Kipp&Zonen UVS-E-T UV Radiometers - Manual (Page 39)

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12. Theory
UVIATOR software manual
This part shows the theoretical background of the UVIATOR software. Reading is only meant for those who are interested and
not required for use of the UVIATOR software.
Atmospheric ultraviolet radiation measurements are difficult to perform due to the drastic decrease of UV-B irradiance towards
shorter wavelengths, caused by the strong stratospheric ozone absorption. Besides the extinction of UV radiation due to ozone,
Rayleigh scattering also affects the UV radiation, especially in the UV-B spectral region. As UV radiation represents a small portion
of the solar spectrum only, broadband UV radiometers contain filters and use signal amplifiers to measure the UV irradiance in the
appropriate spectral region. On top of the filters used to measure the UV irradiance in the entire UV-A or UV-B spectral region,
dedicated weighting functions require to use additional filters which change the transmission function of the UV radiometer such
that the spectral response function of the UV radiometer corresponds as much as possible to the theoretical weighting function. As
none of these spectral response functions correspond exactly to the theoretical weighting functions, even not for the radiometers
measuring only UV-A or UV-B irradiances, the measurements are affected by a systematic error.
The UV-S-X is suitable to measure UV irradiance according to the theoretically defined UV-B, E and A spectra. In general all
broadband filter instruments have limited performance due to the intrinsic spectral mismatch of each sensor with respect to the
theoretical definition of UV-B, -E and -A.
By knowing the spectral mismatch in detail one could compensate the instrument effects for different measurement conditions.
Kipp & Zonen developed a software program for post processing and extensive analyses of UV data. The UVIATOR program
performs automatically a number of UV measurement corrections and improves thereby the measurement quality significantly.
The so-called spectral mismatch error correction is based on the correction method described in the WMO report No. 141 [2].
Further explanations and discussions of the spectral mismatch error are presented in a number of publications listed at the end
of this section.
12.1 UV Radiometer Calibration and Correction Method
To achieve the most accurate measurement result with broadband UV radiometers, the raw signals must be transformed into UV
irradiances using two calibration steps (A and B), and an adjustment step:
• Calibration step A: The raw signal of the instrument (in units of Volts) has to be transformed into an irradiance (in units of
W/m²). To achieve this transformation a so-called “radiometric calibration factor”, denoted as ρ (in units of V/W/m²), has to be
determined.
• Calibration step B: The irradiances have to be corrected for the so-called spectral mismatch error with the “conversion
factors”, denotes as γ (no units). The conversion factors are determined using modelled UV irradiances as a function of various
total ozone column densities and solar zenith angles.
• Adjustment step: The corrected and final UV measurements are obtained by multiplying the raw UV radiometer reading under
outdoor measurement conditions with an appropriate adjustment factor, χ, defined as 1/(ρ•γ). The appropriate adjustment
factor has to be chosen according to the measurement condition at the time of the UV radiometer reading.
The “adjustment step” which provides the final, spectral mismatch corrected, UV irradiance (in units of W/m²), is carried out by
the UVIATOR program for each individual UV radiometer reading. Before the broadband UV radiometer can be used in the field,
however, it must be calibrated according to the “calibration steps” A and B, which provide the calibration and correction factors
for a particular instrument.
The next two paragraphs describe the calibration steps A and B as they are performed at Kipp & Zonen. The final paragraph of
this chapter describes the adjustment procedure as implemented in the UVIATOR program.