Transmittance of the surface of a material is its effectiveness in transmitting radiant energy. It is the fraction of incident electromagnetic power that is transmitted through a sample, in contrast to the transmission coefficient, which is the ratio of the transmitted to incident electric field.^{[2]}
Internal transmittance refers to energy loss by absorption, whereas (total) transmittance is that due to absorption, scattering, reflection, etc.
Hemispherical transmittance of a surface, denoted T, is defined as^{[3]}
where
Spectral hemispherical transmittance in frequency and spectral hemispherical transmittance in wavelength of a surface, denoted T_{?} and T_{?} respectively, are defined as^{[3]}
where
Directional transmittance of a surface, denoted T_{?}, is defined as^{[3]}
where
Spectral directional transmittance in frequency and spectral directional transmittance in wavelength of a surface, denoted T_{?,?} and T_{?,?} respectively, are defined as^{[3]}
where
By definition, internal transmittance is related to optical depth and to absorbance as
where
The Beer-Lambert law states that, for N attenuating species in the material sample,
or equivalently that
where
Attenuation cross section and molar attenuation coefficient are related by
and number density and amount concentration by
where N_{A} is the Avogadro constant.
In case of uniform attenuation, these relations become^{[4]}
or equivalently
Cases of non-uniform attenuation occur in atmospheric science applications and radiation shielding theory for instance.
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 | ?_{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 | ?_{e,?}^{[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}. | |||
?_{e,?}^{[nb 4]} | watt per metre | W/m | M?L?T^{-3} | |||||
Radiant intensity | I_{e,?}^{[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,?,?}^{[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,?,?}^{[nb 4]} | watt per steradian per metre | W?sr^{-1}?m^{-1} | M?L?T^{-3} | |||||
Radiance | L_{e,?}^{[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,?,?}^{[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,?,?}^{[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,?}^{[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". Non-SI 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,?}^{[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,?}^{[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,?}^{[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,?}^{[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,?}^{[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,?}^{[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,?}^{[nb 4]} | joule per square metre, per metre | J/m^{3} | M?L^{-1}?T^{-2} | |||||
Hemispherical emissivity | ? | 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 | ?_{?} or ?_{?} |
N/A | 1 | Spectral exitance of a surface, divided by that of a black body at the same temperature as that surface. | ||||
Directional emissivity | ?_{?} | 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 | ?_{?,?} or ?_{?,?} |
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_{?} or A_{?} |
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_{?} | 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_{?,?} or A_{?,?} |
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_{?} or R_{?} |
N/A | 1 | Spectral flux reflected by a surface, divided by that received by that surface. | ||||
Directional reflectance | R_{?} | N/A | 1 | Radiance reflected by a surface, divided by that received by that surface. | ||||
Spectral directional reflectance | R_{?,?} or R_{?,?} |
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_{?} or T_{?} |
N/A | 1 | Spectral flux transmitted by a surface, divided by that received by that surface. | ||||
Directional transmittance | T_{?} | N/A | 1 | Radiance transmitted by a surface, divided by that received by that surface. | ||||
Spectral directional transmittance | T_{?,?} or T_{?,?} |
N/A | 1 | Spectral radiance transmitted by a surface, divided by that received by that surface. | ||||
Hemispherical attenuation coefficient | ? | 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 | ?_{?} or ?_{?} |
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 | ?_{?} | 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 | ?_{?,?} or ?_{?,?} |
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 |