GENEQ > Meteorology > Spectroradiometers
Rotating Shadowband Spectroradiometer
|The Rotating Shadowband Spectroradiometer (RSS)
combines a high-performance 1024-pixel Charge Coupled Device spectrograph
with an external rotating shadowband. It provides automatic direct, diffuse,
and total spectral irradiance measurements at high resolution.
|Two variants of the RSS are available:|
The unique optical design and state-of-the-art
astronomical-grade slow scan CCD array results in a very stable instrument,
with no moving parts to control wavelength accuracy or throughput. This provides
both high speed performance and long term stability superior to other spectroradiometers.
The simple design approach helps the RSS maintain its calibration while in long-term
operation in the field. Because the RSS samples all wavelengths simultaneously,
spectra taken under varying sky conditions are an accurate time-average over
the sampling period at all wavelengths.
|Direct-normal solar irradiances allow calculations such as:|
|PRINCIPLE OF OPERATION|
Light enters the RSS through an optimized cosine response-optimized diffuser fore optic. An external shadowband alternately shades and exposes the diffuser, making sub one second speed direct-normal, diffuse-horizontal, and total-horizontal spectral irradiance measurements. Smapling rates can be as fast as once every 30 seconds. The technique provides direct-diffuse and direct-total ratios that are fully independent of calibration.
Once inside the diffuser, light passes through an entrance slit, through an entrance lens into dual prisms which refract the light. Finally, a camera lens focuses the spectrum onto a Peltier-cooled, 1024x256 pixel element astronomy-grade slow scan CCD. Careful attention to anti reflection coatings and a large volume of space around the spectrograph itself helps give the system state-of-the- art out of bandwidth performance.
The CCD analog subsystem employs dual slopeintegration and column binning. CCD signals are acquired by a precision 16-bit analog-to-digital processing subsystem controlled by an embedded CPU. A second CPU manages the shadowband, and a third 32 bit CPU manages data calibration, database storage and the web server.
A precision multi-channel analog proportional-differential-derivative thermal management subsystem precisely maintains the temperature of five different thermal zones within the spectrograph. Its job is to keep the temperature of internal optics held stable and above ambient temperature fluctuations. Any environmentally-induced temperature fluctuations would serve to "tune" the wavelength response. In addition, this subsystem also controls the shutter and maintains the CCDs thermoelectric coolers at about 15°C on the RSS and near 0° on the UVRSS. For data presentation, the Data Visualization Engine and YESDAQ database permit data access via TCP/IP to any web browser.
Side cut-away view of UVRSS
|PRISM VS GRATING INSTRUMENTS|
|Traditionally, ruled optical diffraction gratings
have been the wavelength selection mechanism of choice for narrowband spectral
irradiance measurements at
sub-nanometer slit widths. In the RSS, prisms were chosen as the primary dispersion mechanism for the following important design reasons:
|The RSS was designed for long term scientific climatic research of the earths atmosphere, which requires excellent long-term calibration stability in order to produce useful data. The locked-down internal optics are the key to the system's excellent long term stability. Extreme care was given to all aspects of the optical design to reduce the chance of mechanical dimensional changes that could adversely affect the instruments spectral, angular, or absolute response. An extensive field-test program at the US Department of Energy's ARM site is ongoing to track and verify this stability.|
|The initial concept for the RSS instrument was driven by a team of scientists at the Atmospheric Sciences Research Center at the State University of New York/Albany. YES worked closely with this team during the R&D of the commercial version. Because the RSS instrument is important to climate change research and is used by the US Department of Energy, it funded a portion of its development.|
|COMMUNICATION LINK METHODS|
|Data can be viewed on any browser-equipped workstation for real-time display and analysis. For communications, the built in 10/100 BaseT Ethernet connections is preferred. For remote sites, a dial in PPP (V.90) modem is provided. for off-grid remote measurements. Both 10/100BaseT Ethernet and V.90 modem for PPP are provided. As with the Model TSI-880, acquired data scans are initially stored internally via YESDAQ. The Data Visualization Engine package is included with the system and permits viewing of YESDAQ data, permitting users to rapidly browse data using a web browser. Also, database connections via ODBC, or JDBC to 3rd party applications (e.g. MS-Excel, Matlab or Splus) are supported. The DVE package provides wavelength and absolute-calibrated spectral scans and produces fully calibrated spectral irradiance data.|
|ARCHITECTURAL DIFFERENCES BETWEEN THE UVRSS AND VISIBLE RSS SYSTEMS|
|Both the visible RSS and UVRSS versions of the instrument use identical electronic systems and firmware; however, the geometry of the optical elements and detector are carefully optimized for each domain.|
|The UVRSS instrument is configured with a visible
pre-filter installed; outdoor applications require this filter to block
the strong visible solar signal, however
some laboratory applications may not; the system can be supplied without it as an option.
|Optimal Slit Size|
A precision laser machined slit is supplied, and is externally
located to permit exchanges by the user. The supplied slit is optimized
for outdoor atmospheric work, and may be exchanged if more or less signal
is available. The general engineering tradeoff is spectral resolution
vs. light gathering (for SNR). However, because the aberration limit for
the UVRSS is approximately 0.2 nm FWHM, attempts to drop below approximately
0.3 nm FWHM by decreasing the slit size only results in a loss of light
and subsequent loss of system SNR. As the slit size is made larger to
gain throughput the expense is a
Optical components within the system operate in a dry nitrogen purged environment to eliminate potential condensation on the surface of the cooled internal CCD that would blur the refracted light and ultimately contaminate it. Internal humidity and pressure are continuously monitored to ensure the integrity of the seal over its lifetime.
|This section discusses using the instrument
as a spectrograph/CCD detector for laboratory radiance and irradiance measurements.
As every optical measurement setup tends to be application-specific, the
following discussion is intended to cover some of the design tradeoffs involved
in attaining adequate
SNR and sampling performance.
|Spectroradiometric Method||Dual Prism w/1024 Si CCD detector, cooled to 5°C||Dual Prism w/1024 Si CCD detector, cooled to 5°C|
|Spectral Accuracy||0.1 pixel (±0.2nm)||0.1 pixel (±0.06nm)|
|Spectral Repeatability||Too small to be measured, all temps||0.005 nm RMS|
|Sampling Interval||0.75 nm||0.07 nm (0.3nm effective slit width)|
|Spectral Range||360-1100nm continuous||288-365 nm continuous*|
|Absolute calibration over entire range?||5% (over full temperature range)||5% (over full temperature range)|
|Scanning time||1 second||1 second|
|Internal moving parts that control the wavelength selection||No||No|
|Has shadowband for automated direct/diffuse measurements?||Yes||Yes**|
|Can be calibrated with standard FEL lamps?||Yes||Yes|
|Can be calibrated on its side for base-down FEL?||Yes||Yes|
|Thermally stabilized?||Yes, @ 50°C||Yes, @ 50°C|
|Temperature Range||-50°C to +50°C||-50°C to +50°C|
|Waterproof?||Yes, NEMA-4X||Yes, NEMA-4X|
|Software support||MS-Windows 95/NT, Mac, & Unix||MS-Windows 95/NT, Mac, & Unix|
|RSS-1024: Other Specifications||UVRSS-1024: Other Specifications|
|Effective Slit Bandwidth||0.6 nm @ 360 nm up to 4nm @1100nm, 2.25 pixels FWHM||Dispersion Optic||Transmission, fused-silica prism spectrograph|
|ADC @ CCD detector (bits)||16||Spectral Resolution||0.3 nm FWHM @300nm; optional 0.6 nm slit width available but not recommended for most applications.|
|Raw Angular Response||<8% over +/- 70°||Wavelength Calibration||Hg lamp|
|Dark Count subtract?||Yes||Dynamic range||65,000 counts|
|Visible Resolution (pix)||1024||Fore-optic raw angular response error ration from ideal cos(z)||<5% over +/- 70°, correctable to 0.2%|
|Internal 2nd order sorting filter required?||No|
|* The upper wavelength limit is not hard, but instead a consequence of intentionally declining responsivity to longer wavelengths achieved by band-blocking elements and dynamic range compression done in the fore-optic, all to minimize stray light contribution at the shortest wavelengths. The UVRSS lower limit is set to capture the 289 nm Hg emission line for wavelength calibrations.|
|** The UVRSS makes measurements of the direct-normal, diffuse-horizontal, and total-horizontal spectral irradiances routinely and synchronously via the automated shadowband method similar to the RSS. This is possible only because of the simultaneous acquisition of the spectrum at all wavelengths. These irradiance components automatically share the same calibration coefficients.|