Electromagnetic propertiesElectrical and magnetic propertiesFundamentals of high frequency electromagnetic waves (Light)The electromagnetic spectrumRelationship between frequency and wavelengthSlide 6Plants light harvesting structure - modelLight emission / absorption governed by quantum effectsFrequency bands and photon energyChanges in energy states of matter are quantitizedMeasurement of reflected intensity – Typical Multi-Spectral Sensor ConstructionMeasurement of reflected intensity - Fiber-Optic SpectrometerVisual reception of colorQuantification of colorCIE XYZ modelCIE Lab modelPhoto-ChemistrySilicon ResponsivityPrimary and secondary absorbers in plantsChlorophyll absorbanceRadiation Energy BalanceInternal Absorbance (Ai)ReflectanceSolar IrradianceSoil and crop reflectanceSoil Reflectances - OklahomaElectrical properties - Current and VoltageResistors and Ohms LawResistivityElectromagnetic propertiesPart IElectrical 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” propertiesFundamentals 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 spectrumRelationship between frequency and wavelengthPlusMinus MinusPlusWavelength = 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 vacuumcRelationship between frequency and wavelengthPlusMinus MinusPlusAntenna+ - 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 mPlants light harvesting structure - modelJungas et. al. 1999Light emission / absorption governed by quantum effectsPlanck - 1900E nhE is light energy fluxn is an integer (quantum)h is Planck’s constant is frequency E hpEinstein - 1905One “photon”Frequency bands and photon energyChanges in energy states of matter are quantitizedBohr - 1913h E Ek j Where Ek, Ej are energy states (electron shell states etc.) and frequency, , is proportional to a change of stateand hence color of light. Bohr explained the emission spectrum of hydrogen. Hydrogen Emission Spectra (partial representation)WavelengthMeasurement of reflected intensity –Typical Multi-Spectral Sensor ConstructionAnalog toDigitalConverterComputerOne Spectral ChannelPhoto-Diode detector/ AmplifierOptical FilterCollimatorTargetIlluminationCPURadiometerMeasurement of reflected intensity - Fiber-Optic SpectrometerOpticalGlass FiberPhoto Diode ArrayOptical GratingAnalog toDigitalConverterComputerCPUElement selectionOne Spectral Channel at a timeVisual 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)Wavelength400 nm700 nm500 nmQuantification 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 broadcastsCIE XYZ model•Attempts to describe perceived color with a three coordinate system modelXYZ= luminanceCIE 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 modelsPhoto-Chemistry•Light may be absorbed and participate (drive) a chemical reaction. Example: Photosynthesis in plants6 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 structureSilicon ResponsivityPrimary and secondary absorbers in plants•Primary–Chlorophyll-a–Chlorophyll-b•Secondary–Carotenoids–Phycobilins–AnthocyaninsChlorophyll absorbanceChla: blackChlb: redBChla: magentaBChlb: orangeBChlc: cyanBChld: bueBChle: greenSource: Frigaard et al. (1996), FEMS Microbiol. Ecol. 20: 69-77Radiation Energy BalanceIncoming 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 respectivelyI0I0 I0Iout = I0Internal 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 pathk = extinction coefficient of medium•Normally in absorbance measurements the measurement is structured so that reflectance is zeroklIIAouti)1(log0klTIIAouti1loglog0Reflectance–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 characteristicsSolar IrradianceNIRUVSoil and crop reflectance00.10.20.30.40.50.6300 400 500 600 700 800 900 1000 1100Wavelength (nm)Fractional Reflectance43 Soils27 Soybeans25 Potatoes9 Sunflower73 Cotton17 CornP. S. ThenkabailR. B. SmithE. De PauwYale Center for Earth
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