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How to Characterize Far-Field Color Mixing with the Source Illumination Map

This article explains how the Source Illumination Map can be used to read angular data from a source and display a true color image of the resulting illumination pattern in the far-field.
 
Dr. Sanjay Gangadhara
07/10/2012
Illumination & Stray Light

Introduction

Many lighting applications involve the use of multiple sources for generating a particular color output in the far-field. The spectral distribution for each of the sources may be simple (e.g. monochromatic) or complex. The traditional approach for simulating the far-field color generated by mixing these sources would be to trace a large number of rays from each source onto a far-field detector.

However, if we are only interested in the far-field behavior of each source, this information may be obtained directly from the angular distribution present in the source data file, since the spatial distribution of each source can be neglected in the far-field. Thus, in the absence of optical components that can modify the angular distributions, ray-tracing is not needed to characterize the far-field mixing of sources.

The Source Illumination Map (found under the Analysis…Source Viewers menu) can be used to read angular data from a source and display a true color image of the resulting illumination pattern in the far-field. The input source data for this feature can be in the RSMX format that is provided when the source is measured by Radiant Zemax (the RSMX format represents the most complete description of the source). The feature can also read from input source data present in spectral color format (SDF) files as well as in the standard IESNA (IES) or EULUMDAT (LDT) format (the latter of which only contain far-field data for the source).

A number of lighting vendors (e.g. Osram, Opsira) provide data files in SDF, IES, and/or LDT format for many of their sources, if they do not already have RSMX measurements for their sources. For a list of vendors that provide RSMX files for their sources as a part of the Zemax installation, look at the Download Radiant Source Model Data tool (under the Tools…Sources menu).

The Source Illumination Map allows you to look at the illumination pattern not just from a single source, but from up to 8 sources. Each source can be characterized by a different data file, have a different spectral distribution (which may be user-specified if this data is not already contained in the source file), and have a different position and orientation with respect to the far-field detector. Based on all inputs, Zemax will determine what the final illumination pattern will look like without needing to trace rays. As a result, this analysis provides a very fast tool for determining the far-field color output from a number of sources.

Color Mixing the Traditional Way

To illustrate use of the Source Illumination Map, we’ll look at color mixing from an Osram Multiled package. More information on obtaining source data from the Osram website may be found in the article entitled “How to Use Osram LED data with Zemax”.

The particular package we will investigate is part number LRTB C9TP, which is a 6 lead multiled (picture courtesy of Osram):


This package contains 3 dies used to generate light in the blue, green, and red (picture courtesy of Osram):



Osram has provided the ray data for each color in a flux only format (DAT) file, along with spectrum (SPCD) files for each color. They have also supplied a representation of the package geometry as a SolidWorks part file. Although the rays contained in the source file are measured outside of the package, it is useful to include the package geometry in your Zemax model if you are concerned about light reflecting back towards - and therefore interacting with - the package. You may include the SolidWorks part file in Zemax via the PartLink technology (see the article entitled “How to Use the PartLink Object”).

The traditional method for determining the far-field color output from this package would be ray-tracing. A Zemax file illustrating the setup is provided as a part of the Osram download. Each of the sources is brought into Zemax as a Source File object. Since the source files are flux only, the spectral distribution of each source is specified via the SPCD file, e.g. for the blue source:



Each source file contains 5 million rays. We trace the rays onto a 2 M x 2M Detector Color object that is 500 mm away from the package, and find:



On a 12-core computer the ray-trace takes about 10 seconds. Not bad considering that we are tracing 15 million rays! This speed comes about from Zemax being fully multi-threaded. Nonetheless, these same results can be obtained more quickly via the Source Illumination Map.

Color Mixing with the Source Illumination Map

Before we can use the Source Illumination Map, the flux only format files need to be converted into spectral color format (SDF) files. This can be done easily using the “Convert to Spectral Source File” tool under the Tools…Sources menu:



The input file is the flux only file (shown for the blue LED in above). The output file will have the same name as the input (by default, though it can be changed) but with a different extension (SDF vs. DAT). The tool will add a spectral distribution to the flux only file based on the specified input for the Source Color. For this example, we choose “Spectral File”, and then specify the name of the SPCD file that we wish to use (again, given to us by Osram). To create the SDF file, simply select the “Convert” button.

The same procedure is repeated for the green and red LED sources. These three SDF files may then be used in the Source Illumination Map. First, we launch the analysis (again, under the Analysis…Source Viewers menu) and open the settings:



Under the “No. of Sources” setting, select 3 (as shown above). The properties of each source are then specified by changing the “Active Source” input:



We wish to select each SDF file (blue, green, red) for each source:







Although in principle the position and orientation of each source may be set separately, for this example we set them all identically, since the ray data contained in the source file assumes a common origin for all sources (as defined by Osram). Thus the Z position of each source represents the distance from the source to the detector. Finally, the detector size and sampling are set once, at the bottom of the dialog.

The first time that an SDF file is used by Zemax it will take some time to load, as Zemax is generating a far-field distribution (FFD) dataset for that file (the FFD file will be placed in the same file as the SDF, i.e. the <Objects>\Sources\Source Files\ folder). Thus, when we first hit “OK” to generate the Source Illumination Map it may take a few seconds. But any subsequent updates to the analysis will be nearly instantaneous – you could change the position or orientation of each source and re-generate the illumination pattern instantly.

For the default settings used above, we find very good agreement between the Source Illumination Map and the results found from ray-tracing:



Now tilt the red source by 45 degrees about X:



Note the color separation that occurs:



Again, we find the above result almost instantly, which ray-tracing would take some number of seconds to reproduce.


Summary

The Source Illumination Map provides a fast and accurate method for characterizing the far-field color output resulting from mixing multiple sources. While this tool does not account for optical components that are located after the source package, it does allow the user to determine how to position and orient sources prior to those components, and/or in systems that do not require these components.