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