tri-stimulus models

17
Photo-Chemistry
  • Light may be absorbed and participate (drive) a
    chemical reaction. Example Photosynthesis in
    plants
  • The wavelength must be correct to be absorbed by
    some participant(s) in the reaction
  • Some structure must be present to allow the
    reaction to occur
  • Chlorophyll
  • Plant physical and chemical structure

18
Silicon Responsivity
19
Primary and secondary absorbers in plants
  • Primary
  • Chlorophyll-a
  • Chlorophyll-b
  • Secondary
  • Carotenoids
  • Phycobilins
  • Anthocyanins

20
Chlorophyll absorbance
Chla black Chlb red BChla magenta BChlb
orange BChlc cyan BChld bue BChle green
Source Frigaard et al. (1996), FEMS Microbiol.
Ecol. 20 69-77
21
Radiation Energy Balance
  • Incoming radiation interacts with an object
  • and may follow three exit paths
  • Reflection
  • Absorption
  • Transmission
  • a t r 1.0
  • a, t, and r are the
  • fractions taking each path
  • Known as
  • fractional absorption coefficient,
  • fractional transmittance, and
  • reflectance respectively

Il0
Il0 r
Il0 a
Iout Il0 t
22
Internal Absorbance (Ai)
  • Lambert's Law - The amount of light absorbed is
    directly proportional to the logarithm of the
    length of the light path or the thickness of the
    absorbing medium. Thus
  • l length of light path
  • k extinction coefficient of medium
  • Normally in absorbance measurements the
    measurement is structured so that reflectance is
    zero

23
Reflectance
  • Ratio of incoming to reflected irradiance
  • Incoming can be measured using a white
    reflectance target
  • Reflectance is not a function of incoming
    irradiance level or spectral content, but of
    target characteristics

24
Solar Irradiance
NIR
UV
25
Soil and crop reflectance
26
Soil Reflectances - Oklahoma
27
Electrical properties - Current and Voltage
  • Current
  • Flow of electrons
  • The quantity of electrons per unit time flowing
    through a conducting medium
  • Units Amperes (A), abbreviated amps or
    fundamentally coulombs per second
    (coulomb6.03x1023 electrons)
  • Voltage
  • Electromotive force (EMF)
  • A potential or tension between two points of a
    conducting medium that can drive the flow of
    electrons through the medium expressed as work
    per number of electrons
  • Analogous to pressure in a fluid that can drive
    flow of fluid through a pipe
  • Units of Volts (V) or fundamentally joules per
    coulomb, the energy (potential) per unit of
    electrons.

28
Resistors and Ohms Law
  • Property of a resistor Flow of current is
    proportional to voltage (or vice versa). The
    proportionality constant is known as resistance
  • For the following circuit
  • Resistance has units of Ohms (W)
  • (fundamentally, volts per amp)
  • The current could be computed in the circuit
    above given Vsupply and R i 5V / 10,000W
    0.0005 V 0.5 mV

29
Resistivity
  • The fundamental property of materials defining
    resistance is resistivity
  • Where
  • L length of conductive path
  • A Crossectional area of conductive path
  • R Resistance
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