Description

This tutorial describes the creation of a simple plano-plano lens with scatter properties assigned to it such that an incoming ray scatters back towards its origin.

The accompanying FRED file, tutorialBackScatter.frd, is located in the <install dir>\Resources\Samples\Tutorials & Examples\ directory.

Implementation

Step 1 - Open a document and add a lens

First, create a new FRED document and add a lens to the system with the default settings. This will create a plano-plano lens at the center of the global origin.

Step 2 - Add an optical source

Right click on the Optical Sources folder and select “Create New Detailed Optical Source” from the list menu.  On the Location/Orientation tab, right click in the operations list and select “Append”.  Change the action for this modifier to “Rotate about X-axis”. Set the X angle to 45°. Append another modifier and choose Shift for the action. Set the Y value to 0.5 and the Z value to –0.5. The dialog should look like like the following (press OK):

Step 3 - Modify the scatter model

Open the Scatterers folder, right click on the White Lambertian model and select “Edit/View Scatterer…”.  In the resulting dialog check “Apply On Reflection”, uncheck both “Apply On Transmission” and “Halt Incident Ray” at the bottom of the dialog, and then press OK.  With these settings, the model will only generate scatter in the reflected hemisphere and the specular ray will continue propagating through the system after it intersects the scattering surface (assuming the coating and raytrace property of the surfaces allow it).  These settings are shown below:

Step 4 - Apply a coating and raytrace control to the lens surface

Expand the Geometry folder and the Lens 1 node. Right mouse click on Surface 2 and select “Edit/View Surface…” from the context menu. Switch to the Coating/RayControl tab and select "Standard Coating" in the list of available coatings and then press the Assign button.  Similarly, select the Allow All raytrace control from the list of available raytrace controls and hit the Assign button.  Press the Apply button on the dialog.  The Coating/RayControl tab of the Surface 2 dialog is shown below:

Step 5 - Assign surface scatter and importance sampling

Switch to the Scatter tab of the Surface 2 dialog, select the White Lambertian scatter model from the list of available scatter properties and press the Assign button. The bottom portion of the dialog defines any Scatter Direction Region(s) of interest, which specifies the directions into which rays should scatter from the surface.  Every surface has a "Default" scatter direction region of interest, shown in the dialog below, which will generate scattering into the forward or reverse hemisphere relative to the surface normal if the scatter model allows it.  No modification to the default importance sample needs to be made, though the importance sample definition can be viewed by selecting the "Default" entry in the Scatter Direction Region(s) of Interest and then pressing the Edit/View button.

Step 6 - Trace and render

Press the Trace and Render button (  ) on the toolbar, or go to Raytrace > Trace and Render.  In the 3D view, shown below, a solid ray is seen traveling into the lens from the upper left and then 10 scattered rays are generated in reflection at Surface 2.  The dashed lines on the screen indicate that rays are propagating along those directions out to infinity (i.e. they are not stopped from propagation by any objects in the model).  It can also be seen that some scattered rays return back to Surface 1 and are halted.  This is because the Raytrace Property on Surface 1 of the lens element does not allow TIR to occur.

Related Topics

Tutorial Summary