Sensors for the verification

The two available sensors at DLR Oberpfaffenhofen are the AISA 1710 (Airborne Imaging Spectrometer for different Applications) and the ROSIS-03 (Reflective Optics System Imaging Spectrometer) are used for the verification of the methods. These sensors are pushbroom scanners measuring light in the wavelength range between 430 nm and 900 nm. The specifications of the sensors are explained in the following section.

AISA model 1 (ID 1710)

The model 1 AISA with the ID 1710 is a compact light-weight imaging spectrometer which is being operated since 1996 [36]. The sensor is equipped with a Thomson TH7863 CCD silicon detector array of 288 * 384 pixels [74]. Each pixel has a size of 23 µm * 23 µm. The instrument records the spectral range from 430 nm to 900 nm in up to 286 pixels (spectral axis). The imaging spectrometer has 364 spatial pixel and 20 FODIS (fiberoptic downwelling irradiance sensor) pixels. Additional to the 384 pixels the signals of 8 masked dark reference pixels are collected for each channel (spatial axis).

Optical system

The fore optics is a standard C-mount camera lens. Two lenses [93, 94] from Schneider Optics are available with a focal length of 8.3 (Cinegon f/1.4 [95]) and 22.5 mm (Xenoplan f/1.4 [96]). The dispersion component is a prism-grating-prism (PGP) element [68]. A PGP element uses a holographic transmission grating between two protective glass plates with high diffraction efficiencies. The typical absolute diffraction efficiencies of gratings (without optics) for non polarised light are listed in App. A, Table A.2 [68]. The radiation entering the spectrometer slit is collimated by the first lens and refracts at the prism surface (see Figure 6.1). The grating disperses the light so that the central wavelength passes symmetrically through the PGP (see Figure 6.2).

Four neutral density filters are available which can be mounted in front of the lens. These filters reduce the incoming signal by factors of 2, 4, 8 or 64.

6.1 Sensors for the verification S p e c tr a l a x is

Figure 6.1: Scheme of the components of an imaging spectrometer and mapping to the CCD pixels [68]

Figure 6.2: Basic principle of the prism-grating-prism element [68]

Characterisation

The former owner (Airobotics GmbH) delivered AISA with calibration coefficients from SPECIM. The last wavelength and radiometric characterisations for the two lenses were performed in July 2000 [93, 94]. Afterwards the sensor was not any more at SPECIM for calibration purposes. For the wavelength calibration a Helium-Neon laser and a fluorescent lamp (20 W) were used. The laser produces a monochromatic light at 632.8 nm. The light was projected on different spatial pixels, with the result that the illuminated channel differs around 1 channel from the centre to the border (channel no. 124 at the borders and 125 in the middle). The centre pixel of the spatial plane is pixel no. 192. A two-piece linear model, split at 611 nm, was used for the wavelength calculation of all channels. For radiometric characterisation a radiance standard was used [97].

Measurement process aspects

For all performed measurements the data acquisition mode A has been selected. This mode records the information of all spatial and spectral pixels (see chapter 4.4). During long series measurements the data files are transferred to the computer with the Master software. Therefore the measurement process contains in addition to the Slave settings, two actions (see chapter 4.3). A FTP data transfer and a file delete command are performed directly after each measurement.

6 Verification of the Characterisation Methods

Sensor parameters

Table 6.1 summarizes the sensor parameters for the AISA 1710 given by the manufacturer (SPECIM Ltd.) [36, 74, 95]. The table contains the specification of the 8 mm lens since this has been used for the performed measurements.

Table 6.1: AISA sensor parameters

Spectral Geometric

Parameter Value Parameter Value

No. of bands 286 No. of spatial pixel 364

Wavelength range (nm) 430 – 900 FOV (°) 57.6

Spectral sampling interval (nm) 1.63 IFOV (mrad / °) 2.6 / 0.15 No. of centre pixel 192 Focal length (mm) 8.3

ROSIS-03

ROSIS was developed in a cooperation between German industry (EADS, Ottobrunn), DLR in Oberpfaffenhofen and GKSS in Geesthacht. ROSIS is a light-weight and compact instrument. The sensor has a tiltable mirror capability in flight direction of ± 20° and binning functionality. The Thomson THP 7895 detector is a two-dimensional CCD array [31]. The rows contain the spatial and the columns the spectral information. A single row of the detector consists of 552 elements of which 512 are used for spatial information and the other pixels are foreseen for system corrections [14, 31].

Optical System

The optical assembly of ROSIS is shown in Figure 6.3. The mirror (2) can be tilted to different positions to record dark current, spectral or nadir data information. Since ROSIS has no shutter dark current measurements are performed by tilting the mirror in the opposite viewing direction to the CCD. To perform in-flight calibration (IFC) measurements the mirror moves to the position where the mercury vapour lamp (3) illuminates the CCD (13). Between the tilt mirror and the second mirror is a long pass filter (5) which transmits only wavelengths greater than 430 nm (Schott GG 420) [30]. For reduction of the intensity neutral density filters can be used in the filter holder (4). Available are 10%, 31% and 63% transmittance filters.

6.1 Sensors for the verification

Figure 6.3: ROSIS optical assembly [98] Characterisation

ROSIS is inhouse characterised at DLR Oberpfaffenhofen. The spectral, geometric and radiometric measurements were performed in the second laboratory [4, 21]. The spectral measurements were executed with a monochromator and the radiometric calibration with the small integrating sphere. First spectral resolution measurements where performed for the wavelength 600 nm in 2001 and continued in 2002 for the wavelengths 500, 600, 700 and 800 nm [99]. Further measurements are made in 2003 for defined wavelengths (450nm, 500nm, 550nm, 600nm, 650nm, 700nm, 762nm, 800nm) [100]. The radiometric calibration measurements are executed close before or after flight activities. The geometric characterisation measurements were performed on the tilt and rotation stage for sensors with a weight up to 50 kg [101].

Measurement process aspects

Two different types of measurement modes (automatic and nadir) were used for the execution of the characterisation measurements. The automatic mode performs measurements at five mirror positions by tilting the mirror to desired positions. In the nadir mode the sensor records only data at the actual manually adjusted mirror position. The different acquisitions modes are explained in chapter 4.4. For the execution of automatic measurements with the Master software the nadir mode of the DSU software is required.

Sensor parameters

The necessary sensor parameters for the characterisation measurements and analysis are listed in Table 6.2.

(1) Baffle

(2) Movable mirror (3) Mercury lamp (4) Filter holder (5) Long pass filter (6,7) Telescope (8) Slit (9,10) Collimator (11) Grating (12) Mirror (13) CCD 1 2 4 5 6 7 8 9 10 11 13 12 3

6 Verification of the Characterisation Methods

Table 6.2: ROSIS sensor parameters

Spectral Geometric

Parameter Value Parameter Value

No. of bands 115 No. of spatial pixel 512

Wavelength range (nm) 430 – 860 FOV (°) 16.0

Bandwidth (nm) 4 IFOV (mrad / °) 0.54 / 0.03

No. of centre pixel 258