In physics, optical depth or optical thickness, is the natural logarithm of the ratio of incident to transmitted radiant power through a material, and spectral optical depth or spectral optical thickness is the natural logarithm of the ratio of incident to transmitted spectral radiant power through a material.^{[1]} Optical depth is dimensionless, and in particular is not a length, though it is a monotonically increasing function of optical path length, and approaches zero as the path length approaches zero. The use of the term "optical density" for optical depth is discouraged.^{[1]}
In chemistry, a closely related quantity called "absorbance" or "decadic absorbance" is used instead of optical depth: the common logarithm of the ratio of incident to transmitted radiant power through a material, that is the optical depth divided by ln 10.
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Transcription
Contents
Mathematical definitions
Optical depth
Optical depth of a material, denoted , is given by:^{[2]}
where
 Φ_{e}^{i} is the radiant flux received by that material;
 Φ_{e}^{t} is the radiant flux transmitted by that material;
 T is the transmittance of that material.
Absorbance is related to optical depth by:
where A is the absorbance.
Spectral optical depth
Spectral optical depth in frequency and spectral optical depth in wavelength of a material, denoted τ_{ν} and τ_{λ} respectively, are given by:^{[1]}
where
 Φ_{e,ν}^{t} is the spectral radiant flux in frequency transmitted by that material;
 Φ_{e,ν}^{i} is the spectral radiant flux in frequency received by that material;
 T_{ν} is the spectral transmittance in frequency of that material;
 Φ_{e,λ}^{t} is the spectral radiant flux in wavelength transmitted by that material;
 Φ_{e,λ}^{i} is the spectral radiant flux in wavelength received by that material;
 T_{λ} is the spectral transmittance in wavelength of that material.
Spectral absorbance is related to spectral optical depth by:
where
 A_{ν} is the spectral absorbance in frequency;
 A_{λ} is the spectral absorbance in wavelength.
Relationship with attenuation
Attenuation
Optical depth measures the attenuation of the transmitted radiant power in a material. Attenuation can be caused by absorption, but also reflection, scattering, and other physical processes. Optical depth of a material is approximately equal to its attenuation when both the absorbance is much less than 1 and the emittance of that material (not to be confused with radiant exitance or emissivity) is much less than the optical depth:
where
 Φ_{e}^{t} is the radiant power transmitted by that material;
 Φ_{e}^{att} is the radiant power attenuated by that material;
 Φ_{e}^{i} is the radiant power received by that material;
 Φ_{e}^{e} is the radiant power emitted by that material;
 T = Φ_{e}^{t}/Φ_{e}^{i} is the transmittance of that material;
 ATT = Φ_{e}^{att}/Φ_{e}^{i} is the attenuance of that material;
 E = Φ_{e}^{e}/Φ_{e}^{i} is the emittance of that material,
and according to Beer–Lambert law,
so:
Attenuation coefficient
Optical depth of a material is also related to its attenuation coefficient by:
where
 l is the thickness of that material through which the light travels;
 α(z) is the attenuation coefficient or Napierian attenuation coefficient of that material at z,
and if α(z) is uniform along the path, the attenuation is said to be a linear attenuation and the relation becomes:
Sometimes the relation is given using the attenuation cross section of the material, that is its attenuation coefficient divided by its number density:
where
 σ is the attenuation cross section of that material;
 n(z) is the number density of that material at z,
and if is uniform along the path, i.e., , the relation becomes:
Applications
Atomic physics
In atomic physics, the spectral optical depth of a cloud of atoms can be calculated from the quantummechanical properties of the atoms. It is given by
where
 d is the transition dipole moment;
 n is the number of atoms;
 ν is the frequency of the beam;
 c is the speed of light;
 ħ is Planck's constant;
 ε_{0} is the vacuum permittivity;
 σ the cross section of the beam;
 γ the natural linewidth of the transition.
Atmospheric sciences
In atmospheric sciences, one often refers to the optical depth of the atmosphere as corresponding to the vertical path from Earth's surface to outer space; at other times the optical path is from the observer's altitude to outer space. The optical depth for a slant path is τ = mτ′, where τ′ refers to a vertical path, m is called the relative airmass, and for a planeparallel atmosphere it is determined as m = sec θ where θ is the zenith angle corresponding to the given path. Therefore,
The optical depth of the atmosphere can be divided into several components, ascribed to Rayleigh scattering, aerosols, and gaseous absorption. The optical depth of the atmosphere can be measured with a sun photometer.
The optical depth with respect to the height within the atmosphere is given by
^{[3]}
and it follows that the total atmospheric optical depth is given by
^{[3]}
In both equations:
 k_{a} is the absorption coefficient
 w_{1} is the mixing ratio
 ρ_{0} is the density of air at sea level
 H is the scale height of the atmosphere
 z is the height in question
The optical depth of a plane parallel cloud layer is given by
^{[3]}
where:
 Q_{e} is the extinction efficiency
 L is the liquid water path
 H is the geometrical thickness
 N is the concentration of droplets
 ρ_{l} is the density of liquid water
So, with a fixed depth and total liquid water path,
^{[3]}
Astronomy
In astronomy, the photosphere of a star is defined as the surface where its optical depth is 2/3. This means that each photon emitted at the photosphere suffers an average of less than one scattering before it reaches the observer. At the temperature at optical depth 2/3, the energy emitted by the star (the original derivation is for the Sun) matches the observed total energy emitted.^{[citation needed]}^{[clarification needed]}
Note that the optical depth of a given medium will be different for different colors (wavelengths) of light.
For planetary rings, the optical depth is the (negative logarithm of the) proportion of light blocked by the ring when it lies between the source and the observer. This is usually obtained by observation of stellar occultations.
Quantity  Unit  Dimension  Notes  

Name  Symbol^{[nb 1]}  Name  Symbol  Symbol  
Radiant energy  Q_{e}^{[nb 2]}  joule  J  M⋅L^{2}⋅T^{−2}  Energy of electromagnetic radiation.  
Radiant energy density  w_{e}  joule per cubic metre  J/m^{3}  M⋅L^{−1}⋅T^{−2}  Radiant energy per unit volume.  
Radiant flux  Φ_{e}^{[nb 2]}  watt  W = J/s  M⋅L^{2}⋅T^{−3}  Radiant energy emitted, reflected, transmitted or received, per unit time. This is sometimes also called "radiant power".  
Spectral flux  Φ_{e,ν}^{[nb 3]}  watt per hertz  W/Hz  M⋅L^{2}⋅T^{−2}  Radiant flux per unit frequency or wavelength. The latter is commonly measured in W⋅nm^{−1}.  
Φ_{e,λ}^{[nb 4]}  watt per metre  W/m  M⋅L⋅T^{−3}  
Radiant intensity  I_{e,Ω}^{[nb 5]}  watt per steradian  W/sr  M⋅L^{2}⋅T^{−3}  Radiant flux emitted, reflected, transmitted or received, per unit solid angle. This is a directional quantity.  
Spectral intensity  I_{e,Ω,ν}^{[nb 3]}  watt per steradian per hertz  W⋅sr^{−1}⋅Hz^{−1}  M⋅L^{2}⋅T^{−2}  Radiant intensity per unit frequency or wavelength. The latter is commonly measured in W⋅sr^{−1}⋅nm^{−1}. This is a directional quantity.  
I_{e,Ω,λ}^{[nb 4]}  watt per steradian per metre  W⋅sr^{−1}⋅m^{−1}  M⋅L⋅T^{−3}  
Radiance  L_{e,Ω}^{[nb 5]}  watt per steradian per square metre  W⋅sr^{−1}⋅m^{−2}  M⋅T^{−3}  Radiant flux emitted, reflected, transmitted or received by a surface, per unit solid angle per unit projected area. This is a directional quantity. This is sometimes also confusingly called "intensity".  
Spectral radiance  L_{e,Ω,ν}^{[nb 3]}  watt per steradian per square metre per hertz  W⋅sr^{−1}⋅m^{−2}⋅Hz^{−1}  M⋅T^{−2}  Radiance of a surface per unit frequency or wavelength. The latter is commonly measured in W⋅sr^{−1}⋅m^{−2}⋅nm^{−1}. This is a directional quantity. This is sometimes also confusingly called "spectral intensity".  
L_{e,Ω,λ}^{[nb 4]}  watt per steradian per square metre, per metre  W⋅sr^{−1}⋅m^{−3}  M⋅L^{−1}⋅T^{−3}  
Irradiance Flux density 
E_{e}^{[nb 2]}  watt per square metre  W/m^{2}  M⋅T^{−3}  Radiant flux received by a surface per unit area. This is sometimes also confusingly called "intensity".  
Spectral irradiance Spectral flux density 
E_{e,ν}^{[nb 3]}  watt per square metre per hertz  W⋅m^{−2}⋅Hz^{−1}  M⋅T^{−2}  Irradiance of a surface per unit frequency or wavelength. This is sometimes also confusingly called "spectral intensity". NonSI units of spectral flux density include jansky (1 Jy = 10^{−26} W⋅m^{−2}⋅Hz^{−1}) and solar flux unit (1 sfu = 10^{−22} W⋅m^{−2}⋅Hz^{−1} = 10^{4} Jy).  
E_{e,λ}^{[nb 4]}  watt per square metre, per metre  W/m^{3}  M⋅L^{−1}⋅T^{−3}  
Radiosity  J_{e}^{[nb 2]}  watt per square metre  W/m^{2}  M⋅T^{−3}  Radiant flux leaving (emitted, reflected and transmitted by) a surface per unit area. This is sometimes also confusingly called "intensity".  
Spectral radiosity  J_{e,ν}^{[nb 3]}  watt per square metre per hertz  W⋅m^{−2}⋅Hz^{−1}  M⋅T^{−2}  Radiosity of a surface per unit frequency or wavelength. The latter is commonly measured in W⋅m^{−2}⋅nm^{−1}. This is sometimes also confusingly called "spectral intensity".  
J_{e,λ}^{[nb 4]}  watt per square metre, per metre  W/m^{3}  M⋅L^{−1}⋅T^{−3}  
Radiant exitance  M_{e}^{[nb 2]}  watt per square metre  W/m^{2}  M⋅T^{−3}  Radiant flux emitted by a surface per unit area. This is the emitted component of radiosity. "Radiant emittance" is an old term for this quantity. This is sometimes also confusingly called "intensity".  
Spectral exitance  M_{e,ν}^{[nb 3]}  watt per square metre per hertz  W⋅m^{−2}⋅Hz^{−1}  M⋅T^{−2}  Radiant exitance of a surface per unit frequency or wavelength. The latter is commonly measured in W⋅m^{−2}⋅nm^{−1}. "Spectral emittance" is an old term for this quantity. This is sometimes also confusingly called "spectral intensity".  
M_{e,λ}^{[nb 4]}  watt per square metre, per metre  W/m^{3}  M⋅L^{−1}⋅T^{−3}  
Radiant exposure  H_{e}  joule per square metre  J/m^{2}  M⋅T^{−2}  Radiant energy received by a surface per unit area, or equivalently irradiance of a surface integrated over time of irradiation. This is sometimes also called "radiant fluence".  
Spectral exposure  H_{e,ν}^{[nb 3]}  joule per square metre per hertz  J⋅m^{−2}⋅Hz^{−1}  M⋅T^{−1}  Radiant exposure of a surface per unit frequency or wavelength. The latter is commonly measured in J⋅m^{−2}⋅nm^{−1}. This is sometimes also called "spectral fluence".  
H_{e,λ}^{[nb 4]}  joule per square metre, per metre  J/m^{3}  M⋅L^{−1}⋅T^{−2}  
Hemispherical emissivity  ε  N/A  1  Radiant exitance of a surface, divided by that of a black body at the same temperature as that surface.  
Spectral hemispherical emissivity  ε_{ν} or ε_{λ} 
N/A  1  Spectral exitance of a surface, divided by that of a black body at the same temperature as that surface.  
Directional emissivity  ε_{Ω}  N/A  1  Radiance emitted by a surface, divided by that emitted by a black body at the same temperature as that surface.  
Spectral directional emissivity  ε_{Ω,ν} or ε_{Ω,λ} 
N/A  1  Spectral radiance emitted by a surface, divided by that of a black body at the same temperature as that surface.  
Hemispherical absorptance  A  N/A  1  Radiant flux absorbed by a surface, divided by that received by that surface. This should not be confused with "absorbance".  
Spectral hemispherical absorptance  A_{ν} or A_{λ} 
N/A  1  Spectral flux absorbed by a surface, divided by that received by that surface. This should not be confused with "spectral absorbance".  
Directional absorptance  A_{Ω}  N/A  1  Radiance absorbed by a surface, divided by the radiance incident onto that surface. This should not be confused with "absorbance".  
Spectral directional absorptance  A_{Ω,ν} or A_{Ω,λ} 
N/A  1  Spectral radiance absorbed by a surface, divided by the spectral radiance incident onto that surface. This should not be confused with "spectral absorbance".  
Hemispherical reflectance  R  N/A  1  Radiant flux reflected by a surface, divided by that received by that surface.  
Spectral hemispherical reflectance  R_{ν} or R_{λ} 
N/A  1  Spectral flux reflected by a surface, divided by that received by that surface.  
Directional reflectance  R_{Ω}  N/A  1  Radiance reflected by a surface, divided by that received by that surface.  
Spectral directional reflectance  R_{Ω,ν} or R_{Ω,λ} 
N/A  1  Spectral radiance reflected by a surface, divided by that received by that surface.  
Hemispherical transmittance  T  N/A  1  Radiant flux transmitted by a surface, divided by that received by that surface.  
Spectral hemispherical transmittance  T_{ν} or T_{λ} 
N/A  1  Spectral flux transmitted by a surface, divided by that received by that surface.  
Directional transmittance  T_{Ω}  N/A  1  Radiance transmitted by a surface, divided by that received by that surface.  
Spectral directional transmittance  T_{Ω,ν} or T_{Ω,λ} 
N/A  1  Spectral radiance transmitted by a surface, divided by that received by that surface.  
Hemispherical attenuation coefficient  μ  reciprocal metre  m^{−1}  L^{−1}  Radiant flux absorbed and scattered by a volume per unit length, divided by that received by that volume.  
Spectral hemispherical attenuation coefficient  μ_{ν} or μ_{λ} 
reciprocal metre  m^{−1}  L^{−1}  Spectral radiant flux absorbed and scattered by a volume per unit length, divided by that received by that volume.  
Directional attenuation coefficient  μ_{Ω}  reciprocal metre  m^{−1}  L^{−1}  Radiance absorbed and scattered by a volume per unit length, divided by that received by that volume.  
Spectral directional attenuation coefficient  μ_{Ω,ν} or μ_{Ω,λ} 
reciprocal metre  m^{−1}  L^{−1}  Spectral radiance absorbed and scattered by a volume per unit length, divided by that received by that volume.  
See also: SI · Radiometry · Photometry 
 ^ Standards organizations recommend that radiometric quantities should be denoted with suffix "e" (for "energetic") to avoid confusion with photometric or photon quantities.
 ^ ^{a} ^{b} ^{c} ^{d} ^{e} Alternative symbols sometimes seen: W or E for radiant energy, P or F for radiant flux, I for irradiance, W for radiant exitance.
 ^ ^{a} ^{b} ^{c} ^{d} ^{e} ^{f} ^{g} Spectral quantities given per unit frequency are denoted with suffix "ν" (Greek)—not to be confused with suffix "v" (for "visual") indicating a photometric quantity.
 ^ ^{a} ^{b} ^{c} ^{d} ^{e} ^{f} ^{g} Spectral quantities given per unit wavelength are denoted with suffix "λ" (Greek).
 ^ ^{a} ^{b} Directional quantities are denoted with suffix "Ω" (Greek).
See also
 Air mass (astronomy)
 Absorbance
 Absorptance
 Actinometer
 Aerosol
 Angstrom exponent
 Attenuation coefficient
 Beer–Lambert law
 Pyranometer
 Radiative transfer
 Sun photometer
 Transmittance
 Transparency and translucency
References
 ^ ^{a} ^{b} ^{c} IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Absorbance". doi:10.1351/goldbook.A00028
 ^ Christopher Robert Kitchin (1987). Stars, Nebulae and the Interstellar Medium: Observational Physics and Astrophysics. CRC Press.
 ^ ^{a} ^{b} ^{c} ^{d} W., Petty, Grant (2006). A first course in atmospheric radiation. Sundog Pub. ISBN 9780972903318. OCLC 932561283.