photon_properties command

Purpose

Command for defining additional photon properties for the according custom material type (examples).

Warning

GPU support for this command has not been tested and may not work as expected.

Syntax

photon_properties keyword value

With the following keyword argument pairs:

Keywords	Description
minimum_photon_energy	the energy below which photons are deleted default: 1e-8; units: [energy]
optical_density	the optical density or refractive index of the medium filling the simulation domain default: 1, i.e. air; units: [dimensionless]
reflection_law	the reflection law to use, either `Schlick` or `Fresnel` or `Fresnel_unpolarized` default: `Schlick`

Examples

photon_properties reflection_law Fresnel minimum_photon_energy 1e-16

Description

Customize photon properties like reflection law, etc.

Note

This command is not required. If omitted the default values will be used.

The reflection law calculates the energy retained by the photon after reflection on mesh or particle surfaces. In terms of surface reflectivity as a function of incident angle $R(\theta)$ the three availble options read:

Schlick

$R_0 &= \left( \frac{n_1 - n_2}{n1 + n_2} \right)^2 \\ R(\theta) &= R_0 + (1 - R_0)(1 - \cos\theta)^5$
Fresnel Depending on the contributions in (subscript $\mathrm{s}$ ) and normal to (subscript $\mathrm{p}$ ) the plane of incidence:

$R_\mathrm{s} &= \left( \frac{n_1 \cos\theta - n_2 \sqrt{1 - \left( \frac{n_1}{n_2} \sin\theta \right)^2}} {n_1 \cos\theta + n_2 \sqrt{1 - \left( \frac{n_1}{n_2} \sin\theta \right)^2}} \right)^2 \\ R_\mathrm{p} &= \left( \frac{n_1 \sqrt{1 - \left( \frac{n_1}{n_2} \sin\theta \right)^2} - n_2 \cos\theta} {n_1 \sqrt{1 - \left( \frac{n_1}{n_2} \sin\theta \right)^2} + n_2 \cos\theta} \right)^2 \\ R(\theta) &= |p_\mathrm{s}|^2 R_\mathrm{s} + |p_\mathrm{p}|^2 R_\mathrm{p}$

with $p_\mathrm{s}$ and $p_\mathrm{p}$ the polarization split into its component in and normal to the plane of incidence, respectively.
Fresnel_unpolarized This reflection law works in the same manner as the Fresnel law but calculates the reflectivity without considering the polarization:

$R(\theta) = \frac{1}{2} (R_\mathrm{s} + R_\mathrm{p})$

In the above equations $n_1$ is the refractive index of the medium set by the optical_density keyword and $n_2$ that of the wall material set by that material’s refractiveIndex property.

Restrictions

Must be called after setting up the particle_contact_model command.

Default

reflection_law : Schlick
optical_density : 1.
minimum_photon_energy : 1e-8