Selecting Solar Instruments
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.
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| 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.
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| 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.
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| 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 -
3µm). Pyrgeometers measure terrestrial infrared total-horizontal irradiance from 3µm to approximately
20µm. 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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