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 ?
The structure of the 3-quarter film system is exactly like that of the 1-quarter film system, with the optical thickness of each layer three quarters of the aimed reflective wavelength rather than one quarter of the wavelength.
5] concentrations using Multiangle Imaging Spectroradiometer aerosol optical thickness over the contiguous United States.
This disturbing force is negligible at normal pressure, but when the pressure within the vacuum chamber is reduced beyond the millibar (for instance to avoid other disturbances due to air convection or to minimize the air friction on the oscillating pendulum) the meatus optical thickness further reduces, so as to attain the above condition about 1 mean free path.
In 1991 Tarasova and Yarkho from the University of Moscow published a model for the determination of atmospheric aerosol optical thickness, AOT550, i.
2 and wherein at least one layer has an optical thickness which is different from all of the other layers, whereby the pigment is not a quarter-wave stack.
Based on the white-light interference effects occurring at thin transparent films, RIfS has shown high precision and high stability in monitoring the change of optical thickness attributable to interfacial molecular interaction and has been shown to be a useful approach for the detection of herbicides in river water and hydrocarbons in air (1, 2).
The instrument calculates thickness by dividing optical thickness by the material's refractive index.
General Uniaxial Medium - Use the new General Uniaxial Medium (GUM) to define uniaxial materials by physical thickness rather than optical thickness, and with optional reflectance specified using plane-of-incidence polarization states rather than device eigenmodes - useful for film modeling where optical thickness is not strictly controlled or where fractional-wave retardance variations are not of concern.
Several models have been created for transition from broadband irradiance to spectral aerosol optical thickness (AOT[lambda]).
MODIS measures aerosol optical thickness (AOT), which indicates how much sunlight is prevented from traveling through the atmosphere.
Any change in the optical thickness of the thin film, such as one caused by the deposition of target molecules on the surface, will produce a visible color change from gold to purple.
Until now, chipmakers have normally purchased separate analytical and traditional optical thickness metrology tools to provide thickness and composition measurements.

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