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Unformatted text preview: ing space. Reflectance in Band 2 page 10 Introduction to Hyperspectral Imaging Radiance and Reflectance
From the discussions on the preceding pages, it should be clear that spectral reflectance is a property of ground features that we would like to be able to measure precisely and accurately using an airborne or satellite hyperspectral sensor. But look at the brightness spectrum in the illustration below. This is the average of 25 image spectra measured by the AVIRIS sensor over a bright dry lake bed surface in the Cuprite, Nevada scene. The input spectra have been adjusted for sensor effects using on-board calibration data, but no other transformations have been applied.
Relative Brightness at Sensor
Averaged measured brightness for a portion of playa surface (red square at right). 0.5 1.0 1.5 2.0 2.5 Wavelength, (micrometers) This spectrum does not bear much resemblance to the reflectance spectra illustrated previously. This is because the sensor has simply measured the amount of reflected light reaching it in each wavelength band (spectral radiance), in this case from an altitude of 20 kilometers. The spectral reflectance of the surface materials is only one of the factors affecting these measured values. The spectral reflectance curve for the sample area is actually relatively flat and featureless. In addition to surface reflectance, the spectral radiance measured by a remote sensor depends on the spectrum of the input solar energy, interactions of this energy during its downward and upward passages through the atmosphere, the geometry of illumination for individual areas on the ground, and characteristics of the sensor system. These additional factors not only affect our ability to retrieve accurate spectral reflectance values for ground features, but also introduce additional within-scene variability which hampers comparisons between individual image cells. These factors are discussed in more detail on the next two pages. page 11 Introduction to Hyperspectral Imaging Illumination Factors
Source Illumination The figure below shows a typical solar irradiance curve for the top of the Earth’s atmosphere. The incoming solar energy varies greatly with wavelength, peaking in the range of visible light. The spectrum of incoming solar energy at the time an image was acquired must be known, assumed, or derived indirectly from other measurements in order to convert image radiance values to reflectance.
2.5 Spectral Irradiance ( kilowatts / m2 • µ m) 2.0 1.5 1.0 0.5 1.0 1.5 2.0 Solar energy arriving at the top of the atmosphere 0 0.5 Wavelength (micrometers, µm) 2.5 3.0 Illumination Geometry The amount of energy reflected by an area on the ground depends on the amount of solar energy illuminating the area, which in turn depends on the angle of incidence: the angle between the path of the incoming energy and a line perpendicular to the ground surface. Specifically, the energy received at each wavelength (Eg) varies as the cosine of the angle of incidence (θ): Eg = Eo x cos θ, where Eo is the amount of incoming energy. The energy received by any ground area therefore varies as the sun’s height changes with time of day and season. If the terrain is not flat, the energy received also varies instantaneously across a scene because θ of differences in slope angle and direcθ tion. Shadowing The amount of illumination received by an area can also be reduced by shadows. Shadows cast by Illumination differences can arise from topographic features or clouds can afdiffering incidence angles (θ ) as for A fect areas including many contiguous and B, or from shadowing ( C). image cells. Trees, crop rows, rock outcrops, or other small objects can also cast shadows that are confined to an individual image cell. Both types of shadows have the effect of lowering the measured brightness across all wavelengths for the affected pixels.
A B C page 12 Introduction to Hyperspectral Imaging Atmospheric and Sensor Effects
Atmospheric Effects Even a relatively clear atmosphere interacts with incoming and reflected solar energy. For certain wavelengths these interactions reduce the amount of incoming energy reaching the ground and further reduce the amount of reflected energy reaching an airborne or satellite sensor. The transmittance of the atmosphere is reduced by absorption by certain gases and by scattering by gas molecules and particulates. These effects combine to produce the transmittance curve illustrated below. The pronounced absorption features near 1.4 and 1.9 µm, caused by water vapor and carbon dioxide, reduce incident and reflected energy almost completely, so little useful information can be obtained from image bands in these regions. Not shown by this curve is the effect of light scattered upward by the atmosphere. This scattered light adds to the radiance measured by the sensor in the visible and near-infrared wavelengths, and is called path radiance. Atmospheric effects may also differ between areas in a single scene if atmospheric conditions are spatially variable or if there are significant ground elevation differences that vary the path length of radiation through the atmosphere.
1.0 0.8 Visible Nea...
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This note was uploaded on 12/16/2010 for the course ENV 148 taught by Professor Chang during the Spring '10 term at APU Japan.
- Spring '10