OSRAM TOPLED LS-T676

The goal of this Advanced Level Tutorial is to introduce advanced concepts and techniques including 1) constructing a source model from vendor ray data and mechanical CAD, and 2) configuration and optimization of a freeform lens [FRED Optimum Only].

Note: The videos included in this tutorial are hosted by YouTube and an internet connection is required to view them.  Controls embedded into the video player will allow you to view the videos in full screen and/or change the video resolution.

Sources from Vendor Data

In the previous sections of this tutorial we represented the LED source is idealized Lambertian emitters using the Lambertian Plane and LED (far-field) type Source Primitive specifications.  Manufacturers of LEDs, such as OSRAM and LumiLeds, do provide resources for optical simulations on their websites.  These resources include ray data files, which are based on luminance measurements of actual sources, spectrum data files and mechanical CAD files.

In this section of the tutorial, we will upgrade the source model representation by using the rayfile data describing the source's radiance and the mechanical CAD model describing the physical structure of the LED.

To begin, download the ZIP archive, "rayfile_LS_T676_20170118_ASAP.zip", from the OSRAM website.  When the archive is extracted you will find three rayfiles with the *.DIS file extension, one each describing 100K, 500K and 5M total rays.  These binary DIS files describe the starting power, position and direction of individual rays emitting from the LED.  Additionally, the STEP file describes the physical geometry of the LED.

Next, delete from the model the prism that was glued to the exit face of the lightpipe and make both of the existing source nodes Not Traceable by right mouse clicking on each and selecting the "Traceable" option at the top of the context menu.

Our first step in building the new source model will be to import the LED package geometry into our existing FRED model. CAD Import is initiated by one of the following methods:

1.From the Main Menu, File>Import>Import CAD

2.Keystroke Ctrl+Shft+J

3.File Toolbar button

Initiate an import of CAD geometry and select the file "LS_T676_20141217_geometry.STEP". The CAD Import dialog should appear as shown in Figure 3-1. The Destination option should be set to Import into Current Document and the Surface Drawing Mode left in its default setting Wire frame (fast). Select the Create button to import the geometry. The Dismiss button must be pressed once the import is complete to close the dialog box. For further information concerning the options in this dialog, consult the Help manual under the topic CAD Import. The LED package will be imported as a subassembly with its local origin located at the Geometry coordinate system origin.

Figure 3-1. CAD Import dialog for import of LS_T676 packaging.

Right mouse click on the "Geometry.ls_t676_20141217_geometry" subassembly node and select the Position/Orientation option from the context menu.  In the resulting position/orientation dialog, right mouse click on row 0 in the Location section and choose the "Append" option.  Set the Z component of the new Shift operation type to -0.1 and then hit OK on the dialog.  This shift, as shown in figure 3-2 below, has the effect of moving the CAD model of the LED geometry away from the origin by -0.1 mm.

Figure 3-2.  Position/Orientation modifications of the LED geometry subassembly node.

The next step is to create a new source representation of the LED emission by using the rayfile data downloaded from the OSRAM website.  Create a new Source Primitive node of the "Rayfile Source" type by using one of the following options:

          1. Right Mouse Click on the Optical Sources folder, select Create New Source Primitive > Rayfile Source

          2. Menu option, Create > Source Primitive > Rayfile Source

Name the new source, OSRAM LS-T676 Rayfile, and then press the "Select" button on parameter #2 of the source primitive.  Select the rayfile_LS_T676_100k_20170118_ASAP.DIS rayfile that was downloaded from the OSRAM website.  Note that FRED will present the dialog shown below in Figure 3-3, indicating that the ray data has a total power of 0.004 Watts.  Press OK on this dialog.  Keep in mind that although the rayfiles do come with a specified total ray flux, the specification sheet for the LED being represented should be consulted to determine whether the supplied value is appropriate.  The appropriate power value will depend on the spectral variant of the LED, operating conditions, quality bin, etc.

Figure 3-3. Source power warning upon loading of rayset.

We will be representing the Super-Red variant of the LS-T676 package, which was previously set to emit 0.61 Lumens in the Intermediate tutorial.  We will use the same power specification and spectrum for the rayfile source as was used in the previous tutorial sections.

Update the following settings of the source node, as shown in Figure 3-4:

•Power = 0.61

•Power Units = Lumens

•Wavelength Attributes - Spectrum = OSRAM LS-T676

•Source Draw Color = Red

•Starting Location/Orientation = Geometry.ls_t676_20141217_geometry  (this is the Subassembly node of the CAD import)

Figure 3-4: Configured Rayfile Source dialog

Figure 3-5 shows the starting ray position distribution sitting just in front of the LED geometry.  Although some number of rays can be seen in the 3D view to start from outside the nominal emitting aperture of the LED geometry, these rays will contribute negligible power to the total power of the source.  These outlier rays are typical of rayfile data and their contributions to the resulting analyses will be inconsequential.

Figure 3-5. LS_T676 package and rayset.

In some applications, the mechanical model of the LED may be relevant.  In such a scenario, the appropriate properties should be assigned to the surfaces of the imported CAD geometry so that the physical interactions between the LED mechanical structure and the rest of the system can be captured.  For our purposes, the CAD representation is primarily cosmetic and its properties will be left as the default - Absorbing.

As the final exercise in this section, we will perform a raytrace of the new rayfile source and the Lightpipe, and then calculate the Illuminance on the exit face of the Lightpipe.  That the following actions after double checking that the only traceable source in your Optical Sources folder is the OSRAM LS-T676 Rayfile source node:

•Menu item, Raytrace > Trace All Sources

•Menu item, Analyses > Illuminance.  Use the "Irradiance at exit face" analysis surface and choose the Output Units as LUX (lm/m^2).

Figure 3-6 shows the resulting Illuminance distribution at the output of the Lightpipe, which is nearly identical the Illuminance calculated in the Intermediate section of this tutorial using the LED (far-field) source representation.  Additionally, data printed to the Output Window as part of the Illuminance analysis indicates that the total integrated power in the Illuminance distribution is 0.52 Lumens.

Figure 3-6. Illuminance distribution at the Lightpipe exit face using the rayfile source model

Freeform Lens Construction

In this section, we turn away from the lightpipe model and begin to focus on the design of a freeform lens with the intention of mapping the Lambertian emission of the LS-T676 LED into a square irradiance distribution on a plane some distance away from the light source.

Start by deleting the Lightpipe node from the Geometry folder as well as the analysis surface and directional analysis entity nodes from the Analysis Surface(s) folder.  The model should be reduced so that it contains only the LED source nodes and the imported CAD geometry of the LED, as shown in Figure 3-7.

Figure 3-7.  Model view and object tree at the start of the freeform lens construction.

The freeform lens will be defined as a Custom Element in the Geometry folder of the object tree, meaning that we must define the individual surfaces of the freeform lens manually.  Create a Custom Element node on the object tree using one of the following methods:

•Menu bar, Create > Custom Element

•Right mouse click on the Geometry folder and select, Create New Custom Element from the context menu

Name the element, "Freeform Lens" and shift the element by Z=5 mm the Global Coordinate System as shown in Figure 3-8.

Figure 3-8.  Custom Element creation for the Freeform Lens.

Create the first surface of the freeform lens by right mouse clicking on the new Freeform Lens Custom Element node and selecting Create New Surface from the context menu.  The surface being created will have a 20 mm diameter elliptical aperture and be of the surface type, Super-Gaussians Super Position Surface (referred to as the SGSS from here on).  The surface profile of the SGSS is formed by adding super-gaussian functions together, where each function is allowed to have varying amplitude, centration on the surface, width, exponents and orientation.  In our application, we will use three rectangular super-gaussian functions of orders 2, 4 and 6 and ultimately allow FRED's optimization engine to determine the appropriate amplitude and widths of the different terms.  The video clip in Figure 3-9 shows the construction of the first SGSS in the Freeform Lens element, where the initial amplitudes of all terms are set to 0 and the initial widths are set to 5.

Figure 3-9. Construction of the first surface of the Freeform Lens.

Note that in the above video, the Z semi-aperture of the surface was set to 5mm.  Loosely bounding the surface with a +/-5mm possible surface sag in the Z direction allows the surface to bend with aggressive curvature values without risking being clipped by the Z semi-aperture.  The second surface of the Freeform Lens will be a duplicate of the first surface, shifted in Z by 5 mm.  To accomplish this:

1. Right mouse click on Surf 1 of the Freeform Lens and choose the Copy option.

2. Right mouse click on the Freeform Lens custom element and choose the Paste option.

3. Right mouse click on the new Surf 2 node of the Freeform Lens and choose the Position/Orientation option.  Append on an operation and enter a Z shift of 5.

The image in Figure 3-10 shows the object tree and 3D view at this point in the Freeform Lens construction process after the second surface is added.

Figure 3-10.  Object tree and 3D view of the Freeform lens with front and back surfaces.

The last piece of geometry needed for the Freeform Lens is a cylindrical surface that will serve as the lens edge connecting the front and back surfaces.  In the construction of this edge cylinder, the length will be dramatically oversized to account for the fact that the front and back surface curvatures will be varying during optimization and the final shape of the lens surfaces are unknown.  To accommodate the variation in the surface shapes, surface trimming operations will be used to cut the cylinder in such a way that only the region of the cylinder between the front and back surfaces is kept.

Right mouse click on the Freeform Lens custom element and select the Create New Surface option from the context menu.  The name of the surface will be "Edge" and the surface type will be set to the Cylinder option, with 10 mm semi-apertures for the cylinder's Front End and Back End.  On the Aperture tab of the surface dialog, the X and Y semi-apertures of the Trimming Volume Outer Boundary will be set to 11 mm and the Z semi-aperture will be set to 7.5 mm with its center at 2.5 mm.  In other words, the center of the cylinder length is halfway between the first and second surfaces of the Freeform Lens and it extends 7.5 mm in either direction from that point.  The Trimming Volume Inner Hole of the cylinder will be set to a 9 mm semi-aperture in X and Y.  Press Apply on the surface dialog to create the initial cylinder surface, but leave the surface dialog open.

The video in Figure 3-11 shows the initial construction of the edge surface as described above.

Figure 3-11.  Construction of the Edge surface of the Freeform Lens.

The resulting cylinder lens is clearly oversized in its length relative to the thickness of the Freeform Lens.  To address this, we turn to the bottom portion of the surface dialog labeled, "Surface Trimming Specification".  In this portion of the dialog, we specify how the Edge surface will be trimmed (i.e. cut) by other surfaces or curves in the model.  The small spreadsheet labeled, "Operation List View", in this portion of the dialog is where we will build up our surface trimming specification.

Right mouse click on row 0 in the operation list view and choose the "Select Entity" option.  In the resulting entity picker control, expand the mini-tree view and select Surf 1 of the Freeform lens node.  Hit OK on the entity picker control to complete the selection.  Hit Apply on the surface dialog to commit the change and then inspect the updated 3D view.  Note that the portion of the cylinder to the left of Surf 1 is now trimmed away.  The current object tree, 3D view, and Edge surface dialog is shown in Figure 3-12.

Figure 3-12. Edge surface being trimmed by Surf 1 of the Freeform Lens.

Next, we return to the Edge surface dialog trimming specification in order to remove the portion of the cylinder surface that lies to the right side of Surf 2 of the Freeform Lens.  To accomplish this, we append on three more items to the trimming operation list view.

•Right mouse click on Row 1 and choose the "AND" operation

•Right mouse click on Row 2 and choose the "NOT" operation

•Right mouse click on Row 3 and choose the "Select Entity" operation, and then choose Surf 2 of the Freeform Lens node.  Hit OK on the entity picker control

After hitting Apply on the surface dialog and inspecting the 3D view, you should see that only the portion of the Edge surface between the front and back surfaces of the lens remains.  The video in Figure 3-13 demonstrates the second trimming sequence for the Edge surface.

Figure 3-13. Completion of the Edge surface trimming to include Surf 2 of the Freeform Lens.

To complete the setup of our custom Freeform Lens, we will configure the surface and material properties and also set the Visualization Attributes.  Take the following actions to setup the surface properties on the Freeform Lens surfaces.

•Drag and drop the PMMA material node onto the Freeform Lens custom element.  Press OK on the resulting dialog, which has the effect of replacing one of the existing Air materials assignments at each surface with the PMMA material.

•Drag and drop the Transmit coating node onto Surf 1 of the Freeform Lens custom element.

•Drag and drop the Transmit coating node onto Surf 2 of the Freeform Lens custom element.

•Drag and drop the Transmit Specular Raytrace Property node onto Surf 1 of the Freeform Lens custom element.

•Drag and drop the Transmit Specular Raytrace Property node onto Surf 2 of the Freeform Lens custom element.

Confirm that the appropriate surface properties are assigned to each of the surfaces in the Freeform Lens custom element by hovering your mouse over the icon of each surface on the tree and inspecting the popup summary, as shown in Figure 3-14.  The Edge surface should be left with the Absorb coating and Halt All raytrace property.

Figure 3-14. Surface property summary for Surf 2 of the Freeform Lens.

Finally, right mouse click on the Freeform Lens custom element node and select the Visualization Attributes option from the context menu.  In the resulting dialog, toggle the "Reset Draw Mode" box on the upper left and also toggle the "Reset Opacity" toggle on the middle right.  Hit OK on the Visualization Attributes dialog to commit the changes.  The updated 3D view should appear as shown in Figure 3-15.

Figure 3-15.  Freeform Lens with updated visualization attributes.

Illumination Plane and Analysis Surface

The goal of the optimization is to find the freeform lens parameters which (a) maximize the power on a distant plane surface, and (b) produce an irradiance distribution with the minimum variance (i.e. best uniformity).  In this section, we define the distant plane and the analysis surface that will be used to evaluate the irradiance during optimization.

Right mouse click on the Geometry folder and choose, Create Element Primitive > Plane.  Give this plane the name, "Illumination Plane", set its X and Y semi-apertures to 1,000 mm, and shift the plane in Z by 1000 mm.  The final dialog for the illumination plane is shown in Figure 3-16.

Figure 3-16.  Illumination Plane primitive.

Next, right mouse click on the Analysis Surface(s) folder and select, New Analysis Surface.  In the resulting dialog, name the analysis surface "Illumination Analysis", set the aperture limits to go from -1000 to +1000 mm, set the location to be at Geometry.Illumination Plane.Surface, and set the ray selection filter to isolate only "Rays on surface Geometry.Illumination Plane.Surface".  The configured analysis surface dialog is shown in Figure 3-17.

Figure 3-17.  Configured Analysis Surface dialog at the Illumination Plane.

Configuration of Parameter Pickups [FRED Optimum Only]

The goal of the optimization will be to convert the lambertian output of the LED into a square irradiance distribution on a plane some distance away.  Rather than allowing the X and Y terms in the SGSS surface to vary independently during optimization, we take advantage of the symmetry in the source and optimization target by establishing a dependent link of the Y parameter values on the X parameter values.  This link between model parameters is called a Parameter Pickup.  If, for example, the value of an X parameter in the SGSS surface is modified, the dependent Y parameter that is linked via a parameter pickup will be automatically updated.  Establishing this link between X and Y parameters will reduce the number of optimization variables required and lead to faster optimization convergence.

Figure 3-18 below shows the SGSS parameters for Surf 1 of the Freeform Lens element.  There are three terms in the SGSS surface, indexed 0 to 3 in the left most column under the Term # header.  During optimization, the coefficients (i.e. the amplitude of the gaussian functions) will all be allowed to vary.  Additionally, the Sx values (width in X) will be allowed to vary during optimization.  The Sy values, however, will be linked to the Sx values via parameter pickups and therefore update automatically during optimization.

Figure 3-18.  SGSS parameters for Surf 1 of the Freeform Lens.

Open the Parameter Pickups interface by navigating to the menu item Optimize > Parameter Pickups.  For the purpose of this example, it will be sufficient to configure only the columns underneath the Destination Variable and Source Variable headings.  As an example, consider Figure 3-18 above.  To access parameter Sx of Term #1 from Surf 1 of the Freeform Lens, we would use the Entity specification to select Surf 1, choose the Type specification to be "SuperGaussian width Y" and then set the Index specification to be 1 (corresponding to the Term #).  The video in Figure 3-19 shows the procedure for specifying the Sx parameters of Surf 1 as the Source Variables and then specifying the Sy parameters of Surf 1 as Destination Variables.

Figure 3-19.  Configuring the Sx and Sy parameter pickups for Surf 1.

Repeat the process shown in Figure 3-19 for the Sx and Sy parameters of Surf 2.  The final Parameter Pickup configurations are shown below in Figure 3-20.

Figure 3-20.  Fully configured parameter pickups dialog, linking the Sy widths of the SGSS surfaces to the Sx widths.

Configuration of the Optimization [FRED Optimum Only]

Three sets of inputs are required in order to configure the optimization engine in FRED:

1. Variables: This set of information indicates the parameters of the model that will be varied during optimization.  In our application, the variables will be the amplitude coefficients and X semi-widths of the terms on the front and back SGS surfaces of the freeform lens.

2. Aberrations: A merit function will be formed during optimization and used to determine whether the optimization process is moving the variables in the direction of the best configuration.  The merit function itself is formed by defining individual aberrations to be evaluated (ex. RMS spot size, total power, irradiance variance) and then taking the weighted sum of the squares of the aberrations as the final merit function value.  In our application, the aberrations will be configured to maximize the total power on the illumination plane and to minimize the variance in the irradiance distribution at the illumination plane.

3. Method: The optimization engine has three modes which could be used for multi-variable optimization; simplex, simplex with multiple restarts, and simplex with simulated annealing.  The mode to be used must be specified in addition to the convergence criteria that will determine when the optimization is converged.  In our application, the Simplex mode will be used and the optimization stopping/convergence criteria will be set to allow 15 iterations of the Simplex.

Open the optimization configuration dialog by going to Optimize > Define/Edit.  Expand the dialog horizontally to view all columns by grabbing the right edge of the dialog with your mouse and stretching its width.  The initial dialog is shown in Figure 3-21.

Figure 3-21.  Initial appearance of the Optimization Define/Edit dialog.

The general work flow for defining optimization variables is the sequence below.  The optimization variables help topic contains specific information regarding the different variable types.

•Click in the Entity column and then use the entity picker control to select the node whose parameter is being varied.

•Use the Type column to specify the parameter type that is being varied.

•Enter the Index # and Subindex # as appropriate (refer to the optimization variables help for specific information) for the variable type

•Enter a Lower Limit and Upper Limit, between with the variable will be constrained

•Right mouse click on the Current Value cell and choose, "Retrieve Current Value(s) from Doc"

As used previously in the configuration of the Parameter Pickups, the Index # in the optimization variables dialog is used to point to the Term # on the SGSS whose parameter is being varied.  The video in Figure 3-22 demonstrates how to configure the amplitude coefficients of the three terms on the first SGSS of the freeform lens.  The amplitude coefficients will be allowed to vary over a range from -2 to +2.  The Frac Step parameter, which lies between 0 and 1, can be used to control the size of the initial search of the Simplex and while it is not required to be modified form the default value of 0.1, some experimentation may lead to a value which allows for faster convergence.  With the benefit of foresight, we will set the Frac Step parameter to 0.5.

Figure 3-22.  Configuring optimization variables for the coefficients of the three SGSS terms on the first surface of the freeform lens.

Repeat the process above in order to configure optimization variables for the SuperGaussian width X parameters of the three terms on the first SGS surface of the freeform lens.  The parameter limits should be set to range from 1 to 10, with a Frac Step of 0.5.  After setting up the variables, right mouse click in the Current Value column and select the option, "Retrieve Current Value(s) from Doc".  The optimization variables tab should appear as shown in Figure 3-23 up to this point.

Figure 3-23.  Optimization variables configured for the first surface of the freeform lens.

Lastly, define the SuperGaussian coefficient and SuperGaussian width X variables for the second surface of the freeform lens as shown in Figure 3-24.  In addition, toggle the "Soft limits (apply penalty)" option at the bottom of the dialog.  If, during optimization, the variables wander outside of the allowed range established by Lower Limit and Upper Limit, this option will apply a penalty to the merit function in order to drive the optimization variable back into the allowed range.  The Hard limits option will simply halt the optimization process if the variables wander outside of their allowed range.

Figure 3-24.  Completed optimization variables dialog for both surfaces of the freeform lens.

Next, we move to the Merit Function Aberrations tab of the optimization dialog, which is shown in Figure 3-25.

Figure 3-25. Initial appearance of the Merit Function Aberrations tab on the optimization dialog.

Two aberrations will be defined in the merit function, one which maximizes the power on the illumination plane and one which minimizes the variance of the irradiance distribution on the illumination plane as a metric driving the uniformity of the illumination.

An aberration's contribution to the merit function is given by w * (A - T)² , where w is a weighting factor, A is the computed aberration value, and T is the target value that you would like the aberration to achieve.

When maximizing power on a surface the aberration value is computed as A = 1/P, where P is the power on the target surface.  The OSRAM LS-T676 LED has a radiometric power of 0.00463 Watts.  If all of the power in the source were to illuminate the analysis plane, the value of the aberration would then be 1/0.00463 = 216, and we enter this value as the Target of the aberration.  The video in Figure 3-26 shows how to add this aberration to the Merit Function Aberrations tab of the optimization dialog.

Figure 3-26.  Configuring the aberration for maximizing total power on the illumination plane.

Additionally, we wish to minimize the variance of the irradiance distribution at the illumination plane in order to drive the optimization towards uniform illumination.  This can be accomplished by clicking on the empty row at the bottom of the aberrations list in the dialog, setting the Aberration Definition of row #1 to "Irradiance Variance", selecting the Analysis Surface to Evaluate to be "Analysis Surface(s).Illumination Analysis", and then setting the Target to 0.

Lastly, we will apply a Weight of 0.0001 for the first aberration and a Weight of 1E+19 for the second aberration, which has the effect of making each aberration contribute roughly equally to the total Merit Function at the start of optimization.  These values can be determined by hitting Apply on the optimization definition dialog and then going to Optimize > Evaluate Once and then inspecting the output window for the evaluated aberration values.  Additionally, toggle the "Apply penalty and continue" option at the bottom of the dialog.  If the variables are driven towards values which prevent the merit function from being evaluated, this option will apply a penalty so that the variables are driven back into a range for which the merit function can be computed.  If the "Immediately halt" option is used, the optimization will immediately halt when the merit function cannot be computed.

The final configuration of the Merit Function Aberrations tab is shown below in Figure 3-27.

Figure 3-27. Final configuration of the Merit Function Aberrations tab.

To complete the optimization configuration, move to the Method tab.  The default settings of the Method tab are shown below in Figure 3-28, which are sufficient for the purposes of this example.  Note that the optimization method being used is the Simplex option and that the optimization will run for 15 iterations. 

Figure 3-28.  Method tab settings for the optimization configuration.

The Output/Results tab, shown below in Figure 3-29, contains options for redrawing the 3D view during optimization iterations and reporting information about the merit function and aberrations to the output window.  The spreadsheet at the top of the dialog will store up to the last 20 optimization results, with right mouse click context menu options available to organize the stored results, apply them to the FRED document, or apply them to the Variables tab of the optimization dialog.  The default settings on this tab of the optimization configuration are sufficient for the purposes of this example.

Figure 3-29. Output/Results tab settings for the optimization configuration.

Optimization [FRED Optimum Only]

Before running the optimization we should raytrace the OSRAM LS-T676 Rayfile source and calculate the Irradiance distribution using the "Illumination Analysis" grid.  The starting irradiance distribution, prior to optimization, is shown in Figure 3-30.

Figure 3-30.  Irradiance distribution on the Illumination Analysis grid prior to optimization.

Start the optimization by going to Optimize > Optimize.

During optimization, the value of the merit function for each iteration of the Simplex is reported to the output window.  Optimization will stop once 15 iterations of the Simplex have been evaluated and the final merit function, aberrations, and variable values will be printed to the output window, as shown in Figure 3-31.  Additionally, a summary comparison of the variables and aberrations before and after optimization is printed to the output window.

Figure 3-31. Information printed to the output window during optimization.

Perform a raytrace and irradiance spread function using the "Illumination Analysis" grid again, following the optimization.  As can be seen in Figure 3-32 below, the illumination is already beginning to be stretched into a square distribution filling the analysis grid after just 15 iterations of the simplex.  Increasing the number of allowed iterations, or specifying one of the other stopping/convergence criteria on the Method tab of the optimization dialog, and using the OSRAM LS-T676 rayfile with 500Kk rays can improve the quality of the optimized distribution at the expense of longer computation time.

Figure 3-32. Irradiance distribution on the "Illumination Analysis" grid after optimization with 15 iterations and the 100K rayfile source.