Radiation Basics
out excess heat from lamps in growth chambers so that leaves can absorb
high levels of PAR but not be simultaneously exposed to high levels of
radiant heat not useful in photosynthesis. This permits better control of
leaf temperature. In Case 3, the human eye just happens to have its greatest
sensitivity in the green wavelengths (0.55 pm) where the absorption of
visible radiation by leaves is minimal (See Fig. 11.5); perhaps this is a
useful adaptation for survival. Case 4 is interesting because the infrared
thermometer has a response of 1.0 in the 8 to 14 pm wavelength band
(R(8-14 pm) = 1.0). In this wavelength band, thermal radiation emitted
by the soil is completely absorbed by the glass because the transmissivity
of the glass is zero in this wavelength band. Although the glass transmits
90 percent of the visible radiation "seen" by human eyes, allowing the soil
to be seen clearly through the glass, it does not transmit any of the thermal
radiation emitted by the soil to the infrared thermometer. Because the glass
emits thermal radiation, the infrared thermometer actually measures the
temperature of the glass not the soil.
We have shown that emission and absorption of radiation are linked by
the same procesethat of changing the energy status of the emitting or
absorbing atoms or molecules. Thus we would expect the emissivity and
absorptivity of a material at a given wavelength to be equal, that is, &(A) =
a@), which is a statement of a principle due to Kirchhoff. It is important
to recognize that the absorptivity or emissivity values represent only the
fraction ofpossible absorption or emission at a particular wavelength, and
say nothing about whether or not radiation is actually being absorbed or
emitted at that wavelength. For example, carbon black has an emissivity
and absorptivity for visible radiation of nearly unity. When carbon black
is at room temperature, it may absorb radiation in the solar waveband
but emits negligible quantities in that waveband. The radiant emittance
at such short wavelengths is near zero not because the emissivity is low,
but because there is no energy to be emitted in the solar waveband from
such a cold surface. Methods for computing the spectral emittance at a
particular wavelength will be given in the next section.
Remote sensing is playing an increasingly important role in plant biophysics. To analyze the interaction of radiation with plant canopies and
soil surfaces we need to specifically incorporate directionality into the
more general definitions we have just given. The following four definitions are for reflectivity, but corresponding ones could be given for
transmissivity.
Bi-directional reflectance (sr-I): The ratio of the reflected radiance
from a single view direction to the irradiance from some incident view
direction that is confined to a very narrow range of incident angles.
Directional-hemispherical reJlectance: The ratio of the reflected radiance integrated over the entire view hemisphere to the irradiance
from a single view direction that is confined to a very narrow range
of incident angles.
out excess heat from lamps in growth chambers so that leaves can absorb
high levels of PAR but not be simultaneously exposed to high levels of
radiant heat not useful in photosynthesis. This permits better control of
leaf temperature. In Case 3, the human eye just happens to have its greatest
sensitivity in the green wavelengths (0.55 pm) where the absorption of
visible radiation by leaves is minimal (See Fig. 11.5); perhaps this is a
useful adaptation for survival. Case 4 is interesting because the infrared
thermometer has a response of 1.0 in the 8 to 14 pm wavelength band
(R(8-14 pm) = 1.0). In this wavelength band, thermal radiation emitted
by the soil is completely absorbed by the glass because the transmissivity
of the glass is zero in this wavelength band. Although the glass transmits
90 percent of the visible radiation "seen" by human eyes, allowing the soil
to be seen clearly through the glass, it does not transmit any of the thermal
radiation emitted by the soil to the infrared thermometer. Because the glass
emits thermal radiation, the infrared thermometer actually measures the
temperature of the glass not the soil.
We have shown that emission and absorption of radiation are linked by
the same procesethat of changing the energy status of the emitting or
absorbing atoms or molecules. Thus we would expect the emissivity and
absorptivity of a material at a given wavelength to be equal, that is, &(A) =
a@), which is a statement of a principle due to Kirchhoff. It is important
to recognize that the absorptivity or emissivity values represent only the
fraction ofpossible absorption or emission at a particular wavelength, and
say nothing about whether or not radiation is actually being absorbed or
emitted at that wavelength. For example, carbon black has an emissivity
and absorptivity for visible radiation of nearly unity. When carbon black
is at room temperature, it may absorb radiation in the solar waveband
but emits negligible quantities in that waveband. The radiant emittance
at such short wavelengths is near zero not because the emissivity is low,
but because there is no energy to be emitted in the solar waveband from
such a cold surface. Methods for computing the spectral emittance at a
particular wavelength will be given in the next section.
Remote sensing is playing an increasingly important role in plant biophysics. To analyze the interaction of radiation with plant canopies and
soil surfaces we need to specifically incorporate directionality into the
more general definitions we have just given. The following four definitions are for reflectivity, but corresponding ones could be given for
transmissivity.
Bi-directional reflectance (sr-I): The ratio of the reflected radiance
from a single view direction to the irradiance from some incident view
direction that is confined to a very narrow range of incident angles.
Directional-hemispherical reJlectance: The ratio of the reflected radiance integrated over the entire view hemisphere to the irradiance
from a single view direction that is confined to a very narrow range
of incident angles.