Calibration Home Base (CHB)

The Calibration Home Base (CHB) is a unique facility located at DLR Oberpfaffenhofen that is dedicated to perform laboratory measurements of airborne optical sensors and hyperspectral field instruments in a wide spectral range from 380 nm to 14 µm. A folding mirror design facilitates geometric and spectral measurements of heavy instruments up to 350 kg. The CHB was partly funded by ESA to establish a calibration facility for the airborne imaging spectrometer APEX, but it is used for other optical sensors as well.

Premises

The CHB is located close to DLR's airfield in Oberpfaffenhofen and accessible to bulky instruments. It is installed in a large room (12.8 x 9.0 m², 8 m height). A crane is mounted on the ceiling for handling of heavy components. The room can be darkened, the walls are painted black. An air condition keeps the temperature at 22±1 C. Relevant environmental parameters (temperature, pressure, humidity, ambient light) are monitored and included automatically to each measurement protocol.

Folding mirror concept

To avoid instrument moves at angle-dependent geometric and spectral measurements, the instrument is mounted in fixed position on a massive optical bench. Oriented like in the airplane, the sensor is downward looking to a flat mirror (12 x 18 cm²), which reflects either the beam for spectral or for geometric measurements at well-defined angle (±0.007 mrad uncertainty) to the sensor entrance. This 'folding mirror' can be tilted in order to set the angle of incidence, and it can be moved in horizontal direction (±37 cm) to meet the entrance aperture. A mechanical interface, which is attached on top of the bench's pillars, bears the sensor and allows to align its optical axis relative to the folding mirror axis.

System corrections

The sensitivity of imaging sensors is usually not constant across the field of view due to response differences of individual pixels, variations of the entrance slit width, and vignetting of optical components. To measure these variations quantitatively, a large integrating sphere (1.65 m diameter) is available, which is illuminated from the interior by up to 18 stabilised lamps. It provides a homogeneous radiance distribution (variations smaller 0.5% rms) over a large area (55 x 40 cm²). The radiant exitance can be changed from 20 to 1640 W m-2 for adjustment to instrument sensitivity and to measure detector linearity. A photodiode inside the sphere measures the relative intensity changes.

Radiometric calibration

Absolute radiometric calibration requires a source of well-known spectral radiance. CHB operates two sources which are certified against German national standard (PTB) for the spectral range 0.35-2.5 µm:

• Integrating sphere (50 cm diameter, 4 x 20 cm² opening). The uncertainty is % from 390-1700 nm; it increases towards shorter and longer wavelengths.
• Halogen lamp in combination with a reflectance panel (18 x18 cm²). The uncertainty is 2.3% from 410-1700 nm; it increases towards shorter and longer wavelengths.

Both sources are monitored at each usage by means of a stable spectrometer. When the measured radiance differs more than 3% from the initial value in the range 450-900 nm, the source is re-calibrated at PTB.

Spectral measurements

For spectral instrument characterisation a spectrally narrow-band beam of radiation is required which overfills both the entrance aperture and instantaneous field of view of the pixel under investigation. Such a beam is provided at CHB by means of a monochromator Oriel MS257. After passing a parabolic mirror (focal length 119 mm) it is directed at well-defined angles to the sensor via reflection at the folding mirror. The monochromator covers the wavelength range 0.214 µm. A quartz tungsten halogen lamp provides for most instruments sufficient intensity from 380-2500 nm, and a ceramic element from 314 µm. Wavelength uncertainty is ±0.1 nm. Spectral bandwidth, beam divergence and geometric cross-section at sensor entrance are set by tuning width and height of the monochromator exit slit. Spectral bandwidth depends further on the selected grating. The minimum spectral bandwidth is 0.1 nm from 0.21.4 µm, 0.25 nm from 1.43 µm, and 2 nm from 4.5-14 µm. The divergence of the beam is typically 0.8 mrad, the geometric cross section at sensor entrance in the order of 3 x 15 cm² (at 100 µm slit width, 10 mm slit height, 1.5 m distance between parabolic mirror and sensor).

Geometric measurements

Geometric characterisation is relevant for imaging sensors e.g. to assign coordinates to each image pixel and to quantify image sharpness by MTF measurements, and for spectrometers to determine the optical axis and the field of view. Depending on the measurement task certain sensor pixels have to be illuminated with spectrally broad-band radiation either completely or very narrow compared to pixel size. A suitable beam is formed at CHB by means of a lamp-slit-collimator combination. Three horizontal and three vertical slits of different width (50, 100, 1000 µm), mounted on a turnable wheel in the focal plane of the collimator, are used to form a beam of selectable divergence in the sensor's along track or across track direction. The beam is directed to the sensor at well-defined angle by means of the folding mirror. Its incident angle can be changed in small intervals either by stepping the folding mirror (minimum step width 0.0017 mrad) or the slit wheel (0.0078 mrad). Due to the collimator's focal length of 750 mm the beam has a divergence of 0.067, 0.13 or 1.3 mrad in its narrow direction. The geometric cross section is 12 cm.

Auxiliary measurements

The above-mentioned measurements are relevant to determine basic parameters required for calibration of optical sensors. Calibration can be improved using auxiliary measurements of the following instrument properties:

• Detector linearity: radiation is attenuated in well-defined steps using neutral density filters.
• Spectral stray light: spectral filters block radiation in well-defined spectral intervals. Sensor signals in these regions are caused by stray light inside the instrument.
• Spatial stray light: selected pixels are illuminated using the set-up for geometric measurements. Signals of non-illuminated pixels are caused by stray light inside the instrument.
• Polarisation sensitivity: a set of 3 linear polarisers is available to characterise the dependency of sensor response on polarisation from 0.47-2.5 µm.

Calibration can be checked by measuring the radiance of coloured reflectance panels which are illuminated by a calibrated halogen lamp.

Computer control

Since a full measurement cycle may last many hours or even some days, major CHB components are computer controlled, and time-consuming steps can be done automatically without operator interaction. Two software packages enable automated measurements:

• CHB slave software. Controls CHB devices.
• CHB master software. Implements the measurement concept, commands both the CHB slave software and the sensor software. Can be adapted to any sensor that supports external control by XML commands.

Services

CHB is accessible to customers. DLR provides the laboratory equipment required for calibration measurements. Trained staff is available to adapt the customer's instrument mechanically to CHB, set-up its communication with CHB Master software, operate CHB hardware and software, assist the measurements, and support in data interpretation.

Calibration and system characterisation of optical sensors is also performed in dedicated labs of the Optical Sensorics and Electronics Department at DLR's premises in Berlin-Adlerhof. More information can be found here.

References

Gege, P., Fries, J., Haschberger, P., Lenhard, L., Schötz, P., Schwarz, Ch. J., Schwarzmaier, T.: „Concept for improved radiometric calibration of radiance sources at the CHB facility“, In: Proc. Hyperspectral Workshop, ESRIN, Frascati, Italy (2010).

Lenhard, K., Baumgartner, A.: “Estimation of radiometric uncertainty after smile correction”, In: Proc. Hyperspectral Workshop, ESRIN, Frascati, Italy (2010).

Schwarz, Ch. J., Lenhard, K., Gege, P.: “Concept for fast spectral characterisation of imaging spectrometers”, In: Proc. Hyperspectral Workshop, ESRIN, Frascati, Ital (2010).

Suhr, B., “A Sensor independent Concept for the Characterisation of Imaging Spectrometers”, Dissertation, Universität Zürich/CH, 141 pp. (2010).

Gege, P., J. Fries, P. Haschberger, P. Schötz, H. Schwarzer, P.Strobl, B. Suhr, G. Ulbrich, W. J. Vreeling, "Calibration facility for airborne imaging spectrometers", ISPRS Journal of Photogrammetry & Remote Sensing 64, 387-397 (2009).

Gege, P., J. Fries, P. Haschberger, P. Schötz, B. Suhr, W. Vreeling, H. Schwarzer, P. Strobl, G. Ulbrich, "A new laboratory for the characterisation of hyperspectral airborne sensors", In: Proc. 6th EARSeL SIG IS Workshop, Tel Aviv, Israel (2009).

Nieke, J.; Itten, K.I.; Meulemann, K.; Gege, P.; Dell'Endice, F.; Hueni, A.; Alberti, E.; Ulbrich, G.; Meynart, R. and the APEX team, "Supporting Facilities of the Airborne Imaging Spectrometer APEX", Proc. IGARSS, July 6-11, Boston, USA (2008).

Downloadable CHB Flyer