Spectrophotometer SP1A

General
The multi-channel Spectrophotometer SP1A 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 SP1A, corresponding with today s QA/QC-concepts. The Langley method and comparison with calibrated standard equipment also can be used for calibration of the SP1A with the sun as the calibration source for irradiance.

Instrument function
The spektrophotometer SP1A works completely program controled. The desired wavelengths are automaticaly selected by the measuring program which can configurate before starting the measurement. The control of the filter wheel motion, of the measurement and the data stoage takes place by different programs which can also select bevor starting the measurement. 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 internal amplifiers. The optimal amplification (factor 10⁰...10⁸) is automatically adjusted depending on the strength of the incoming radiation. After the A/D-convertion of the measuring voltage this signal is then passed on to a computer via a RS 232 interface. Within 8 seconds a full measuring cycle can be completed; i.e., taking the data of 18 channels, storing them in the memory and calling up the next cycle. However, the program allows not only automatic collection and treatment of data from a single preselected channel (e.g. 500 nm following the WMO-recommendations /3/) but also the measurement in all 18 channals in a preselected temporal sequence. But this requires an additional tracking device which continually directs the spectrophotometer toward the sun. After ending of eatch measurement the present data can be shown as a graph or in alpha numerical form for an immediate appreciation as well as for further calculations or for transfer to other special processing systems using the data record formats ASCII . Other standard formats can be substituted upon the user's request. The mechanical construction of the spectrophotometer SP1A allows either to mount it on a manually driven parallactic mounting or on an automatic tracking device (solar tracker).

Specifications

Filter selection

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

Data Processing
The program system SPMH which also controls 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

Calibration procedures
The parameters relevant to the results of measurements are carefully measured for every spectrophotometer SP1A 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 special calibrating method /2/.
Upon delivery, the spectrophotometers SP1A are normally calibrated in the laboratory and compared with standard equipment.

Service
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 SP1A is under study as a basic update for high quality SP1A-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 SP1A or its basic-type SP2H (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
• Institute for Space Research, Berlin
• German Research Council, Bonn
• Institute of Troposphere Research, Leipzig
• University of Leipzig
• University of Mainz
• University of Oujda, Marokko
• University of Potsdam
• National Institute of Polar Research, Tokyo
• Alice Springs
• NASA Langley Research Center, Hampton

• Chesapeak Lighthouse

Optional available

• fully automatic Suntracker with viewfinder
• Control PC (processor, memory, HDD depending on agreement)
• Tripods
• Parallactic mountings (hand controled, motor controled)
• Transport and storage case
• Power supply units / charging units
• Accumulators 12V/5,5Ah
• Accumulator bag

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)