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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).

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Optical sensing in precision farming techniques
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)


  • Purpose for use of optical sensing in precision farming
    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)


  • Issues what is being measured
    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


    Typical multi spectral sensor construction
    Typical Multi-Spectral Sensor Construction

    One Spectral Channel

    Photo-Diode

    Amplifier

    Analog to

    Digital

    Converter

    CPU

    Filter

    Illumination

    Collimator

    Radiometer

    Computer

    Target


    Fiber optic spectrometer
    Fiber-Optic Spectrometer

    Optical

    Glass Fiber

    Optical Grating

    Analog to

    Digital

    Converter

    CPU

    Computer

    Photo Diode Array


    Fundamentals of light
    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)



    Relationship between frequency and wavelength1
    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 wavelength2
    Relationship between frequency and wavelength

    l

    +

    -

    lKOSU= 3 x 108 / 97.1 x 106

    lKOSU= 3 m

    lred= 6.40 x 10- 7 m = 640 nm

    Bohr’s Hydrogen = 5 x 10 - 11 m

    Antenna

    Plus

    Plus

    Minus

    Minus


    Light emission absorption governed by quantum effects
    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 Planck’s constant

    n is frequency


    Changes in energy states of matter are quantitized
    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


    Photo chemistry
    Photo-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


    Visual reception of color
    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


    Primary and secondary absorbers in plants
    Primary and secondary absorbers in plants

    • Primary

      • Chlorophyll-a

      • Chlorophyll-b

    • Secondary

      • Carotenoids

      • Phycobilins

      • Anthocyanins


    Absorption of Visible Light

    by Photopigments

    Sunlight

    Chlorophyll b

    Phycocyanin

    Absorption

    B-Carotene

    Chlorophyll a

    300 400 500 600 700 800

    Wavelength, nm

    Lehninger, Nelson and Cox


    0.5

    Visible

    Near Infrared

    Reflectance (%)

    0.25

    Plant Reflectance

    0.00

    450

    500

    550

    600

    650

    700

    750

    800

    850

    900

    950

    1000

    1050

    1100

    1150

    Wavelength (nm)




    Thermal nature of the emission of radiation
    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


    Wien s displacement law
    Wien’s Displacement Law

    lpeak= 2,897,000 / T

    where: T = [0K ]

    l= [ nm]

    Hot metal example

    lpeak-sun = 2,897,000/6000 = 475nm

    lpeak-plant = 2,897,000/300 = 9700nm

    Point: Emission “color = f(T of emitter)


    Planck s law
    Planck’s Law

    Equation:

    Point: Emission “color = f(T of emitter)


    Sun vs plant soil radiation
    Sun vs. Plant / Soil radiation

    SUN

    6000K

    Terrestrial

    300K


    Radiation energy balance
    Radiation Energy Balance

    SUN

    Earth

    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
    Nature of absorption by the atmosphere

    Reflected

    Transmitted

    Incident

    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

    Earth's

    surface

    Atmosphere



    Radiation energy balance1
    Radiation Energy Balance

    • Incoming radiation interacts with an object

    • and may follow three exit paths:

      • Reflection

      • Absorption

      • Transmission

    • a + t + rf = 1.0

    • a, t, and rfare the

    • fractions taking each path

    Rl0

    Rl0 rf

    Rl0 a

    Rl0 t


    Reflectance
    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


    Diffuse and specular radiation
    Diffuse and Specular Radiation

    Multiple reflections in the atmosphere

    cause diffuse radiation


    Measurement of light
    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]


    Measurement of light1
    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)


    Terminology
    Terminology

    • 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


    Radiance
    Radiance

    Energy Flux through a surface per unit of solid angle

    per unit area of source

    Watts

    Solid Angle

    Steridian [St]

    per meter square of source


    Irradiance
    Irradiance

    Unit Area (m2)

    Energy Flux through

    a surface per unit of area

    Power = Energy / Time [Joules / Second] = [Watts]

    Power = DE / Time

    Power = Photons / Time

    Power = nhn /Time

    Irradiance = Power / Area = (Photons / Time) / Area

    Irradiance = [Watts / Square Meter]


    Irradiance and reflectance
    Irradiance and Reflectance

    • Irradiance (Il0) a measure of power per unit area

    • Reflectance (rf ) is the ratio of reflected to incident Irradiance rf = Il0 rf / Il0


    Spectral irradiance
    Spectral Irradiance

    • Power per unit spectral width

    Area = [ W/m2 ] = Irradiance

    height = [ W/m2 nm ] = Spectral Irradiance

    width = [ nm ] = Bandwidth

    Spectral Irradiance

    Bandwidth


    Computation of irradiance from spectral irradiance
    Computation of Irradiance from Spectral Irradiance

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


    NDVI

    • 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



    Irradiance indices
    Irradiance Indices

    Based on ratios of reflected

    Red and NIR intensity

    Example Index:

    Rred / Rnir

    Spectral shift in illumination

    prevents use of

    simple irradiance sensing


    Reflectance indices
    Reflectance Indices

    Based on ratios of

    Red and NIR Reflectance

    Red Reflectance:

    r = Rred / Ired

    Example Index:

    rred / rnir

    Reflectance is primarily

    a function of target


    NDVI

    • Developed as an irradiance Index for application to remote sensing

    • Normalized Difference Vegetative Index

    • Varies from -1 to 1

      • Soil NDVI = -0.05 to .05

      • Plant NDVI = 0.6 to 0.9

      • Typical plants with soil background NDVI=0.3-0.8

  • OSU sensors

    • narrow-bandreflectance based NDVI



  • Osu reflectance sensor1

    Natural Illumination

    Battery powered

    Wide dynamic range

    Low noise

    0.75 x 0.25 m field of view

    OSU Reflectance Sensor


    NDVI

    INIR = 780 ±6 nm

    IRED =671 ±6 nm


    Photo diode detector
    Photo Diode Detector

    Opto 202

    Die Topography

    Photo Diode Area

    2.29mm x 2.29mm

    5.2e-6 m2



    Calculation of irradiance from detector output
    Calculation of Irradiance from Detector output

    Responsivity: rl [V/uW]

    for a particular wavelength, output in volts, V is the product of

    Responsivity times the Irradience I times sensor area.

    [ W/m2 ] [V/uW] [m2]

    For a wide band,


    Calculation of irradiance from sensor output cont
    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,rl

    for the detector must be used


    Calculation of irradiance from sensor output cont1
    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

    VRange input range of the amplifier-A/D circuit

    n binary word width of the A/D converter


    Calculation of irradiance from sensor output cont2
    Calculation of Irradiance from Sensor output -cont

    Example:

    Let I = 1 W/m2

    A = 5.2e-6 m2 (for the Burr-Brown 201)

    rl= 0.5 V/mW (for l = red)

    VRange = 5 V

    n = 12 bits


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