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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One Spectral Channel
PhotoDiode
Amplifier
Analog to
Digital
Converter
CPU
Filter
Illumination
Collimator
Radiometer
Computer
Target
Optical
Glass Fiber
Optical Grating
Analog to
Digital
Converter
CPU
Computer
Photo Diode Array
l
Plus
Plus
Minus
Minus
Wavelength = speed of light divided by frequency
(miles between bumps = miles per hour / bumps per hour)
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
Planck  1900
Einstein  1905
One “photon”
DE is light energy flux
n is an integer (quantum)
h is Planck’s constant
n is frequency
Bohr  1913
Hydrogen Emission Spectra (partial representation)
Wavelength
Wavelength
Absorption of Visible Light
by Photopigments
Sunlight
Chlorophyll b
Phycocyanin
Absorption
BCarotene
Chlorophyll a
300400 500600 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)
lpeak= 2,897,000 / T
where:T = [0K ]
l= [ nm]
Hot metal example
lpeaksun = 2,897,000/6000 = 475nm
lpeakplant = 2,897,000/300 = 9700nm
Point: Emission “color = f(T of emitter)
Equation:
Point: Emission “color = f(T of emitter)
SUN
6000K
Terrestrial
300K
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
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
NIR
UV
Rl0
Rl0 rf
Rl0 a
Rl0 t
Multiple reflections in the atmosphere
cause diffuse radiation
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
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]
Area = [ W/m2 ] = Irradiance
height = [ W/m2 nm ] = Spectral Irradiance
width = [ nm ] = Bandwidth
Spectral Irradiance
Bandwidth
Based on ratios of reflected
Red and NIR intensity
Example Index:
Rred / Rnir
Spectral shift in illumination
prevents use of
simple irradiance sensing
Based on ratios of
Red and NIR Reflectance
Red Reflectance:
r = Rred / Ired
Example Index:
rred / rnir
Reflectance is primarily
a function of target
Natural Illumination
Battery powered
Wide dynamic range
Low noise
0.75 x 0.25 m field of view
INIR = 780 ±6 nm
IRED =671 ±6 nm
Opto 202
Die Topography
Photo Diode Area
2.29mm x 2.29mm
5.2e6 m2
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,
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
Sensor reading, S, is normally an amplified and
digitized numeric value
Where:
V voltage output of the sensor
VRange input range of the amplifierA/D circuit
n binary word width of the A/D converter
Example:
LetI = 1 W/m2
A = 5.2e6 m2 (for the BurrBrown 201)
rl= 0.5 V/mW (for l = red)
VRange = 5 V
n = 12 bits