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 Plus Wavelength = speed of light divided by frequency (miles between bumps = miles per hour / bumps per hour)  = Wavelength [m]  = Frequency [Hz] c = 3x10 8 m/s in a vacuum

Relationship between frequency and wavelength Plus Minus Plus Antenna +- KOSU = 3 x 10 8 / 97.1 x 10 6 KOSU = 3 m red = 6.40 x m = 640 nm Bohr’s Hydrogen = 5 x m

Plants light harvesting structure - model Jungas et. al. 1999

Light emission / absorption governed by quantum effects Planck  E is light energy flux n is an integer (quantum) h is Planck’s constant is frequency Einstein One “photon”

Frequency bands and photon energy

Changes in energy states of matter are quantitized Bohr Where E k, E j 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 to Digital Converter Computer One Spectral Channel Photo-Diode detector / Amplifier Optical Filter Collimator Target Illumination CPU Radiometer

Measurement of reflected intensity - Fiber-Optic Spectrometer Optical Glass Fiber Photo Diode Array Optical Grating Analog to Digital Converter 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 nm 500 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 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: black Chlb: red BChla: magenta BChlb: orange BChlc: cyan BChld: bue BChle: 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 the fractions taking each path Known as: fractional absorption coefficient, fractional transmittance, and reflectance respectively I 0  I 0  I 0 I out = I 0 

Internal Absorbance ( A i ) 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

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

Solar Irradiance NIR UV

Soil and crop reflectance

Soil Reflectances - Oklahoma

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=6.03x10 23 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 V supply and R: i = 5V / 10,000  = V = 0.5 mV

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