Optical sensing in Precision Farming (Techniques) Aerial remote sensing Film (visible/NIR/IR) and digitization Direct Digital recording Field machine based remote sensing Direct Digital recording Manual crop survey methods Direct Digital (manual recording /logging) Aerial remote sensing Film (visible/NIR/IR) and digitization Direct Digital recording Field machine based remote sensing Direct Digital recording Manual crop survey methods Direct Digital (manual recording /logging)

Purpose for use of optical sensing in Precision Farming Used to characterize plant or soil status Requirement: Calibration of spectral parameters to status Used to characterize boundaries –Physical –Morphological Requirement: Accurate spatial calibration (1m actual = 1 pixel) Lat/Lon = f(pixel position) Used to characterize plant or soil status Requirement: Calibration of spectral parameters to status Used to characterize boundaries –Physical –Morphological Requirement: Accurate spatial calibration (1m actual = 1 pixel) Lat/Lon = f(pixel position)

Issues - What is being measured? –Variability in light source –Filtering of light along path –Measuring units/calibration of sensing system –Geometry –Spatial and temporal frequency of measurements –Variability in light source –Filtering of light along path –Measuring units/calibration of sensing system –Geometry –Spatial and temporal frequency of measurements

Typical Multi-Spectral Sensor Construction Analog to Digital Converter Computer One Spectral Channel Photo-Diode Amplifier Filter Collimator Target Illumination CPU Radiometer

Fiber-Optic Spectrometer Optical Glass Fiber Photo Diode Array Optical Grating Analog to Digital Converter Computer CPU

Fundamentals of 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) 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)

Relationship between frequency and wavelength Plus Minus Plus

Relationship between frequency and wavelength Wavelength = speed of light divided by frequency (miles between bumps = miles per hour / bumps per hour)

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

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”

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. 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

Photo-ChemistryPhoto-Chemistry Light may be absorbed and participate (drive) a chemical reaction. Example: Photosynthesis in plants The frequency (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 The frequency (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

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) 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

Primary and secondary absorbers in plants Primary –Chlorophyll-a –Chlorophyll-b Secondary –Carotenoids –Phycobilins –Anthocyanins Primary –Chlorophyll-a –Chlorophyll-b Secondary –Carotenoids –Phycobilins –Anthocyanins

Sunlight Chlorophyll b B-Carotene Phycocyanin Chlorophyll a Wavelength, nm Absorption Lehninger, Nelson and Cox Absorption of Visible Light by Photopigments Absorption of Visible Light by Photopigments

Wavelength (nm) Reflectance (%) Visible Near Infrared Plant Reflectance

Soil and crop reflectance

Soil Reflectances - Oklahoma

Thermal Nature of the Emission of Radiation Black-body radiation –Matter is made up of inter-related particles which may be considered to vibrate or change energy state –A distribution of energy states exists within a blackbody –Matter emits radiation in proportion to the energy state changes Black-body radiation –Matter is made up of inter-related particles which may be considered to vibrate or change energy state –A distribution of energy states exists within a blackbody –Matter emits radiation in proportion to the energy state changes

Wien’s Displacement Law peak = 2,897,000 / T where:T = [ 0 K ] = [ nm] Hot metal example peak-sun = 2,897,000/6000 = 475nm peak-plant = 2,897,000/300 = 9700nm Point: Emission “color = f(T of emitter) peak = 2,897,000 / T where:T = [ 0 K ] = [ nm] Hot metal example peak-sun = 2,897,000/6000 = 475nm peak-plant = 2,897,000/300 = 9700nm Point: Emission “color = f(T of emitter)

Planck’s Law Equation: Point: Emission “color = f(T of emitter)

Sun vs. Plant / Soil radiation 6000K 300K SUN Terrestrial

Radiation Energy Balance Earth SUN Temperature of the earth is set by the difference between absorbed and emitted energy If no energy was emitted by the earth, The earth’s temperature would eventually rise to that of the sun Temperature of the earth is set by the difference between absorbed and emitted energy If no energy was emitted by the earth, The earth’s temperature would eventually rise to that of the sun

Nature of absorption by the atmosphere Incident Reflected Transmitted Absorbed Radiant energy balance must be computed for each component of the atmosphere and for each wavelength to estimate the radiation incident on the earth's surface Radiant energy balance must be computed for each component of the atmosphere and for each wavelength to estimate the radiation incident on the earth's surface Earth's surface Atmosphere

Solar Irradiance NIR UV

Radiation Energy Balance Incoming radiation interacts with an object and may follow three exit paths: Reflection Absorption Transmission  +  + r f = 1.0 , , and r f are the fractions taking each path Incoming radiation interacts with an object and may follow three exit paths: Reflection Absorption Transmission  +  + r f = 1.0 , , and r f are the fractions taking each path R 0  R 0  R 0 r f R 0

ReflectanceReflectance 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 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

Diffuse and Specular Radiation Multiple reflections in the atmosphere cause diffuse radiation Multiple reflections in the atmosphere cause diffuse radiation

Measurement of Light Photometry Measurement of visible radiation in terms of sensitivity of the human eye. Used in photography and in lighting performance Photometric measures –Luminous intensity - Candela [cd] –Luminous Flux - Lumen [lm] –Luminance (cd/m2) - [nit] –Illuminance (lm/m2) - [lx] Photometry Measurement of visible radiation in terms of sensitivity of the human eye. Used in photography and in lighting performance Photometric measures –Luminous intensity - Candela [cd] –Luminous Flux - Lumen [lm] –Luminance (cd/m2) - [nit] –Illuminance (lm/m2) - [lx]

Measurement of Light Radiometry –Measurement of the properties of light without regard to human perception –Used for quantifying energy in radiation –Radiometric Measures Radiant Flux - Watt (W) (rate of energy from source) Radiometry –Measurement of the properties of light without regard to human perception –Used for quantifying energy in radiation –Radiometric Measures Radiant Flux - Watt (W) (rate of energy from source)

TerminologyTerminology Radiant flux –Energy in the form of radiation from a source per unit time units passing through a surface = Watt [W] –irradiance irradiate - to have light radiating on to an object irradiance - the light emitted from an object surface that is being irradiated Radiant flux –Energy in the form of radiation from a source per unit time units passing through a surface = Watt [W] –irradiance irradiate - to have light radiating on to an object irradiance - the light emitted from an object surface that is being irradiated

RadianceRadiance Energy Flux through a surface per unit of solid angle per unit area of source Energy Flux through a surface per unit of solid angle per unit area of source Solid Angle Steridian [St] Solid Angle Steridian [St] Watts per meter square of source

IrradianceIrradiance Energy Flux through a surface per unit of area Energy Flux through a surface per unit of area Unit Area (m 2 ) Power = Energy / Time [Joules / Second] = [Watts] Power =  E / Time Power = Photons / Time Power = nh  /  Time Irradiance = Power / Area = (Photons / Time) / Area Irradiance = [Watts / Square Meter]

Irradiance and Reflectance Irradiance (I 0 ) a measure of power per unit area Reflectance (r f ) is the ratio of reflected to incident Irradiance r f = I 0 r f / I 0 Irradiance (I 0 ) a measure of power per unit area Reflectance (r f ) is the ratio of reflected to incident Irradiance r f = I 0 r f / I 0

Area = [ W/m 2 ] = Irradiance height = [ W/m 2 nm ] = Spectral Irradiance width = [ nm ] = Bandwidth Spectral Irradiance Bandwidth Spectral Irradiance –Power per unit spectral width

Computation of Irradiance from Spectral Irradiance Irradiance for a particular band is the “sum” of Spectral Irradiance across the band times the wavelength

NDVINDVI –Normalized Difference Vegetative Index Difference increases with greater red absorption Increase or decrease in total irradiance does not effect NDVI Typically computed with irradiances, use of reflectance eliminates spectral shift sensitivity –Normalized Difference Vegetative Index Difference increases with greater red absorption Increase or decrease in total irradiance does not effect NDVI Typically computed with irradiances, use of reflectance eliminates spectral shift sensitivity

OSU Irradiance ratio sensor

Irradiance Indices Spectral shift in illumination prevents use of simple irradiance sensing Based on ratios of reflected Red and NIR intensity Example Index: R red / R nir

Reflectance Indices Based on ratios of Red and NIR Reflectance Red Reflectance:  = R red / I red Example Index:  red /  nir Reflectance is primarily a function of target

NDVINDVI Developed as an irradiance Index for application to remote sensing Normalized Difference Vegetative Index Varies from -1 to 1 Soil NDVI = to.05 Plant NDVI = 0.6 to 0.9 Typical plants with soil background NDVI= OSU sensors –narrow-band reflectance based NDVI Developed as an irradiance Index for application to remote sensing Normalized Difference Vegetative Index Varies from -1 to 1 Soil NDVI = to.05 Plant NDVI = 0.6 to 0.9 Typical plants with soil background NDVI= OSU sensors –narrow-band reflectance based NDVI

OSU Reflectance Sensor

Natural Illumination Battery powered Wide dynamic range Low noise 0.75 x 0.25 m field of view Natural Illumination Battery powered Wide dynamic range Low noise 0.75 x 0.25 m field of view

NDVINDVI I RED =671  nm I NIR = 780  nm

Photo Diode Detector Photo Diode Area 2.29mm x 2.29mm 5.2e-6 m 2 Opto 202 Die Topography

Silicon Responsivity

Calculation of Irradiance from Detector output Responsivity:  [V/uW] for a particular wavelength, output in volts, V is the product of Responsivity times the Irradience I times sensor area. [ W/m 2 ] [V/uW] [m 2 ] For a wide band,

Calculation of Irradiance from Sensor output -cont- Irradiance may be computed from the voltage reading for a narrow spectral band : The average value of Responsivity,  for the detector must be used

Calculation of Irradiance from Sensor output -cont- Sensor reading, S, is normally an amplified and digitized numeric value Where: V voltage output of the sensor V Range input range of the amplifier-A/D circuit n binary word width of the A/D converter

Calculation of Irradiance from Sensor output -cont Example: Let I = 1 W/m 2 A = 5.2e-6 m 2 (for the Burr-Brown 201)   = 0.5 V/  W (for = red) V Range = 5 V n = 12 bits