Using Spectroscopy for Solar Irradiance and UV Measurements

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The Sun is the main source of energy on Earth. The sun’s radiation hits the Earth’s atmosphere which is absorbed by gases and air particles, while the remaining energy reaches the surface of the Earth. Earth absorbs most of this radiation, but it also reflects and releases radiation.

Even slight deviations to the equilibrium of this energy system are thought to be capable of causing extreme changes to Earth’s climate.

Although solar irradiance data is an important component of environmental and climate research, solar radiation can impact many other areas of life on Earth, from health and agriculture to manufacturing and technology. Scientists and researchers from industry and academia require access to consistent, repeatable solar characterization data. Avantes, the world leader in the manufacture and design of optical spectroscopy equipment, can be relied upon for solar spectra collection and solar characterization systems.

Defining Solar Irradiance

Total solar irradiance is defined as the amount of solar energy, over all wavelengths, per meter squared at the point of incidence upon the Earth’s outer atmosphere. This total solar irradiance is about 1,360.8 W/m2; however, the sun’s energy is not a homogenous constant. In fact, the sun’s energy varies slightly as it produces radiation across the whole electromagnetic spectrum. This spectrum includes visible light, Infrared, and Ultraviolet radiation.

A solar spectra measurement varies from the total solar irradiance in that it provides an intensity measurement for each wavelength. The characterization of the total spectrum of sunlight offers crucial data points for researchers. As the sun’s energy passes via the layers of Earth’s atmosphere, a part of that energy is scattered, reflected, or absorbed by water vapor, gases, or airborne particulates. This scattering is what causes the sky to have blue appearance. The shorter wavelengths, like those in the blue range, are dispersed more than longer wavelengths.

It is a fact that all the elements in the periodic table absorb light at a particular wavelength. Each element has a unique spectral signature like a fingerprint enabling researchers to identify materials with spectral analysis. Measuring the spectral irradiance of the sun’s light reveals the chemical characteristic of Earth’s atmosphere and helps predict how climate will react to variations in Earth’s energy system.

Measurement Parameters

There are many factors that influence the characteristics and behavior of light from the sun as seen on Earth. The sun’s energy does differ, and solar storms and other phenomena can impact solar output. Water vapor, air pollution, and cloud coverage can also influence the quality of light reaching the Earth as a result of absorption and scattering of light.

Furthermore, solar position is a key factor in solar irradiance measurements. Parameters such as time of day, the observer’s geographic location, and season influence solar measurements.

The Sun’s radiation is strongest close to the equator during summer when the Earth’s tilt brings the observer’s hemisphere nearest to the sun, and at noon when the sun is nearest to its zenith. At this point, the energy from the sun has the smallest distance to travel through atmosphere. At sea level, this is denoted as 1 atmosphere (1 atm). When measurements are collected at other latitudes, or other times of day, the angle of the sun will influence solar irradiance.

The quantity of atmosphere that energy must travel through before reaching the point of observation is known as the air mass index. This is denoted in ratio to the constant 1 atm so that for most latitudes of North American the Air Mass index is considered to be about 1.5 atm.

Capturing Solar Spectra

The Colorado state in the U.S. Rocky Mountains has a healthy solar research community because the mile-high altitude is a benefit for solar measurement applications. Avantes engineers at the U.S. offices near Boulder, Colorado made the most of the location at 1,626 m above sea level to complete some solar irradiance tests recently. The higher altitude offers the Boulder, Colorado region a noon-time air mass index slightly lower than the most of the United States. Colorado also averages about 300 days of sunshine annually.

Over many days, Avantes engineers collected broadband spectra of universal horizontal irradiance using the Avantes cosine corrector (CC-UV/VIS) to compensate for solar zenith angle and any other characteristic sampling geometry. The cosine corrector was coupled to the Avantes AvaSpec-ULS2048x16-USB2 spectrometer using a solarization resistant FC/PC 400?m core UV fiber.

The Avantes AvaSpec-ULS2048x16 was calibrated for irradiance with a NIST traceable source over the range from 300 to 1100 nm. Following such calibration, the spectrometer was used to gather solar spectra using a laptop running AvaSoft 8.6.1.0 in Absolute Irradiance mode.

Avantes engineers installed the monitoring station in the same geographic location for each series of solar irradiance spectra measurements. In the first series, measurements were taken at approximately the same time of day over the course of a number of days. Extra atmospheric data was gathered for reference.

Avantes Offices Location

  • Longitude&Latitude 39°57'46.1"N 105°07'24.0"W
  • Elevation: 5,335’ a.s.l. (1,626 m)

Solar Spectra Data

Date Time Spectral Integer Humidity % Visibility mi Pressure in Zenith angle° Air mass index GHI record
3/29/17 12:08 49,475.88 66 25 30.09 35.69 1.23 1050.8
3/30/17 12:15 37,307.73 30 50 29.76 35.38 1.23 1051.2
3/31/17 8:35 1.151.90 93 0.1 29.78 58.37 1.9 927.6
4/3/17 12:20 43,968.43 19 50 29.78 34.19 1.21 1052.1
4/4/17 12:01 42,496.18 81 7 29.97 33.60 1.2 1053.2
4/6/17 12:21 46,645.97 29 50 30.19 33.12 1.19 1053.2
4/7/17 12:02 37,169.57 23 50 29.96 32.47 1.18 1054.3
4/10/17 12:15 45,702.08 24 50 30.18 31.52 1.17 1054.9
4/11/17 12:16 45,542.88 17 35 30.16 31.19 1.17 1055.1

In the second set of measurements, spectra data was gathered every hour during the course of the workday. The same geographic location was used during the whole sampling process. For reference, hourly atmospheric data was also gathered.

Time Date Temp °F Dew point °F Humidity % Pressure in Zenith angle°
7:51 AM 4/11/17 39.2 17.6 49% 30.2 15.15
8:50 AM 4/11/17 44.6 21.2 34% 30.21 26.36
9:50 AM 4/11/17 50 19.4 30% 30.21 37.32
10:50 AM 4/11/17 55.4 19.4 25% 30.2 47.22
11:49 AM 4/11/17 59 14 17% 30.16 54.79
12:50 PM 4/11/17 62.6 10.4 13% 30.18 58.55
1:47 PM 4/11/17 64.4 10.4 12% 30.14 56.98
2:50 PM 4/11/17 66.2 12.2 12% 30.1 50.30

Solar Irradiance Applications-RADIANCE Team

The CU Boulder RADIANCE senior design team in Aerospace Engineering is working to develop affordable, mass-producible solar irradiance cube satellites to make it easier for researchers to gather solar data and help close gaps in the universal irradiance records. These skilled, young engineers are preparing the cube satellite for a circumpolar test flight aboard the Arctic Hi-Wind Gondola with the AvaSpec-MINI onboard.

The RADIANCE team has been excited with the performance of the AvaSpec-Mini. The high-powered compact spectrometer is surpassing the team’s expectations as they consistently obtain full spectra measurement ranging from 200 to 1100 nm at the rate of 1 spectra per minute. To match the requirements of CubeSat design, the system was designed to fit in a package that is approximately the size of lunch box. The AvaSpec-Mini, itself, weighs just 174 g and is about the size of a deck of cards.

Solar Irradiance- Tartu Observatory, Estonia

For over two decades, Avantes has been a trusted source for solar measurements. The Tartu Observatory, in Tartu, Estonia, contributes to the European Database for UV Climatology and Evaluation (EDUCE). This administrative body keeps a database of solar spectra data and fixes standards for data quality control. In 2005 Tartu Observatory set up an Avantes system to provide a nonstop backup redundancy UV monitoring system supporting the present pyrheliometer and pyranometer systems to meet data handling standards for EDUCE.

Solar Simulators

A solar simulator is a light source that mimics natural sunlight in controllable laboratory testing conditions. Solar simulators are important in the certification and testing of photovoltaic cells and modules.

The International Electrotechnical Commission and other organizations fix standards for solar simulator performance based on three parameters: spatial non-uniformity, spectral match, and temporal instability. Each parameter receives a distinct letter grade of A, B, or C. The quality, and thus price, of a solar simulator relies on the class rating of the light source. The difference of a few points in performance can mean a variance of hundreds of thousands, even millions of dollars in revenue.

These robust light sources are used in a number of industries to test solar responsiveness of a number of materials. They can be used to measure material degradation, to describe all manner of photoresponsive devices, and to examine and rate photovoltaic cells for use in solar panels. The industries that employ solar simulators are at danger of substantial losses if solar simulator systems are not ideal, and yet these systems, whether continuous or pulsed light, have a limited bulb life which may merit frequent calibration.

This in turns means that the characterization and systematic maintenance of solar simulator light sources is a mission critical technology for a number of organizations. For solar simulator manufacturers and users, Avantes provides the AvaSpec-SolarXM spectroradiometer system for easy solar simulator characterization. This ASTM 927-05 compliant calibrated system includes the AvaSpec-ULS2048x16-USB2 spectrometer, a 90° cosine corrector, and fiber optic cables along with a dedicated software application for NIST traceable calibrated measurements.

The AvaSpec-SolarXM system is ideal for steady state and pulsed solar systems, and quantifies spectral match according to the A, B, or C classification through six spectral bands.

Avantes, Your Trusted Solar Measurement Source

For more than two decades, Avantes has been on the cutting edge of light measuring technology. The company’s team has helped countless number of customers find the right solutions to their spectroscopy requirements. Avantes’ experience in solar characterization and solar irradiance measurements is what sets its service and support stand out from other competitors. Customers can contact the team to discuss their specific application needs with an expert distributor or sales engineer.

Sources and Further Reading

This information has been sourced, reviewed and adapted from materials provided by Avantes BV.

For more information on this source, please visit Avantes BV.

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