# ARTS built-in documentation server

## Workspace Method surfaceSemiSpecularBy3beams

### Description

A simplistic treatment of semi-specular surfaces. This method has no strong physical basis but could be used for simpler testing or as starting point for more advanced methods. This method assumes that the surface can be treated to have three facets, all lacking surface roughness. One facet is assumed to give standard specular reflection, while the two other facets are tilted with +dza and -dza, respectively. The tilt is assumed to only affect the zenith angle of the reflected direction (azimuth same as for specular direction). The area ratio of the non-tilted facet is set by *specular_factor*. That is, the specular beam is given weight w, while the other two beams each get weight (1-w)/2. If a facet tilts away from the viewing direction in such way that the surface is observed from below, the tilt of the facet is decreased in steps of 1 degree until a successful calculation is obtained. If this turns out to require a tilt of zero, this facete is merged with the specular direction. The pure specular properties of the surface shall be described by surface_rtprop_sub_agenda. That is, if you have specular surface described and you want to make it semi-specular by this method, you move the content of the existing surface_rtprop_agenda to surface_rtprop_sub_agenda and instead fill surface_rtprop_agenda with this method.

**Authors: **Patrick Eriksson

### Synopsis

surfaceSemiSpecularBy3beams( | surface_skin_t, surface_los, surface_rmatrix, surface_emission, atmosphere_dim, f_grid, rtp_pos, rtp_los, surface_rtprop_sub_agenda, specular_factor, dza ) |

### Variables

OUT | surface_skin_t | (Numeric) | Surface skin temperature. |

OUT | surface_los | (Matrix) | Downwelling radiation directions to consider in surface reflection. |

OUT | surface_rmatrix | (Tensor4) | The reflection coefficients for the directions given by surface_los to the direction of interest. |

OUT | surface_emission | (Matrix) | The emission from the surface. |

IN | atmosphere_dim | (Index) | The atmospheric dimensionality (1-3). |

IN | f_grid | (Vector) | The frequency grid for monochromatic pencil beam calculations. |

IN | rtp_pos | (Vector) | Position of a radiative transfer point. |

IN | rtp_los | (Vector) | Line-of-sight at a radiative transfer point. |

IN | surface_rtprop_sub_agenda | (Agenda) | Sub-agenda to surface_rtprop_agenda. |

GIN | specular_factor | (Numeric) | The weight given to the specular direction. Denoted as w above. A value between 1/3 and 1. |

GIN | dza | (Numeric) | Zenith angle seperation to each secondary direction. A between 0 and 45 degrees. |