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The on-ground calibration of an VIS / TIR imaging spectrometer consists
of a sequence of procedures providing the calibration data files and ensuring
the radiometric, spectrometric and geometric stability of the instrument:
- measurement of noise characteristics of the sensor channels,
- measurement of the dark current of the channels,
- measurement of the relative spectral response function of the channels,
- derivation of the effective spectral bandwidth of the channels,
- definition of the spectral separation of the channels, measurement
of the absolute radiometric calibration coefficients (the "transfer
functions" between absolute spectral radiance at the entrance of the
aperture of the sensor and the measured radiation dependent part of the
output signal of each channel; the definition of the scan-angle radiance
sensitivity is a part of these measurements);
- definition of the Noise Equivalent Radiance (NER) and the Noise Equivalent
Temperature Difference (NE DeltaT) of the channels,
- measurement of the Instantaneous Field of View (IFOV), the spectral
resolution and the deviations in the band-to-band registration.
The on ground calibration data are used for the systematic correction
of the hyper spectral imaging spectrometer data obtained during its in-flight
data registration.
The Laboratory Calibration Facility ( LCF ) for VIS - TIR wide - angle
video spectrometric airborne sensors has been developed, taking into account
the specific local situation with the close proximity of the calibration
lab and the aircraft hangar at DLR-Oberpfaffenhofen, where the DAIS-7915
is operated and calibrated.
The units of the LCF are used in the laboratory ( and two of them as well
in the aircraft hangar for on-ground calibration checks of the sensor,
installed on board of an aircraft ). The LCF consists of the following
four main parts:
- Spectrometric/geometric Calibration Part (SCP)
- Relative Radiometric Calibration Part (RRCP)
- Thermal Absolute Calibration Part (TACP)
- Absolute Radiometric Calibration Part (ARCP)
Part 1 (SCP) comprises a mirror collimator with a flat folding
mirror at the parallel-light output of the collimator. The mirror collimator
has a focal length of 150 cm and a circular aperture of 30 cm in diameter.
At the focus position of the mirror collimator can be installed:
- the output slit of a monochromator for the 400-2500 nm spectral region,
- the interference filter arrangement,
- different illuminated point or slit targets (for geometric calibration).
The flat folding mirror at the collimator output allows to realise different
illumination directions of the parallel light from the collimator output
to the sensor entrance.
Part 2 (RRCP) is mobile and included in the LCF equipment, because
it is impossible to check the absolute radiometric calibration of the installed
sensor by means of an integrating sphere. The RDRS consists of the following
parts: optical module and power supply unit. The optical module contains:
4 halogen lamps of the type KGM-12-100 (12V, 100W) and several dissipative
panels made by ground glass. The dimensions of the illuminated surface
are 40 cm by 55 cm.
Part 3 (TACP) is mobile and consists of 2 water-conditioned blackbodies,
each with the dimensions of 100 x 100 cm2 - one at ambient or up to 5 C
below ambient temperature, the other at 25 - 30 C above ambient temperature
- and the supply module. The TACP is used for the radiometric calibration
of the mid-infrared and the thermal channels of the sensor in the laboratory
and in the hangar.
Part 4 ( ARCP ) is used for the absolute on-ground
calibration of the sensor in the VIS-SWIR spectral region (400 - 2500 nm)
by means of an Integrating Sphere (IS). The interior of the Integrating
Sphere (1.65 m in diameter) is coated with barium sulphate. The eighteen
200-Watt lamps are mounted internally. Each lamp has its own individual
power supply, so that each may be lightened independently, providing 18
equal steps of radiance. The maximum radiance of the IS is 0.08 mW/sr*cm²*nm
at the wavelength of 1000 nm. The homogenity of the radiance at the 40
cm x 55 cm rectangular output part of the sphere is better than 1 %, if
at least 4 lamps are lightened.
The IS has to be calibrated from time to time by establishing the ratio
between the output of the sphere and a well known radiance standard - the
so called Absolute Diffuse Source (ADS) - a separate unit of the LCF.
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