Data Processing Methods

The Langley calibration method can be used to convert bias- and cosine- corrected MFRSR voltages to irradiances (watts m-2 nm-1) by:

irradiance = mV measured x ET/Langley Voltage Intercept

ET (extra-terrestrial irradiance) is calculated for each channel of the instrument for each deployment period by weighting the Susim 3 spectrum by the most recent filter function determined by the calibration facility (YES or CUCF) prior to the deployment. For more information about the Langley calibration method, see Slusser, J.R., J.H. Gibson, D.S. Bigelow, D. Kolinski, P. Disterhoft, K. Lantz and A. Beaubien, 2000, Langley Method of Calibrating UV Filter Radiometers, J. Geophys. Res. 105, 4841-4849.

For most time periods, data calibrated by the Langley method are now available. Whereas lamp-calibrated data typically become available the next day after collection, you can expect a 1 to 5 week delay for Langley-calibrated data. Langley calibrations are updated monthly, usually after the first week of each month. During the time immediately following an instrument rotation, the lag time may be greater than 5 weeks if sufficient calibration information has not yet been collected. This will affect the relatively turbid sites more frequently than sites that are typically dry and sunny.

If Langley-calibrated data are available for a site and time period in which you are interested, please use the following guidelines in deciding which calibration method provides the more accurate information.

Due to lack of funds for calibrating the Vis-MFRSR instruments, their lamp-calibration information is very outdated. Given that situation, the Langley-calibrated data from those instruments should always be considered more accurate than the lamp-calibrated data. There may, however, be periods for some sites when Langley calibration is not possible due to very short instrument residence times and/or too few clear days during the time a given instrument was operating.

UV-MFRSR (300 - 368 nm):

In the ultraviolet, the question of whether lamp calibration or Langley calibration is better is not so straightforward. For instance, at sites west of the Mississippi River, which are usually sunny and dry, Langley calibration is probably more accurate because of better signal-to-noise ratios. Even so, at the shortest wavelengths (300 - 311 nm), variations in ozone during the Langley calibration period of two to three hours make these calibrations less accurate. At turbid sites which have few Langley events, it is difficult to say which is more accurate.