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The existence of a continuous size spectrum, characterized by an exponent in the vicinity of
4, is an important result for the prediction of the optical properties of such an assemblage of
particles, with interesting consequences examined later on. It is acknowledged that local and
temporary deviations from this somewhat ideal distribution may occur. As an operative
hypothesis, however, this size spectrum can provide a useful tool.
OPTICAL PROPERTIES OF INDIVIDUAL CELLS OR PARTICLES
Some definitions
The interaction between incident radiations and a particle considered as an isolated body has
two aspects. The first one, already described when defining the complex refractive index,
corresponds to the phenomenon of absorption and affects the amount of energy carried by the
radiation, whereas the second process deals with the direction of propagation of the radiation
and is called scattering (or "elastic" scattering, as there is no loss of energy). Absorption
results mostly or exclusively in heating and providing energy for chemical reactions (such as
photosynthesis), or possibly for re-emission of radiation at other generally larger wavelengths
(Raman "inelastic" scattering and fluorescence emission). A proper definition of absorption
thus implies that the wavelength be fixed.
If we assume that the incident electromagnetic radiation propagates as a plane wave, it is
possible to define the geometrical cross section, Sg, of any particle as being the projected
("shadow") area of this particle on the plane of the wave. The above processes (absorption
and scattering) are conveniently described by two dimensionless "efficiency factors", Qa and
Qb' respectively defined as the ratios of the radiative energy absorbed within, and the energy
scattered by, the particle, to the energy impinging on its geometrical cross section. The sum,
(Qa + Qb) = Qc' defines the efficiency factor for attenuation, which expresses the global
removal of energy from the incident plane wave (note that the scattered energy is diverted
from its initial direction, but not lost for the medium). The products Sg Qa and Sg Qb
(dimension U) are the absorption and scattering cross sections of the particle, denoted Sa and
Sb' respectively.
The existence of a continuous size spectrum, characterized by an exponent in the vicinity of
4, is an important result for the prediction of the optical properties of such an assemblage of
particles, with interesting consequences examined later on. It is acknowledged that local and
temporary deviations from this somewhat ideal distribution may occur. As an operative
hypothesis, however, this size spectrum can provide a useful tool.
OPTICAL PROPERTIES OF INDIVIDUAL CELLS OR PARTICLES
Some definitions
The interaction between incident radiations and a particle considered as an isolated body has
two aspects. The first one, already described when defining the complex refractive index,
corresponds to the phenomenon of absorption and affects the amount of energy carried by the
radiation, whereas the second process deals with the direction of propagation of the radiation
and is called scattering (or "elastic" scattering, as there is no loss of energy). Absorption
results mostly or exclusively in heating and providing energy for chemical reactions (such as
photosynthesis), or possibly for re-emission of radiation at other generally larger wavelengths
(Raman "inelastic" scattering and fluorescence emission). A proper definition of absorption
thus implies that the wavelength be fixed.
If we assume that the incident electromagnetic radiation propagates as a plane wave, it is
possible to define the geometrical cross section, Sg, of any particle as being the projected
("shadow") area of this particle on the plane of the wave. The above processes (absorption
and scattering) are conveniently described by two dimensionless "efficiency factors", Qa and
Qb' respectively defined as the ratios of the radiative energy absorbed within, and the energy
scattered by, the particle, to the energy impinging on its geometrical cross section. The sum,
(Qa + Qb) = Qc' defines the efficiency factor for attenuation, which expresses the global
removal of energy from the incident plane wave (note that the scattered energy is diverted
from its initial direction, but not lost for the medium). The products Sg Qa and Sg Qb
(dimension U) are the absorption and scattering cross sections of the particle, denoted Sa and
Sb' respectively.