Spectrophotometer SPGS

General

The Spectrophotometer SPGS may be used as mobile or stationary equipment for spectral measurements in a range between 350 nm and 1050nm.

The spectrophotometers are acurately calibrated using laboratory equipment (climate chamber, monochromator, effective solid angle scanner, stabil standard lamp, mechanical adjustment). This method is also the base for the long-term radiometric stability control of the SPGS, corresponding with todayīs QA/QC-concepts. The Langley method and comparison with calibrated standard equipment also can be used for calibration of the SPGS with the sun as the calibration source for irradiance.

Principle

The spectrophotometer SPGS measures the irradiance of the direct solar radiation and the radiance of surfaces at predetermined wavelengths. The simultaneous measurement makes it possible to retrive optical properties of the atmosphere. Among them are the aerosolparameters, such as, the total optical depth, 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 sunīs aureole and in the azimuthal angle of the sunīs elevation (almucantar).

Instrument function

The spectrophotometer SPGS works completely program controlled. The desired wavelengths are automaticaly selected by the measuring program which can configurate before starting the measurement. After directing the objective toward the sun using the view finder (mechanical and elektronical), the solar radiation passes through the objective and is directed to a circular quartz glass fiber bundle mounted in an SMA connector. The opposite end of this bundle is formed like a rectangular slit and is optical connected to a diode array with 256 pixels This sensor converts the incident radiation meeting on every pixel into an electrical voltage in a automatically preselectable integration time. After the A/D-convertion of the measuring voltage this signal is then passed on to a computer via a RS 232 interface. In less than 1 second a full measuring cycle can be completed. After storing the data in the memory the next cycle can be started. 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) but also the measurement at all wavelength in a preselected temporal sequence. After ending of eatch measurement the present data can be shown in a graphical or 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 SPGS allows 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 SPGS 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.

Upon delivery, the spectrophotometers SPGS are normally calibrated in the laboratory and compared with standard equipment

Data Processing

The program system SPMHWin which also controlled 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 aerosol optical depth of the atmosphere 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 re-calibration 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 SPGS is under study as a basic update for high quality SGSA-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 re-calibration purposes. Dr. Schulz & Partner Ltd. offers maintenance of the spectrophotometer at intervals appropriate to the user. The company also offers a cyclic recalibration of the photometers.

Installation of the measuring system

Assembly of the Mounting and the Measuring Equipment

For installation a tripod or a mounting console in connection with a paralactic mounting can be used. The spectrophotometer gets fixed to the dovetail mount. The connection has to fit tightly to prevent a slippage of the spectrophotometer. The spectrophotometer gets directed toward the sun with the aid of the adjustment screws on the mounting. It is also possible to use other tracking units to follow the sun's position.

Installation of the Power Supply and Computer

Either a battery or the power supply unit can be set up between the tripod legs or o to another place. The installation of the data processing and control computer depends on local conditions. It should be placed at a distance of not more than 15 m (49 ft) from the tripod with the measuring equipment if using a RS232 interface.. The monitor should preferably be in sight of the operator in the case of adjusting the measuring system.

Connecting the Units

It is only one cable necessarily for the connection between PC and the measuring equipment. Note, however, that the connecting cable must not be bent. The connection should be made in the following order:

1. Connect Spectrophotometer SPGS with the main cable.
2. Connect the interface cable to the control computer COMx.
3. Connect the power supply cable to the power supply unit. Before doing this, ensure that the power supply has been turned off.

1. Turn on the power supply. Check the indicator light on the measuring unit.
2. Turn on the computer and start the program SPMHWin.
3. The system is prepared for measurement.

• The spectrophotometer must not be exposed to mechanical stress (e.g. strong vibration). It is full sealed against dust and liquids.
• If the entrance window becomes dirty, it should be cleaned carefully with pure alcohol and a non-fibrous cloth.
• When required, the equipment case should be cleaned carefully using a non-aggressive cleaning agent and water.

Specifications SPGS