Description

This feature adds absorption to a material definition using either absorption coefficients or internal transmittance.  Automatic data conversion options are available by right mouse clicking in the data spreadsheet area.

NOTE: Absorption entered by this method overrides imaginary refractive indicies.

Navigation

This feature can be accessed in the following ways:

•In a materials dialog select the "Absorption" tab.

Controls

Application Notes

Absorption model

This absorption model follows Beer's Law; I/I_o= Exp[-aL] and data is specified in inverse system units.  In this model, a is the absorption coefficient, I/I_o is the internal transmittance and L is the reference distance.  Note that as the absorption coefficient becomes large (the internal transmission going towards zero) the material behaves like a metal and the reflection/transmission properties of objects whose coatings depend on the material model will change accordingly.

Active option

If the active box is checked, the imaginary indicies specified in the Materials tab of the dialog are overridden. If there are no entries on the absorption Tab and the active box is checked, FRED assumes zero absorption and still overrides the imaginary refractive indicies.

Reading data from file

Internal Transmittance or Absorption coefficient data can be read from text files by right mouse clicking in the data table area and selecting "Replace Data with File Data" from the list menu.

Data files have a two line header followed by two columns data (wavelength  value) delimited by a tab or a space.  The first header line must read "type materialabsorption". The second header line specifies the data type. For Internal Transmittance, the second line must read "format internal thk", where thk is the reference thickness in system units.  For absorption coefficient data, the second line must read "format absorption". Example file formats are shown below.

Copying data from spreadsheet

Internal Transmittance or Absorption coefficient data can also be copied directly from an Excel spreadsheet into the data area.  The data should be formated as two columns; column 1 being wavelength in microns, and column two being Internal Transmittance or Absorption coefficient (system units). Consider the following example. The data shown below in Excel has been highlighted and copied to the clipboard (Ctrl+C).

In FRED, left mouse click in the empty data table area such that the entire empty row is highlighted.

Now Paste the data using Ctrl + V into the table (Note: It may be necessary to click once on the scroll bar at the right of the data table to display the data if the list is longer than the current table area):

Converting absorption to internal transmittance

Right mouse clicking in the data spreadsheet area of the absorption tab provides access to two menu options, "Convert internal transmittance to absorption" and "Convert absorption to internal transmittance".  Depending on the specification of the data as Internal Transmittance or as Absorption Coefficient (inverse sys units) only one of these options will be available.  These menu options allow automatic conversion of the entered data into either format using Beer's law.

When converting from absorption to internal transmittance a dialog will be shown prompting the user for the reference distance to be used in the conversion.

Related Topics

Create / Edit Material

Volume Scatter