The following calibration services can be performed at the WCC-UV.
A description of the calibration facility is given in section WCC-UV Facility.
Stationary spectroradiometer-systems can be calibrated by a visit of the Qasume spectroradiometer (see section Qasume Audits). The visit usually lasts 5 days with two days needed for travel and setup time and 3 days for intercomparison between the host and the reference instrument. The guideline of the audit is described in section 'Qasume audits'. The audit will result in a detailed report of the intercomparison and a calibration certificate (see certificate C1 and report R1 and report R2). It is recommended that a visit is repeated annually to ensure the long term stability of the system.
Any type of reference standard lamp can be calibrated at the WCC-UV relative to the Qasume irradiance scale. The usual procedure is the delivery of the owners standard lamp to PMOD/WRC, where it is then calibrated. For the return of the lamp special care is needed. Personal delivery is recommended. Standard lamps suitable for the calibration unit from CMS Schreder (250 W) can be bought at the WCC-UV, including calibration certificate. Examples of calibration certificates can be downloaded here (certificate C2 and certificate C3).
The full calibration of a filter radiometer includes the characterization of the instrument and its absolute calibration.
Radiation measurements with broadband filter radiometers fundamentally depend on the relationship between the measured spectral radiation spectrum and the spectral responsivity of the radiometer. Indeed, different spectral radiation distributions will produce results which not only depend on the amount of radiation received by the detector, but also on their relative spectral shape. Furthermore, the detector spectral response will usually not be identical to the nominal spectral sensitivity for which the radiometer was designed, for example the erythemal action spectrum. This implies that the calibration of such a radiometer will depend on the spectral source function and the spectral filter response. In addition, suitable correction functions will be required to convert from the detector weighted radiation to the one representative for the desired weighting function. Measurements of the spectral response function for the three major type of radiometers can be seen in Figure 1.
Examples of spectral response functions. Displayed are the mean and standard
deviation of the SRFs of 5 Kipp & Zonen and 2 Scintec, 9 YES UVB-1 and 10 Solar Light SL501A radiometers.
The SRF characterization results in a calibration certificate (certificate C4).
In a similar context as described for the SRF measurement, irradiance measurements require the detector to weight incoming radiation with the cosine of the incoming angle relative to normal incidence. Deviations from this angular response will usually result in diurnal variations and may also depend on the atmospheric state. This is especially true for measurements in the ultraviolet wavelength region where the relative proportions of the direct un-scattered solar radiation to the diffuse radiation changes substantially during the day and depends significantly on the solar zenith angle. The deviation of the actual angular response function from the ideal cosine function for the three major types of radiometers can be seen in Figure 2 and 3.
Examples of angular response functions. a) mean and standard deviation of the ARFs of sets of different radiometer types
(same as in Figure 1); b) cosine error of these instruments.
The ARF characterization results in a calibration certificate showing the angular response for the four planes of the radiometer filter and the resulting cosine error (see certificate C5).
The absolute calibration factor of the instrument is obtained during an outdoor calibration. The signal of the radiometer is recorded for several days and compared to the reference spectroradiometer.
The resulting certificate will provide all the necessary calibration factors to convert from the raw signal to weighted irradiance (see certificate C6). These factors are first, the absolute calibration factor; second, a function to convert from the irradiance weighted with the detectors' specific function to the desired weighting function; third, a suitable cosine correction function.
The picture below shows a map of all broadband and multichannel radiometer locations which have been calibrated at PMOD/WRC - or where the calibration of the instrument is directly traceable to the WCC-UV.
Locations of all broadband radiometers calibrated at PMOD/WRC or traceable to the WCC-UV