How to model LEDs and other complex sources

When working in Non-Sequential Mode, the first step in building an accurate simulation of an illumination system is often to correctly model the source. OpticStudio offers several ways to generate accurate source models. This article discusses how to model LEDs or other complex sources in Non-Sequential Mode using the Source Radial, Source File, and by building complex geometries around other source objects.

Authored By Mark Nicholson

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Introduction

Zemax, LLC thanks Radiant Imaging, Opsira and Lumileds for the experimental data used in this article. Luxeon is a trademark of Lumileds, Inc.

Accurate source modelling is the key to accurate illumination system modelling. OpticStudio can split, scatter, diffract, refract, reflect, etc. a ray, but this article discusses how to launch a bundle of rays initially, so that they are an accurate representation of the source's spatial and angular flux distributions. We will discuss modelling multiple Luxeon LEDs in this article, but the design approach can be used for any complex source such as arc lamps or incandescent lights.

Modelling LEDs and other complex sources

OpticStudio contains many source objects that can be used as initial approximations to a source's properties. For example the Source Filament is a good first approximation to an incandescent lamp, the Source Volume Cylinder is useful for modelling fluorescent tubes. The key to our approach in this article is to get as close as possible to experimental, measured data in both the spatial (near field) and angular (far-field) distributions.

As the LED we are modelling is specified in photometric units, we will use photometric units also in our simulation. Under System Explorer...Units, select Lumens.

The settings for photometric analysis

As a result, illuminance will be in units of Lux (Lumens.m-2), luminous intensity will be in Candela (Lumens.steradian-1) and radiance will be in Candela.m-2.

Using the Source Radial

The Source Radial is the simplest way to enter data from a manufacturer's datasheet. Here for example is the luminous intensity of the Luxeon Emitter Red (LXHL-BD01), as provided on the product datasheet. The "batwing" nature of the angular profile can be clearly seen.

TableLXHL-BD01_data_from_manufacturer's_angular_datasheet

In the Article Attachments there is a file (radial_source.zmx) that contains just two objects: a Source Radial and a Detector Rectangle. The Source Radial is a flat, rectangular or elliptical shaped object which emits rays with an angular distribution given by the supplied data.

The angular distribution of the source_radial

Note that the Source Radial allows the angular data to be made variable, and hence we can optimize to identify the desired angular performance for a given application. We will not use this capability in this article, as we are aiming to describe as fully as possible what the experimental data is. But this is a fantastically useful capability when we are at an early stage of design and need to know what kind of distribution we need.

The manufacturer's data sheet tells us that the source is 6 mm diameter and has a typical output of 27 lumens: this data has been added to the Source Radial as well. With 30 Layout Rays, we can see the basic positioning and distribution of the source in a layout:

NSC Shaded model 2

Analysis Rays allow us to run a more robust ray trace and view the results on the detector object. With 10 million Analysis Rays, we can look at the data in more detail we can see the spatial and angular performance of the source.

Illuminance and Luminous Intensity

Illuminance and Luminous Intensity Cross-section

There is excellent agreement between the computed Luminous Intensity and the manufacturer's datasheet, but there was no data provided by the manufacturer on the illuminance of the LED (the spatial structure of the LED). Therefore, OpticStudio has assumed that the source is uniformly bright across its 3 mm radial aperture. In the absence of better data, this is all we can do. To improve the quality of the simulation, we need both the angular and the spatial data: this is referred to as source radiance or source luminance.

Using Radiant Source Models in OpticStudio

Note: the tools described in this section are only available in the Premium edition of OpticStudio.

Radiant Imaging (www.radiantimaging.com) makes measurements of the radiance (or luminance) of a source by taking a series of calibrated 16-bit photographs of the source using a high-linearity, low noise camera and combining them into a database. Their ProSource software allows this data to be viewed in many ways and for rays to be generated that represent the full radiance (angular and spatial distributions) of the source, making this a much more complete representation of the source than the previously discussed Source Radial model.

OpticStudio Premium License holders can take advantage of these source modeling capabilities, via the Radiant Source Models Catalog. From this catalog, Radiant Source Model files (RSMX) can be downloaded from a large set of vendors who have made this data available to OpticStudio users. Within OpticStudio, you can then utilize tools to generate raysets based on this data. A demonstration follows, using the Lumileds LXML-PD01 (red) LED.

To begin, we can download the RSMX file from the Radiant Source Models Catalog. The catalog is opened via the Libraries...Radiant Source Models...Download Radiant Source Models. We can then download the desired model. The downloader menu also provides some data about the source. When you download the source, you will be asked to accept a license agreement, and then the RSMX file will be downloaded with the name shown to the {Zemax root}\Objects\Sources\Radiant Source Model Files folder.

Download radiant source model data

The key benefit in using Radiant Sources is that as the full measured data is available, effects due to reflection, scattering, total internal reflection can be seen. We can examine the RSMX file by opening the Radiant Source Model Viewer, available from Libraries...Radiant Source Models...View Radiant Source Models. Here for example are photographs of the unilluminated LED and the illuminated LED. Typically, there are several alignment photographs available, which allow the user to gain some knowledge of the LED geometry.

The unilluminated LED and the illuminated LED

The measured data is shown here in false color, to improve feature visibility. This shows detailed optical effects due to the structure of the source. For example, the contact electrode partially occludes the emitted light, and there is a crisscross pattern blocking a portion of the emitting surface. A large number of photographs are taken at different angles, which are used to compute a model of the radiance (or luminance) of the source.

The next step is to convert this source model into something that we can use in an OpticStudio simulation. There is a built-in tool for converting the RSMX file to a ray file called the Generate Rays tool, which is again available in the Libraries...Radiant Source Modeling drop-down.

The ProSource ray-generation dialog

The ringed settings indicate that:

  • We will generate 1 million rays from the RSMX file we downloaded.
  • A narrow-band Gaussian distribution, centered at 0.6325 microns with min/max bounds at 0.62 and 0.645 microns (as the 2-sigma points of the distribution), will be used as spectral data for the rays.
  • The rays will be generated over 2p steradians.
  • As the LED body has a spherical extent of (roughly) 1.5 mm, we choose to generate rays initially on a sphere of radius 1.5 mm.

Note that the ray file must be saved in the {Zemax}/Objects folder. Because wavelength data is to be added to the ray file at this stage, the file must have the extension .sdf.

The datafile generated is essentially a listing of rays, with data XYZ (starting position), LMN (starting direction cosines), and wavelength. To use this datafile, change the Source Radial to a Source File and select the LUMI_LXHL_PD01_red.SDF source file that was just generated. We will set the # Layout Rays parameter to 500, and the # Analysis Rays to 1 million (the maximum allowed value, as the source file only contains 1 million rays).

Note that the rays in the rayfile are not in random order, and so we need to randomize them in order to view an accurate sample in the layout plot. In order to see random rays in the layout plots, first navigate to System Explorer...Non-Sequential...Maximum Source File Rays in Memory and set it to a value larger than 1 million.

The General...Non Sequential dialog box

Then, in the Non-Sequential Component Editor, set the Randomize? parameter of the Source File to a non-zero value. This will make OpticStudio choose a random subset of rays for the layout plot:

Non-Sequential Component Editor

Note that to randomize rays, we must hold all rays in the rayfile in memory; the memory requirements are high for large rayfiles. When we trace the rays, the spatial and angular performance of the source is as follows.

Spatial and angular distribution

This data file is much more richly detailed in both the spatial and angular domains than a Source Radial can be. A demonstration of the spatial performance that is maintained with the Radiant Source Model is to compare the measurement image with a back-tracing of the generated rays. One method to view the spatial resolution of the source is to generate the rays on a planar surface, and then simply place them just behind a detector. We can improve the resolution by reducing the cone angle to a few degrees, and using a large number (10 million) of rays:Trace rays in a small cone angle

Placing this source file just behind a detector yields the following spatial distribution at the virtual image.

Luminance in position space

compare this back to the original measurement photograph in the RSMX file shows great agreement between the two.

Using Opsira's Luca Raymaker

Opsira (www.opsira.de) offer a similar capability with their Luca Raymaker software, which produces ray sets from measurements made in a goniometer. The ray-generation is made as follows:

The-settings-for-Luca-Raymaker

The file produced is a binary .dat which is then read into OpticStudio as a Source File object and then traced in the same way as the Radiant Sources described in the previous page.

Building a complex geometric model

The final technique we will discuss is making up a complex geometric source model. This is a "mini-model" of the source, and uses the geometric sources supplied with OpticStudio along with a series of other objects intended to represent the internal construction of the source. For example, look in the folder {Zemax}/Samples/Non-Sequential/Sources/led_model.zmx.

A complex geometric source

This object is made up of a series of smaller internal objects.

The inner construction of an LED

which can represent the LED die, electrode wires, mounting points etc. Then detailed optical properties can be applied to the faces of the various objects, and then a large number of rays traced.

These source models can be traced directly, or the rays produced can then be saved to a ray database. By navigating to Analyze...Database...Ray Database Viewer, you may select a test object and save all rays that land on that object as a new source file.

The settings on the ray database viewer

We can also do this during the initial ray trace, using the Save Rays functionality to save to an SDF or DAT file. In order to save the rays which are incident on object 4, for example, the syntax for the save name is as shown (where the “4-“indicates the object number).

Ray trace control

This new datafile can then be used in subsequent ray traces by importing it into the NSCE as a Source File object.

Complex geometrical models suffer from one big problem: you need to know what values to enter! For example, what scattering function should be used on the electrode wires? What is the reflectivity of the support structure? This kind of data is not easy to obtain. Finally, the complex model must be validated against experimental measurements: which begs the question, why not just use the experimental data in the first place?

Generally speaking, experimental data is more accurate and easier to work with. But in some systems, especially where light from the source is re-imaged onto the source, the effort required to construct a good complex source is worth it. It is also possible to get the best of both worlds, by defining a complex object, but to launch rays initially from a measured source file:

Combining Geometric Modelling and Measured source data

This figure is copyright Radiant Imaging and is used with permission.

KA-01590

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