# Optical Thickness

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## optical thickness

[′äp·tə·kəl ′thik·nəs]
(meteorology)
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.
(optics)
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 ?
* ShadowMike is a structured-light optical thickness sensor for sheet and foam.
The Navy Atmospheric Variational Data Assimilation System for aerosol optical thickness (NAVDAS-AOT; Zhang et al.
And on the BIOMEDevice section of the exhibit floor, Bristol Instruments highlighted its Model 157 optical thickness gauge, which employs optical interferometer-based technology to measure the thickness of a variety of transparent and semitransparent materials.
We would notice here that the results of numerical study presented, in particular, in , were obtained for the layer of optical thickness [bar.h] = 0.1,that is, for opticallythin medium.
This attenuation is directly related to the optical thickness of the vegetation, [tau] [17, 19], measuring the degree of transparency of vegetation and then the fraction of electromagnetic radiation scattered or absorbed by the vegetation layer, using
where [bar.[C.sub.v]] is the vapor concentration (mol/[m.sup.3]); [epsilon]([[lambda].sub.UV]) is the absorption coefficient of vaporized fuel at wavelength [[lambda].sub.UV] ([m.sup.2]/niol); [L.sub.v] is the optical path length of light beam in the vapor phase (m); log[[I.sub.0]([[lambda].sub.UV])/I([[lambda].sub.UV])] is the transmissivity of UV light; G is the ratio of the optical thickness of the droplet cloud for UV and visible light, and log[[I.sub.0]([[lambda].sub.V])/I([[lambda].sub.V])] is the transmissivity of visible light.
Given the same substrates and light incident angles, the reflectance just depends on the optical thickness of films.
The study looked at the observed contribution of different gases to the infrared (IR) optical thickness of the atmosphere.
Optical thickness of L and H Layers in bandpass interference filter should be equal to one- fourth of peak wavelength:
In order to achieve a deeper insight into this aspect, we analyzed the evolution of diffraction efficiency as a function of exposure and used the fitting of the angular response of the hologram around the first Bragg angle to calculate the physical and optical thickness  and attenuation of refractive index modulation in depth.
The thickness of the well ([Ga.sub.0.847][In.sub.0.153]As) has been chosen as 7 nm which corresponds to an optical thickness of 25.34 nm.

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