Creating a lightguide

Overview

A light guide is a pipe guiding the light while extracting a ratio of this light in a specific direction using prisms. Line-shaped lit appearance is achieved by this element. You cannot extract light without a prism. The light continues to propagate (it is just a rod).
Total Internal Reflection happens when the light is incident to a surface at an angle greater than the critical angle.

$$ \theta_{crit} = sin^{-1} \left( \frac{n_2}{n_1} \right) $$

Adding a prism to extract light:
The effect of the prism is to make the incident angle smaller than the critical angle. Thanks to this, some part of the light is extracted. The optical shape of the prism depends on styling and regulation.

With the lightguide feature of Speos we will create a prsimatic lightguide.

OPD Lightguide creation

1. Open the Ansys Speos model "Headlamp_LightGuide.scdocx”
2. From the Design tab, click Light Guide
3. As guide curve for the light guide body select “Headlamp Demo Highest Ver_stp-FreeParts/Spline” and validate the selection
4. Define Lightguide General – Body with
   • Type: Circular shape
   • Profile diameter: 6 mm
5. In Lightguide definition General - Guide Curve set Reverse direction True to reverse direction of Light Direction. You can also reverse it by clicking on its arrow in the 3D view.

6. In General - Prisms Orientation choose Type Direction and as Optical axis select Y Axis. This will be the main direction we expect the light to be extracted from the light guide body by the prisms.
7. Set General – Distances as follows
   • Type: Curvilinear
   • Start: 30 mm
   • End: 16 mm
8. Set the following parameters for Prism Geometries
   • Step: Constant
   • Step value: 2mm
   • Trimming ratio: Control points
     Click "+" to add as many control points as needed.
     Set their Position along the curve. The position is expressed in % (a position of 50 places the control point in the middle of the guide). Define the parameter's value for each control point.

• • Offset: Constant
  • Offset Value: 2.7 mm
  • Width value: 2 mm
  • Start angle: Constant
  • Start angle value: 85°
  • End angle: Control points

9. From Design panel click Compute to build the lightguide geometry. The Light Guide is created and appears in the 3D view.
10. In Light Simulation tab double click on material “Light pipe”
11. Apply the material to "Light Guide.1"
12. Doubleclick the predefined "Lightguide" simulation and add "Lightguide.1" as Geometry to it
13. Click on Compute to run the simulation and check the results

Important settings

The source position impacts the following Prisms tab parameters: Start, End and Length.

The side of the source is indicated in the 3D view with a yellow arrow called Light Direction. You can also reverse the direction of Light Direction by clicking on its arrow in the 3D view.

For small prisms it is important to adjust the Meshing Parameters accordingly.

Light Guide Parameters

Lightguide Body Type

The body type allows you to determine what to base the body construction on.

Prism Orientation

From the Prisms Orientation Type drop-down list, select how the prism should be oriented in relation to the light guide body.

• Select Direction if you want all prisms to have the same orientation all along the guide curve. Light extracted through the prisms has the same direction. This type is particularly suited for linear light guides.

• Select Normal to surface if you want prisms to potentially have different orientations. Light extracted through the prisms has a different direction all along the guide curve. This type is suited for non-linear light guides.

Prism Geometries

You can adjust the geometries of the prisms with the following parameters.

For more information: Ansys User Guide/ Lightguide Parameters

Distances

Start and End define the size of the prism-free zones at the beginning and at the end of the guide curve.

• Start: enters a value to set the size of the prism-free zone at the beginning of the guide curve where the source is located.
• End: enters a value to set the size of the prism-free zone at the end of the guide curve.

Design Table

You can manage the following parameters, related to prisms, with a design table: Step, Length, Offset, Width, Start Angle and End Angle. A design table is an ANSYS SPEOS feature linking SPEOS parameters to values from an external file. This can be an Excel file (xls, xlsx, xlsm) or a text file (txt). You can use a Design Table to create a new Table, use an Existing Table or modify a Table.

1. From Design panel export Light Guide as CSV file
2. Modify the table for each prism and wanted parameter
3. Set required Prism Geometries parameter to Input file mode
4. Browse and load the generated table as Input.

Additional resources

• Ansys Speos Optical Part Design Advanced/ M05_LightGuide
• Application Gallery: Exterior Lightguide Optimization