A sunphotometer is a class of photometers designed for observing certain narrow spectral bands of sunlight. A typical instrument will produce a output signal proportional to the solar irradiance within the intended spectral band. The instruments typically used by ESRL/GML are intended to deduce spectral atmospheric transmission, or optical depth, from which the contributions of various atmospheric constituents can be calculated, most commonly aerosol, but water vapor and ozone are also potentially derived. The spectral discrimination of the sunphotometer is often accomplished with a narrowband interference glass filter which is the source of considerable uncertainty in the typical measurement.
There have been numerous commercial sources of sunphotometers but in most cases ESRL/GML has designed and built their own for various field programs. Calibration of the ESRL/GML sunphtometers is accomplished by the common Langley technique at our field site at Mauna Loa. The filters used in past instruments have proven to typically be too unstable for long-term measurements at remote sites. Therefore, ESRL/GML has subsequently utilized sunphotometers for short-term expeditionary programs or for ongoing observations at Mauna Loa where they are frequently calibrated. Techniques for obtaining, reducing and utilizing sunphotometer observations, as practiced by ESRL/GML are given in related papers.
The Precision Spectral Pyranometer is a World Meteorological Organization First Class Radiometer designed for the measurement of sun and sky radiation, totally or in defined broad wavelength bands. It comprises a circular multi-junction wire-wound Eppley thermopile which has the ability to withstand severe mechanical vibration and shock. Its receiver is coated with Parson's black lacquer (non wavelength selective absorption). This instrument is supplied with a pair of removable precision ground and polished hemispheres of Schott optical glass. Both hemispheres are made of clear WG295 glass which is uniformly transparent to energy between 0.285 to 2.8µm. For special applications, other Schott glasses and Infrasil II quartz hemispheres are available. Included is a spirit level, adjustable leveling screws and a desiccator which can be readily inspected. The instrument has a cast bronze body with a white enameled guard disk (shield) and comes with a transit/storage case. A calibration certificate traceable to the World Radiation Reference and a temperature compensation curve is included.
The Precision Infrared Radiometer, Pyrgeometer, is intended for unidirectional
operation in the measurement, separately, of incoming or outgoing terrestrial
radiation as distinct from net long-wave flux. The PIR comprises a circular
multi-junction wire-wound Eppley thermopile which has the ability to withstand
severe mechanical vibration and shock. Its receiver is coated with Parson's
black lacquer (non-wavelength selective absorption). Temperature compensation
of detector response is incorporated. Radiation emitted by the detector
in its corresponding orientation is automatically compensated, eliminating
that portion of the signal. A battery voltage, precisely controlled by a
thermistor which senses detector temperature continuously, is introduced
into the principle electrical circuit.
Isolation of long-wave radiation from solar short-wave radiation in daytime is accomplished by using a silicone dome. The inner surface of this hemisphere has a vacuum-deposited interference filter with a transmission range of approximately 3.5 to 50 µm.
The Eppley Normal Incidence Pyrheliometer is a World Meteorological Organization
First Class Pyrheliometer designed, as its name implies, for the measurement
of solar radiation at normal incidence.
The pyrheliometer is mounted on a power-driven equatorial mount for continuous readings. Please see Solar Trackers.
A calibration certificate traceable to the World Radiation Reference and a temperature compensation curve are included.
The self-calibrating Absolute Cavity Pyrheliometer, Model HF, has been a reference standard level device for many years. The sensor consists of a balanced cavity receiver pair attached to a circular wire-wound and plated thermopile. The blackened cavity receivers are fitted with heater windings which allow for absolute operation using the electrical substitution method, which relates radiant power to electrical power in SI units. The forward cavity views the direct beam through a precision aperture. The precision aperture area is nominally 50 mm2 and is measured for each unit. The rear receiver views an ambient temperature blackbody. The HF radiometer element with baffle tube and blackbody are fitted into an outer tube which acts as the enclosure of the instrument. The Model AHF has an automatic shutter attached to the outer tube.
Model HF Control The operation of the cavity radiometer, and the measurement of the required parameters is performed using an appropriate control box. The control functions include setting of the calibration heater power level, activation of the calibration heater, selection of the signals to be measured and control of the meter measurement functions and ranges. The measured parameters include the thermopile signal, the heater voltage and the heater current which is measured as the voltage drop across a 10 Ohms precision resistor. The instrument temperature may also be measured using an internally mounted thermistor. The meter resolution of 100 nV allows for a thermopile signal equivalent in radiation of approximately 0.1 Wm-2.
Control boxes for manual or manual/automatic are available. The control box can operate either one radiometer in the measurement mode or two radiometers in the comparison mode. Automatic operation allows for computer control of shuttering, calibration heating and measurement functions. Calculation operations and data storage are also possible under computer control. Programs for independent, automatic measurement and cavity radiometer comparison are supplied with automatic units.
Although these are absolute devices, the radiometers are compared with the EPLAB reference cavity radiometers which have participated in the International Pyrheliometric Comparison (IPC) and other intercomparisons and are directly traceable to the World Radiation Reference (WRR).
The Multifilter Rotating Shadowband Radiometer (MFR-7) is a field instrument that measures the global, direct, and diffuse components of solar irradiance at up to seven wavelengths. A microprocessor- controlled shadowband alternately shades and exposes the instrument diffuser, enabling the system to measure all three irradiance components with only one detector.
In addition to a broadband channel, this instrument has six narrowband channels.
|CHANNEL||WAVELENGTH (10 NM EFFECTIVE BANDWIDTH)|
|1||Broadband (silicon pyranometer)|
The narrowband wavelengths can be used in various atmospheric studies. For example, certain wavelengths are affected by aerosols, so by comparing the ratios of data from various channels, the presence of aerosols can be detected.
The 940 nm channel can be used to measure column water vapor; the 415 and 500 nm channels can be used to extract column ozone data.
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 .4· by a microprocessor controlled stepper motor. The motor housing is adjusted for the latitude of the instrument and is azimuthally aligned to the North or South pole, depending on the hemisphere.
A microprocessor in the Yankee Environmental Systems Data Aquisition System (YESDAS), a 32-channel datalogger, controls the instrument. At each measurement interval, the instrument computes the solar position using an approximation of the solar ephemeris. The first measurement is made with the band rotated to its nadir position (also called the home position) to obtain the global or total irradiance. The band is then rotated to make three more measurements. The first measurement (the diffuse horizontal irradiance) is made with the sun completely blocked; the other two are made with the band rotated 9· to either side of the sun. These side measurements permit the system to correct for the excess sky that is blocked by the shadowband during the sun-blocked measurement.
YESDAS subtracts the corrected diffuse component value from the global irradiance to obtain the direct horizontal component. It then divides the direct horizontal component by the cosine of the solar zenith angle (available from the ephemeris calculation) to compute the direct normal component. This entire sequence completes in less than 15 seconds on an MFR and can be programmed to occur up to 4 times per minute. (The sequence takes 20 seconds on a high-latitude instrument and can occur up to 3 times per minute.)
|Spectral response||The MFR has one broadband channel and the following additional wavelengths: 10 nm FWHM centered on 415, 500, 615, 673. 870, and 940 nm*|
|Cosine response||Better than 5% for 0 - 80 deg zenith angle; better than 1% with corrections|
|Temperature range||-30C to 50C (All photodiodes, interference filters, and sensitive electronic components are held inside a temperature controlled enclosure.)|
|Power requirement||110/220 VAC,50/60 Hz (50 watts maximum) or 12 VDC at 1.5 amp 50% duty cycle and 3 amps 50% duty cycle|
|Weight||11 lb. (4.55 kg)|
|Packaging||The instrument is designed for continuous outdoor (Above water) use: all fasteners are stainless steel and connectors are potted and weatherproof|
The UVB-1 Ultraviolet Pyranometer is a precision radiometer that measures biologically-effective solar ultraviolet-B (UV-B) radiation. The instrument uses colored glass filters and a UV-B phosphor to convert incoming UV-B radiation to green light, which is then measured by a calibrated solid state photodetector.
Measures global solar irradiance
The UVB-1 measures global solar UV-B irradiance or the power per unit area of UV-B radiation received by a horizontal surface from the entire hemisphere of the sky. Global radiation includes both light transmitted directly through the atmosphere and light scattered by atmospheric gases and particulate matter in the atmosphere. The UVB-1 measures both the direct and diffuse components of global radiation.
The spectral response of the instrument is similar to the erythemal and DNA damage spectra, making it ideal for climatological data gathering and ozone layer depletion impact studies.
Because ozone in the stratosphere strongly absorbs energy in the UV-B portion of the solar spectrum (280 to 320 nm), any changes in the total amount of ozone affect the levels of UV-B radiation reaching the ground.
The operating characteristics for the UVB-1 pyranometer are summarized below.
||280 to 320 nm
|Cosine response||±5% for 0 - 60 degree solar zenith angle
|Sensitivity||1.97 volt/(watt/m2) of total UV-B irradiance
|Output signals||0 to 4 VDC, low impedance (single-ended) output for each channel
|Operating temperature||Thermally regulated for operation over an ambient temperature range
of -40°C to +40°C; an internal YSI #44011 monitor thermistor (100
K ohms @ 25°C) is also provided.
|Response time||Approximately 0.1 second|
|Power requirement||-12 VDC @ 5 mA; +12 VDC load varies with ambient temperature: 120
mA at +20°C, 500 mA at -40°C; the maximum allowable supply voltage
range is 11 to 14 VDC
|Electrical connections||Amphenol #165-15 weatherproof connector, and mating connector, Amphenol
#165-14, prewired with 32 feet (10 meters) Belden cable; the opposite
end is terminated in pigtail leads for a terminal board or connector,
|Size||5.06", (12.9 cm) high; 5.75", (14.6 cm) diameter base|
|Weight||3 lb (1.3 kg)|
All optical components, the detector, and phosphor are thermally stabilized at +45°C.
The Eppley Laboratory manufactures two types of Solar Trackers to be used with the Normal Incidence Pyrheliometer and the Cavity Radiometer.
The Automatic Solar Tracker, Model SMT-3 is a 2 axis, azimuth/elevation device programmed to align direct beam instruments with the normal incidence of the sun from any position on the earths surface. Tracking is achieved using a computer program which calculates the solar position for the time and location and transmits pulses to the drives, which then operate the 2 stepping motors. The stepping motors move the elevation and azimuth axes to the correct position. After initial installation, the tracker will continue to track the sun and reset during darkness. Only periodic resetting of the system clock is required.
The Models ST-1 and ST-3, "green tracker" are electrically driven
using a clock-based motor which makes one revolution every 24 hours. The
ST-1 is designed for use with a single instrument while the ST-3 accommodates
up to three. The ST-3 also incorporates worm and gear fine adjustments for
declination and equation of time.
A modified version of the Brusag Tracker brochure.
The Eppley Ventilator is designed to be used with the Precision Spectral Pyranometer, Model PSP or the Precision Infrared Radiometer, Model PIR.
A "muffin" fan in the base continuously blows air over both the instrument case and the instrument dome, keeping the hemispheres cleaner from frost, snow, dew and moisture buildup. The fan provides approximately 30 C FM of ventilation and draws approximately 0.15 amps, or 11 watts, in the 115 volt configuration. The clear plastic upper housing allows the instrument, connector, and desiccator window to be easily viewed. A white enameled guard disk, leveling screws and hold down holes are provided. The 8 inch diameter, 5.75 inch high ventilator weighs 5.5 pounds.
Diffuse Solar Radiation can be determined by measuring the Direct Beam Radiation using a Normal Incidence Pyrheliometer on a Solar Tracker and the Total Solar Radiation using a Precision Spectral Pyranometer and computing the difference. However, the Diffuse measurement can be taken by shading the Direct Beam from the pyranometer measuring Total Radiation. The Eppley Laboratory manufactures two standard types of Shading Devices to be used to block the Direct Beam Radiation from a receiving sensor, the Shadow Band Stand and the Shade Disk Kit (to be used on the Automatic Solar Tracker).
The Shadow Band Stand, Model SBS, is constructed of anodized aluminum, weighs approximately 24 pounds and uses a 3" band of approximately 25" diameter to shade the pyranometer. The declination setting must be adjusted regularly.
Many National Authorities wished to shade the Precision Infrared Radiometer to reduce the heating effect of the silicon dome. Since they already owned the Automatic Solar Tracker, an adaption kit was designed to shade and ventilate the PIR while the tracker aligned NIP for direct measurement. This Shade Disk Kit, Model SDK also allows for the measuring of Diffuse and Direct Solar Radiation simultaneously. The SDK attaches onto the SMT-3 Tracker and includes a mounting plate with built-in ventilators that allows one or two instruments to be shaded by a shade disks extended over the radiometers on an arm. The standard 1:10 ratio is maintained by using a 6 cm diameter disk at a distance of 60 cm from the receivers of the instruments. This picture shows the SMT-3 Tracker equipped with the SDK and an HF Cavity Radiometer, a Normal Incidence Pyrheliometer, a Precision Spectral Pyranometer and a Precision Infrared Radiometer.
Note: All information and b/w photos of Eppley instrumentation have been taken directly from the Eppley home page. Information and photos for YES instrumentation adapted from the associated user manuals and/or the Yankee Environmental Systems homepage. Instrumentation from Biospherical Instruments is to be added to this page.