introduction to optical/thermal remote sensing - esa...
TRANSCRIPT
Introduction to Optical/Thermal Remote Sensing
J. A. Sobrino, J. C. Jiménez-Muñoz
(University of Valencia, Spain)
OUTLINE
Basic radiometric magnitudes
Basic laws of radiation
Radiative transfer equation
Optical remote sensing: reflectivity
Thermal remote sensing: thermal properties
Atmospheric correction: solar & thermal
Measurements
Spectral regions
Spectral range (mm) Term
0.4-0.7 Visible (VIS)
0.7-1.5 Near Infrared (NIR)
1.5-2.5 Short-Wave Infrared (SWIR)
3-5 Mid-Infrared (MIR)
8-14 Thermal Infrared (TIR)
VIS, NIR, SWIR: dominated by reflected solar energy TIR: dominated by surface emission MIR: reflected solar energy + surface emission
RADIOMETRIC MAGNITUDES
Radiative field: EM field of the wave propaging between the source and the
detector.
Energy of the EM wave Radiant energy: Q [J]
Radiant energy/time: Radiant flux f = dQ/dt [W] (W=J·s)
(f: flux of the Poynting vector)
f Does not provide information about distribution/direction of radiation
Basic magnitudes
Conservation of energy
fi = ft + fr + fa
i: incident
t:transmitedo
r:reflected
a:absorbed
Radiant flux density: F=df/dS [Wm-2] (f through an element dS)
Irradiance (E): F when the surface receives the radiation
Emittance (M): F when radiation is emitted by a source
F does not include information about direction
Radiant intensity I = df/dW [Wsr-1] : radiante flux in a given dW
I : used to characterize emission from point sources
Basic magnitudes
Radiance
Radiant flux (f) in a given dW through a
perpendicular surface (dScosq) to the
propagation direction.
22 1d dI
L [Wm sr ]d dScos dScos
f
q q
W
Basic magnitudes
2
dAd Esfera : 4 [sr]
r
Fuente puntual isotropa : I4
Superficie Lambertiana :
L( ) L(0º ) I( ) I(0º )cos (Ley coseno)
Radiancia / Emi tancia :
M L( , )d cos L( , )cos sen d d
Superficie Lambertiana : M L
f
q q q
q f q q f q q q f
W W
W
Sphere
Isotropic point source
Lambertian surface:
(cosine law)
Radiance/Emittance
Lambertian surface:
Spectral magnitudes
The interaction mechanisms are dependent on l
The previous magnitudes are spectral magnitudes!!!
(Ql, fl, Il, El, etc)
2ll m
dL WL
d m sr mSpectral radiance:
(In practice spectral magnitudes are measured over a certain band witdh)
MAGNITUDE SYMBOL DEFINITION UNITS
Radiant energy Q - J
Radiant flux f dQ/dt W
Emittance M df/dS W m-2
Irradiance E df/dS W m-2
Radiant intensity I df/dW W sr-1
Radiance L d2f/(dWdScosq) W m-2 sr-1
Spectral radiance Ll dL/dl W m-2 sr-1 mm-1
Summary of radiometric magnitudes
REFLECTIVITY:
Ratio between reflected and incident fluxes = fr/fi
ABSORTIVITY: Ratio between absorbed and incident fluxes
= fa/fi
TRANSMISIVITY: Ratio between transmitted and incident fluxes
t = ft/fi
EMISSIVITY Ratio between the emittance of a body and the emittance of a blackbody at the same temperature = M/Mn
Adimensional magnitudes
Conservation of energy
fi = ft + fr + fa + + t = 1
Spectral magnitudes
l, l, tl, l
Basic laws of radiation
Planck law
Planck (1900) found a mathematical description for the spectra distribution of the radiance emitted by a blackbody:
k: Stefan-Boltzmann constant (1.38·10-23 J K-1)
c: speed of light (2.9979·108 m s-1)
T: absolute temperature (K)
c1 , c2 : radiation constants
c1 = 2hc2 = 1.19104·108 W mm4 m-2 sr-1 ; c2 = hc/k = 14387.7·104 mm K
(l in mm, then Planck’s function in W m-2 mm-1 sr-1)
5
1
2
( )
exp 1
cB T
c
T
l
l
l
3 22( )
exp 1
h cB T
h
kT
2
cc d dl l
l
ll
l
1kT
hcexp
hc2M
5
2
,n
Blackbody is lambertian
M(T) = L(T)
Earth: 300 K
Sun: 6000 K
Planck’s law is fundamental in thermal remote
sensing. It related emitted radiance with body
temperature from the measured radiance in a
given sensor bands, we can obtain the radiant
temperature
2
15
cT
cln 1
Bl
ll
Stefan-Boltzmann law
Example: emittance for 300 K
(Earth) and 6000 K (Sun).
The figure includes the bands of
the NOAA/AVHRR sensor.
48 2 4
4
TL(T) L (T)d ( 5.67·10 Wm K )
M(T) T
l
l
l
Total emissive power of a blackbody (no consideration of the spectral distribution) is
a function of T4
Stefan-Boltzmann constant ()
Wien’s displacement law
For an object at a constant kinetic temperature, the radiant energy (flux) varies as a function of wavelength (see Planck’s law).
The radiant energy peak (lmax) is the wavelength at which the maximum amount of energy is radiated.
As temperature increases, the radiant energy peak shifts to shorter wavelengths. This shift is described by Wien’s displacement law:
max
3
max
dB(T)0 T 2.8975·10 m·K
d l l
ll
max 6 3 1 5
5
B4.7961·10 (Wm sr K )
T
l
When l=lmax:
Wien’s displacement law
Sun (6000 K): lmax=0.5 mm (VIS) Earth (300 K): lmax=9.7 mm (TIR)
Radiation laws need to be corrected for the emissivity, since real materials are not perfect blackbodies:
B(T) B(T)
Approximations (Planck’s law)
2 2 2c c c1 ( cortas) T 5000 m·K exp 1 exp
T T Tl l m
l l l
Wien’s radiation law
52 2 2c c c1 ( largas) T 10 m·K exp 1
T T Tl l m
l l l
Rayleigh-Jeans’ law
(In this case, blackbody radiance and temperature show a linear relationship)
Examples:
5 m WienT 1000K
100 m R J
16.6 m WienT 300K
333.3 m R J
l m
l m
l m
l m
Kirchhoff’s law
When a radiation source is surrounded by other radiation sources, it emits
radiation but also absorbs radiation.
The temperature will depend on the balance between emitted and absorbed
energy.
Radiation equilibrium: emitted energy = absorbed energy (all l)
absorbida emitidaL B (T) L B (T)l l l l l l l l
Kirchhoff’s law: for a given l, the absorptivity of the surface equals
the emissivity of the surface, at the same temperature.
Superficies opacas ( 0) 1l l lt Opaque surfaces
Fundamental approach in
radiometry, since emissivity
values can be obtained from
reflectivity measurements
Radiative transfer
It describes the processes involved in the propagation of the radiation from
the surface to the sensor (or sun-to-sensor) through the atmosphere
RADIATIVE TRANSFER EQUATION (RTE)
RTE allows to perform atmospheric corrections and also
forward simulations
“CLASSIC” THEORY OF RADIATIVE TRANSFER:
RTE is obtained from an energy balance (conservation of energy)
(Chandrasekhar, 1960; Lenoble, 1993)
“MODERN” THEORY OF RADIATIVE TRANSFER:
RTE obtaiend from wave theory (Maxwell Eqs. + statistical considerations)
(Apresyan y Kravtsov, 1996).
Radiative Processes
a
s
a s e
dLL
dn
dLL
dn
dLL L
dn
e
sc em
dLL J
dn
J J J
L
L+dL
dn
absorption
scattering
abs+scat
abs+scat+sources
Source function
n
(Spectral magnitudes)
dL = -a L dx L(x2) = L(x1) exp(-da)
2
1
( )x
a ax
x dxd
)exp()(
)(
1
2a
xL
xLdt
)(
)(lnln
2
1
xL
xLa td
t
1)(
)()(
1
21
xL
xLxL
Absorption
Path between x1 and x2
Beer-Lambert-Bouguer law
Volumetric
absorption coefficient (m-1) Optical depth (absorption)
Homogeneous media: a·Dx
Transmissivity
da
Absorptivity (with reflexion: + + t = 1)
dL = -s L dx 2
1
( )d x
s sx
x dx
Scattering (similar to absorption, more complex because of the scattering function)
Let us to consider a volume element dv. The incident ray is characterized by its irradiance E over dv. The scattered radiant flux by dv in a given direction n and angle q inside a solid angle dW is given by
d2f = f(q) E dv dW f(q) (m-1sr-1): scattering function (characterizes the angular distribution of scattered photons)
Total flux lost in the scattering (integration over all the space):
Wespacio
dfEdvd )(qf
For a given volume (dV (dv = dS dx), the incident flux is given by f = E dS
df = -s f dx = -s E dv Wespacio
s df )(q
)(4
)( q
q fp
s
q 4)( Wespaciodp
mmqqq0
1
12)()( dpdsenp
Normalized phase
function
;
General case: Absorption + Scattering
e = a + s d = da + ds
e
s
s e
a e1
Similar to the previous cases
Extinction coefficient (e) and Total optical depth (d)
Single-scattering albedo
(ratio between absorption and scattering processes)
Conservative case: single-scat. albedo = 1 No loss of radiative energy
(no absorption)
Radiative Transfer Equation
“CLASSIC” THEORY OF RADIATIVE TRANSFER:
-Phenomenological considerations
-Based on the conservation of energy
-Radiance is an energetic variable. Energy balance:
Variation
of L(R,n)
along the
direction n
Loss of energetic flux
because of absorption
and scattering from
direction n to direction
n’
Increase of energy
flux in the direction n
because of scattering
from other directions
n’
Increase in the
flux because of
the sources
= - + +
dn
d
dV=ddn
Ll(R,n)ddW [Ll(R,n)+dLl(R,n)]ddW f1
f2
-el(R)Ll(R,n)dnddW
f3
el(R)Jl (R)dndd
dW
n
dW
f4
e
( , )= - ( ) ( , ) ( , )l
l l l dL
L Jdn
R nR R n R n
Radiative Balance
R.T.E.
OPTICAL REMOTE SENSING
REFLECTIVITY
Definitions of reflectivity
a) lambertian (perfectly diffuse) b) non-lambertian (directional or diffuse) c) Specular d) Hot spot
r
i
d
d
f
f Adimensional magnitude. Ratio between reflected
and incident fluxes
Hemispherical reflectance rh
=df
h
dfi
d2f = LdWdScosq
dW =dA
r2=
rsenq (rdq )dj
r2
ü
ýï
þïÞ df
h= dj
0
2p
ò LdScosqsenq dq0
p
2ò
dfi= EdSÞ r
hq ,j( ) =
1
Edj
0
2p
ò LdScosqsenq dq0
p
2ò
Para una superficie lambertiana :
dfh
= LdS dj0
2p
ò cosqsenq dq0
p
2ò = pLdS Þ rh
=pL
E=
M
E
Reflectividad hemisferica espectral : rh,l
=M
l
El
(Ratio between emittance and irradiance)
Lambertian surface
BIDIRECTIONAL REFLECTANCE DISTRIBUTION FACTOR: BRDF NATURAL SURFACES ARE NOT LAMBERTIAN distribution of radiance shows a significant angular dependence
r i i r r i r i i r r i 1
r i i r r
i i i i i i i i
dL , , , ,E dL , , , ,Ef , , , sr
dE ( , ) L ( , )cos d
q f q f q f q fq f q f
q f q f q
Nicodemus et al. (1977):
r i
i
ri i r r i
i
r i i r r i i i i r
i i i i
d( , ; , ;L )
d
f ( , ; , )L ( , )d d
L ( , )d
f q f q f
f
q f q f q f
q f
W W
W
Definition of reflectivity from BRDF:
r
r,id
dBRF
d
f
f
Bidirectional Reflectance Factor (BRF):
Ratio between reflected flux and reflected flux of a perfectly diffuse (ideal) surface
Different definitions of reflectivity (Sobrino, 2000)
Reflectivity of natural surfaces Soils
Reflectivity increases with increasing wavelength Depends on: -texture -water content -organic matter -iron oxides -roughness
Reflectivity of natural surfaces Water and Snow
Water: low reflectivity Depends on: -depth -chlorophyll, nutrients… -roughness Snow: high reflectivity l<0.8 mm, low reflectivity l>1.5 mm Decreases with the snow age Fresh snow > frozen snow
Reflectivity of natural surfaces Green Vegetation (Leaves)
THERMAL REMOTE SENSING
Thermal properties & Emissivity
Thermal Remote Sensing
Less attention has been given (in comparison to VNIR-SWIR data exploitation)
Necessary for a better understanding of land surface processes and land-atmosphere interactions (most of the fluxes at the surface/atmosphere interface can only be parameterized through the use of TIR data).
Key parameters:
LAND SURFACE TEMPERATURE (LST)
LAND SURFACE EMISSIVITY (LSE, ) – spectral magnitude!
Low Resolution:
LST: climatology, land cover change, SST
LSE: not exploited. Used as input to LST algorithms.
High Resolution:
LST: energy balance (heat fluxes => ET => water management)
LSE: geology (mineral mapping), identification of characteristic feautres in the spectrum… and also land cover change
Thermal Remote Sensing
Heat energy is transferred from one place to another by 3 means: Conduction, Convection and Radiation Materials at the surface of the earth receive thermal energy primarily in the form of radiation from the sun. All matter radiates energy at thermal IR wavelengths (around 3 to 15 mm) both day and night (VNIR data only available at day!). Kinetic heat: energy of particles of matter in a random motion, which causes particles to collide, resulting in changes of energy state and the emission of electromagnetic radiation form the surface of materials (radiant energy). Kinetic temperature: concentration of kinetic heat of a material. Measured with a thermometer (direct contact). Radiant temperature: concentration of radiant flux of a body. Measured remotely by radiometers.
Reflectivity, absorptivity, transmissivity
Radiante energy striking the surface of a material is partly reflected, partly absorbed, and partly transmitted (conservation of energy)
reflectivity + absorptivity + transmissivity=1 Spectral magnitudes determined by properties of matter Opaque materials: reflectivity + absorptivity = 1
Blackbody. Emissivity.
The concept of blackbody is fundamental to understanding heat radiation.
Blackbody: theoretical material that absorbs all the radiant energy that strikes it (absorptivity=1). It also radiates all of its energy in a wavelength distribution pattern that is dependent only on the kinetic temperature.
A blackbody is a physical abstraction: no material has an absorptivity of 1 and no material radiates the full amount of energy.
Real materials: a property called emissivity is introduced, defined as the ratio bewteen the real radiant flux and the radiant flux from a blackbody. It is a spectral magnitude.
Blackbody: emissivity=1. Real material: emissivity<1.
Effect of emissivity on measured radiant temperature: “Real” temperature: 10 ºC Radiometer 1: Emis=0.06. T= -133 ºC Radiometer 2: Emis=0.97. T=8 ºC The difference for the radiometer 1 is more than 140 ºC!!! The difference for the radiometer 2 is only 2 ºC becuase emissivity is near to 1 (blackbody).
RADIOMETER 1 RADIOMETER 2
Other thermal properties
Thermal conductivity: rate at which heat will pass through a material. Rocks and soils are relatively poor conductors of heat.
Thermal capacity: ability of a material to store heat. Water has a high value of thermal capacity.
Thermal inertia: measure of the thermal response of a material to temperature changes. It depends on thermal conductivity, density and thermal capacity. Materials with a high thermal inertia (sandstone, basalt) strongly resist temperature changes and have low DT (maximum-minimum T in the daily cycle).
Apparent Thermal Inertia: it can be measured from remote sensing methods, and depends on DT and albedo (measurements of thermal inertia require contact methods).
Thermal diffusivity: rate at which temperature changes within a substance.
Emissivity spectra of natural surfaces
Surface emissivity is a key variable in thermal remote sensing. Accurate retrievals of LST from remote sensing techniques rely on accurate estimations of surface emissivity.
Natural (and manmade) surfaces are not perfect blackbodies. The emissivity spectra needs to be characterized.
Spectral libraries include several emissivity spectra measured in the laboratory for different surfaces. These spectra are useful for simulation purposes.
MODIS Emissivity Library
http://www.icess.ucsb.edu/modis/EMIS/html/em.html
ASTER Spectral Library
http://speclib.jpl.nasa.gov/
Natural surfaces: Rocks
0.80
0.82
0.84
0.86
0.88
0.90
0.92
0.94
0.96
0.98
1.00
7 8 9 10 11 12 13 14 15
longitud de onda (mm)
em
isiv
idad
basalto
diorita
cuarzo
mármol
0.80
0.82
0.84
0.86
0.88
0.90
0.92
0.94
0.96
0.98
1.00
7 8 9 10 11 12 13 14 15
longitud de onda (mm)
em
isiv
idad
Agua
hierba seca
hierba verde
Natural surfaces: Water and Vegetation
Spectral features
A sample and the medium in which it is embedded for the purpose of taking and infrared spectrum will always having differing dispersions, although their indices of refraction may match at a center frequency of interest. In general, there will therefore be a wavelength at which the sample+medium have a maximum transparency. Christiansen frequency: the frequency of maximum transparency.
Reststrhalen bands: reflectance phenomenon in which EM radiation within a narrow energy cannot propagate within a given medium due to a change in refractive index concurrent with the specific absorbance band of the medium. Normally-inciden Reststrahlen band radiation experiences strong-reflection or total-reflection from that medium.
Reststrahlen bands manifest as a minima in emissivity.
quartz corn
“a” shows a strong minimum at 9,67 mm, related to the Reststrhalen bands Christiansen bands for quartz
0
10
20
30
40
50
60
70
80
90
100
0.0 0.5 1.0 1.5 2.0 2.5
longitud de ona (mm)
refl
ecti
vid
ad
(%
)
0
10
20
30
40
50
60
70
80
90
100
0.0 0.5 1.0 1.5 2.0 2.5
longitud de onda (mm)
reflecti
vid
ad
(%
)
REFLECTIVITY (VNIR) vs
EMISIVITY (TIR)
Green vegetation
Rock (quartz)
Emissivity/Temperature
What happens if measurements are not corrected for emissivity?
T 1
T 5
D D
T 300K T 60
1% T 0.6K
D D
D D
Sample (8-14 mm) DT (=1)
Sand 0.8-0.9 15-6
Clay 0.94-0.98 4-1
Water, Vegetation 0.97-0.99 2-0.6
Snow 0.99 0.6
(T=300K)
0.84
0.86
0.88
0.90
0.92
0.94
0.96
0.98
1.00
6 7 8 9 10 11 12 13 14 15 16
Longitud de Onda (m)
Em
isiv
idad
Coníferas
Mármol
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
10.5
6 7 8 9 10 11 12 13 14 15 16
Longitud de Onda (m)
Rad
ian
cia
(W
m-2
sr-1
m
-1)
B(Ts)
e1*B(Ts)
e2*B(Ts)
288
290
292
294
296
298
300
302
6 7 8 9 10 11 12 13 14 15 16
Longitud de Onda (m)
Tem
pera
tura
(K
)
Ts
Ts1
Ts2
Emissivity correction Surfaces are not blackbodies: ≠1 y =f(l) Some natural surfaces behave as black/grey-bodies (e.g. water, vegetation)
Emissivity spectra
Radiant energy (radiance)
Temperature
Atmospheric Correction
SURFACE
ATMOSPHERE
The atmosphere mainly absorbs the thermal radiation emitted by the surface, and also scatter in the VNIR (solar) range. This loss of radiation needs to be compensated to recover the at-surface signal (Atmospheric Correction).
LlTOA = Ll
SUN tl lsur + Ll
path
LTOA: At-sensor registered radiance (TOA, Top Of Atmosphere). LSOL: Extraterrestrial solar irradiance (corrected for excentricity and zenithal solar angle). t: Total atmospheric transmissivity: Sun-to-target (t), target-to-sensor (t) t = t·t . sur: at-surface reflectivity. Lpath: path radiance.
Lpath
LSun
tLSun sur(tLSun)
t[sur(tLSun)]
ATMOSPHERIC CORRECTION IN THE SOLAR RANGE: SIMPLIFIED APPROACH
0,2
cosSOL zL Ed
l l
q
At-surface reflectance is obtained after compensation of path radiance and atmospheric transmissivity
ll
lll
t
SOL
ocaTOA
L
LL minsup
In terms of reflectivity the R.T.E. can be expressed as:
lTOA = tl l
sur + lpath
Atmospheric effect
ATMOSPHERIC CORRECTION FROM A SIMPLIFIED APPROACH OF THE R.T.E.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
7 8 9 10 11 12 13 14 15
longitud de onda (mm)
tra
ns
mis
ivid
ad
LMV
LMI
SAV
SAI
TRO
ATMOSPHERIC CORRECTION IN THE THERMAL INFRARED
O3
8-9.4 mm 10-12.5 mm Atmospheric Windows in the TIR
Tran
smis
sivi
ty
Wavelength (mm)
THERMAL INFRARED - TIR (8-14 mm)
( ) (1 )sen atm atm
sL B T L Ll l l l l l l t
B: Planck’s law
Brightness temperature: LsenB(Tsen)
LST: Ts (l → i)
Physical basis: classical radiative transfer
( ) (1 )sen atm atm
sL B T L Ll l l l l l l t
sup ( ) (1 )erficie atm
sL B T Ll l l l l
supsen atmL L Ll l l lt
supsen atmL L
L l ll
lt
Atmospheric Correction (TIR)
Atmospheric Correction:
The signal measured by the
sensor is not the radiance
coming from the surface
because of the perturbation
of the atmosphere
LST retrievals from remote sensing data require an atmospheric correction and
also a previous estimation of the surface emissivity (we need to solve the coupling
between LST & )
Rigorous algorithms are based in the Radiative Transfer Equation. Algorithms
depend mainly on the spectral characteristics of the TIR sensor
METHODS FOR LST RETRIEVAL:
1 BAND: Single-channel (mono-window)
2 BANDS: Two-channel or split-window
1 BANDS and 2 VIEW ANGLES: Bi-angular (dual-angle)
3 BANDS (multispectral): TES algorithm
Other Methods: TISI, day/night combinations, tri-channel, etc…
LST RETRIEVAL
LST: TWO-CHANNEL ALGORITHMS (2 BANDS)
Based on the “differential absorption” concept, also known as “split-
window”
Differential absorption Atmospheric correction (Ti – Tj)
LST = Ti + c1 (Ti-Tj) + c2 (Ti-Tj)2 + c0 + (c3+c4W)(1-) + (c5+c6W)D
= 0.5 (i - j)
D = i – j
Ti,Tj in K
W in g/cm2
Mathematical structure of the algorithm (Sobrino et al. 1996; Sobrino & Raissouni, 2000)
DUAL-ANGLE ALGORITHMS
Same as split-window, but using two different view angles:
Ti Tn (nadir) Tj Tf (forward)
Emissivities are obtained from VNIR data, and not from TIR data! This avoids the
undetermined problem.
Simplified approach
Emissivity as an average of soil and vegetation emissivities according to the Fractional
Vegeetation Cover (FVC).
NDVI Thresholds Method (Sobrino & Raissouni, IJRS, 2000)
Pixels are classified into bare pixels, mixed pixels and fully-vegetated pixels. Cavity effect is
estimated from a geometric model.
, ,(1 )s vFVC FVCl l l
, ,(1 )
0.985 0.005
red
s v
a b NDVI NDVIs
FVC FVC C NDVIs NDVI NDVIv
NDVIv
l l
l l l l
(1 ) '(1 )i si vi vC F P 2
' 1 1H H
FS S
2NDVI NDVIs
FVCNDVIv NDVIs
EMISSIVITY: SINERGY BETWEEN VNIR & TIR
MEASUREMENTS
REFLECTANCE MEASUREMENTS
(GER, ASD, etc.)
REFERENCE WHITE PANEL: Spectralon, highly reflectant and diffuse (hemispherical reflectance > 99%)
THERMAL RADIOMETERS
CALIBRATION SOURCES DIFFUSE PLATE
BOX FOR EMISSIVITY MEASUREMENTS THERMAL CAMERAS
GONIOMETER
Broadband radiometers (8-14 mm)
RAYTEK ST8
-30 to 100 ºC
0.5 ºC
FOV: 8º
RAYTEK MID
-40 to 600 ºC
0.5 ºC
FOV: 20º
EVEREST 3000
-40 to 100 ºC
0.5 ºC
FOV: 4º
Apogee
IR-120 (Campbell)
-25 to 60 ºC
0.2 ºC
FOV: 20º
multiband:
CIMEL
0.0
0.1
0.2
0.3
0.4
0.5
0.6
7 8 9 10 11 12 13 14 15
longitud de onda (mm)
filt
ro
banda 1: 8-14 um
banda 2: 11.5-12.5 um
banda 3: 10.5-11.5 um
banda 4: 8.2-9.2 um
0.0
0.1
0.2
0.3
0.4
0.5
0.6
7 8 9 10 11 12 13 14 15
longitud de onda (mm)
filt
ro
banda 1: 8-13 um
banda 2: 11-11.7 um
banda 3: 10.3-11 um
banda 4: 8.9-9.3 um
banda 5: 8.5-8.9 um
banda 6: 8.1-8.5 um
MODEL BANDS RANGE ACCURACY FOV
CIMEL 312-1
8-13 mm
8.2-9.2 mm
10.3-11.3
mm
11.5-12.5
mm
-80 a 60 ºC 0.1 ºC 10º
CIMEL 312-2
8-13 mm
11-11.7 mm
10.3-11 mm
8.9-9.3 mm
8.5-8.9 mm
8.1-8.5 mm
-80 a 60 ºC 0.1 ºC 10º
The box method: emissivity measurements
(Buettner y Kern, 1965; Dana, 1969; Sobrino y Caselles, 1989; Nerry et al. 1990)
The box method was designed to measure the surface emissivity. It removes the surrounding (or sky) irradiance using a closed lambertian box. It can be used both at the field (outdoors) or in the laboratory (indoors).
The box method Formulation
SENSOR
LID
2 LIDS
FINAL EXPRESION (T removed!)
1
1
2
1
t ss
t s
L L
L L
1
1
2
1
( ) ( )
( ) ( )
b b
t ss b b
t s
T T
T T
Ideal case: t1: blackbody (=1 y =0) t2: perfect reflector (=1 y =0)
Radiance/Temperature approach: (Slater, 1980): L=aTb
Accuracy of the box method
The box is not perfect, so a correction factor is added:
s 0 d d: depends on the box geometry, temperature and emissivity of the lids
RADIOMETRIA PASIVA
1.-MÉTODO DE LA RADIANCIA ATMOSFÉRICA
Utiliza una superficie reflectante cerrada y lambertiana (Caja)3 medidas para obtener la emisividad
1) Radiometro observa muestra a traves orificio practicado en lacaja
LBBcaja =s B(Ts) + (1-s) B(Ts) = B(Ts)
Supone emisividad caja=0
2) Medida muestra sin caja
LBB=s B(Ts) + (1-s) La
3) Medida de la Radiancia de los alrededores, La
La temperatura del suelo no debe cambiar durante el proceso demedida
Combinando 1) y 2) LBB=s (LBBcaja-La)+ La
sl= LBB-La
LBBcaja-La
Método adecuado cuando alrededor=atmosfera
IT IS ALSO POSSIBLE TO MEASURE THE EMISSIVITY WITH A LAMBERTIAN BOX WITH ONLY 1 “COLD” LID
BOX WITH THE COLD LID
BOX WITHOUT THE LID
FINAL EXPRESSION
RECOMMENDATIONS FOR IN-SITU MEASUREMENTS USING THE BOX METHOD 1 LID: clear-sky, or totally cloudy 2 LIDS: T for the “hot” lid should be higher than the sample T Buettner & Kern (1965): hot water flows Nerry (1988): Electrical resistances Sobrino (1989): Heating by sunlight
Measurements should be performed around midday. Our experience shows a reproductibility better than 1% for temperature differences between the lids of aroudn 20 ºC. Measurements in short periods of time with low wind speed. 1 LID: first measurement with box, second measurement without the box, and third measurement the atmospheric contribution 2 LIDS: i) box with the “cold” lid, ii) box with the “hot” lid, and iii) T of the “hot” lid
1
1
2
1
t ss
t s
L L
L L
Lt1: radiance from the “hot” lid L1
s: radiance from the sample with the “cold” lid L2
s: radiance from the sample with the “hot” lid
Box with 2 lids
Angular Variations
Surface emissivity depends on different factors: -Composition of the body and spectral range -Temperature of the body (negligible for ambient temperatures) -Soil moisture (increases with SM) -Direction of emission (angular effects)
ANGULAR MEASUREMENTS
Expresión operativa para la emisividad direccionalabsoluta:
q =exp (- / T ) - 1,3 exp (- / T
exp (- / T ) - 1,3 exp (- / T )rad atm0
S atm0
)
Expresión operativa para la emisividad direccionalrelativa:
qr,
rad atm0
rad0 atm0
=exp (- / T ) - 1,3 exp (- / T
exp (- / T ) - 1,3 exp (- / T )
)
Absolute directional emissivity:
Relative directional emissivity:
(Factor 1.3: depends on the radiometer)
EXPERIMENTAL RESULTS
Semicircular goniometer with R=1.5 m Radiometer Omega OS86 (8-14 mm) with =0.1 K and IFOV=2º Thermopar (water measurements): TES 1310 Type K (=0.1 K) Thermistor calibrated in the laboratory with a Hg thermometer (Siebert&Kuhn, =0.05K) Samples: sand, clay, lime, gravel, water, grass Measurement steps: 5º (t< 2 min, thermal stability)
Samples
HOMOGENEOUS SAMPLES (8-14 mm)
sample
Decrease in
relative
emissivity
(%)
Decrease in
absolute
emissivity(%)
water 3,3 3,3
sand 2,0 1,9
clay 0,5 0,5
slime 0,9 0,9
gravel 1,2 1,2
grass 0 0
Difference at 0º and 55º
canal 1
0,940
0,960
0,980
1,000
0 20 40 60
ángulo (º)
r
ela
tiva
agua
arena
arcilla
limo
gravilla
canal 2
0,940
0,960
0,980
1,000
0 20 40 60
ángulo (º)
r
ela
tiva
agua
arena
arcilla
limo
gravilla
canal 3
0,940
0,960
0,980
1,000
0 20 40 60
ángulo (º)
r
ela
tiva
agua
arena
arcilla
limo
gravilla
canal 4
0,940
0,960
0,980
1,000
0 20 40 60
ángulo (º)
r
ela
tiva
agua
arena
arcilla
limo
gravilla
(Cuenca & Sobrino, 2004, Applied Optics)
HOMOGENEOUS SAMPLES (CIMEL BANDS)
(8-13 um)
(8-9 um) (10-11 um)
(11.5-12.5 um)
RESULTS FOR A WATER SAMPLE
agua: canal 1
0,950
0,960
0,970
0,980
0,990
1,000
1,010
0 20 40 60
ángulo (º)
re
lati
va
Masuda et al.
trabajo presente
agua: canal 2
0,950
0,960
0,970
0,980
0,990
1,000
1,010
0 20 40 60
ángulo (º)
r
ela
tiva
Masuda et al.
trabajo presente
agua: canal 3
0,950
0,960
0,970
0,980
0,990
1,000
1,010
0 20 40 60
ángulo (º)
r
ela
tiva
Masuda et al.
trabajo presente
agua: canal 4
0,950
0,960
0,970
0,980
0,990
1,000
1,010
0 20 40 60
ángulo (º)
r
ela
tiva
Masuda et al.
trabajo presente
Target
Automatic Goniometer
Green grass Wheat Bare soil
Angular measurements over different natural (agricultural) surfaces
Dn
iii LSBTn
LSBTLSBT1i
)( 1
- )( )( qqq
Angular variations on Brihtness Temperatures (LSBT: Land Surface Brightness Temperature)
Bare soil 0.00
2.00
4.00
6.00
8.00
10.00
-60 -40 -20 0 20 40 60
angle (º)
?T
(K
)
45.00
50.00
55.00
60.00
Tra
d (
ºC)
ΔT=T0-Tθ
T1 (ºC) Cim4
-3
-2
-1
0
1
2
3
-60 -40 -20 0 20 40 60
angle (º)
ΔLS
BT (K
)
C1 (8-13μm)
C2 (11,0-11,7μm)
C3 (10,3-11,0μm)
C4 (8,9-9,3μm)
C5 (8,5-8,9μm)
C6 (8,1-8,5μm)