spectral reflectance is not a natural characteristic of the materials, but it is rather
derived as “spectral reflectance factor” R(l), a ratio of the radiance reflected by a
surface to that reflected into the same beam geometry by an ideal (lossless) and diffuse
(isotropic, equally in all directions) standard surface irradiated under the same
conditions (Milton et al., 2009). It is dimensionless and ranges from 0 to 1 but
may reach values beyond 1, especially for highly reflective surfaces, for instance,
snow. In addition, the spectral reflectance depends on the wavelength of the incident
radiation, the properties of the target, and the incident radiation angle.
The bulk solar radiation is concentrated at wavelengths of 100–2500 nm, with high
irradiance intensity in the visible spectrum (VIS) from 400 to 700 nm, which declines
toward the short-wave infrared (SWIR) from 1300 to 2500 nm. That is because of the
complex absorption and light scattering reactions between the solar energy and the
atmosphere constituent gases, primarily H 2 O, CO 2 , O 2 , O 3 , CH 4 , and N 2 O (Avery and
Berlin, 1992). Therefore, atmospheric gases make certain wavelengths more selective
transmission bands (called atmospheric windows) and absorption bands (called
atmospheric blinds) to the incoming solar radiation. The water vapor forms the
major atmospheric blinds (Price, 1998) by strong energy absorption in the near
infrared (NIR) from 700 to 1300 nm through the SWIR from 1300 to 2500 nm, with
broad regions of complete absorption near 1450, 1900, and 2400 nm. The other
atmospheric gases display less strong absorption bands.
Compared to land cover types, such as water or soil, vegetation reflectance is the
most inconsistent in shape and generally looks similar, irrespective of vegetation
type and health status (Hadjimitsis et al., 2009). The typical spectral signature of
vegetation in the VIS spectrum is composed of the maximum reflectance (minimum
absorption) at 550 nm and increased absorption in the red and blue portions of the
VIS due to the selective reflection/absorption properties of chlorophyll and the
other auxiliary pigments such as carotenoids in the leaves. Further, the points of
inflection at about 600 and 630 nm and minimum reflectance (maximum absorption) in the neighborhood of 680 nm are due to the presence of specific accessory
pigments, while beyond 680 nm of the electromagnetic spectrum vegetation
becomes highly reflective. The red edge (680–800 nm) is the region of rapid
change in reflectance before the NIR. Maximum reflectance is reached in the
NIR plateau from 800 to 1300 nm due to the multiple scattered reflectances in the air
spaces—in both the mesophyll cells of the leaves and the canopy. The SWIR
reflectance is characterized by two major water vapor absorption zones: 1450–
1530 nm and 1900–2000 nm.
Finally, the natural anisotropy (direction dependency) of the reflectance introduces
the bidirectional reflectance distribution function (BRDF) of each target (Schaaf,
2009). It is a four-dimensional function with unit-inverse steradian (sr
−1
) which
describes the change of the reflectance with the solar position geometry (Brennan and
Bandeen, 1970). The underlying concept for characterization of the BRDF and the
directional issues of radiation and reflectance are discussed in Nicodemus et al.
(1977). The consideration of BRDF in the field spectroradiometry is discussed later in
this section.
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HYPERSPECTRAL REMOTE SENSING WITH EMPHASIS ON LAND COVER MAPPING
derived as “spectral reflectance factor” R(l), a ratio of the radiance reflected by a
surface to that reflected into the same beam geometry by an ideal (lossless) and diffuse
(isotropic, equally in all directions) standard surface irradiated under the same
conditions (Milton et al., 2009). It is dimensionless and ranges from 0 to 1 but
may reach values beyond 1, especially for highly reflective surfaces, for instance,
snow. In addition, the spectral reflectance depends on the wavelength of the incident
radiation, the properties of the target, and the incident radiation angle.
The bulk solar radiation is concentrated at wavelengths of 100–2500 nm, with high
irradiance intensity in the visible spectrum (VIS) from 400 to 700 nm, which declines
toward the short-wave infrared (SWIR) from 1300 to 2500 nm. That is because of the
complex absorption and light scattering reactions between the solar energy and the
atmosphere constituent gases, primarily H 2 O, CO 2 , O 2 , O 3 , CH 4 , and N 2 O (Avery and
Berlin, 1992). Therefore, atmospheric gases make certain wavelengths more selective
transmission bands (called atmospheric windows) and absorption bands (called
atmospheric blinds) to the incoming solar radiation. The water vapor forms the
major atmospheric blinds (Price, 1998) by strong energy absorption in the near
infrared (NIR) from 700 to 1300 nm through the SWIR from 1300 to 2500 nm, with
broad regions of complete absorption near 1450, 1900, and 2400 nm. The other
atmospheric gases display less strong absorption bands.
Compared to land cover types, such as water or soil, vegetation reflectance is the
most inconsistent in shape and generally looks similar, irrespective of vegetation
type and health status (Hadjimitsis et al., 2009). The typical spectral signature of
vegetation in the VIS spectrum is composed of the maximum reflectance (minimum
absorption) at 550 nm and increased absorption in the red and blue portions of the
VIS due to the selective reflection/absorption properties of chlorophyll and the
other auxiliary pigments such as carotenoids in the leaves. Further, the points of
inflection at about 600 and 630 nm and minimum reflectance (maximum absorption) in the neighborhood of 680 nm are due to the presence of specific accessory
pigments, while beyond 680 nm of the electromagnetic spectrum vegetation
becomes highly reflective. The red edge (680–800 nm) is the region of rapid
change in reflectance before the NIR. Maximum reflectance is reached in the
NIR plateau from 800 to 1300 nm due to the multiple scattered reflectances in the air
spaces—in both the mesophyll cells of the leaves and the canopy. The SWIR
reflectance is characterized by two major water vapor absorption zones: 1450–
1530 nm and 1900–2000 nm.
Finally, the natural anisotropy (direction dependency) of the reflectance introduces
the bidirectional reflectance distribution function (BRDF) of each target (Schaaf,
2009). It is a four-dimensional function with unit-inverse steradian (sr
−1
) which
describes the change of the reflectance with the solar position geometry (Brennan and
Bandeen, 1970). The underlying concept for characterization of the BRDF and the
directional issues of radiation and reflectance are discussed in Nicodemus et al.
(1977). The consideration of BRDF in the field spectroradiometry is discussed later in
this section.
288
HYPERSPECTRAL REMOTE SENSING WITH EMPHASIS ON LAND COVER MAPPING