361
14.7.3 Epidermal Cell Index of Refraction
As light passes into and through the epidermal cell walls and into mesophyll cells,
it is scattered in new directions, based on differences in the velocity with which different wavelengths move between the cell solution and the air spaces. The refractive
index determines how much the light is bent between different media, as described
by the Snell–Descartes’ law, which says that the biochemical constituents of the leaf
determine the speed of light passing through it relative to its speed in a vacuum. The
effective refractive index of leaves varies with the biochemical composition of different species. It also varies with the wavelength of light, as seen when white light
is split into its individual colors by a prism. Each wavelength is bent at a different
angle, from 40° to 42°, causing separation of the colors. The angles of incidence and
refraction at the interfaces between the mesophyll and the cell walls influence the
leaf’s optical properties by affecting the probability that light is multiply scattered
through the cell interior, escapes directly out of the leaf after the first interaction
with a surface, or is absorbed.
The refractive index is a complex number in which the real part is the refractive
index and the imaginary part is related to the extinction coefficient (also called the
mass absorption coefficient), which accounts for light attenuation when photons
pass through a medium. These values change across the optical spectrum, and it has
not been easy to determine the refractive index for most plant compounds; only pure
liquid water has been fully characterized. Thus, in most cases, the refractive index
and extinction coefficient cannot be measured directly and must be estimated from
measurements of properties that depend on them, such as reflectance and transmittance. Since the 1950s, a long list of investigators have improved knowledge of the
real and imaginary parts of the refractive index for water in different regions of the
electromagnetic spectrum (e.g., Segelstein 1981; Hale and Querry 1973; Wieliczka
et al. 1989) as shown in Fig. 14.6. Because these coefficients change with the phase
of water (vapor, liquid, or solid) and its temperature, the phase needs to be specified.
Curcio and Petty (1951) were among the first to accurately measure absorption
coefficients for liquid water between 700 nm and 2500 nm at 20 °C; they identified
absorptions at 760, 970, 1190, 1450, and 1940 nm. Except for the water absorption
feature at 760 nm, the liquid water bands at 970 and 1190 nm are readily observed
in canopy and leaf spectra. In the laboratory, we observe the much stronger liquid
water absorptions at 1450 and 1940 nm; however, in satellite and airborne imagery,
these wavelengths are usually saturated with atmospheric water vapor, which has
significantly higher concentration over the full atmospheric column than does liquid
water in the leaf. As with other molecular absorptions (pigments, liquid water, dry
biomass, etc.), as the amount of water vapor increases, the wavelength bands on the
shoulders of the absorption maximum also absorb energy, and the feature expands
over more wavelengths. Generally, acquisition of airborne imagery is avoided under
rainfall or high water vapor conditions because clouds obscure the ground in optical
imagery; hence, archives have little data acquired under high liquid water atmospheric conditions.
14 How the Optical Properties of Leaves Modify the Absorption and Scattering…
14.7.3 Epidermal Cell Index of Refraction
As light passes into and through the epidermal cell walls and into mesophyll cells,
it is scattered in new directions, based on differences in the velocity with which different wavelengths move between the cell solution and the air spaces. The refractive
index determines how much the light is bent between different media, as described
by the Snell–Descartes’ law, which says that the biochemical constituents of the leaf
determine the speed of light passing through it relative to its speed in a vacuum. The
effective refractive index of leaves varies with the biochemical composition of different species. It also varies with the wavelength of light, as seen when white light
is split into its individual colors by a prism. Each wavelength is bent at a different
angle, from 40° to 42°, causing separation of the colors. The angles of incidence and
refraction at the interfaces between the mesophyll and the cell walls influence the
leaf’s optical properties by affecting the probability that light is multiply scattered
through the cell interior, escapes directly out of the leaf after the first interaction
with a surface, or is absorbed.
The refractive index is a complex number in which the real part is the refractive
index and the imaginary part is related to the extinction coefficient (also called the
mass absorption coefficient), which accounts for light attenuation when photons
pass through a medium. These values change across the optical spectrum, and it has
not been easy to determine the refractive index for most plant compounds; only pure
liquid water has been fully characterized. Thus, in most cases, the refractive index
and extinction coefficient cannot be measured directly and must be estimated from
measurements of properties that depend on them, such as reflectance and transmittance. Since the 1950s, a long list of investigators have improved knowledge of the
real and imaginary parts of the refractive index for water in different regions of the
electromagnetic spectrum (e.g., Segelstein 1981; Hale and Querry 1973; Wieliczka
et al. 1989) as shown in Fig. 14.6. Because these coefficients change with the phase
of water (vapor, liquid, or solid) and its temperature, the phase needs to be specified.
Curcio and Petty (1951) were among the first to accurately measure absorption
coefficients for liquid water between 700 nm and 2500 nm at 20 °C; they identified
absorptions at 760, 970, 1190, 1450, and 1940 nm. Except for the water absorption
feature at 760 nm, the liquid water bands at 970 and 1190 nm are readily observed
in canopy and leaf spectra. In the laboratory, we observe the much stronger liquid
water absorptions at 1450 and 1940 nm; however, in satellite and airborne imagery,
these wavelengths are usually saturated with atmospheric water vapor, which has
significantly higher concentration over the full atmospheric column than does liquid
water in the leaf. As with other molecular absorptions (pigments, liquid water, dry
biomass, etc.), as the amount of water vapor increases, the wavelength bands on the
shoulders of the absorption maximum also absorb energy, and the feature expands
over more wavelengths. Generally, acquisition of airborne imagery is avoided under
rainfall or high water vapor conditions because clouds obscure the ground in optical
imagery; hence, archives have little data acquired under high liquid water atmospheric conditions.
14 How the Optical Properties of Leaves Modify the Absorption and Scattering…