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):

1. Calibration of Shortwave Radiometers
2. Calibration of Longwave Radiometers
3. Calibration of Spectral Radiometers
4. References
5. Shipping Instructions
6. General Conditions of Trade 2011

The following is a short description of each category:
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 with pyranometers 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 and pyrradiometers. 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 (SPM), 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.

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1. Calibration of Shortwave Radiometers

Instrument	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 please contact
Dr. Wolfgang Finsterle, e-mail: w.finsterle(at)pmodwrc.ch

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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

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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 are: 368, 412, 450, 500, 610, 675, 719, 778, 817, 862, 946, and 1024 nm (primary wavelength underlined).

Instrument	Type of calibration
Sunphotometer	- 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

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4. References

Fröhlich, C., 1977, 'World Radiometric Reference', in: WMO/CIMO Final Report, WMO No. 490, 97-100. (PMOD/WRC intern: 545a)

Fröhlich, C., 1991, History of solar radiometry and the World Radiation Reference, Metrologia 28, 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, 57-76.

Philipona, R., Fröhlich, C., and Betz, Ch, 1995, Characterization of pyrgeometers and the accuracy of atmospheric long-wave radiation measurements, Applied Optics, 34, 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)