# Optical depth

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 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.

## Mathematical definitions

### Optical depth

**Optical depth** of a material, denoted *τ*, is given by:^{[2]}

where

- Φ
_{e}^{t}is the radiant flux*transmitted*by that material; - Φ
_{e}^{i}is the radiant flux received 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 absorbance in frequency** and **spectral absorbance 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

### Attenuance

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 *attenuance* 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 *N*(*z*) is uniform along the path, the relation becomes:

## Applications

### Atomic physics

In atomic physics, the spectral optical depth of a cloud of atoms can be calculated from the quantum-mechanical 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 plane-parallel 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.

### 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.

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 or 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]}or Φ _{e,λ}^{[nb 4]} |
watt per hertzorwatt per metre |
W/HzorW/m |
M⋅L^{2}⋅T^{−2}orM⋅L⋅T^{−3} |
Radiant flux per unit frequency or wavelength. The latter is commonly measured in W⋅sr^{−1}⋅m^{−2}⋅nm^{−1}. | |||

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]}or I_{e,Ω,λ}^{[nb 4]} |
watt per steradian per hertzorwatt per steradian per metre |
W⋅sr^{−1}⋅Hz^{−1}orW⋅sr ^{−1}⋅m^{−1} |
M⋅L^{2}⋅T^{−2}orM⋅L⋅T^{−3} |
Radiant intensity per unit frequency or wavelength. The latter is commonly measured in W⋅sr^{−1}⋅m^{−2}⋅nm^{−1}. This is a directional quantity. | |||

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]}or L_{e,Ω,λ}^{[nb 4]} |
watt per steradian per square metre per hertzorwatt per steradian per square metre, per metre |
W⋅sr^{−1}⋅m^{−2}⋅Hz^{−1}orW⋅sr ^{−1}⋅m^{−3} |
M⋅T^{−2}orM⋅L^{−1}⋅T^{−3} |
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". | |||

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]}or E_{e,λ}^{[nb 4]} |
watt per square metre per hertzorwatt per square metre, per metre |
W⋅m^{−2}⋅Hz^{−1}orW/m ^{3} |
M⋅T^{−2}orM⋅L^{−1}⋅T^{−3} |
Irradiance of a surface per unit frequency or wavelength. This is sometimes also confusingly called "spectral intensity". Non-SI units of spectral flux density include Jansky = 10^{−26} W⋅m^{−2}⋅Hz^{−1} and solar flux unit (1SFU = 10^{−22} W⋅m^{−2}⋅Hz^{−1}=10^{4}Jy). | |||

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]}or J_{e,λ}^{[nb 4]} |
watt per square metre per hertzorwatt per square metre, per metre |
W⋅m^{−2}⋅Hz^{−1}orW/m ^{3} |
M⋅T^{−2}orM⋅L^{−1}⋅T^{−3} |
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". | |||

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]}or M_{e,λ}^{[nb 4]} |
watt per square metre per hertzorwatt per square metre, per metre |
W⋅m^{−2}⋅Hz^{−1}orW/m ^{3} |
M⋅T^{−2}orM⋅L^{−1}⋅T^{−3} |
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". | |||

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]}or H_{e,λ}^{[nb 4]} |
joule per square metre per hertzorjoule per square metre, per metre |
J⋅m^{−2}⋅Hz^{−1}orJ/m ^{3} |
M⋅T^{−1}orM⋅L^{−1}⋅T^{−2} |
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". | |||

Hemispherical emissivity | ε |
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} |
1 | Spectral exitance of a surface, divided by that of a black body at the same temperature as that surface. | |||||

Directional emissivity | ε_{Ω} |
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} |
1 | Spectral radiance emitted by a surface, divided by that of a black body at the same temperature as that surface. | |||||

Hemispherical absorptance | 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} |
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} |
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} |
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 |
1 | Radiant flux reflected by a surface, divided by that received by that surface. | |||||

Spectral hemispherical reflectance | R_{ν}or R_{λ} |
1 | Spectral flux reflected by a surface, divided by that received by that surface. | |||||

Directional reflectance | R_{Ω} |
1 | Radiance reflected by a surface, divided by that received by that surface. | |||||

Spectral directional reflectance | R_{Ω,ν}or R_{Ω,λ} |
1 | Spectral radiance reflected by a surface, divided by that received by that surface. | |||||

Hemispherical transmittance | T |
1 | Radiant flux transmitted by a surface, divided by that received by that surface. | |||||

Spectral hemispherical transmittance | T_{ν}or T_{λ} |
1 | Spectral flux transmitted by a surface, divided by that received by that surface. | |||||

Directional transmittance | T_{Ω} |
1 | Radiance transmitted by a surface, divided by that received by that surface. | |||||

Spectral directional transmittance | T_{Ω,ν}or T_{Ω,λ} |
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.
- 1 2 3 4 5 Alternative symbols sometimes seen:
*W*or*E*for radiant energy,*P*or*F*for radiant flux,*I*for irradiance,*W*for radiant exitance. - 1 2 3 4 5 6 7 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.
- 1 2 3 4 5 6 7 Spectral quantities given per unit wavelength are denoted with suffix "λ" (Greek).
- 1 2 Directional quantities are denoted with suffix "Ω" (Greek).

## See also

- Absorbance
- Absorptance
- Transmittance
- Beer–Lambert law
- Attenuation coefficient
- Actinometer
- Aerosol
- Angstrom exponent
- Pyranometer
- Radiative transfer
- Transparency and translucency
- Sun photometer

## References

- 1 2 3 IUPAC,
*Compendium of Chemical Terminology*, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Absorbance". - ↑ Christopher Robert Kitchin (1987).
*Stars, Nebulae and the Interstellar Medium: Observational Physics and Astrophysics*. CRC Press.