SPECTRAL SIGNATURES

dr.ir. Jan Clevers

Centre for Geo-InformationDept. Environmental Sciences

Wageningen UR

Wageningen UR 2010

Subjects Spectral Signatures

• Spectral signature (reflection)

• Temporal signature

• Vegetation indices

Reflective Optical Window

200µµµµm0.3 0.6 1.0 5.0 10 50 10m100 1mm 1cm 1m

200µµµµm0.3 0.6 1.0 5.0 10 50 10m100 1mm 1cm 1m

multispectral scanners

optical

lidar

photography

human eye

mic

row

aves

microwave radiometry

blocking effect of atmosphere

radar

atm

osp

her

ictr

ansm

itta

nce

UV

blue

gree

nre

d

NIR

MIR

MIR

TIR TIR

thermal scanners

VIS

Wavelength

reflective

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100

10-1

10-2

10-3

10-4

0.2 0.5 2 101.0 5 20

radiation (Wcm-2 µm-1)

wavelength (µm)

REFLECTION EMISSION

emitted

thermal radiation

reflected

solar radiation

Source of EM radiation

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curves for an “average”object on earth

Reflection is the process of sending back EM radiation byan object

Reflectance factor [0,1]-coefficient [0,1]-percentage [0%,100%]

1) ρ(λ) = Lr(λ) / Li(λ)= reflected divided by incoming

radiation per wavelength λor wavelength interval ∆λ

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Reflection

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Hand-held radiometer

2) ρobj(λ) = Lobj(λ) / Lref(λ) * ρref(λ)

Reflectance factor: the radiance of an object in a certain direction under certainconditions of irradiation, relative to the radiance of an (idealwhite) diffuse surface in the same direction and the sameirradiation conditions.

3) Atmospheric correctionmodel

Reflection

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Reference targets

λwavelength (µm)

red middle-

infrared

near-

infraredblue

gree

n

dry soil

water

wet soil

vegetation

reflectance (%)

SPOT HRG Panchromatic

Pan: IKONOS, Quickbird, GeoEye, WorldView

Meteosat

NOAA AVHRR

Landsat TM

40

20

00.8

60

0.4 1.2 1.6 2.0 2.4

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SPOT HRG multispectral

Multispectral: IKONOS, Quickbird, GeoEye, WorldView

Spectral band positionsTIRS

TIRS

TIRS

Spectral Signatures: Soil

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0.8%

4.7%

8.3%

12.9%16.9%20.2%

Newtona silt loam

0

20

40

60

1.51.0 2.0 2.50.5

Wavelength (µm)

Example influence of soil moisture content:R

efle

ctan

ce (

%)

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absorptance

(water)abso

rpta

nce

(chl

orop

hyll)

transmittance

reflectance

VIS NIR MIR

100

0

20

40

60

80

2.82.42.01.61.20.80.4

Wavelength (µm)

%

Spectral Signatures: Leaf

Individual leaf

VIS → chlorophyll (absorptance)

NIR → cell walls - air transitions (reflectance)

MIR → water (absorptance)

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Spectral information of a leaf

Spectral Signatures: Leaf

citrustomatosorghumcotton

Wavelength (µm)

0.5 1.0 1.5 2.0 2.5

20

40

60

0

Plant type

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Ref

lect

ance

(%

)

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Ref

lect

ance

(%

)

(1)

(2)

100

0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

20

40

60

80

0

Wavelength (µm)

Disease infection

Spectral Signatures: Vegetation

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1.0

VIS veg. ≈ 1/2 VIS leaf

NIR veg. > NIR leaf

dry soil

wet soil

0.4 0.5 0.6 0.7 0.8 0.9

50

40

30

20

10

0

Reflectance (%)

Wavelength (µm)

barley

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60

50

40

30

20

10

0 50 100 150days after sowing

NIR

redgreen

Reflectance (%)

bare soil

Temporal signatures: Barley Crop

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Field trial with barley

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Single-engine aircraft for 70-mm photography

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Mounts and cameras for 70-mm photography

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Microlight aircraft for 70-mm photography

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Camera mounts for the microlight

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Spectral bands for multispectral photography

1.0

50

40

30

20

10

0

Reflectance (%)

Wavelength (µm)

0.4 0.5 0.6 0.7 0.8 0.9

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Green recording, 27 May

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Red recording, 27 May

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NIR recording, 27 May

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True colour recording, 29 July

False colour recording, 29 July

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Reference targets

Vegetation Indices:

simple band combinations in order to- strenghten the spectral contribution of vegetation- eliminate the disturbing influence of the

atmosphere, soil, etc.

Example:

• simple ratio: NIR/Red

• “Normalized difference vegetation index”:NDVI = (NIR-Red) / (NIR+Red)

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NOAA-NDVI image of Africa

March June

Barley crop: Red, NIR

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Barley crop:

WDVI

50 150100

Ref

lect

ance

(%

)

50

40

30

20

10

0

wet

wet

dry

days after sowing50 150100

days after sowing

WD

VI (

%) 50

0

10

40

30

20 wet

wet

dry

dry

dry

Temporal signatures

LAI = -1/α • ln(1 - WDVI / WDVIω)

LAI

WDVI

6040200

2

4

6

8

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Relationship WDVI - LAI

NDVI = (NIR - Red) / (NIR + Red)

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LAI

10

8

6

4

2

86420

NDVI

Relationship NDVI - LAI

LAI

25 50 75 100 125

8

6

4

2

0

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NIR / Red

Relationship NIR/Red - LAI

LAI R NIR NIR-R NIR/R NDVI

wet 0 10 10 0 1 0dry 0 20 20 0 1 0

2 2 40 38 20 0.9054 2 50 48 25 0.9238 2 60 58 30 0.935

colour 8 1 60 59 60 0.967

NDVI = (NIR - R) / (NIR + R)

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Example vegetation indices