Spectrophotometer SP2H

General
The multi-channel Spectrophotometer SP2H may be used as mobile or stationary equipment for spectral measurements in a range between 350 nm and 1050 nm. The spectrophotometers are acurately calibrated using laboratory equipment (climate chamber, monochromator, effective solid angle scanner, mechanical adjustment). The knowledge of the effective solid angle for each channel allows radiometric connection between the spectrophotometer and the solar radiation tables of Neckel and Labs [1] in the laboratory [2]. This method is also the base for the long-term radiometric stability control of the SP2H, corresponding with today s QA/QC-concepts. The Langley method and comparison with calibrated standard equipment also can be used for calibration of the SP2H with the sun as the calibration source for irradiance.

Principle
The spectrophotometer SP2H using theimaging method measures the irradiance of the direct solar radiation and the radiance of surfaces at predetermined wavelengths. The quasi-simultaneous measurement with different filters makes it possible to retrive optical properties of the atmosphere. Among them are the aerosolparameters, such as, the total optical thickness, the size distribution, the real part of the refractive index, the factor of asymmetry and the phase function. This is done by using mathematical models and by measuring the spectral sky radiance in the sun´s aureole and in the azimuthal angle of the sun´s elevation (Almucantarat).

Imaging Method The sun or an incremental area of the sky is sharply focused in a measuring diaphragm with an objective lens having a plane angle of 1° . This method ensures that the complete image of the radiation source is depicted on the photo detector.

Instrument function
The desired wavelengths are selected by manually turning one of the two mounted filter wheels. After directing the objective toward the sun using the view finder, the solar radiation passes through the selected filter and is directed to the photosensor. This sensor converts the incident radiation into a voltage which is amplified by the amplifiers. The optimal amplification (factor 10⁰...10⁸) is automatically adjusted depending on the strength of the incoming radiation. The signal is then passed on to a computer via the interface module SPI.
Using the preset measuring program selected from the menu, an interactive procedure between the user and the computer becomes feasible. Within 30 seconds a full measuring cycle can be completed; i.e., taking the data of 14 channels, storing them in the memory and calling up the next cycle. Also, the program allows automatic collection and treatment of data from a single preselected channel (e.g. 500 nm following the WMO-recommendations /3/) in a preselected temporal sequence. But this requires an additional tracking device which continually directs the spectrophotometer toward the sun.
The data which were received and precalculated by the PC are stored in a standard file ready for further calculations or for transfer to other special processing systems using the data record format ASCII. Other standard formats can be substituted upon the user's request.
The mechanical construction of the spectrophotometer SP2H allows either to mount it on a manually driven parallactic mounting or on an automatic tracking device (solar tracker).

Calibration precedures
The parameters relevant to the results of measurements are carefully measured for every spectrophotometer SP2H and the corresponding corrections and calibration factors are determined.
For radiometric calibration, direct solar radiation under clear sky conditions can be used by comparison with another spectrophotometer that has been previously calibrated, or the method of Langley-extrapolation can be used in appropriate environments.
Artifical radiation sources can also be used at a specially equipped optical laboratory, either with a standardized radiation normal or with the procedure described in /2/.
Upon delivery, the spectrophotometers SP2H are normally calibrated in the laboratory and compared with standard equipment.

Data Processing
The program system SPMH which also controled the measurement, guarantees fast and convenient processing of the received signal. This program allows the following procedures:

• calculation of the atmospheric transmission
• computation of the optical depth of the atmosphere using 3 free selectable Rayleigh corrections and regressions following the Angstroem formula
• statistical computation of mean values, dispersion, regression analysis and other mathematical operations
• inquiry of the voltage, Ue, corresponding to the extraterrestrial irradiance by the method of Langley extrapolation
• graphical display of data and results
• calculation of the normalised scattering function
• print out and storage of data and results

Service for operation
Correct measurements over long periods of time as well as network operations require stringent the repeated and regular control of calibration and stability of the spectrophotometers. All photometers delivered by Dr. Schulz & Partner Ltd. get a passport containing equipment parameters and basic calibration values, which may be used in case of recalibration and/or repair to ensure long term stability.
A calibration data base as an interactive system between user and Dr. Schulz & Partner Ltd. has been established in order to get a continuous record of all stability and calibration parameters for each instrument delivered. This virtual component of SP2H is under study as a basic update for high quality SP2H-applications, in particular, for network operations and long term observations.
It is recommended that the user should keep a record of every change or irregularity since this information is useful for recalibration purposes. Dr. Schulz & Partner Ltd. offers maintenance of the spectrophotometer at intervals appropriate to the user and yearly recalibration. Special calibrations for use of the spectrophotometer under very clear air conditions (e.g. polar regions, deserts, high moutains) are also offered.

Recent User
The spectrophotometer SP2H or its basic-type ABAS (or BAS) is being used by the following organizations for research projects in atmospheric physics and chemistry and also as a network instrument at polar stations:

• Alfred-Wegener-Institute for Polar und Marineresearch Bremerhaven, Researchdepartment Potsdam
• Arctic and Antarctic Research Institute, St. Petersburg
• German Weather Service
• German Aerospace Research Establishment, Institute for Space Sensor Technology, Berlin
• Baltic Sea Research Institute Warnemünde, Rostock
• Department of the environment
• University of Mainz
• University of Oujda, Marokko
• Institute for Space Research, Berlin
• University of Potsdam

References

• /1/ Neckel, H., D. Labs, 1984: The Solar radiation between 3300 and 12500 Å, Solar Physics 90 (1984) 205-258
• /2/ Leiterer, U., M. Weller, J. Janiak, 1985: Verfahren zur Bestrahlungsstärke- und Strahldichtekalibrierung von Spektrometern. - Patentschrift DD 228 631 A1, WP GO1 D/265 1067 v. 16.10. 1985
• /3/ VDI 3786 Bl. 10 Umweltmeteorologie, Messung der Lufttrübung durch Aerosolpartikel mit Sonnenphotometern, (issue 04/94)

Optional available

• Control PC
• Tripods
• Parallactic mountings (hand controled, motor controled)
• Transport and storage case
• Power supply units / charging units
• Accumulators 12V/5,5 Ah

Specifications

Filter selection

¹) appropriate the recommendation of WMO/1983 and VDI-recommendation VDI 3786/10 /3/