Chlorophyll Estimation Using Multi-spectral
Reflectance and Height Sensing
C. L. Jones Research Engineer N. O. Maness Professor M. L. Stone Regents’ ProfessorR. Jayasekara Research Engineer
2004 ASAE/CSAE Annual International Meeting
Paper Number: 043081
Objectives
• Select “best” wavelength or indices for estimating chlorophyll content and concentration in spinach• Non-destructively quantify chlorophyll content and concentration by combining biomass estimates
with multi-spectral imaging system reflectance data.• Assess the feasibility of reflectance-based sensing in estimating chlorophyll concentration in
spinach
Paper Number: 043081
Let’s Talk Chlorophyll…(Content vs. Concentration)
• Content: related to nitrogen concentration in green vegetation
• Chlorophyll mass per unit ground surface area or per plant (mg/ha or mg/plant)
• Indicator of photosynthetic capacity: productivity of the plant canopy
Paper Number: 043081
Let’s Talk Chlorophyll…(Content vs. Concentration)
• Concentration: estimate of pigment concentration – chlorophyll a and b
• Chlorophyll mass per unit mass of plant material (mg/kg)
• Indicator of plant physiological status: level of stress
Paper Number: 043081
Previous Work
No single spectral approach offers an adequate method of estimating pigment (chlorophyll) concentration in all plants.
Paper Number: 043081
Experimental Plan• Consider spinach plants at the single plant level
• Use chlorophyll content estimates to determine chlorophyll concentration (implies a need for biomass estimate)
• Ccontent (mg) = Cconcentration (mg/kg) x biomass (kg)
• Use ultrasound and digital imagery for biomass estimate
Paper Number: 043081
Experimental PlanEstimators used for chlorophyll content investigation:
– Rgreen : Reflectance at 550 nm ±10 nm
– Rred : Reflectance at 670 nm ±10 nm
– RNIR : Reflectance at 780 nm ±10 nm
– NIR/RED : Ratio of reflectance at 780 and 670 nm ±10 nm
– NIR/GREEN : Ratio of reflectance at 780 and 550 ±10 nm
– NDVI670 = (R780 – R670)/(R780 + R670)
– NDVI550 = (R780 – R550)/(R780 + R550)
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Plant SamplesTwo flats- no fertilizer
Two flats adequate fertilizer
Two flats 2x adequate fertilizer amount
Provided varying biomass and chlorophyll levels
Sampled at six weeks post plant
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Data Gathering Overview
• Plants removed from flats
• Individually placed on turntable (0.91 m diameter) 1.04 m below ultrasonic distance sensor (UDS) for height estimation
• Turntable rotating at 1.06 rpm
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Data Gathering Overview• Plants placed under multispectral
camera mounted on tripod for reflectance data and top view surface area estimate
• Vegetative portion harvested, weighed, and packed on ice for transport to lab
• Chlorophyll analysis per Inskeep and Bloom (1985)
Paper Number: 043081
Ultrasonic Distance Sensor (UDS)
Senix™ Ultra-SPA.(Senix, Bristol, Virginia)• 50 kHz, piezoelectric• 12 degree angle view, signal > 3db• Accuracy: better than 1% of target
distance• Laptop computer• SoftSpan© software
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UDS Response
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Spinach Height Profile - Ultrasound
0 2 4 6 8
10 12 14 16 18
1.37 2.74 4.11 5.49 6.86 8.23 9.6 11 12.3 13.7 15.1 16.5 17.8 19.2 Range, cm
He
igh
t,cm
Multispectral Camera• DuncanTech MS3100• 550, 670, 780 nm ±10 nm FWHM• Resolution: 1392 x 1040 x 3 sensors• Pixel size: 4.65 x 4.65 microns• 14mm focal length lens• RS232 interface with National Instrument’s
PCI-1424 frame grabber• Dolch ruggedized laptop
Paper Number: 043081
Multispectral Camera
• Mounted on tripod 1 m above target• Target illuminated with incandescent lighting• Labsphere Reflectance Calibration Standard
targets: 10%, 50%, 75%, and 99% (Weckler, et al. 2002)
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Multispectral Camera• Images processed with Matlab©
• NIR, green, and red reflectance pixel values identified
• > 0 : plant material, < 0 : background, nonplant material, NDVI averaged
• Image binarized• “bwarea” command to calculate vegetation area
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NIR Green or Red
NIR Green or Red
ρ - ρNDVI =
ρ +ρ
Biomass Estimation
PWC = 0.86
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WEST = AMS x HUDS
Where: WEST = estimated plant biomass
AMS = surface area estimate from camera (pixels)
HUDS = estimated plant height from UDS (cm)
Biomass Estimation
PWC = 0.86
Paper Number: 043081
W = 3E-05W EST + 2.0958
r 2 = 0.884
0
5
10
15
20
25
30
35
0.00E+00 2.00E+05 4.00E+05 6.00E+05 8.00E+05 1.00E+06 1.20E+06 Estimated Biomass (WEST), H UDS * A MS
Act
ual
Bio
mas
s (W
), g
Chlorophyll Content Estimation, r2
Paper Number: 043081
Estimator Ccontent vs. estimator
Rgreen0.21
Rred 0.10
RNIR 0.58
NIR/RED 0.15
NIR/GREEN 0.05
NDVI670 0.75
NDVI550 0.40
C CONT = 1364.5NDVI - 370.41
r 2 = 0.75
0
50
100
150
200
250
300
0 0.1 0.2 0.3 0.4 0.5 NDVI
Ch
l. C
on
ten
t, m
g/p
lan
t
Chlorophyll Content Estimation, r2
Paper Number: 043081
Estimator Ccontent vs. estimator
Ccontent vs. estimator * WEST
Rgreen0.21 0.87
Rred 0.10 0.86
RNIR 0.58 0.90
NIR/RED 0.15 0.90
NIR/GREEN 0.05 0.89
NDVI670 0.75 0.91
NDVI550 0.40 0.89
C
CONT(Estimate) = 17.478(NDVI)(W EST ) + 22.82 r 2 = 0.910
0
50
100
150
200
250
300
350
0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 NDVI*W EST
Chl
. C
onte
nt, m
g/pl
ant
Chlorophyll Concentration Estimation
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•Estimated chlorophyll content calculated from regression equations for each band or ratio
•CCONC = CCONT / WEST
•Compared to actual Chlorophyll concentration
Chlorophyll Concentration Estimation, r2
Paper Number: 043081
Estimator Ccontent vs. estimator
Ccontent vs. estimator *
WEST
Actual Cconc
vs. Cconc
estimate
Rgreen0.21 0.87 0.22
Rred 0.10 0.86 0.30
RNIR 0.58 0.90 0.05
NIR/RED 0.15 0.90 0.04
NIR/GREEN 0.05 0.89 0.03
NDVI670 0.75 0.91 0.08
NDVI550 0.40 0.89 0.000004
Conclusions• Of the indices tested in this study, NDVI670
multiplied by estimated biomass appears to provide best estimate of chlorophyll content in spinach
• Multiplying reflectance ratios and indices by estimated biomass improved chlorophyll content estimations
• Reflectance-based remote sensing may not be the best method for estimating plant pigment concentrations (chl. a and b): RRED best, r2 = 0.30
Paper Number: 043081
QUESTIONS?
AcknowledgmentsSupport through funding from the USDA Special
Research Grant Number 2003-06134
Paper Number: 043081
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