Solar Irradiance Measurements |

Solar Irradiance Measurements

Miniature spectrometers such as our Jaz modular sensing suite are especially suited to solar irradiance measurements, where portability and flexibility make the researcher’s job easier. Determination of absolute and relative solar irradiance is possible, as well as related applications such as measurement of upwelling and downwelling radiation.

Introduction
Irradiance is the amount of energy at each wavelength emitted from a radiant sample. Absolute irradiance is the measure of light in absolute terms. Relative irradiance is a comparison of the fraction of energy the sample emits and the energy the sampling system collects from a lamp with a blackbody energy distribution.

Measurement of absolute solar irradiance is relevant in a number of applications: monitoring the sunlight itself, perhaps in the context of its relationship to greenhouse gases in the atmosphere; investigating the effect of solar radiation on ecological systems and crops; and evaluating the effect of UV sunlight on our skin and eyes.

Solar radiation includes spectral response across the broad UV-NIR region. We offer a number of spectrometer options that will measure solar irradiance over various portions of the solar spectrum between 200-1100 nm. A spectrometer now in development will combine both UV-Vis CCD-array and NIR InGaAs-array detectors in the same unit, extending the possible measurement range considerably.

Experimental Conditions
To measure a series of strong absorption lines in the solar spectrum, we configured our Jaz spectrometer with a grating optimized to 200-850 nm, a detector collection lens to increase light collection, a 50 µm slit and a battery module for portable operation. The Jaz is a self-contained unit that includes a microprocessor and low-power OLED display in place of a PC. Also, we added SpectraSuite spectrometer operating software and our Jaz-A-IRRAD irradiance-measurement application software to complete the basic irradiance setup.

For absolute irradiance measurements, we added a radiometrically calibrated light source (LS-1-CAL) and cosine corrector (several options are available), which also is our sampling optic. The cosine corrector collects sunlight from 180º and attaches directly to the SMA 905 connector on the Jaz. Accessory options such as a fixture for mounting Jaz in various positions and a shoulder holster for carrying Jaz are also available.

The Jaz spectrometer is radiometrically calibrated against the NIST-traceable LS-1-CAL Light Source, with the calibration file stored on an SD card. This SD card comes with the Jaz and fits into a slot in the battery module. We used this calibration file in software to determine the absolute irradiance values of the solar spectrum.

Results
Integration time for the measurement was 4 milliseconds, with spectral averaging set at 10 and boxcar smoothing at 2. The resulting spectrum showed a number of strong absorption lines from atmospheric elements in the range from 300-900 nm, including hydrogen, helium and sodium.

Similar results can be achieved using different spectrometer models with comparable optical bench configurations, including a USB2000+ Spectrometer that has our extended-range (200-1050 nm) grating or our USB2000+RAD, an application-ready setup with all the components needed for absolute solar irradiance measurements.

Conclusions
Because of its form-factor and battery operation, Jaz is a useful option for solar irradiance measurements of all types. The system is easily portable and extremely robust as well, having traveled to the top of Mt. Everest and successfully making solar radiation measurements for researchers studying ozone depletion.

Additional Resources
http://www.oceanoptics.com/Products/jazulm.asp
http://www.oceanoptics.com/Products/usb2000rad_compare.asp
http://www.oceanoptics.com/applications/samplesetups_upwellingdownwelling.asp
http://www.oceanoptics.com/Products/jaz_el350_200.asp
http://www.oceanoptics.com/Products/usb2000+rad.asp