Langley or Lamp Calibration?
Which is Better?

For most time periods, data calibrated by the Langley method are now available. Whereas lamp calibrated or MLO calibrated data typically become available the next day after collection, Langley calibrations are updated monthly, so data is available for periods up to the previous month only. During the time immediately following an instrument rotation, the lag time for Langley calibrated data 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.

Due to lack of funds for calibrating the Vis-MFRSR and UV-MFRSR instruments, their lamp-calibration information is very outdated. Therefore, the Langley calibrated data (in-situ) 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. If available the MLO Langley calibrated could be used. The calibration date for these instruments can be found under Monitoring Network, Sites Information, . If the lamp calibrated is within 18 months of the requested date then the lamp calibrated will also be accurate.

Corrections Applied to UV-MFRSR Voltages

Last Update: January 2016

Total horizontal and diffuse horizontal radiation is measured by the MFRSR instrument as a voltage. The direct normal radiation response is determined by subtraction of the diffuse radiation from the total, followed by division by the cosine of the solar zenith angle.

dark current

Nighttime electronic (bias) offsets are applied only to the diffuse horizontal voltage. This is done by:
  • Determining the time of minimum solar elevation for each day.
  • Averaging the nighttime bias for an hour before and after the time of minimum solar elevation.
  • Subtracting the average bias from the diffuse voltage, only if the diffuse is greater than 1.

Note: The direct normal component is already bias-corrected within the instrument. The direct horizontal is formed as the difference between the total horizontal and diffuse both of which are assumed to have equal bias. The total horizontal is re-calculated as explained below and, therefore, needs no bias correction.

Angular corrections (instrument cosine responses) are applied only to the direct normal. The cosine responses are taken from the most recent laboratory determination prior to the date of the data being corrected. The instrument cosine responses were determined by Yankee Environmental Systems, Inc. (YES) or NOAA's Central UV Calibration Facility (CUCF) or by UVMRP. Older determinations were typically made by YES while more recent ones are from CUCF or here at UVMRP.
Diffuse cosine correction factors, determined using the isotropic sky assumption, are applied to the bias-corrected diffuse horizontal and are equal to

diffuse_bias_corrected/diffuse_cosine_factor

Total horizontal is re-calculated by summing the direct horizontal and the diffuse horizontal:

total_horizontal = (cosine_corrected_direct_normal x cos(zenith_angle)) + cosine_corrected_diffuse

The voltages, as they are at this point in the procedure, are kept 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, which we use to convert our data to irradiances.

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

irradiance = cosine_corrected_voltage / calibration factor

Calibration factors can be laboratory-determined (lamp-calibrated) or calculated from Langley analysis. Due to funding issues most of our current calibration factors are calculated from Langley analysis. A lamp calibration factor is the product of head and board gains. 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.