Electromagnetic properties

Part I

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

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)

The electromagnetic spectrum

Relationship between frequency and wavelength

Plus

Minus Minus

Plus

Wavelength = speed of light divided by frequency

(miles between bumps = miles per hour / bumps per hour)

= Wavelength [m]= Frequency [Hz]c = 3x108 m/s in a vacuum

c

Relationship between frequency and wavelength

Plus

Minus Minus

Plus

Antenna

+ -

KOSU = 3 x 108 / 97.1 x 106

KOSU = 3 m

red = 6.40 x 10- 7 m = 640 nmBohr’s Hydrogen = 5 x 10 - 11 m

Plants light harvesting structure - model

Jungas et. al. 1999

Light emission / absorption governed by quantum effects

Planck - 1900

E nh E is light energy fluxn is an integer (quantum)h is Planck’s constant is frequency

E hp Einstein - 1905

One “photon”

Frequency bands and photon energy

Changes in energy states of matter are quantitized

Bohr - 1913

h E Ek j

Where Ek, Ej are energy states (electron shell states etc.) and frequency, , 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

Measurement of reflected intensity –Typical Multi-Spectral Sensor Construction

Analog toDigitalConverter

Computer

One Spectral Channel

Photo-Diode detector/ Amplifier

Optical Filter

Collimator

Target

Illumination

CPU

Radiometer

Measurement of reflected intensity - Fiber-Optic Spectrometer

OpticalGlass Fiber

Photo Diode Array

Optical GratingAnalog toDigitalConverter

Computer

CPU

Element selection

One Spectral Channel at a time

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

400 nm 700 nm500 nm

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 L*a*b*

• YUV and YIQ - television broadcasts

CIE XYZ model

• Attempts to describe perceived color with a three coordinate system model

X

Y

Z= luminance

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

Photo-Chemistry

• Light may be absorbed and participate (drive) a chemical reaction. Example: Photosynthesis in plants

6 6 62 2 6 12 6 2CO H O h C H O O

• 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

Silicon Responsivity

Primary and secondary absorbers in plants

• Primary– Chlorophyll-a– Chlorophyll-b

• Secondary– Carotenoids– Phycobilins– Anthocyanins

Chlorophyll absorbance

Chla: blackChlb: redBChla: magentaBChlb: orangeBChlc: cyanBChld: bueBChle: green

Source: Frigaard et al. (1996), FEMS Microbiol. Ecol. 20: 69-77

Radiation Energy Balance

Incoming radiation interacts with an object and may follow three exit paths:

• Reflection• Absorption• Transmission

+ + = 1.0, , and are thefractions taking each pathKnown as:

fractional absorption coefficient,fractional transmittance, andreflectance respectively

I0

I0 I0

Iout = I0

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

klI

IA

outi

)1(log 0

klTI

IA

outi

1loglog 0

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

0

100

200

300

400

500

600

700

0 250 500 750 1000 1250 1500 1750 2000Wavelength (nm)

Sp

ectr

al Irr

adie

nce

(w

/m^2

nm

)

Extraterrestrial SolarIrradience

Terrestial SolarIrradience

Adapted from Thekaekara, M. P. 1973.Solar Energy Outside the Earth's Atmosphere.Solar Energy, Vol 14, p 109.

Solar Irradiance

NIRUV

Soil and crop reflectance

0

0.1

0.2

0.3

0.4

0.5

0.6

300 400 500 600 700 800 900 1000 1100

Wavelength (nm)

Fra

cti

on

al

Re

fle

cta

nc

e

43 Soils

27 Soybeans

25 Potatoes

9 Sunflower

73 Cotton17 Corn

P. S. ThenkabailR. B. SmithE. De PauwYale Center for Earth Observation

Soil Reflectances - Oklahoma

0

0.2

0.4

0.6

0.8

1

350 400 450 500 550 600 650 700 750 800

Wavelength (nm)

Ref

lect

ance

(Fra

ctio

n)Tipton Stillwater

Perkins Mangum

Lahoma Haskell

Goodwell Ft. Cobb

Chickasha Altus

Agron. Stwr.

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 (coulomb=6.03x1023 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.

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 ()– (fundamentally, volts per amp)

• The current could be computed in the circuit above given Vsupply and R: i = 5V / 10,000= 0.0005 V = 0.5 mV

Riv

RiV supply

5V

10 kii

Vsupply

R

Resistivity

• The fundamental property of materials defining resistance is resistivity

Where:L = length of conductive path

A= Crossectional area of conductive path

R = Resistance

][ mL

RA