YES Multifilter Rotating Shadowband Radiometer (MFR-7)
The Multifilter Rotating Shadowband Radiometer (MFR-7) measures total, diffuse, and direct irradiance with a broad-band silicon pyranometer and at six wavelengths using sensors with narrow band filters at 415, 500, 615, 673, 870, and 940 nanometers. The full width half maximum width is ten nanometers. The head, where the sensors are located, is thermally stabilized to significantly reduce uncertainties associated with temperature variation. The MFR-7 makes measurements simultaneously across all seven channels. The shadowband enables the instrument to measure the global and diffuse components of solar irradiance and then calculate the beam component. The MFR-7 system includes the YESDAS-2 datalogger.
The shadow band is controlled by a microprocessor. Measurements can be made as frequently as one every 15 seconds and averaged or stored individually. In Eugene, we make one measurement every minute. First the shadowband is positioned to the east side of the entrance aperture, then it completely shades the aperture, and then it moves to the west side of the aperture. The measurements made when the shadowband is to the east or west of the aperture are used to correct for the shading of the sky by the shadowband itself.
Global values are determined when the shadowband is at rest, and diffuse measurements take place when the aperture is shaded. The diffuse values are adjusted by adding back the amount of the sky shaded by the shadowband not blocking the sun. The corrected diffuse is then subtracted from the global and the horizontal beam is projected back onto the direct normal.
The broadband diffuse measurement has the same problem experienced by the sensor in an RSP. Responsivity of the sensor to diffuse radiation is different on cloudy days: it is about 30 percent higher than the responsivity of the sensor to diffuse radiation on clear days. Both sensors are solar cell based and they experience the same problem. The narrowband sensors are spectrally selective and don't experience this problem.
The MFR-7 wavelengths are chosen to allow the reconstruction of the incident solar spectral irradiance and the determination of optical depths of water vapor, aerosols, and ozone (using the Chappuis ozone absorption bands).
MFR-7 Radiometer Assembly
The MFR-7 Radiometer Assembly consists of the MFR-7 sensor head along with a rotating shadowband that is driven by a stepper motor: both are mounted on a common base. An electronics enclosure, mounted below the Detector Assembly, contains a YESDAS microprocessor-driven data acquisition and control system. The system electronics has 13-bit A/D conversion accuracy and has the capability of collecting data from an additional 24 analog and six pulse/counting-type meteorological sensors.
(Above: interior optical detector assembly)
Control and Data Logging System
The MFR-7 instrument operation and data logging are controlled by a microprocessor. This on-board CPU: 1) performs the required ephemeris calculations, 2) controls the stepper motor which positions the shadowband, 3) controls the acquisition, processing and storage of sensor data for the MFR-7 and up to 24 additional analog meteorological sensors, and 4) permits simultaneous data telemetry. The very stable system time keeping (with an accuracy of 1 second per month) ensures that the positioning of the shadowband will be precise over extended time periods, with no need for operator intervention or adjustment.
The system data logger includes a state-of-the-art 13-bit self-calibrating analog-to-digital converter and on-board data storage capability of up to two Mbytes with the PCMCIA-2 memory option. Most MFR-7 users will need the two Mbyte card to permit high time resolution sampling. You can communicate directly from a PC, Macintosh, or UNIX workstation through a serial port (3m cable provided) or over a phone line using a user-supplied modem. The user may download the stored data and has full control over the data acquisition process. Below is a schematic diagram of the control and data logging system.
The usefulness of any instrument depends critically on the quality and long-term stability of its calibration. Yankee Environmental Systems, Inc. has fully equipped optical laboratories to completely characterize the performance and calibrate each MFR-7 instrument. The cosine, spectral, and absolute responses of each wavelength channel of the instrument are measured with NIST-traceable optical and electronic equipment and test results are supplied with each system. Interference filters deteriorate over time and yearly recalibrations of the MFR-7 are recommended.
The spectral response of each wavelength channel of the MFR-7 instrument is measured in YES's laboratory. Below is a graph showing the typical filter passbands of an MFR instrument.
The absolute response of each wavelength channel of the MFR-7 instrument is measured using a 1,000 watt FEL NIST-traceable spectral irradiance standard light source. The spectral response of the channel and the output of the instrument in response to the irradiation by the FEL lamp are used to obtain the channel calibration constant.
The MFR-7 belongs to a class of instruments that measure flux incident on a horizontal surface. The response of such instruments to radiation incident at an angle, q, with respect to the surface normal is called the cosine response. The ideal cosine response is proportional to the cosine of the angle q, and any deviation from this response introduces measurement errors.
The MFR-7 has a novel input optic with superb cosine response and long-term stability. The radiation receiver element is a specially shaped Spectralon diffuser disk that is directly coupled to a Spectralon integrating cavity. Spectralon is a halocarbon with excellent resistance to chemical and ultraviolet degradation, thus ensuring calibration stability in the field. Below is a graph of the ratio of a typical instrument's angular response to the ideal cosine response:
The cosine response of each MFR-7 is fully characterized in YES's angular test facility. Each instrument is tested by placing it on a computer-controlled rotary actuator and measuring the instrument response as a function of its angle with respect to a feedback-stabilized, parallel, uniform light beam. The individual MFR-7 cosine response, supplied with each instrument, is used by YESDAS system software to correct, in real time, for deviations from the ideal cosine response.
© 2000, UO Solar Radiation Monitoring Laboratory.
Last revised: December 11, 2000.
Home page URL: solardat.uoregon.edu