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Instrumentation

The Broadband UVB-1 Pyranometer
The Yankee Environmental Systems UVB-1 Pyranometer measures global irradiance in the UVB spectral range of 280-330 nm. The measurement technique employs a combination of colored glass optical filters to block the sun's visible spectra and a UV sensitive flourescent phosphor to convert the UVB light to visible light. The converted light is then measured with a solid-state photodiode. Signal output is 0-5 volts DC, low Z, corresponding to 0-9.84 Watts/meter^2 of effective UVB irradiance.

  • Broadband Measurement Plan

    The USDA UV-B Monitoring and Research Program recognizes the importance of instrument stability in establishing a long-term climatological record. The network documents the stability of its broadband instruments through annual calibrations and annual re-characterizations of each instrument's spectral response. Initial instrument characterizations were first established by submitting each of the network's initial broadband meters to the National Institute of Standards and Technology (NIST) for an evaluation of their cosine and spectral response. Plots of the NIST characterizations are available.

  • Representativeness

    • Spectral Response

        The spectral response of a broadband meter is a measure of the instrument's response to light at specific wavelengths - typically generated with a high resolution scanning monochrometer and xenon arc source. Two important quality attributes of the instrument can be determined and tracked through this characterization; the instrument's central wavelength stability and stability of its characteristic shape. Because of the importance of documenting uniformity in these parameters over the life of the USDA's monitoring program, spectral characterizations are remeasured approximately annually when each instrument is returned to the manufacturer for re-calibration.

  • Bias

    • Cosine Response

        The cosine response of a broadband meter is a measure of the departure of the angular response of the instrument from that of Lambert's cosine law. This law states that the response of an ideal detector to constant and uniform light source should decrease in proportion to the cosine of the angle of incidence of the light. In practice the response is a function of the design geometry and manufacturing of the meter. It is anticipated that once determined, the cosine response characteristic of an individual instrument will not change unless the instrument becomes damaged. In field applications the stability of cosine response, once characterized, should only be dependent upon maintaining it in a level plane. In the USDA monitoring program this is checked and adjusted annually.

    • Calibration

        Calibration of the USDA broadband meters follows the theory of Grainger, et al., 1993. That is, a calibration constant for a selected broadband meter was derived from a regression of the integrated spectral response of a spectroradiometer against the signal produced by the selected meter. This meter serves as the primary reference for the network. Results of the regression yielded a relationship of 1.968 +/- 0.11 (Watts per meter-squared)/Volt. The relationship represents the irradiance integrated over the range of 280 - 320nm. It should be noted that integrating over other spectral ranges will result in a different relationship (constant).

        Annual calibrations follow ASTM E_824 methodology ending with the adjustment of each test meter's signal to the signal of a reference instrument maintained according to ASTM method E_816 by the calibration facility. The merits of this approach are discussed by Deluisi et al. (1992). Presently, the calibration facility is located at Yankee Environmental Systems, Inc. in Turners Falls, MA.

  • Precision

  • Comparability

    • Traceability to NIST
    • 1994 Boulder Intercomparison
    • 1995 Boulder Intercomparison
    • See Spectral and Cosine Response (above)

  • Additional Information

    • Manufacturer's Specifications
      • Instrument Response Time: approximately 100 ms.
      • Cosine Response: Better than +/- 5% for O-60 degrees zenith angle
      • Active Sensor Area: approximately 2.54 cm iin diameter.
      • Operating Temperature: -40 to +40 degrees C.
    • From the Surface Radiation Research Branch of NOAA

  • References

      • American Society for Testing Materials, 1994. ASTM Method E_824. Standard Test Method for Transfer of Calibration From Reference to Field Radiometers. West Conshohocken, PA 19428.

      American Society for Testing Materials, 1995. ASTM Method E_816. Standard Test Method for Calibration of Pyrheliometers by Comparison to Reference Pyrheliometers. West Conshohocken, PA 19428.

      Deluisi, J., Wendell, J., Kreiner, F. 1992. An Examination of the Spectral Response Characteristics of Seven Robertson-Berger Meters After Long-Term Field Use. Photochemistry and Photobiology 56(1) pp115-122.

      Grainger, R.G., R.E. Basher, R.L. McKenzie 1993. UV-B Robertson-Berger meter characterization and field calibration. Applied Optics, 32(3) pp343-349.

      Green, A.E.S., Cross, K.R., Smith, L.A. 1980. Improved Analytic Characterization of Ultraviolet Sky. Photochemistry and Photobiology 31, pp59-65.

      Yankee Environmental Systems, Inc. Bulletin UVB-1S. PO Box 746, Montague Industrial Park, Turners Falls, MA. 01376 USA.

      Yankee Environmental Systems, Inc. 1995. UVB-1 Ultraviolet Pyranometer Installation and User Guide, Turners Falls, MA 01376