1

orfeo-toolbox.org

Orfeo Toolbox Radiometric corrections

Orfeo Toolbox Radiometric corrections

Stéphane MAY

stephane.may@cnes.fr

2

orfeo-toolbox.org

Radiometry = science of electromagnetic radiations

Interest for Remote Sensing

Physical interpretation of signals

Enhance detection

Estimate physical values related to ground and / or

atmospheric effects

Radiometry Radiometry

3

orfeo-toolbox.org

Goal : using physical measures

TOA reflectance (Top of Atmosphere)

TOC reflectance (Top of Canopy)

Calibration of images : DN (digital numbers) converted into TOA reflectance

Application of calibration coefficient inserted into metadata

files → Luminance

Normalisation with solar effect →

Reflectance

Example for Spot :

Radiometry : from DN to TOA reflectance Radiometry : from DN to TOA reflectance

4

orfeo-toolbox.org

LuminanceLuminance

5

orfeo-toolbox.org

LuminanceLuminance

Definition

Emitted power per unit area and per unit solid angle

Unit

W.m-2.sr-1

Numerical example:

Luminance of the sky (visible): ~ 6 W.m-2.sr-1

6

orfeo-toolbox.org

TOA reflectanceTOA reflectance

7

orfeo-toolbox.org

Atmospheric correction : inversion of surface TOC reflectance

Need to caracterise atmospheric conditions

➢TOMS / TOAST data : (grid : 1,25°lon x 1°lat ~ 135 x 110

km) : stratospheric Ozone amount

NCEP meteo data : (grid : 2,5°lon x 2,5°lat ~ 270 x 270

km) : Atmospheric Pressuree, water vapor, wetness

SeaWiFS or MODIS data : (grid : 5’ x 5’ ~ 9 x 9 km) ) :

Optical aerosols thickness

Cimel of aeronet network : information

about aerosols, and water amount

Radiometry : from TOA reflectance to TOC reflectance Radiometry : from TOA reflectance to TOC reflectance

Cloud

Aerosols

Molecules

Ozone

ScatteringAbsorption

Emission Water vapor

Reflection

8

orfeo-toolbox.org

Atmosphere

Thickness ~100 km

Filter descending and ascending radiations

Composition of the atmosphere:

Molecules

nitrogen (78%), oxygen (21%), argon, carbon oxides,

ozone, water vapor (highly variable, even locally) ...

Aerosols

small solid or liquid particles suspended in the

atmosphere. Microparticles, water particles, ice crystals,

smoke ...

Dimensions: mainly from 0.1 to 10 µm

Abundance and type varies geographically and with time

Atmospheric effects Atmospheric effects

9

orfeo-toolbox.org

85

50

10

Altitude (km)

800

0 Sol

Tropopause

Stratopause

TROPOSPHERE

STRATOSPHERE

THERMOSPHERE

EXOSPHERE

Ozone

Vapeurd'eau

AérosolsNuages

MESOSPHERE

Mesopause

Profil de Températures

-90°C

0°C

-55°C

+15°C

Atmospheric effects Atmospheric effects

10

orfeo-toolbox.org

Atmospheric effects Atmospheric effects

11

orfeo-toolbox.org

Three main phenomena, depending of wavelength

Absorption

Diffusion (in the visible and NIR, up to ~ 3 µm)

Emission (in the thermic IR, from ~ 3 µm)

Optional : refraction, usually neglected for high elevation

acquisition and not included spatial resolutions greater than

10 cm

The contribution of the signal from the ground in the measured signal is disturbed by filtering effects and luminance of the atmosphere

Need to correct measures

Need to understand the phenomena

Atmospheric effectsAtmospheric effects

12

orfeo-toolbox.org

Atmospheric corrections : invert the TOC surface reflectance. Simulation with radiative transfer code

6S code

Diffusion : code of the successive diffusion orders

Absorptions : O2, CO2, H2O, O3, N2O, CH4

Spectro data at 10 cm-1« AFGL atmospheric absorption

line parameters compilation » (1991)

Look-Up Tables : refl. TOC -> TOA

Radiometry : 6S modelRadiometry : 6S model

ρTOC

ρTOA

0 0,02 0,80,01 0,03

Mesure

Valeur sol interpolée

Simulations TOA

Géo d’observationConditions atmosphériques

13

orfeo-toolbox.org

14

orfeo-toolbox.org

Atmospheric parameters => parameters of radiative transfer

The zenithal and azimutal solar angles that describe the

solar incidence configuration (in degrees)

The zenithal and azimuthal viewing angles that describe the

viewing direction (in degrees)

The month and the day of the acquisition

The atmospheric pressure

The water vapor amount, that is, the total water vapor

content over vertical atmospheric column

Radiometry : 6S parametersRadiometry : 6S parameters

15

orfeo-toolbox.org

Atmospheric parameters => parameters of radiative transfer

The ozone amount that is the Stratospheric ozone layer

content;

The aerosol model that is the kind of particles (no

aerosol, continental, maritime, urban, desertic)

The aerosol optical thickness at 550 nm that is the is the

Radiative impact of aerosol for the reference wavelength

550 nm

The filter function that is the values of the filter function

for one spectral band, from λinf to λsup by step of 2.5

nm. One filter function by channel is required. This last

parameter are read in text files, the other one are directly

given to the class.

Radiometry : 6S parametersRadiometry : 6S parameters

16

orfeo-toolbox.org

Spectral sensitivity file

Data provided in OTB-Data (not with Monteverdi)

➢OTB-Data/Input/Radiometry/

For each band with a step of 0.25 nm, the spectral band

sensitivity of the instrument

➢Input file : values in the Atmospheric parameters window

➢No input file : iso sensitivity (1 everywhere) or default file

automatically loaded (>OTB V3.10)

Radiometry : 6S parametersRadiometry : 6S parameters

17

orfeo-toolbox.org

AERONET (AErosol RObotic NETwork)

This program is a federation of ground-based remote sensing

aerosol networks established by NASA and LOA-PHOTONS

(CNRS) and is greatly expanded by collaborators

Aeronet dataAeronet data

18

orfeo-toolbox.org

Aerosol optical depth data are computed for three data quality levels

Level 1.0 (unscreened)

Level 1.5 (cloud-screened)

Level 2.0 (cloud screened and quality-assured).

Extract Aerosol thickness (épaisseur) and Water amount

Download site

http://aeronet.gsfc.nasa.gov/cgi-bin/webtool_opera_v2_new

Aeronet data and OTB Aeronet data and OTB

19

orfeo-toolbox.org

Mesure of the reflectance of a source

Principle : use a source and measure reflected luminance

➢Direct measure : known source

➢ Indirect measure : use of a known reference reflectance

surface (spectralon, or BaSO4)

Measures in laboratory

➢Artificial source : directional, mobile (lamp, laser)

➢Sensor (radiometer or spectro-radiometer) : mobile

Direct measure or indirect measure

Radiometry – Reference reflectancesRadiometry – Reference reflectances

20

orfeo-toolbox.orgLongueur d’onde (µm)

EauChlorophylleABSORPTION due à :

REFLECTION due à : pigmentde la feuille

Structurecellulaire

Teneur en eau

VisibleProche

Infra-RougeMoyen Infra-Rouge

Radiometry Radiometry

Spectral signature of the chlorophyllian vegetation

21

orfeo-toolbox.org

3 processing steps

Digital Number to Luminance

Luminance to TOA reflectance

TOA reflectance to TOC surface reflectance

Many parameters available in the metadata of sensors

Importance of aerosols to quantify the effects of the

atmosphere

Difficulties

Aerosol model used?

Validation of results?

Ground truth?

Radiometry processing - SynthesisRadiometry processing - Synthesis

22

orfeo-toolbox.org

Optical calibration module (1/2)Optical calibration module (1/2)

23

orfeo-toolbox.org

Optical calibration module (2/2)Optical calibration module (2/2)

24

orfeo-toolbox.org

Menu File > Open

./img_kalideos_reunion/2009-03-21/IMAGERY.TIF

Menu Calibration > Optical calibration

Load Aeronet file

➢ ./aeronet/070101_091231_REUNION_ST_DENIS.lev20

Load spectral sensity file

➢ ./OTB-Data/Input/Radiometry/SPOT5/HRG1/rep6S.dat

Change correction parameters

Change Radiative terms

Save / quit

File > Save (Luminance, TOA, TOC, TOA-TOC files)

Repeat the Calibration process with other parameters set

Visualization > Viewer : compare results

Use case 1 : radiometric correctionUse case 1 : radiometric correction

25

orfeo-toolbox.org

Use the commande line tool :

OtbOpticalCalibration-cli -h

otbOpticalCalibration-cli -in IMAGERY.TIF -out img_toc.tif -level TOC

-AerosolModel 0 –OzoneAmount 0 --AtmosphericPressure 1030

--AerosolOptical 0.0329 –WaterVaporAmount 4.226 -aeronet file.lev20

-srs rep6S.dat

Use case 1b : radiometric correctionUse case 1b : radiometric correction

26

orfeo-toolbox.org

Spectral viewer (1/2)Spectral viewer (1/2)

27

orfeo-toolbox.org

Spectral viewer (2/2)Spectral viewer (2/2)

28

orfeo-toolbox.org

Menu File > Open

./img_kalideos_reunion/2009-03-21/IMAGERY.TIF

Menu Viewer > Spectral viewer

Left click : select your area

Right click : select pixel

Spectral angle menu > Select one curve ID

➢Click on Compute button

File > Save (the Spectral angle file). Use Float data type

Create several reference spectral angle images : roads, houses, sea, clouds, vegetation, bare soil

Use case 2 : Spectral viewerUse case 2 : Spectral viewer

29

orfeo-toolbox.org

Thank you for your attention !

Monteverdi Monteverdi