We experimentally investigate the interaction between one and two atoms and the field of a high-finesse optical resonator. Laser-cooled caesium atoms are transported into the cavity using an optical dipole trap. We monitor the interaction dynamics of a single atom strongly coupled to the resonator mode for several hundred milliseconds by observing the cavity transmission. Moreover, we investigate the position-dependent coupling of one and two atoms by shuttling them through the cavity mode. We demonstrate an alternative method, which suppresses heating effects, to analyze the atom-field interaction by retrieving the atom from the cavity and by measuring its final state.
The Remote Sensing Technology Institute (Institut für Methodik der Fernerkundung) of the German Aerospace Agency (DLR) operates two sensors for airborne hyperspectral imaging, i.e., a Norsk Elektro Optikk A/S (NEO) HySpex VNIR-1600 and a NEO HySpex SWIR-320m-e. Since these sensors are used for the development of physically based inversion algorithms, atmospheric correction algorithms and for calibration/ validation activities, their properties need to be characterized in detail, and an accurate calibration is mandatory. The characterization is performed at the calibration laboratory of DLR for imaging spectrometers in Oberpfaffenhofen. Key results of the characterization are assessments of the radiometric, spectral, and geometric performances, including the typical optical distortions prevalent in pushbroom imaging spectrometers, keystone and smile, and the associated measurement uncertainties. Potential sources of systematic error, the detector nonlinearity and the polarization sensitivity are discussed. The radiometric calibration is traceably performed to the German national metrology institute Physikalisch-Technische Bundesanstalt, whereas the spectral measurements can be traced back to the spectral properties of atomic line lamps. The implemented level 0 to level 1 calibration procedure is presented as well.
The German Aerospace Center's (DLR) Remote Sensing Technology Institute (IMF) operates a laboratory for the characterisation of imaging spectrometers. Originally designed as Calibration Home Base (CHB) for the imaging spectrometer APEX, the laboratory can be used to characterise nearly every airborne hyperspectral system. Characterisation methods will be demonstrated exemplarily with HySpex, an airborne imaging spectrometer system from Norsk Elektro Optikks A/S (NEO). Consisting of two separate devices (VNIR-1600 and SWIR-320me) the setup covers the spectral range from 400 nm to 2500 nm. Both airborne sensors have been characterised at NEO. This includes measurement of spectral and spatial resolution and misregistration, polarisation sensitivity, signal to noise ratios and the radiometric response. The same parameters have been examined at the CHB and were used to validate the NEO measurements. Additionally, the line spread functions (LSF) in across and along track direction and the spectral response functions (SRF) for certain detector pixels were measured. The high degree of lab automation allows the determination of the SRFs and LSFs for a large amount of sampling points. Despite this, the measurement of these functions for every detector element would be too time-consuming as typical detectors have 10 5 elements. But with enough sampling points it is possible to interpolate the attributes of the remaining pixels. The knowledge of these properties for every detector element allows the quantification of spectral and spatial misregistration (smile and keystone) and a better calibration of airborne data. Further laboratory measurements are used to validate the models for the spectral and spatial properties of the imaging spectrometers. Compared to the future German spaceborne hyperspectral Imager EnMAP, the HySpex sensors have the same or higher spectral and spatial resolution. Therefore, airborne data will be used to prepare for and validate the spaceborne system's data.
The calibration of remote sensing instruments is a crucial step in the generation of products tied to international reference standards. Calibrating imaging spectrometers is particularly demanding due to the high number of spatiospectral pixels and, consequently, the large amount of data acquired during calibration sequences. Storage of these data and associated meta-data in an organized manner, as well as the provision of efficient tools for the data analysis and fast and repeatable calibration coefficient generation with provenance information, is key to the provision of traceable measurements. The airborne prism experiment (APEX) calibration information system is a multilayered information technology solution comprising a database based on the entity-attribute-value (EAV) paradigm and software written in Java and MATLAB, providing data access, visualization and processing, and handling the data volumes over the expected lifetime of the system. Although developed in the context of APEX, the system is rather generic and may be adapted to other pushbroom-based imagers with little effort. This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING 1 Airborne Prism Experiment Calibration Information SystemAndreas Hueni, Member, IEEE, Karim Lenhard, Andreas Baumgartner, and Michael E. Schaepman, Senior Member, IEEE Abstract-The calibration of remote sensing instruments is a crucial step in the generation of products tied to international reference standards. Calibrating imaging spectrometers is particularly demanding due to the high number of spatiospectral pixels and, consequently, the large amount of data acquired during calibration sequences. Storage of these data and associated meta-data in an organized manner, as well as the provision of efficient tools for the data analysis and fast and repeatable calibration coefficient generation with provenance information, is key to the provision of traceable measurements. The airborne prism experiment (APEX) calibration information system is a multilayered information technology solution comprising a database based on the entity-attribute-value (EAV) paradigm and software written in Java and MATLAB, providing data access, visualization and processing, and handling the data volumes over the expected lifetime of the system. Although developed in the context of APEX, the system is rather generic and may be adapted to other pushbroom-based imagers with little effort.Index Terms-Imaging spectroscopy, relational database, sensor calibration.
This paper investigates at the example of bathymetry how much an application can profit from comprehensive characterizations required for an improved calibration of data from a state-of-the-art commercial hyperspectral sensor. A NEO HySpex VNIR-1600 sensor is used for this paper, and the improvements are based on measurements of sensor properties not covered by the manufacturer, in particular, detector nonlinearity and stray light. This additional knowledge about the instrument is used to implement corrections for nonlinearity, stray light, spectral smile distortion and nonuniform spectral bandwidth and to base the radiometric calibration on a SI-traceable radiance standard. Bathymetry is retrieved from a data take from the lake Starnberg using WASI-2D. The results using the original and improved calibration procedures are compared with ground reference data, with an emphasis on the effect of stray-light correction. For our instrument, stray-light biases the detector response from 416-500 nm up to 8% and from 700-760 nm up to 5%. Stray-light-induced errors affect bathymetry mainly in water deeper than Secchi depth, whereas in shallower water, the dominant error source is the calibration accuracy of the light source used for radiometric calibration. Stray-light correction reduced the systematic error of water depth by 19% from Secchi depth to three times Secchi depth, whereas the relative standard deviation remained stable at 5%.
The German Aerospace Center (DLR) operates the Calibation Home Base (CHB) as a facility for the calibration of airborne imaging spectrometers and for field spectrometers. Until recently, absolute radiometric calibration was based on an integrating sphere that is traceable to SI units through calibration at the German Metrology Institute PTB. However, the stability of the radiance output was not monitored regularly and reliably. This was the motivation to develop a new radiance standard (RASTA) which allows monitoring in the wavelength range from 380 to 2500 nm. Radiance source is a diffuse reflector illuminated by a tungsten halogen lamp. Five radiometers mounted in a special geometry are used for monitoring. This setup improves twofold the uncertainty assessment compared to the previously used integrating sphere. Firstly, lamp irradiance and panel reflectance have been calibrated at PTB additionally to the radiance of the complete system. This calibration redundancy allows to detect systematic errors and to reduce calibration uncertainty. Secondly, the five radiometers form a redundant control system to measure changes of the spectral radiance. This enables long-time monitoring of the radiance source including assessment of the uncertainty caused by aging processes. Further advantages concern the reduction of periods of non-availability, applicability to sensors with larger field of view, and the possibility to alter intensity and spectral shape in a well-known way by exchanging the reflector. RASTA has been calibrated at PTB in November 2011 in the wavelength range from 350 to 2500 nm MECHANICAL SETUPThe mechanical configuration of RASTA is shown in Fig. 1. Light source is a 1000-W FEL tungsten-halogen lamp (Gamma Scientific Model 5000-16C), which is widely used as an irradiance standard. It illuminates perpendicularly a reflectance panel with the dimension 25 cm u 25 cm (1). In order to minimize stray light contamination, the lamp is installed inside a lamp housing (2). The lamp housing is mounted moveably on a bar(3) to adjust the distance between lamp and reflectance panel. The distance can be measured accurately by mounting temporarily a lamp alignment jig instead of the lamp. A mounting adapter for measurement devices (4) is installed in front of the reflectance panel. It provides space for seven instruments arranged on a half circle around the optical axis, which is the line perpendicular to the panel center. This setup allows all instruments to look at the panel at the same spot from the same distance and in the same angle of 45° to avoid differences in the radiance caused by spatial inhomogeneities. Five of the adapter spaces are used for radiometers (5,6) which monitor the stability of the system. The two horizontal spaces left and right of the panel (7) can be used for
Abstract.A dedicated calibration technique was applied for the calibration of the spectral radiance transfer standard (RASTA) of the German Aerospace Center (DLR) at the Physikalisch-Technische Bundesanstalt (PTB), consisting of two independent but complementing calibration procedures to provide redundancy and smallest possible calibration uncertainties. Procedure I included two calibration steps: In a first step the optical radiation source of RASTA, an FEL lamp, was calibrated in terms of its spectral irradiance Eλ(λ) in the wavelength range from 350 nm to 2400 nm using the PTB Spectral Irradiance Calibration Equipment (SPICE), while in a second step the spectral radiance factor β 0°:45°( λ) of the RASTA reflection standard was calibrated in a 0°:45°-viewing geometry in the wavelength range from 350 nm to 1700 nm at the robot-based gonioreflectometer facility of PTB. The achieved relative standard uncertainties (k = 1) range from 0.6 % to 3.2 % and 0.1 % to 0.6 % respectively. Procedure II was completely independent from procedure I and allowed to cover the entire spectral range of RASTA from 350 nm to 2500 nm. In the second procedure, the 0°:45°-viewing geometry spectral radiance Lλ ,0°:45°( λ) of RASTA was directly calibrated at the Spectral Radiance Comparator Facility (SRCF) of PTB. The relative uncertainties for this calibration procedure range from 0.8 % in the visible up to 7.5 % at 2500 nm (k = 1). In the overlapping spectral range of both calibration procedures the calculated spectral radiance Lλ ,0°:45°,calc (λ) from procedure I is in good agreement with the direct measurement of procedure II, i.e. well within the combined expanded uncertainties (k = 2) of both procedures.
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