Optical Thickness

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optical thickness

[′äp·tə·kəl ′thik·nəs]
In calculations of the transfer of radiant energy, the mass of a given absorbing or emitting material lying in a vertical column of unit cross-sectional area and extending between two specified levels. Also known as optical depth.
Subjectively, the degree to which a cloud prevents light from passing through it; depends upon the physical constitution (crystals, drops, droplets), the form, the concentration of particles, and the vertical extent of the cloud.
The thickness of an optical material times its index of refraction.

Optical Thickness


The optical thickness τ of a medium is a dimensionless quantity that characterizes the attenuation of optical radiation in the medium. The attenuation results from the joint action of light absorption and light scattering. The effects of radiation amplification caused by multiple scattering are not taken into account.

For an optically homogeneous medium, τ = ∊l, where ∊ is the medium’s volume extinction coefficient—which is equal to the sum of the absorption and scattering coefficients—and l is the geometric length of the path of the light ray in the medium. In a heterogeneous medium, in which ∊ depends on the coordinates, τ = ∫ ∊ dl, the integration being carried out along the path of the light ray. The modified Bouguer-Lambert-Beer law, which takes into account the scattering as well as the absorption of light, is written in terms of optical thickness as F = F0e, where F0 and F are the radiation flux incident on the medium in the form of a parallel beam of rays and the flux emerging from the medium in the same direction, respectively. A layer of a substance for which τ > 1 is often said to be optically thick, and a layer with τ < 1 to be optically thin; this demarcation, however, is arbitrary.

The relation between the optical thickness of a layer of a medium and the layer’s transmittance T is τ = –In T. The layer’s specular optical density D = —log T is, in terms of r, D = 0.434τ. In general, τ is a function of the frequency v, or of the wavelength λ, of the radiation: τ = τ(ν) = τ*(λ). A single value of the optical thickness, however, is often used when only one radiation frequency is involved—that is, τ is the optical thickness for a monochromatic radiation flux.

The concept of optical thickness is widely used for the description of processes of light scattering and absorption in, for example, the study of turbid mediums and the theory of radiative transfer. With respect to radiative transfer, the concept finds particular application in astrophysics and the physics of the earth’s atmosphere.

References in periodicals archive ?
Finkelnburg [17] advocated that optically thick gases can also produce blackbody radiation [3-6], since he did not properly consider reflection and energy transfer within a gas.
Relative to the claim that optically thick gases [17] can sustain blackbody radiation [2, 3], the arguments advanced [17] fail to properly address the question.
That is why he advocated that optically thick gases could emit as blackbodies.
In [Born and Stosser 2007] and [Kettlitz and others 2007] mercury-free automotive headlight lamps containing scandium were investigated, where plasma temperatures were obtained by numerical plasma simulations in [Born 2007] and by fitting the spectral line shape of optically thick sodium lines in [Kettlitz and others 2007].
Self-reversed spectral lines are observed in spatially inhomogeneous optically thick plasmas.
In the optically thick limit follows the reflection for perpendicular incidence directly from Fresnel equation:
These more optically thick materials (see Figure 1) may exhibit loss of throughcure or adhesion, as energy decreases with increase of speed, for example.
A film can be optically thick to short wavelengths, while being optically thin to longer wavelengths, but the ratio of flux will be fixed for any specific wavelength.
Successful cure speed of an optically thick film of an absorptive material is clearly affected by peak irradiance.
Ultraviolet backscatter sensors such as OMI are sensitive and cannot 'see through' optically thick clouds, indicating the signal of scattering aerosols and sulfur dioxide originated from altitudes above this high cloud top," says Fromm.

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