Speos Scattering Material File Format

Speos Scattering Materials

This article shows how to create a volume scattering file in Speos format based on raw measurement data using Henyey-Greenstein, double Henyey-Greenstein or Gegenbauer formulas.

MIE Theory will be covered in a separate article.

The Speos *.material format models volume/bulk optical properties that are applied to a solid body, not on a surface.

Overview

The following data is needed to create a scattering material file:

the variation of the absorption coefficient [mm^-1] based on the wavelength.
the variation of the refraction index [unitless] based on the wavelength.
the diffusion coefficients [mm^-1] and phase function factor(s) [unitless]

The data can either be processed in the User Material Editor, requiring a Speos license, or in plain text format *.txt without need of a Speos license. Any change in the text file will appear in the User Material Editor interface when saved with the *.material file extension.

The resulting .material file will then be used in Speos as input to model volume optical properties.

Step 1: Index Variation – Defining the dispersion curve

Described in Speos Non-Scattering Material File Format article.

Step 2: Absorption Variation

Described in Speos Non-Scattering Material File Format article.

Step 3: Scattering Properties

The diffusion coefficient [mm-1] is given for each wavelength [nm].

Step 4: Scattering Phase Function

Option 1: by giving the scattering efficiency according to the scattering angle
Option 2: according to the Henyey-Greenstein model
Option 3: according to the double Henyey-Greenstein model
Option 4: according to the Gegenbauer model
Option 5: according to the MIE Theory (detailed in a separate article)

Note:
Isotropic Material Type will be used in this article, but Birefringent, Fluorescent and Metallic types are also possible.

MIE Theory will not be covered in this article.

Prerequisites

More wavelengths will result in a more precise description of the material.
A text editor is required to create the file. A Speos license is not required.

Step 1 and 2 : Dispersion and Absorption Properties

Steps 1 and 2 are covered in the article Speos Non-Scattering Material File Format

Row 1
Row 2
Row 3
---
Row N-2
Row N-1
Row N

OPTIS - Material file v13
Demo material 
Isotropic
--- Dispersion & Absorption ---
1
1
1

Header (not to be changed)
Description
Material Type
--- Dispersion & Absorption ---
Measured concentration
User concentration
1 if material has scattering properties, 0 if not

Once the text file contains N rows based on Steps 1 and 2, the following rows will be added to describe the scattering properties of the material.

Step 3: Scattering Properties

The scattering value for each wavelength is then given, preceded by the number of wavelengths:

Row N
Row N+1
Row N+2
Row N+3
Row N+4
Row N+5
Row N+6

1
OPTIS - Volumic Scattering file v1
0
3
480 1.6e-02
555 0.001
650 2.5e-02

1 if material has scattering properties, 0 if not
Description of the scattering part
Phase function: 0=User-Defined, 1=Henyey-Greenstein, 3=MIE, 5=double Henyey-Greenstein, 6=Gegenbauer
Number of values for the scattering (diffusion)
Wavelength, Diffusion value at the given wavelength 
Wavelength, Diffusion value at the given wavelength 
Wavelength, Diffusion value at the given wavelength

Note: All the data is written in the same file, even if there is a second header "OPTIS - Volumic Scattering file v1" to introduce the scattering description.

More information about is available in the Speos Labs User's Guide: Scattering Properties (ansys.com)

Step 4: Scattering Phase Function

We will describe 4 ways to define the scattering phase function. Only one is required.

More information on the scattering phase function is available in the Speos Labs User's Guide: Scattering Phase Function (ansys.com)

Option 1: by giving the scattering efficiency according to the scattering angle

Row N
Row N+1
Row N+2
Row N+3
Row N+4
Row N+5
Row N+6
Row N+7
Row N+8
Row N+9
Row N+10
Row N+11
Row N+12
Row N+13

1
OPTIS - Volumic Scattering file v1
0
3
480 0.1
555 0.1
650 0.1
3
4
480 550 650
0 0 0 100 
45 0 0 100 
90 50 100 100 
180 100 50 100

1 if material has scattering properties, 0 if not
Description of the scattering part
Phase function: 0 for user defined scattering phase function
Number of values for the scattering (diffusion)
Wavelength, Diffusion value at the given wavelength
Wavelength, Diffusion value at the given wavelength
Wavelength, Diffusion value at the given wavelength
Number of wavelengths for the scattering phase function (1=no wavelength dependency)
Number of Theta angles for which the relative scattering intensity is defined
Wavelengths (skip if no wavelength dependency defined)
Theta angle, relative intensity for each wavelength (only one needed if no wavelength dependency)
Theta angle, relative intensity for each wavelength (only one needed if no wavelength dependency)
Theta angle, relative intensity for each wavelength (only one needed if no wavelength dependency)
Theta angle, relative intensity for each wavelength (only one needed if no wavelength dependency)

Option 2: according to the Henyey-Greenstein model

Row N
Row N+1
Row N+2
Row N+3
Row N+4
Row N+5
Row N+6
Row N+7
Row N+8
Row N+9
Row N+10

1
OPTIS - Volumic Scattering file v1
1
3
480 0.1
555 0.1
650 0.1
3
480 0.8
555 0.9
650 0.91

1 if material has scattering properties, 0 if not
Description of the scattering part
Phase function: 1 for Henyey-Greenstein scattering phase function 
Number of values for the scattering/diffusion
Wavelength, Diffusion value at the given wavelength
Wavelength, Diffusion value at the given wavelength
Wavelength, Diffusion value at the given wavelength
Number of wavelengths for the anisotropy factor(1=no wavelength dependency)                      
Wavelength (skip if no wavelength dependency), anisotropy factor
Wavelength, anisotropy factor (skip line if no wavelength dependency)
Wavelength, anisotropy factor (skip line if no wavelength dependency)

Option 3: according to the double Henyey-Greenstein model

Row N
Row N+1
Row N+2
Row N+3
Row N+4
Row N+5
Row N+6
Row N+7
Row N+8
Row N+9
Row N+10

1
OPTIS - Volumic Scattering file v1
5
3
480 0.1
555 0.1
650 0.1
3
480 0.91 0.9 0.45
555 0.8 0.95 0.5
650 0.8 0.96 0.51

1 if material has scattering properties, 0 if not
Description of the scattering part
Phase function: 5 for double Henyey-Greenstein scattering phase function 
Number of values for the scattering/diffusion
Wavelength, Diffusion value at the given wavelength
Wavelength, Diffusion value at the given wavelength
Wavelength, Diffusion value at the given wavelength
Number of wavelengths for the anisotropy factors and ratio (1=no wavelength dependency)          
Wavelength (skip if no wavelength dependency), anisotropy factor 1 & 2, ratio 
Wavelength (skip if no wavelength dependency), anisotropy factor 1 & 2, ratio
Wavelength (skip if no wavelength dependency), anisotropy factor 1 & 2, ratio

The ratio is the weight between anisotropy factor 1 and 2.

Option 4: according to the Gegenbauer model

Row N
Row N+1
Row N+2
Row N+3
Row N+4
Row N+5
Row N+6
Row N+7
Row N+8
Row N+9
Row N+10

1
OPTIS - Volumic Scattering file v3.0
6
3
480 0.1
555 0.1
650 0.1
3
480 0.9 0.45
555 0.95 0.5
650 0.96 0.51

1 if material has scattering properties, 0 if not
Description of the scattering part (v3 needed here)
Phase function: 6 for Gegenbauer scattering phase function 
Number of values for the scattering/diffusion
Wavelength, Diffusion value at the given wavelength
Wavelength, Diffusion value at the given wavelength
Wavelength, Diffusion value at the given wavelength
Number of wavelengths for the anisotropy factor and alpha coefficient (1=no wavelength dependency)
Wavelength (skip if no wavelength dependency), anisotropy factor, alpha coefficient 
Wavelength (skip if no wavelength dependency), anisotropy factor, alpha coefficient
Wavelength (skip if no wavelength dependency), anisotropy factor, alpha coefficient

Updating the model with your parameters

Once all four steps have been implemented in the *.txt file, the extension of the file can be changed to *.material which can be opened using the User Material Editor, requiring a Speos license.

The curves, comments and parameters entered in the text file can be viewed and modified in the User Material interface. For example, modify the way the scattering coefficient and phase function are defined:

Speos - User Defined Bulk Scattering File.png

Taking the model further

The material types can be set to Birefringent, Fluorescent and Metallic. More information about these types is available in the Speos Labs User's Guide: General (ansys.com)
The scattering properties can be defined according to MIE theory. More information about this is available in the Speos Labs User's Guide: Particles (ansys.com)

Additional Resources