Kipp&Zonen UVS-E-T UV Radiometers - Manual (Page 40)

40
UVIATOR software manual
12.2 Calibration Step A: Determination of the Radiometric Calibration Factor
The calibration of the broadband UV radiometers is performed with a Xe-lamp system, a monochromator (ORIEL CornerStone
MS257), and a calibrated Si-photodiode detector. The Si-photodiode detector and the broadband UV radiometers, mounted
behind the exit slit of the monochromator, are exposed to spectral irradiances between 280 nm and 400 nm (step increments:
1 nm; slit width: 2 nm at FWHM). The spectral measurements are performed sequentially as the monochromator has one exit slit
only. Nevertheless, identical monochromator output signals can be achieved for the Si-photodiode and the broadband UV
radiometers by positioning the sensitive surfaces of both detectors at the same distance from the monochromator’s exit slit(1).
A calibration factor is defined as the ratio between radiometer output and radiation input, i.e. the radiometer reading in Volts
divided by the UV irradiance. To obtain the radiometric calibration factor, ρ, in the laboratory, the UV radiometer output and
the UV irradiance input are determined using the monochromatic measurements. As only monochromatic measurements are
performed the total UV radiometer output must be calculated according to
xxxxxxxxxxxxxxxxxx , where xxxxx x are the spectrally
measured UV radiometer readings. The index UVS denotes the variable of a broadband UV radiometer.
The radiometer-weighted irradiance, i.e. the UV irradiance input, has to be calculated similarly, according to
where eSi(l) is the irradiance (in units of W/nm) of the monochromator output (measured with the Si-photodiode), is the
normalized spectral response function of the broadband UV radiometer, i.e. (l /max ( (l)) , and is the effective surface
area (in m²) of the broadband UV radiometer.
Finally, the radiometric calibration factor is obtained from the two monochromator based measurements ( UVS and UVS ) according
to ρ= / . The units of ρ are V/(W/m²).
12.3 Calibration step B: Determination of the Conversion Factor Table
Without any measurement correction, the broadband UV radiometer can provide results that deviate by a factor of 2 or more from the
true values. The magnitude of the deviation depends mainly on the extent of the spectral mismatch and the measurement condition.
The measurement conditions for which correction factors are calculated are obtained by varying the solar zenith angle, Θ₀, and the
total ozone column density, [O₃] in the radiative transfer model TUV [3]. Other atmospheric parameters affecting UV irradiances,
such as extinction of UV radiation due to aerosols, are not explicitly included as they are assumed to be small compared to the
effects that the solar zenith angle and the ozone column density have on the spectral distribution of UV radiation.
The modelled UV spectra are used to determine the conversion factors, γ(Θ₀, O₃), which are defined as γ= T
UVS
/ T
UVX
, where T
UVS
and T
UVX
denote the normalized spectral response function-weighted irradiance and the ‘true’ irradiance, respectively:
and
where eTUV(l,Θ0,O3) denotes the TUV modelled irradiance as a function of the variable input parameters Θ₀ and O₃. Note, that the
‘true’ irradiance, T
UVX
, represents the modelled irradiance weighted with a theoretical spectral response function, s
UVX
(l) . Such a
theoretical spectral response function could be the Erythema weighting function CIE-1987 [4]. The conversion factors calculated
with the Erythema weighting function provide the corrections of the E-type UV radiometers.
(1) However, the exact distance between the exit slit and the sensitive surface is not very critical, as the spot of the exit slit is smaller than the sensitive surface of
the detector, both detectors capture i.e. the entire radiant energy.