Optical sensing in Precision Farming (Techniques)

Optical sensing in Precision Farming Optical sensing in Precision Farming (Techniques)(Techniques)Optical sensing in Precision Farming Optical sensing in Precision Farming (Techniques)(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 Purpose for use of optical sensing in Precision FarmingPrecision FarmingPurpose for use of optical sensing in Purpose for use of optical sensing in Precision FarmingPrecision 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?Issues - What is being measured?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

LightSource

Plant or SoilSurface

Reflected Light

SensingSystem

Typical Multi-Spectral Sensor Typical Multi-Spectral Sensor ConstructionConstructionTypical Multi-Spectral Sensor Typical Multi-Spectral Sensor ConstructionConstruction

Analog toDigitalConverter

Computer

One Spectral Channel

Photo-DiodeAmplifier

Filter

Collimator

Target

Illumination

CPU

Radiometer

Fiber-Optic SpectrometerFiber-Optic SpectrometerFiber-Optic SpectrometerFiber-Optic Spectrometer

OpticalGlass Fiber

Photo Diode Array

Optical GratingAnalog toDigitalConverter

Computer

CPU

Fundamentals of LightFundamentals of LightFundamentals of LightFundamentals 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 Relationship between frequency and wavelengthwavelengthRelationship between frequency and Relationship between frequency and wavelengthwavelength

Plus

Minus Minus

Plus

Relationship between frequency and Relationship between frequency and wavelengthwavelength

c

Wavelength = speed of light divided by frequency

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

Relationship between frequency and Relationship between frequency and wavelengthwavelengthRelationship between frequency and Relationship between frequency and wavelengthwavelength

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

Light emission / absorption governed Light emission / absorption governed by quantum effectsby quantum effects

Planck - 1900Planck - 1900

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

E hp Einstein - 1905Einstein - 1905

One “photon”

Changes in energy states of matter are Changes in energy states of matter are quantitizedquantitized

Bohr - 1913Bohr - 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.

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

Photo-ChemistryPhoto-ChemistryPhoto-ChemistryPhoto-Chemistry

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

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 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 colorVisual reception of colorVisual reception of colorVisual 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 Primary and secondary absorbers in plantsplantsPrimary and secondary absorbers in Primary and secondary absorbers in plantsplants

Primary– Chlorophyll-a– Chlorophyll-b

Secondary– Carotenoids– Phycobilins– Anthocyanins

Primary– Chlorophyll-a– Chlorophyll-b

Secondary– Carotenoids– Phycobilins– Anthocyanins

SunlightSunlight

Chlorophyll bChlorophyll b

B-CaroteneB-Carotene

PhycocyaninPhycocyanin

Chlorophyll aChlorophyll a

300 400 500 600 700 800 300 400 500 600 700 800

Wavelength, nmWavelength, nm

Ab

sorp

tio

nA

bso

rpti

on

Lehninger, Nelson and CoxLehninger, Nelson and Cox

Absorption of Visible Lightby Photopigments

Absorption of Visible Lightby Photopigments

0.25

0.5

Wavelength (nm)

Ref

lect

ance

(%

)R

efle

ctan

ce

(%)

VisibleVisible Near InfraredNear Infrared

450 550 650 750 850 950 1050 1150500 600 700 1000900800 1100

0.00

Plant Reflectance

Soil and crop reflectanceSoil and crop reflectanceSoil and crop reflectanceSoil 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 - OklahomaSoil Reflectances - OklahomaSoil Reflectances - OklahomaSoil 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.

Thermal Nature of the Emission of Thermal Nature of the Emission of RadiationRadiationThermal Nature of the Emission of Thermal Nature of the Emission of RadiationRadiation

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 LawWien’s Displacement LawWien’s Displacement LawWien’s Displacement Law

peak = 2,897,000 / Twhere: T = [ 0K ] = [ nm]Hot metal examplepeak-sun = 2,897,000/6000 = 475nmpeak-plant = 2,897,000/300 = 9700nm

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

peak = 2,897,000 / Twhere: T = [ 0K ] = [ nm]Hot metal examplepeak-sun = 2,897,000/6000 = 475nmpeak-plant = 2,897,000/300 = 9700nm

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

Planck’s LawPlanck’s Law

Rc h

ef Thc

k T

2 1

1

2

5 ( , )

Equation:

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

Sun vs. Plant / Soil radiationSun vs. Plant / Soil radiation

0

25

50

75

100

0 2500 5000 7500 10000 12500 15000

Wavelength (nm)

Rad

ian

ce (

%)

Radiance of 6000 K Object

Radiance of 300 K Object

6000K

300K

SUN

Terrestrial

Radiation Energy BalanceRadiation Energy BalanceRadiation Energy BalanceRadiation Energy Balance

Earth

SUN

Temperature of the earth is set bythe difference betweenabsorbed 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 bythe difference betweenabsorbed 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 atmosphereNature of absorption by the atmosphereNature of absorption by the atmosphereNature of absorption by the atmosphere

IncidentIncidentReflectedReflected

TransmittedTransmitted

AbsorbedAbsorbedRadiant energy balance mustbe computed for eachcomponent of the atmosphereand for each wavelengthto estimate the radiationincident on the earth's surface

Radiant energy balance mustbe computed for eachcomponent of the atmosphereand for each wavelengthto estimate the radiationincident on the earth's surface

Earth'ssurface

Atmosphere

550 650450

0

100

200

300

400

500

600

700

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

Sp

ectr

al

Irra

die

nce (

w/m

^2 n

m)

Extraterrestrial SolarIrradience

Terrestial SolarIrradience

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

Solar IrradianceSolar Irradiance

NIRUV

Radiation Energy BalanceRadiation Energy BalanceRadiation Energy BalanceRadiation Energy Balance

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

• Reflection• Absorption• Transmission

+ + rf = 1.0, , and rf are the

fractions taking each path

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

• Reflection• Absorption• Transmission

+ + rf = 1.0, , and rf are the

fractions taking each path

R0

R0

R0 rf

R0

ReflectanceReflectanceReflectanceReflectance

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 RadiationDiffuse and Specular Radiation

Multiple reflections in the atmospherecause diffuse radiationMultiple reflections in the atmospherecause diffuse radiation

SpecularTarget

Source

Measurement of LightMeasurement of LightMeasurement of LightMeasurement 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 LightMeasurement of LightMeasurement of LightMeasurement 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)

TerminologyTerminologyTerminologyTerminology

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

RadianceRadianceRadianceRadiance

Energy Flux through a surface per unit of solid angleper unit area of sourceEnergy Flux through a surface per unit of solid angleper unit area of source

Solid AngleSteridian [St]Solid AngleSteridian [St]

WattsWatts

per meter square of sourceper meter square of source

Stm

WR

2

IrradianceIrradiance

Energy Flux through a surface per unit of areaEnergy Flux through a surface per unit of area

Unit Area (m2)

Power = Energy / Time [Joules / Second] = [Watts]Power = E / TimePower = Photons / TimePower = nh/TimeIrradiance = Power / Area = (Photons / Time) / AreaIrradiance = [Watts / Square Meter]

2m

WI

Irradiance and ReflectanceIrradiance and ReflectanceIrradiance and ReflectanceIrradiance and Reflectance

Irradiance (I0) a measure of power per unit area

Reflectance (rf ) is the ratio of reflected to incident Irradiance rf = I0 rf / I0

Irradiance (I0) a measure of power per unit area

Reflectance (rf ) is the ratio of reflected to incident Irradiance rf = I0 rf / I0

0

100

200

300

400

500

600

700

0 250 500 750 1000 1250 1500 1750 2000

Wavelength (nm)

Sp

ec

tra

l Irr

ad

ian

ce

(w

/m^

2 n

m)

Area = [ W/m2 ] = Irradianceheight = [ W/m2 nm ] = Spectral Irradiancewidth = [ nm ] = Bandwidth

Sp

ectr

al

Irra

dia

nce

Bandwidth

Spectral IrradianceSpectral IrradianceSpectral IrradianceSpectral Irradiance

– Power per unit spectral width– Power per unit spectral width

max

min

dII

Computation of Irradiance from Spectral Computation of Irradiance from Spectral IrradianceIrradianceComputation of Irradiance from Spectral Computation of Irradiance from Spectral IrradianceIrradiance

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

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

RedNIR

RedNIR

II

IINDVI

NDVINDVINDVINDVI

– 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 sensorOSU Irradiance ratio sensorOSU Irradiance ratio sensorOSU Irradiance ratio sensor

Plant and Soil target

Micro-Processor, A/D Conversion, and Signal Processing

Ultra-SonicSensor

Photo-Detector

Optical Filters

Collimation

Inir

Rnir

Ired

Rred

Irradiance IndicesIrradiance IndicesIrradiance IndicesIrradiance Indices

Spectral shift in illuminationprevents use ofsimple irradiance sensing

Based on ratios of reflectedRed and NIR intensity

Example Index:Rred / Rnir

Inir

Rnir

Ired

R red

Reflectance IndicesReflectance IndicesReflectance IndicesReflectance Indices

Based on ratios ofRed and NIR Reflectance

Red Reflectance: = Rred / Ired

Example Index:red / nir

Reflectance is primarilya function of target

NDVINDVINDVINDVI

dNIR

dNIRNDVIRe

Re

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-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 = -0.05 to .05• Plant NDVI = 0.6 to 0.9• Typical plants with

soil background NDVI=0.3-0.8

OSU sensors– narrow-band

reflectance based NDVI

Photo-Detector

Turf target

Optical Filters

Fiber OpticLight Guides

GlassCover

Collimation

Sensor/AmplifierIntegrated Circuit

OSU Reflectance SensorOSU Reflectance SensorOSU Reflectance SensorOSU Reflectance Sensor

OSU Reflectance SensorOSU Reflectance SensorOSU Reflectance SensorOSU 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

NDVINDVINDVINDVI

0

50

100

150

200

250

300

350

400

450

0 500 1000 1500 2000 2500

Wavelength (nm)

Te

rre

sti

al S

ola

r Ir

rad

ien

ce

(W

/M^

2-n

m)

0

10

20

30

40

50

60

70

80

90

100

Co

mp

ute

d P

lan

t A

bs

orb

an

ce

Sp

ec

tra

(%

)

Terrestial Solar Irradience

Computed PlantReflectance Spectra

Plant with lowerphotosynthetic activity

IRED =671 nm

INIR = 780 nm

Photo Diode DetectorPhoto Diode Detector

Photo Diode Area2.29mm x 2.29mm

5.2e-6 m2

Opto 202Die Topography

Silicon ResponsivitySilicon Responsivity

Calculation of Irradiance from Detector Calculation of Irradiance from Detector outputoutputCalculation of Irradiance from Detector Calculation of Irradiance from Detector outputoutput

Responsivity: [V/uW]for a particular wavelength, output in volts, V is the product ofResponsivity times the Irradience I times sensor area.

V I A106 [ W/m2 ] [V/uW] [m2]

For a wide band,

V I Ad 106

min

max

Calculation of Irradiance from Sensor Calculation of Irradiance from Sensor output -cont-output -cont-Calculation of Irradiance from Sensor Calculation of Irradiance from Sensor output -cont-output -cont-

Irradiance may be computed from thevoltage reading for a narrow spectral band :

KVA

VI

610

The average value of Responsivity, for the detector must be used

Calculation of Irradiance from Sensor Calculation of Irradiance from Sensor output -cont-output -cont-Calculation of Irradiance from Sensor Calculation of Irradiance from Sensor output -cont-output -cont-

Sensor reading, S, is normally an amplified anddigitized numeric value

SV

VRange

n 2 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

SI A

VRange

n10

26

Calculation of Irradiance from Sensor Calculation of Irradiance from Sensor output -contoutput -contCalculation of Irradiance from Sensor Calculation of Irradiance from Sensor output -contoutput -cont

Example:Let I = 1 W/m2

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

= 0.5 V/W (for = red)VRange = 5 Vn = 12 bits

SI A

VRange

n

10

210 1 05 52 10

52 2130

6 6 612 . .

S kI