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Selecting Solar Instruments

Solar Instrument Selection Three broad classes of instruments are used for solar irradiance measurements. Depending on the application, instruments from one or from a combination of the following classes may be appropriate.
Spectroradiometers and Spectrographs Spectroradiometers measure the solar spectrum in small wavelength increments over a fixed region of the spectrum, typically with resolutions of a few nanometers or less, and produce output in terms of spectral irradiance--usually in W/m2-nm. A spectroradiometer offers the highest spectral resolution and, when calibrated against an irradiance standard such as an FEL lamp, can be used to calibrate simpler instruments such as broadband UV pyranometers. Although it is a valuable research instrument, the high cost (> $100K) and high maintenance overhead of a scanning spectroradiometer has limited its deployment in atmospheric research programs. A conventional scanning spectroradiometer is essentially an indoor laboratory optical UV monochromator packaged to withstand the elements.

Traditionally, optical diffraction gratings have been used as the wavelength dispersion mechanism in spectroradiometers. In mechanical-grating spectroradiometers such as the Brewer and Bentham, a motor moves a grating along a translation stage (a track) and images the spectrum onto a single detector. The low spectral irradiance levels in the UV combined with the high spectral resolution and slow-scanning mechanism of the instrument require several minutes to make a full measurement. During this time, the motion of clouds across the sky can strongly affect the spectral measurement. For this reason, meaningful spectral measurements must be carried out under clear-sky conditions so other instruments are needed for continuous monitoring applications. The motion of the sun itself can also affect the measurement, resulting in a different spectral response due to solar zenith angle changes throughout the scan. Usually, many scans are made and then averaged to produce a statistically representative picture of spectral levels. The use of moving mechanisms eventually leads to wavelength alignment problems and failures in the field. Spectroradiometers' complexity also makes them difficult to operate in a network since they require almost constant attention from skilled technicians and scientists.

In contrast, a spectrograph instrument uses a fixed dispersing element such as a prism or ruled grating to image the spectrum onto a linear array of detectors. In the case of the Rotating Shadowband Spectroradiometer (RSS/UVRSS), prisms and lenses are used to refract the light and then illuminate a charge-coupled device (CCD). This configuration results in a nearly instantaneous measurement--the entire spectrum can be imaged in a few seconds and then integrated over time, so many scans are not required to produce an accurate picture of sky conditions. Also, the elimination of the complex motorized scanning mechanisms and optical gratings results in lower maintenance costs--except for an internal shutter and external shadowband, the RSS has no moving parts. Finally, the prism-based instrument does not use filters, which are subject to variations in absolute throughput over time.

Narrowband Instruments Optical interference-filter-based narrowband instruments or sun photometers, measure discrete spectral regions in the spectrum that are typically 2 nm wide in the UV region and 10 nm in the visible/NIR. They are useful for applications that require better resolution than offered by broadband instruments, but do not require the full capabilities of a spectroradiometer. Narrowband instruments use interference filters to measure the specific spectral regions. The filters are selected based on areas of interest: atmospheric turbidity, total column ozone, aerosols, column water vapor, and UV monitoring.

The YES Multifilter Rotating Shadowband Radiometer (MFR) and UVMFR offer seven channels in the visible/NIR and UV ranges, respectively. A computer-controlled shading band provides total, diffuse, and direct irradiance with one detector. Careful calibration techniques and software-applied angular correction tables provide a superior cosine response. The YES UV Sun Photometer (SPUV) has six or ten channels in the UV and visible/NIR regions that measure direct-normal irradiance. The SPUV must be mounted on an automatic sun tracker. Generally, direct-beam instruments such as the SPUV have a well-defined field of view controlled by mechanical apertures and provide a slightly more accurate direct-normal measurement than shadowband radiometers, assuming the tracker is properly aligned and checked daily.

Narrowband instruments are more expensive than broadband instruments but still are a fraction of the cost of spectroradiometers.

Broadband Instruments Broadband instruments measure the total-horizontal irradiance over a broad region of the spectrum. UV pyranometers such as the UVB-1 and UVA-1 integrate over the 280 - 320 nm and 280 - 400 nm range, respectively. Total solar pyranometers such as the TSP-700 measure total-horizontal irradiance from 300 - 3000 nm (.3 - 3m). Pyrgeometers measure terrestrial infrared total-horizontal irradiance from 3m to approximately 20m. Generally, broadband instruments are relatively inexpensive and easy to operate and maintain. The fast response time (typically on the order of milliseconds to seconds) of these instruments makes their measurements accurate even in the presence of clouds. Their reliability and low maintenance requirements make them well suited to continuous monitoring applications.

Since broadband data is integrated over a broad wavelength region, you should ensure that the instrument’s spectral sensitivity corresponds to the wavelength band of interest: i.e., total solar, UV-B, or UV-A. Many broadband UV instruments have spectral responses that are similar to the erythemal action spectrum, which describes the response of the human skin to solar radiation. By applying a correction factor to the instrument’s output, you can calculate the actual erythemal irradiance in effective W/m2.

Research programs over the last several years have shown good correlation between UV broadband data such as that collected by the YES UVB-1 and spectroradiometer data (Bais et al, Laboratory of Atmospheric Physics, University of Thessalonki, Greece) and conclusions show that broadband instruments can be useful in expanding our knowledge of UV solar irradiance, especially since their relatively low initial and operating costs make them practical instruments for large geographically widespread monitoring networks.

 

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This page was last updated on Monday, September 11, 2006 .