Semitransparent Surface (Surface-to-Surface Radiation Interface)
This node is a variant of the Opaque Surface (Surface-to-Surface Radiation Interface) node. In addition to specular and diffuse reflection, specular transmission, and refraction can also be considered on the surface. It is available when the Surface-to-surface radiation method is set to Ray shooting in the Surface-to-Surface Radiation interface settings. The node adds a radiosity shape function for each spectral band to its selection and uses it as surface radiosity.
If pure specular reflection should be considered, use the Specular Mirror (Surface-to-Surface Radiation Interface) node instead.
If pure specular reflection and specular transmission should be considered, use the Radiation Shield, Upside (Surface-to-Surface Radiation Interface) node instead.
If no radiation is transmitted through the surface, use the Opaque Surface (Surface-to-Surface Radiation Interface) node instead.
Radiation is assumed to occur on both sides of the surface.
This feature accounts for refraction such that if the adjacent domains are of different refractive indices, the angle of the rays after transmission is calculated from Snell’s law. The directional dependence of reflection, transmission and emission can be manually set up in the Surface Radiative Properties section. For automatic calculation of these functions with Fresnel’s relations, use the Radiation Shield, Upside (Surface-to-Surface Radiation Interface) or Dielectric Window (Surface-to-Surface Radiation Interface) nodes instead.
Model Input
This section has fields and values that are inputs to expressions that define material properties. If such user-defined property groups have been added, the model inputs are included here.
There is one standard model input: the Temperature T is used in the expression of the blackbody radiation intensity and when multiple wavelength intervals are used, for the fractional emissive power. The temperature model input is also used to determine the variable that receives the radiative heat source. When the model input does not contain a dependent variable, the radiative heat source is ignored.
The default Temperature is User defined. When additional physics interfaces are added to the model, the temperature variables defined by these physics interfaces can also be selected from the list. The Common model input option corresponds to the minput.T variable, set to 293.15 K by default. To edit the minput.T variable, click the Go to Source button (), and in the Default Model Inputs node under Global Definitions, set a value for the Temperature in the Expression for remaining selection section.
Coordinate System Selection
Select a boundary system from the Coordinate system list to define the reference vector for the azimuthal angle. The first component of the boundary system selected is the reference vector used for directional surface properties. The default is the boundary system of the model, and the list contains any additional boundary system added under the Definitions node.
See Coordinate Systems in the COMSOL Multiphysics Reference Manual for more details.
Ambient
These settings are the same as for the Diffuse Surface (Surface-to-Surface Radiation Interface) node.
Fractional Emissive Power
These settings are the same as for the Diffuse Surface (Surface-to-Surface Radiation Interface) node.
Surface Radiative Properties
Directional Dependence
By default, the Directional dependence of the surface radiative properties (emissivity and transmissivity) is set to Constant. The Emissivity and Transmissivity can be set to expressions depending on other variables like the temperature or the spatial coordinates. Change to Polar or Polar and azimuthal options to define dependencies on the polar and azimuthal angles, θ and φ, through the specification of a Directional emissivity function, fε and a Directional transmissivity function, fτ. When the Polar option is used, select in the list any function of one argument already defined in the component, under the Definitions node. For the Polar and azimuthal option, select among the functions of two arguments. By default, only the Zero function is available in the list, defining fε(θ) = 0, fτ(θ) = 0, fε(θ, φ) = 0, or fτ(θ, φ) = 0.
The other dependencies (temperature T, spatial coordinates x) are handled separately in the Emissivity and Transmissivity expressions, and used to define the total properties:
Emissivity, Diffuse Reflectivity, Transmissivity, and Critical Angle
If Wavelength dependence of radiative properties is Constant:
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By default, the Emissivity ε (dimensionless), Diffuse reflectivity ρd (dimensionless), and Transmissivity τ (dimensionless) use values From material. These are properties of the material surface that depend both on the material itself and the structure of the surface. Make sure that a material is defined at the boundary level (by default materials are defined at the domain level).
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For User defined, set values or expressions. You can set temperature-dependent emissivity and reflectivity using the variable rad.T.
Select the Define properties on each side checkbox to set specific values on each side. Select the Boundary material, upside and Boundary material, downside to have different material properties on each side. The boundary material specified is used only when Emissivity, upside, Emissivity, downside, Diffuse reflectivity, upside, Diffuse reflectivity, downside, Transmissivity, upside, Transmissivity, downside are From material.
If Wavelength dependence of radiative properties is Solar and ambient or Multiple spectral bands:
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By default, the Emissivity ε (dimensionless), Diffuse reflectivity ρd (dimensionless), and Transmissivity τ (dimensionless) use values From material.
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When Emissivity is set to User defined, enter a value or expression for the Emissivity ε. The wavelength can be accessed through the rad.lambda variable. Any expression set for the emissivity is then averaged on each spectral band to obtain a piecewise constant emissivity. If the average value of the emissivity on each band is known, you can instead use the User defined for each band option to avoid the evaluation of the average.
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When Diffuse reflectivity is set to User defined, enter a value or expression for the Diffuse reflectivity ρd. The wavelength can be accessed through the rad.lambda variable. Any expression set for the reflectivity is then averaged on each spectral band to obtain a piecewise constant reflectivity. If the average value of the reflectivity on each band is known, you can instead use the User defined for each band option to avoid the evaluation of the average.
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When Transmissivity is set to User defined, enter a value or expression for the Transmissivity τ. The wavelength can be accessed through the rad.lambda variable. Any expression set for the transmissivity is then averaged on each spectral band to obtain a piecewise constant transmissivity. If the average value of the transmissivity on each band is known, you can instead use the User defined for each band option to avoid the evaluation of the average.
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When Emissivity is set to User defined for each band, enter a value for the Emissivity for each spectral band. Within a spectral band, each value is assumed to be independent of wavelength. By default, the same emissivity is defined on both sides. Select the Define properties on each side checkbox and fill the Upside and Downside columns of the table for a specific definition on each side.
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When Diffuse reflectivity is set to User defined for each band, enter a value for the Diffuse reflectivity for each spectral band. Within a spectral band, each value is assumed to be independent of wavelength. By default, the same reflectivity is defined on both sides. Select the Define diffuse reflectivity on each side checkbox and fill the Upside and Downside columns of the table for a specific definition on each side.
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When Transmissivity is set to User defined for each band, enter a value for the Transmissivity for each spectral band. Within a spectral band, each value is assumed to be independent of wavelength. By default, the same transmissivity is defined on both sides. Select the Define properties on each side checkbox and fill the Upside and Downside columns of the table for a specific definition on each side.
Set the surface emissivity  to a number between 0 and 1, where 0 represents a diffuse mirror and 1 is appropriate for a perfect blackbody. The proper value for a physical material lies somewhere in between and can be found from tables or measurements.
Set the surface diffuse reflectivity  to a number between 0 and 1, where 1 applies to a perfect diffuse surface and 0 is appropriate when reflection is only specular.
Set the surface transmissivity  to a number between 0 and 1, where 0 applies to an opaque surface and 1 is appropriate for a fully transparent surface.
The surface specular reflectivity, ρs, is then defined as
Set a value or expression for the Surface critical angle θc, which is the minimum angle of incidence (measured between the ray and the normal to the surface) for transmission to occur. Below this angle, all the radiation is reflected. The pi/2 default value ensures that total reflection does not occur by default.
When the Radiating side is set to None for a spectral band, the information set for this spectral band in this section is not used.
When this feature is applied on a boundary between two domains or between a domain and a void of different refractive indices, a critical angle is calculated from the Snell’s law. The effective critical angle used in computation is the minimum between the one from Snell’s law and the Surface critical angle.
Location in User Interface
Context Menus
Surface-to-Surface Radiation > Semitransparent Surface
Ribbon
Physics tab with Surface-to-Surface Radiation selected:
Boundaries > Surface-to-Surface Radiation > Semitransparent Surface