Calibration of Radiation Instruments at PMOD/WRC
Radiation instruments can be separated into three different categories: Shortwave
Radiometers, Longwave Radiometers, and Spectral Radiometers. The calibration of radiometers
is routinely performed at PMOD/WRC and is described in the sections (Index):
The following is a short description of each category:
- Calibration of Shortwave Radiometers
- Calibration of Longwave Radiometers
- Calibration of Spectral Radiometers
- Shipping Instructions
- General Conditions of Trade 2011
Shortwave Radiometers measure direct irradiance from the sun, which approximately
corresponds to the Planck-spectrum at 5512°C. Short-wave radiation
is measured with pyrheliometers
for direct solar radiation and
for global radiation (unshaded pyranometer:
Direct solar radiation and diffuse radiation from the ground and the atmosphere)
or diffuse radiation (shaded pyranometer). Pyrheliometers are absolute radiometers
and have a narrow field of view which measure radiation coming directly from the sun
only. Solar radiation is absorbed in a cavity, which ensures a high absorptivity
over the spectral range of interest (Fröhlich
, 1991). Pyranometers
have glass domes to protect the blackened thermopile detector; these domes are
transparent from approximately 300 nm to 2.75 um (50% transmission points).
Longwave Radiometers measure terrestrial and atmospheric thermal radiation
covering the spectrum from 4 - 100 mm. Longwave downwelling and/or upwelling
radiation is measured by instruments such as pyrgeometers
Pyrgeometers (Philipona et al.
, 1995) have the advantage of only measuring longwave
radiation, whereas pyrradiometers (Ohmura and Schroff
, 1983) measure
both, short- and longwave radiation. Hence, to calculate the longwave radiation
of the pyrradiometer, the shortwave radiation measured by pyranometer has to
be subtracted. Netradiometers
measure the net radiation flux (radiation
balance) between down- and upwelling shortwave and longwave radiation.
Spectral Radiometers are instruments such as a sunphotometer
which consist of interference filters and silicon detectors to measure solar
radiation in a narrow spectral band (typically 5 nm FWHM). SPMs are used to
determine atmospheric turbidity (aerosol optical depth) and the concentration
of trace gases such as ozone or water vapor. A SPM is calibrated in terms of
its extraterrestrial signal at 1 AU (astronomical unit) distance from the sun.
When calibrated in absolute units, the SPM can also be used to determine the
solar spectral irradiance from stratospheric balloons, rockets or satellites.
1. Calibration of Shortwave Radiometers
||Type of calibration
|Absolute Radiometer / Normal Incidence Pyrheliometer
||Comparison with the World Standard Group (WSG) using the
sun as source (Direct Radiation).
|Pyranometer (with Thermopile)
||Comparison with the sun and the sky as source (Global Radiation).
The direct solar radiation is obtained from the World Standard Group and
the diffuse radiation is determined from a shaded standard pyranometer.
For further information about calibration and shipping of instruments to PMOD/WRC
Dr. Wolfgang Finsterle, e-mail: w.finsterle(at)pmodwrc.ch
2. Calibration of Longwave Radiometers
Figure 4. Pyrgeometer comparison to the WISG. The upper figure shows the longwave irradiance during nighttime, while the lower figure shows the
residuals between the test pyrgeometer and the WISG. The sensitivity C is determined so as to minimize these residuals
Test pyrgeometers are inspected on arrival and desiccant changed if necessary. They are first characterised in the black-body to retrieve the characterisation constants as described previously. Then, they are installed on the measurement platform for the determination of the sensitivity by comparison to the WISG. Only nighttime measurements are used so far. A nominal calibration procedure requires about 24 hours for the laboratory characterisation and several days with clear and cloudy skies to determine the sensitivity C and check the stability of the test pyrgeometer. Typical variabilities between the test pyrgeometer and the WISG are usually less than ±1 W/m^2 during the whole measurement period (see Figure 4).
For further information about longwave radiation instruments and their calibration
please contact Dr.Julian Gröbner
at PMOD/WRC, e-mail: julian.groebner(at)pmodwrc.ch
3. Calibration of Spectral Radiometers
Sunphotometers can be calibrated either by classical extrapolation through the
atmosphere (Langley method) or by comparison with an irradiance standard lamp
and the solar spectrum or by in situ
measurements from stratospheric balloons
or rockets (Schmid and Wehrli
, 1995). The 12 WMO standard wavelengths for SPMs
412, 450, 500
, 610, 675, 719, 778
, 817, 862, 946,
and 1024 nm (primary wavelength underlined).
||Type of calibration
||- Comparison with WRC SPMs at any of 12 standard wavelengths using the
sun as source.
- Absolute calibration against NIST traceable standard lamp and extraterrestrial
value via standard solar spectrum.
- Optional: optical bandpass determination with monochromator.
For further information please contact Christoph Wehrli
at PMOD/WRC, email: c.wehrli(at)pmodwrc.ch
Fröhlich, C., 1977, 'World Radiometric Reference', in: WMO/CIMO Final
, WMO No. 490, 97-100. (PMOD/WRC intern: 545a)
Fröhlich, C., 1991, History of solar radiometry and the World Radiation Reference,
, 111-115. (PMOD/WRC intern: 650)
Fröhlich, C., Philipona, R., Romero, J., and Wehrli, C., 1995, Radiometry
at the Physikalisch-Meteorologisches Observatorium Davos and the World Radiation
Center, Optical Engineering 34
, 2757-2766. (PMOD/WRC intern: 674)
Ohmura, A., and Schroff, K., 1983, Physical characteristics of the Davos-type
pyrradiometer for short- and longwave radiation, Arch.Met.Geoph.Biocl., B 33
Philipona, R., Fröhlich, C., and Betz, Ch, 1995, Characterization of pyrgeometers
and the accuracy of atmospheric long-wave radiation measurements, Applied Optics,
, 1598-1605. (PMOD/WRC intern: 676)
Philipona, R., 2001, Sky-scanning radiometer for absolute measurements
of atmospheric long-wave radiation, Applied Optics, 40, 2376-2383. (PMOD/WRC intern: 792)
Schmid, B., and Wehrli, Ch., 1995, Comparison of sun photometer calibration by
use of the Langley technique and the standard lamp, Applied Optics 34
4500-4512. (PMOD/WRC intern: 680)