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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, and by building complex geometries around other source objects.
Authored By Mark Nicholson
Introduction
Zemax, LLC thanks, 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.
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.
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.
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:
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.
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 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 file produced is a binary .dat which is then read into OpticStudio as a Source File object. OpticStudio then traces rays based on the measured data contained in the file.
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.
This object is made up of a series of smaller internal objects.
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.
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).
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:
This figure is copyright Radiant Imaging and is used with permission.
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