ASD Inc. application note on "Measuring Solar Spectral Energy"

1. Measuring Solar Spectral Energy
The value of Full-Range Spectroradiometers for measuring
Solar Spectral Energy:
• Many climate and surface energy balance studies involve
monitoring the direct, diffuse and total components
of solar spectral irradiance as they relate to the Earth’s
atmosphere and surface (ASD Inc., n.d.).
• Outdoors, the primary source of spectral irradiance is the
sun. The diffuse radiation from the sky is found primarily in
the visible and UV portions of the spectrum (ASD Inc., 2012).
• “Because the solar radiation on the sun-surface-sensor
path in the 0.4–2.5 µm visible and near-IR spectral regions
is subject to absorption and scattering by atmospheric
gases and aerosols, hyperspectral imaging data contains
atmospheric effects. In order to use hyperspectral imaging
data for quantitative remote sensing of land surfaces and
ocean color, the atmospheric effects must be removed.”
(Gao et al., 2009)
• “Atmospheric turbidity generally inhibits reliable measures
of vegetation and sometimes renders atmosphere-induced
variations on canopy spectra to exceed those due to
vegetation development. These effects make the accurate
and quantitative translation of [vegetation indices] more
difficult and complicated.“ (Gao et al., 2000)
• “The illumination and appearance of the solar/skydome
is critical for many applications in computer graphics,
computer vision, and daylighting studies. Unfortunately,
physically accurate measurements of this rapidly changing
illumination source are difficult to achieve, but necessary
for the development of accurate physically-based sky
illumination models and comparison studies of existing
simulation models.” (Kider et al., 2014)
• “Radiance is normally obtained in the field by nadir
measurement of specific ground targets, with incoming
solar irradiance spectra typically acquired from coincident
measurement of reflected energy flux from reference
panels with known spectral and angular scattering
properties.” (Peddle et al., 2001)
Figure 2. “This figure shows the solar radiation spectrum for direct light at
both the top of the Earth’s atmosphere and at sea level. The sun produces
light with a distribution similar to what would be expected from a 5525 K
(5250 °C) blackbody, which is approximately the sun’s surface temperature.
As light passes through the atmosphere, some is absorbed by gases with
specific absorption bands. Additional light is redistributed by Rayleigh
scattering, which is responsible for the atmosphere’s blue color.” (© Nick84
/ Wikimedia Commons / CC-BY-SA-3.0). Figure by Robert A. Rhode, licensed
under CC-BY 4.0.
Figure 1. Sources of Illumination (ASD Inc., n.d.)

2. The Solution:
ASD spectroradiometers are portable with optimal signal-to-noise design for faster measurements. The wavelength range
of ASD’s full-range spectroradiometers, 350–2500 nm, enables calculation of a wide range of parameters; including: aerosol
optical depth, ozone and water-vapor amount, aerosol size distribution, direct, diffuse and total irradiance.
ASD instruments and accessory options offer a practical solution to analyze and measure Solar Spectral Energy.
References
ASD Inc. (2012). Making Accurate Field Spectral Reflectance Measurements. Boulder,
CO: Alexander Goetz.
File:Solar spectrum en.svg. (2013), In Wikimedia Commons. Retrieved April 20, 2016,
from https://commons.wikimedia.org/wiki/File:Solar_spectrum_en.svg.
Gao, X., Huete, A.R., Ni, W., & Miura, T. (2000). Optical–biophysical relationships
of vegetation spectra without background contamination.Remote Sensing of
Environment, 74(3), 609-620.
Gao, B.C., Montes, M.J., Davis, C.O., & Goetz, A.F. (2009). Atmospheric correction
algorithms for hyperspectral remote sensing data of land and ocean. Remote Sensing of
Environment, 113, S17-S24.
Kider, J.T., Knowlton, D., Newlin, J., Li, Y.K., Greenberg, D.P. (2014). A Framework for the
Experimental Comparison of Solar and Skydome Illumination. Retrieved March 31, 2016
from www.graphics.cornell.edu/resources/clearsky/Kider-ClearSkyCapture(2014).pdf.
Peddle, D.R., White, H.P., Soffer, R.J., Miller, J.R., & LeDrew, E.F. (2001). Reflectance
processing of remote sensing spectroradiometer data. Computers & Geosciences, 27(2),
203-213.
FieldSpec® 4 full-range spectroradiometers designed
originally and primarily for the measurement of radiance,
irradiance, and surface spectral reflectance, and specifically
around the challenges researchers face when performing
spectral measurements in the field.
FieldSpec 4 full range spectroradiometers are calibrated for
radiometric collection using NIST traceable standards and
methods.
ASD’s RS3™ Spectral Acquisition Software has a user interface
optimized for field data collection with easy configuration
of averaging, field-of-view, storage, and display of raw,
reflectance, radiance and irradiance spectra in ‘real-time’.
Built-in Fiber Optic Cables:
• Hard mounted at the factory, continuous (not jumpered),
direct to dispersion elements for optimal signal transmission
and radiometric calibration accuracy.
ASD pioneered the science of field spectroscopy over 25
years ago and continues to lead the industry with the
world’s most trusted field-portable spectroradiometers.
ASD has several types of irradiance fore optics, for use with
the FieldSpec spectroradiometer, that include:
• Diffuse transmission and reflective type cosine receptors
and radiometric calibrations for measuring full sky
irradiance (W/m2/nm).
All ASD Remote Cosine Receptors (RCRs) interface with the
FieldSpec 4 fiber optic inputs.
Direct Irradiance Attachment used for
measuring the direct component of
solar irradiance (left).
Equatorial Mount used to
maintain the orientation
of the Direct Irradiance
Attachment relative to the
sun (right).
Although diligent care has been used to ensure that the information herein is accurate, nothing contained herein can be construed to imply any representation or warranty as to the accuracy, currency or completeness of
this information. The content hereof is subject to change without further notice. Please contact us for the latest version of this document or further information. © PANalytical B.V. 2015. 9498 708 04211 PN10661
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