General DescriptionThe Model UVMFR-7 Ultraviolet Multi-Filter Rotating Shadowband Radiometer is
an instrument that measures diffuse and total global irradiance, and computes
direct irradiance at four or seven narrow-bandwidth wavelengths in the UV-B and
UV-A regions. It extends the YES shadowband instrument family into the UV-B,
providing the atmospheric scientist with a low-cost precision tool for measuring
narrowband UV-B total, direct, and diffuse spectral irradiance. Real-time
UVMFR-7 data is available at sites across the US by browsing http://uvb.nrel.colostate.edu
The UVMFR-7 provides spectral irradiance data with stability equal to that of
spectroradiometers but at a fraction of the cost. In addition, unlike most
global spectroradiometers that only provide total horizontal irradiance, the
UVMFR-7 makes diffuse, and direct normal measurements. Direct beam spectral
irradiance data provides optical depth information that can be used to track the
absolute calibration stability of the instrument.
Most UV-B field spectroradiometers for making narrowband irradiance
measurements demand constant attention by highly skilled personnel. The UVMFR-7
is designed to operate automatically and autonomously in remote locations with
only periodic manual cleaning of the fore optic. A unique optical design coupled
with the flexible and powerful YESDAS-2 data acquisition and control system,
gives the instrument high performance at a modest price.
Applications
Compared to scanning spectroradiometers the UVMFR is a cost-effective
solution for measuring UV-B spectral irradiance in a number of important
applications:
- Meteorological networks used
for prediction of UV-B hazards to the public
- Global climate change and ozone
studies
- UV-B and aerosol research
- Biological effects studies
Model UVMFR-7 Radiometer Detector Assembly
The UVMFR-7 consists of two basic components: a Detector Assembly and an
Electronics Enclosure. The Detector Assembly consists of the UVMFR-7 sensor head
and the stepper-motor-driven rotating shadowband, which are mounted on a common
base. An electronics enclosure, mounted below the Detector Assembly, contains a
YESDAS-2 microprocessor-driven data acquisition and control system with 13-bit
A/D conversion accuracy and the capability of collecting data from an additional
24 analog and 6 pulse/counting-type met sensors.
Features
- All radiation components are
measured with the same photodetector
- Thermally controlled sensor head
eliminates ambient temperature-induced errors, and prevents dew/snow/ice buildup
on aperture
- Data acquisition system has
inputs for up to 24 other analog and 6 pulse-type met sensors
- Durable, state-of-the-art design
Principle of Operation
The UVMFR-7 uses an automated rotating shadowband to make measurements of the
global and diffuse components of solar irradiance. Once these two components are
known, a CPU can readily compute the direct-normal component.
The geometry of the rotating shadowband instrument is shown in the figure
below. The shadowband is a strip of metal formed into a circular arc and mounted
along a celestial meridian with the instrument's entrance aperture at the center
of the arc. The shadowband blocks a strip of sky with a 3.3°
umbral angle, sufficient to block the sun. It can be positioned with an accuracy
of 0.4° by the microprocessor-controlled stepper
motor. The motor housing is adjusted for the latitude of the instrument. When
the instrument is installed at the field site, it must be azimuthally aligned to
the Earth's pole (North or South, depending on the hemisphere). Once aligned, no
further mechanical adjustment is necessary and the instrument may operate for
extended periods without any operator intervention, unlike trackers that require
nearly constant attention.
UVMFR-7 Detector Assembly Side View
The operation of the instrument is controlled by its microprocessor. At each
measurement interval, the microprocessor computes the solar position using an
approximation for the solar ephemeris. The first measurement is made with the
band rotated to its nadir position (global or total horizontal irradiance). The
band is then rotated to make three more measurements. One measurement is made
with the sun completely blocked (diffuse horizontal irradiance) and the other
two are made with the band rotated to 9° on either
side of the sun. The side measurements permit a correction for the "excess
sky" that is blocked by the shadowband when the sun-blocking measurement is
made. The microprocessor then subtracts the corrected diffuse component value
from the global irradiance to obtain the direct-horizontal component. Finally,
division of the direct- horizontal component value by the cosine of the solar
zenith angle (available from the ephemeris calculation) results in the value of
the direct-normal component. The entire sequence is completed in less than 20
seconds and can be programmed to occur up to 3 times per minute.
UVMFR-7 Diffuser Shading View
The use of the computed solar ephemeris in the UVMFR-7 instrument gives it a
significant advantage over instruments that use a continuously moving shadowband
to make the global and diffuse measurements.The UVMFR-7 method
permits much longer integration time for each measurement because it requires
measurements at only four shadowband positions rather than making many
measurements during a continuous scan across the sky. Longer integration time
substantially improves measurement precision and permits operation at
wavelengths and passbands that would not be possible with a continuously moving
shadowband. The excess sky blockage correction significantly improves
measurement accuracy, particularly under skies with fractional cloud cover.
The filter wavelengths were carefully chosen by the atmospheric research
community to maximize the utility of the data. For example, the 311.5 nm filter
was chosen to land on a small but somewhat "flat" area of the
spectrum, and the 332.4 nm was selected as a Dobson reference. Each
filter-detector channel is individually characterized, yielding spectral
irradiance in W/m2-nm.
The UVMFR-7's unique ability to make simultaneous spectral measurements of
the three solar irradiance components makes it an extremely versatile
instrument. For example, in order to obtain the data collected at least two
instruments would have to be used: a global filter radiometer, and either a
tracker-mounted normal incidence sun photometer or a second, shaded global
radiometer. When making spectral measurements, the UVMFR-7 has the advantage of sensing
the global and diffuse irradiance components with the same detector.This
eliminates the error introduced into the measurement of solar irradiance
components by the use of different detectors to measure the different
components, reducing concerns about the intercalibration of multiple sensors.
The UVMFR-7 is simpler, less expensive and more robust. Finally, the UVMFR-7
instrument ensures that the measurements of the irradiance components are
synchronous in time.
Changing ambient temperatures can cause unwanted response changes in
semiconductor detectors, resulting in measurement errors. To avoid these errors
the instrument utilizes a computer-controlled thermal regulation circuit that
maintains the detectors and filters above ambient temperature, thereby
preventing thermal transients from affecting the solid state detectors. A side
benefit is that the additional thermal energy helps to keep the sensor free of
dew, ice, and snow. A separate thermistor permits the user to monitor the
temperature. The solid state photodiodes, interference filters, and sensitive
electronic components are all held inside a desiccated enclosure, further
eliminating environmentally-induced measurement errors.
UVMFR-7 System Electronics
Control and Data Logging System
The UVMFR-7 instrument operation and data logging are controlled by a
microprocessor. This on-board CPU, 1) performs the required ephemeris
calculations, 2) controls the stepper motor which positions the shadowband, 3)
controls the acquisition, processing and storage of sensor data for the MFR-7
and up to 24 additional analog meteorological sensors,and 4) permits
simultaneous data telemetry.The ultra-stable system time-keeping (with
an accuracy of 1 second per month) ensures that the positioning of the
shadowband will be precise over extended time periods, with no need for operator
intervention or adjustment. The system data logger includes a state-of-the-art
13 bit self-calibrating analog-to-digital converter and on-board data storage
capability of up to 2 Mbytes via the PCMCIA-2 memory option (most users want
more memory to permit higher time-resolution sampling.) You can communicate
directly through the 3m serial cable provided, or over a telephone lines using a
user-supplied modem. Using YESDAS Manager software, data are automatically
downloaded, corrected, calibrated and presented to the web. Modem communication
allows a remote instrument to be controlled from a laboratory or office without
interrupting its data acquisition, making it ideal for large, geographically
widespread networks.
Calibration
Each UVMFR-7 instrument is individually tested and characterized before
shipment. First, the response to a direct beam at different angles is measured
in our cosine facility, where an instrument-specific cosine correction file is
generated. Next, each head is spectrally scanned with a narrowband spectrometer
to determine the precise full width at half-maximum (FWHM) bandwidth and center
wavelength. Finally, each head is calibrated against a NIST-traceable FEL lamp
to determine its absolute response.
Calibration Facilities
The usefulness of any instrument depends critically on the quality and
long-term stability of its calibration. YES has fully-equipped optical
laboratories to completely characterize the performance and calibrate each
instrument. The cosine, spectral and absolute responses of each wavelength
channel of the instrument are measured with NIST-traceable optical and
electronic equipment and test results are supplied with each system. A
comprehensive document is available on our web site (Calibration Services.)
Typical Filter Passbands of an MFR-7
Spectral Response
The interference filters and photodiodes used in the UVMFR-7 are of the
highest quality and result in an instrument with exceptional ruggedness, long
term stability of calibration, and excellent unit-to-unit repeatability of
response. The spectral response of each wavelength channel of the UVMFR-7
instrument is measured using an Acton Research AM-511 one meter monochrometer.
Absolute Response
The unique spectral irradiance voltage transfer function calibration data
provided by the FEL lamp is convolved (in conjunction with each channel's
individually measured relative spectral response) to calculate each channel′s spectral
irradiance in W/m2-nm units, as well as identify effective
bandwidth and effective center wavelength. By doing this, unlike the integrated
dose output from broadband UV-B instruments, the UVMFR-7′s spectral irradiance
output data is readily comparable to spectroradiometer data.
The ability to measure direct and diffuse spectral irradiance allows the user
to conduct automated Langley analysis with the software tools provided. As in
the visible wavelength MFR instrument, the automated Langley analysis results
can be used to track the UVMFR-7′s absolute calibration, since the
direct-to-diffuse ratio is independent of the UVMFR′s absolute calibration.
As filter solarization gradually occurs under normal exposure, these shifts can
be monitored and accounted for. Using the same method, the calibration of
other more sophisticated UV-B instruments (such as spectroradiometers) can be
checked against the UVMFR-7's direct-to-diffuse ratios.
Cosine Response
The UVMFR-7 belongs to a class of instruments that measure flux incident on a
horizontal surface from a moving light source. Ideally, one would like these
instruments to be uniformly sensitive to incident radiation coming from any
direction. The response of such instruments to radiation incident at an angle, θ
, with respect to the surface normal is called the cosine response. The ideal
cosine response is proportional to the cosine of the angle θ
, and any deviation from this response introduces measurement errors.
The UVMFR-7 uses a novel diffuser input optic that provides superb cosine
response and long-term stability. The radiation receiver element is a
computer-designed, specially shaped Teflon diffuser
disk that is directly coupled to an optical integrating cavity. Teflon
is a halocarbon with excellent resistance to chemical and ultraviolet
degradation, thus ensuring calibration stability in the field. In addition,
instruments with diffusing input optics are inherently less sensitive to surface
soiling than are instruments with transmission windows such as domes.
Ratio of typical instrument's angular response to the
ideal cosine response
The cosine response of each UVMFR-7 is fully characterized in our angular
test facility. Each instrument is placed on a computer- controlled rotary
actuator and the instrument's relative response as a function of its angle
measured using a feedback-stabilized, parallel, uniform light beam. The
individually characterized cosine response, supplied with each instrument, is
used by system software to correct, in real time, for deviations from the ideal
cosine response.
Using the Data
A primary use of UV-B irradiance data is for tracking long-term changes in
column ozone. The calculated direct beam data is stored and later used for
optical depth determination. Optical depth is a dimensionless
quantity closely related to the extinction coefficient used in visibility
studies. An automatic angular correction procedure corrects raw data for cosine
errors based on the factory-measured, instrument-specific angular
response. Next, factory-determined spectral irradiance calibration constants are
applied to the data to derive engineering units. The software also performs
automated Langley analysis on the data for retrieval of optical depth
information.
Once calibration constants are applied to the data via the host software,
spectral irradiance data can be used for studies such as determining scattering
due to aerosols. Aerosols have recently been shown to have a great effect on
UV-B radiation at the earth′s surface, possibly impacting total irradiance as
much as cloud conditions do.
The direct-normal UVMFR-7 data can be used to measure both ozone and
aerosols. In addition, by using the automated Langley analysis software
(provided with the system) a self-check on the absolute calibration can be
performed based on the stability of the extraterrestrial solar constant and the
known optical depth.
1. J. Michalsky and J. Berndt. "Automated Multifilter
Rotating Shadow-band Radiometer: an Instrument for Optical Depth and Radiation
Measurements." Applied Optics, Vol.33, No.22. pp 5118-5125.
2. J. Michalsky, "Objective Algorithms for the Retrieval
of Optical Depths from Ground-Based Measurements." Applied Optics,
Vol.33, No.22. pp 5126-5132.
Data Analysis and Software
UVMFR-7 instruments are controlled by an included YESDAS-2 data acquisition
and control system that controls the shadowband and thermal subsystems, stores
and telemeters the data. Data are angle-corrected, calibrated and analyzed via
the YESDAS Manager software running on a user-supplied Windows 9x/NT PC. This
system is a reliable and well tested remote data acquisition and display
platform that even provides web access to the data. Calibration information is
applied to the data in a foolproof and flexible architecture yet permits users
to reconstruct data streams when instruments are recalibrated. Multiple systems
can be automatically polled and the data provided on the web letting users
efficiently manage networks with many field sites.
Installation Requirements
The UVMFR-7 instrument is easy to install; it needs a user-provided level
mounting platform approximately 17" in diameter. Set the latitude
adjustment on the shadowband motor assembly (this is usually done at the
factory). Place the Detector Assembly on a flat stable platform. Orient the
instrument with the shadowband support post toward geographic north in
the Northern hemisphere (south in the southern). Step-by-step instructions for
this procedure are in the instrument manual. You need to perform this
orientation procedure only once, at the time of installation; no further
orientation adjustments are necessary.
The UVMFR-7 detector assembly is fabricated from anodized aluminum and
stainless steel; all hardware is stainless steel and all connectors are potted
and weatherproof. The system electronics (including the DC power supply, data
acquisition board and ancillary electronics is housed in a NEMA-4X enclosure.
The DC power supply is high quality a medical-grade, linear type, with a dual
bobbin transformer for maximum power line transient signal and noise rejection.
A 3m null modem cable is provided for direct connection with a local PC or
laptop. A telephone line connection terminal with spark gap lightning arrestor
is provided for connecting a user-supplied Hayes-compatible modem to the system.
Screw terminal connections are provided for the 16 auxiliary sensor connections.
Use the three leveling screws and the precision bubble level to position the
instrument with the receiver surface in the horizontal plane. Once you have
completed this step, secure the instrument to your mounting platform via a #10
screw through-hole located in the center of the base. Connect the two cables
from the system electronics enclosure to the mating receptacles on the Detector
Assembly. Connect the DC standby battery and apply AC power. Depending on
whether the system will be local or remote, either connect the serial cable to
the PC running YESDAS Manager or connect a modem to the system serial port. The
instrument is now installed and ready to take data.
You will need to provide a deep cycle marine/RV 12 Vdc standby battery, as
well as 115/230 Vac 50/60 Hz power to the instrument. A 2m power cord is
provided (please specify 115 or 230 Vac when ordering the instrument.) The
entire system can also optionally be configured to operate from a 12 Vdc solar
panel/battery system. You must also furnish a ground rod to ensure a static
discharge path independent from the path through the power grounding system to
help protect against lightning.
Required Maintenance
The UVMFR-7 is designed for long-term continuous field use. The entire
housing is O-ring sealed and polyester-powder-coated for long life. A
user-removable desiccant is provided to ensure that any water entering the
housing is absorbed. The only required maintenance is periodic cleaning of the
optical
diffuser to remove aerosols and other particulates. The action of the
shadowband coupled with the internal heater help to reduce bird landings and
snow and ice build up. UVMFR-7 heads can be swapped without shipping the entire
system back to the factory. Alternatively, the instrument can be calibrated
against a co-located research grade UV-B spectroradiometer such as a YES Model
UVRSS-1024. Calibration should be performed at least every 12 months.
About Optical Interference Filters
Historically, optical thin film interference filter-based instruments have
gained a reputation for relatively poor long-term stability, especially in the
UV-B region. Some manufacturers have responded by using wideband channels (e.g.
280-305 nm bandpass) that produce data that cannot be separated into the
constituent wavelengths. The UVMFR-7 reverses this trend by offering narrow band
channels deep into the UV-B region at 317, 311, 305 and 300 nm. Nevertheless,
rigid quality control standards are necessary during the filter fabrication
steps as well as strict handling precautions during the final assembly process.
For long life, environmental thermal stabilization is necessary. Earlier
filter-photometer instruments often did not provide adequate thermal
stabilization or humidity protection for their filters.
Recently, advances in interference filter technology have resulted in
next-generation coatings that make them much more mechanically durable and
resistant to changes in throughput due to solarization. The UVMFR-7 uses these
s-called Ion-Assisted Deposition filters, as well as a diffuser fore
optic to reduce the flux level the filters are exposed to by an order of
magnitude. This diffuser design, similar to that used in the MFR instrument has
demonstrated greatly improved calibration stability over earlier instruments.
Also Available
The UVMFR-7 is part of the MFR family of solar radiation instrumentation that
includes the Model SDR-1, MFR-7 and RSS-1024. The UVMFR-4 is a four channel
version of the UVMFR-7 that contains only the UV-B channels at slightly reduced
cost. For applications requiring visible spectral irradiance, the Model MFR-7
instrument has a broadband channel plus six 10 nm wide (FWHM) visible channels
extending from 415 to 940 nm. The broadband channel measures the global,
diffuse, and direct-normal components of total solar radiation making it
equivalent to a total solar pyranometer and a solar tracker-mounted normal
incidence pyrheliometer. A full suite of UVMFR-7, MFR-7 and UVB-1 instruments
forms the backbone of several national monitoring networks.
Typical remote field installation, (shown here with UVB-1 and MFR-7
radiometers)
The UVMFR-7's data acquisition and control system can also be purchased
separately. A YESDAS-2 system collects and store data from up to 32
meteorological sensors while giving the user total flexibility in controlling
the data acquisition. For more information, refer to the YESDAS-2 and YESDAS
Manager data sheets.
YEDSAS-2 (shown with cover cut away)
Instrument Development and History
The visible Multi-Filter Rotating Shadowband Radiometer was initially
developed for the U.S. Department of Energy (DOE) Atmospheric Radiation
Measurement (ARM) program by researchers at the State University of New York at
Albany and the Battelle Memorial Institute at DOE's PNNL. It was designed as a
rugged field instrument to perform the required spectral measurements of solar
irradiance components and to serve as the primary data logging station for a
suite of associated meteorological sensors. In 1993, YES was granted an
exclusive worldwide license by the State University of New York at Albany to
manufacture the UVMFR-7 instrument.
Mechanical Interface
UVMFR Detector Assembly
System Electronics
Specifications
Spectral Response |
UVMFR-4: 300, 305.5, 311.4 and 317.6 nm, (2nm FWHM)
UVMFR-7: 300, 305.5, 311.4, 317.6, 325.4, 332.4, 368
nm, (2nm FWHM). |
Radiometric Accuracy |
2-3%, with angle corrections applied. |
Electrical Interface |
YESDAS-2 system interfaces via RS-232 DB-9 Male (DTE)
or modem. Telephone input is lightning protected. Cable lengths are 2m
Detector Assembly, 2m AC power |
Power Requirements |
Can be operated from 110/220 VAC 50/60 Hz or 12
VDC @ 3A. Customer-supplied 12 VDC deep-cycle battery is required, and a
ground rod |
Weight |
YESDAS: 10 kg; UVMFR-7 Asy: 5 kg |
Mechanical |
Center bolt hold-down, compatible with all MFR mounts.
A level and stable 40 cm x 40 cm platform is required for mounting. |
Note: Included MFR-to-YESDAS system cables are
fixed length and cannot be lengthened.
1. J. Michalsky and J. Berndt. "Automated Multifilter
Rotating Shadow-band Radiometer: an Instrument for Optical Depth and Radiation
Measurements." Applied Optics, Vol.33, No.22. pp 5118-5125.
2. J. Michalsky, "Objective Algorithms for the Retrieval of Optical
Depths from Ground-Based Measurements." Applied Optics, Vol.33, No.22.
pp 5126-5132.
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