Data Processing Methods

Langley or Lamp Calibration?

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.
Vis-MFRSR (415 - 940 nm):
Due to lack of funds for calibrating the Vis-MFRSR instruments, their lamp-calibration information is very outdated.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.
This is an active research area; therefore, we will continue to report data calibrated by both methods. Any questions regarding this topic should be directed to our program.

Corrections Applied to UV-MFRSR Voltages

Last Updated: September 12, 2008

The total horizontal and diffuse horizontal voltages are measured by the MFRSR instrument. The direct normal voltage is determined within the data logger by subtraction of the diffuse voltage from the total, followed by correction for solar zenith angle. We refer to these voltages as raw voltages.

Nighttime bias corrections are applied only to the diffuse horizontal voltages. This is done by:

  • Determining the time of minimum solar elevation for the set of data currently residing in the database table temp_data_poll (amount of data varies, usually 1-3 days)
  • Averaging the nighttime bias voltages from 1 hour prior to 1 hour after the time of minimum solar elevation
  • Subtracting the average bias from the diffuse voltages, only if the diffuse voltage is greater than 1.

    Note: The direct normal component is effectively bias-corrected during the subtraction in the data logger; the total horizontal is re-calculated as explained below and, therefore, needs no bias correction in this step.

Angular corrections (instrument cosine responses) are applied only to the direct normal voltages. The cosine responses are taken from the most recent laboratory determination that was prior to the date of the data being corrected. The instrument cosine responses were determined either by Yankee Environmental Systems, Inc. (YES) or by NOAA's Central UV Calibration Facility (CUCF). Older determinations were typically made by YES while more recent ones are from CUCF.

Diffuse cosine correction factors, determined using the isotropic sky assumption, are applied to the bias-corrected diffuse horizontal voltages:

diffuse_bias_corrected_voltage/diffuse_cosine_factor

Total horizontal voltages are re-calculated by summing the direct horizontal voltage and the diffuse horizontal voltage:

total_horizontal = (cosine_corrected_direct_normal x cos(zenith_angle x pi/180)) + cosine_corrected_diffuse

The voltages, as they are at this point in the procedure, are stored in the database as cosine-corrected voltages.

Note: The cosine-corrected direct normal voltages are later input to the Langley Analyzer for the generation of Langley voltage intercepts for the Langley voltage intercepts time series.

Calibrations are applied to the cosine-corrected direct normal, diffuse horizontal, and total horizontal voltages to convert them to irradiances:

irradiance = cosine_corrected_voltage / calibration factor

Calibration factors can be laboratory-determined (lamp-calibrated) or calculated from Langley analyses. A lamp calibration factor is the product of head and board gains (head_gain x board_gain). The gain values used are linear interpolations of the two determinations that surround the date of the data being corrected. If there is no closing determination, the most recent determination before the date of the data being corrected is used.

Head gains are from lamp calibrations performed either by Yankee Environmental Systems, Inc. (YES) or by NOAA's Central UV Calibration Facility (CUCF). Older determinations were typically made by YES while more recent ones are from CUCF. Board gains are recorded by UVB Program personnel. Langley calibration is discussed in the last section of this document.