Folie 1AITA 9, MERTIS Team – MERTIS Calibration, Oct 2007

Development of a calibration concept for the MErcury Thermal Infrared Spectrometer

Thomas Saeuberlich, Eckehard Lorenz, Wolfgang Skrbek, Ingo Walter,

Carsten Paproth, Joern Helbert

German Aerospace Center (DLR) Optical Information Systems

Rutherfordstr. 2, 12489 Berlin, Germany

AITA 9, MERTIS Team – MERTIS Calibration, Oct 2007

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MERTIS Calibration

(a) determination of coefficients for the conversion of the digital number output DNij of each spectro-spatial channel (i,j) into radiance values Lij

(b) characterization of the optical, thermal and electronical status of the instrument

(c) determination of the accuracy of the acquired values

(d) characterization of system stability

Calibration On-Ground In-Flight

spectral wavelength assignment, SSD, spectral resolution

-

radiometric sensitivity, offset, noise offset, sensitivity, noise

geometric line of sight of pixels, geometric resolution

-

Goals:

AITA 9, MERTIS Team – MERTIS Calibration, Oct 2007

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MERTIS Calibration – Experimental Setup & Procedure

1. 200 object-images (shutter open) for each blackbody temperature TBB in [50°C….1000°C] with TBB = 50 K and an additional measurement of dark-images (shutter closed)

2. calculation of the difference image from the averaged images for object- and dark-measurements

1. assignment of CWL to each channel (i,j) using narrow band passes (FWHM = 100nm) in the spectral range [7 µm…14 µm] and = 0.5 µm

2. calculation of the spectral sampling distance SSD = lj+1-lj = 88 nm

Spectral Calibration:

Radiometric Measurement:

Instrument Housing

TMADetector +

FEECollimator

mirror

Blackbody 50°C – 1000°C

Data Acq. - & Processing

SoftwareShutter

Offner Spectrometer Optics

Slit 3 mm x

0,125 mm

MERTIS optics including TMA, Shutter, Slit, Offner-Spectrometer, Bolometer Detector Array

AITA 9, MERTIS Team – MERTIS Calibration, Oct 2007

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MERTIS Calibration – Radiation Signal Transformation I

(i,j) - single channel; i corresponds to the spatial and j to the spectral directionUij,opened - measured and averaged voltage for channel (i,j) with shutter openUij,closed - measured and averaged voltage for channel (i,j) with shutter closedUij,diff - difference voltage calculated from the measured voltages with opened and closed shutterU0,ij - residual offsetrij - channel sensitivityL - spectral radiance according to the Planck LawTBB - blackbody temperatureSSD - spectral sampling distance (SSD=88 nm for current breadboard)j - spectral interval that corresponds to a single channel (i,j)

dTLrUTUTUTUSSD

BBijijBBclosedijBBopenedijBBdiffij

jjj

]..[

,0,,, ),(*)()()(

• linear approach• model does currently not include the spectral and spatial smearing

AITA 9, MERTIS Team – MERTIS Calibration, Oct 2007

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MERTIS Calibration – Radiation Signal Transformation II

spatial direction (index i)

spec

tral

dir

ectio

n (i

ndex

j)

object te

mperature

blackbody @ 50°C

blackbody @ 1000°C

The cross marks channel (i,j) = (88,88) that corresponds to a spectral position of 9µm within the area illuminated by the blackbody

• verification of linear dependence between the incoming (integrated) radiance and the measured voltages for channel (88,88)

• calculation of the calibration coefficients (residual offset U0 and sensitivity rij) by a linear fit

6.3µm

16.8 µm

37 < i < 137 slit height

AITA 9, MERTIS Team – MERTIS Calibration, Oct 2007

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MERTIS Calibration – Calibration Coefficients

spectral distribution of the sensitivity r88,j

spectral distribution of the residual offset U0,88,j

AITA 9, MERTIS Team – MERTIS Calibration, Oct 2007

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MERTIS Calibration – Application of Calibration Coefficients Irij and U0,ij can be applied to dark-corrected raw images in order to calculate the (integrated) radiances L int,ij having an error eij:

ij

SSD

BBij

ijdiffij

ij edTLr

UUL

jjj

]..[

,0,int, ),(

AITA 9, MERTIS Team – MERTIS Calibration, Oct 2007

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MERTIS Calibration – Application of Calibration Coefficients II

The radiances Lint,ij calculated from a dark-corrected raw measurement can be used to calculate temperatures T ij using Planck´s law and assuming a small SSD (which actually is only 88 nm):

;;)1

2ln(

int,

21

25

1

1

SSD

Lchccwith

c

ccc

T ij

ij

+/- 3 K

+/- 7 K

+/- 15 K

AITA 9, MERTIS Team – MERTIS Calibration, Oct 2007

Folie 9

• a spectro-radiometric calibration procedure for 1st MERTIS prototype has been developed

• 2-step background correction used

• linear approach for modeling the signal transformation from an incoming radiance into a measured voltage

• approach verified by calculating the temperature of the reference radiator for each spectral channel in the spectral interval [7 µm…14 µm]

• the temperature error is ± 3 K…± 15 K depending on the object temperature (1000°C – 100°C)

• topics of the following investigations:

• model has to be extended by the spectral and spatial smearing effects caused by the optics

• determination of the spectral and spatial resolution

MERTIS Calibration – Conclusion