LED Modeling Using Raysets

Overview

Producing accurate simulations of LED based optical systems requires accurate source models. This means the source models must not only produce the correct distribution of light in a far field measurement, they must also produce the correct near field behavior since secondary LED optics are often employed in very close proximity to the LED. Accurate simulations are vital to the design process especially with lens optics commonly used on LEDs given the high cost and long lead times for lens tooling. The data presented in the case studies below is a direct result of the lessons learned by one manufacturer about the importance of simulation accuracy.

What Are Raysets?

Raysets have become a convenient way to model light source behavior, and are commonly provided by LED vendors. A rayset is simply a collection of rays (vectors) that describe the initial emanation of light from a source. Each ray consists of a start point, direction, and magnitude (in lumens). Rayset files are ususally provided in multiple formats for compatability with most optical simulation software.

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What Are Raysets Missing?

Raysets are missing one key componenet of a lamp model, geometry. Raysets only describe the exiting light and do not contain any geometric data about the light source. Since there is no geometric information, there is nothing for the light to interact with if it comes back towards the light source. Some vendors provide 3D cad models as a supplement to their raysets, but this relies on the users to assign appropriate materials to each part of the LED. Additionally, many of these CAD models don't contain enough detail but only describe the bounding volume of the package. This lack of geometry also causes an issue with accurate ray emanation points. Since there is no geometry, the ray emanation points are not coming from geometric locations, but just points in space. The process of combining the 2D images to create raysets does not seem to provide an accurate way to determine correct ray emanation points.

The images below show a cross section of the CREE XR-E LED. The image on the left shows the light field for a rayset based model. The geometry is included only as a reference since the rayset based model doesn't contain any geometric information. The image on the right shows a Photopia lamp model, with the associated geometry. The light field for the Photopia model looks more coherent and seems to match the geometry better than the rayset based model, which seems to indicate light coming directly behind the LED, through the package, which is a physical impossibility.

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Using Raysets in Photopia

Since the original beta release, Photopia v3.0 has supported the use of raysets. You can see how to use these in Appendix B of the v3 User's Guide. Photopia uses raysets that end in a .rir extension. Radiant Imaging's ProSource software exports this format natively, and other lamp manufacturers are working on providing this format of their raysets. Our format is identical to the TracePro Binary format, so if you see this format (which ends in a .ray extension) you can use these in Photopia simply by changing the extension to .rir. When you have a .rir rayset, you'll simpy import a simple lamp model from the Photopia lamp library, and then in the Property control, choose the rayset by browsing to it. You'll also need to set the appropriate lumen value for the source model.

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Rayset Accuracy

Raysets seem to have a good potential of capturing near field photometric data, however, they do have some significant limitations. Their lack of CAD geometry puts the responsibility on the user of the software to import and assign materials in order to achieve an accurate output. Additionally, because the data is compiled from a series of 2D images, raysets often don't contain accurate 3D emanation points. Of the two aspects of lamp modeling, where is the light going and where is it coming from, raysets only accurately adddress where the light is going. Without proper material assignments to the 3D geometry as well as the proper emanation points, photometric simuations can be very inaccurate. Appendix B of the Photopia User's Guide illustrates some of these issues.

In conjunction with Ruud Lighting, we've also done a series of studies that compared simulations with raysets and simulations with Photopia lamp models to measured photometry. We consistently saw very strong correlation between the Photopia lamp models and measured photometry. The match between raysets based models and measured photometry was less consistent. In any application where secondary optics will be placed close to an LED, the Photopia lamp model provides the best correlation. Since these raysets are typically generated from 2D images, they have no way to accurately determine the 3D ray emanation points, which are critical when an optic is placed close to or directly on the LED. Additionally, if an index matching gel is used to join the LED and the optic, raysets provide no way to account for this surface interaction. Case studes are included below as well as in the following papers:

LED Source Modeling Method Evaluations in the November/December 2008 issue of LED Professional Review and LED Source Models in the January/February 2009 issue of LED Journal both cover this study of physical measurements.

These case studies use data collected by BetaLEDTM during the development of their NanoOpticTM LED outdoor area lighting lens optics. The data includes measured luminous intensity distributions along with simulations using both Type 1 and Type 3 source models in Photopia. The optics were measured at Independent Testing Laboratories, Inc. (ITL) in Boulder, Colorado, USA. The simulations used lens geometry that was scanned from the physical as-built parts. This is important since the as-built parts did not always perfectly match the intended design, which removes a potential source of difference between measured and simulated performance.

Case 1: Roadway Type 5 Lens with Index Matching Gel between LED and Lens

The image on the left shows the measured (blue) versus rayset predicted (red) candela plot. The image on the right shows the measured (blue) versus Photopia model predicted (red) candela plot. The rayset based models are called Type 1, the Photopia models are called Type 3 based on the terminology in this paper on lamp modeling.

The Photopia (Type 3) model on the right predicts the beam more closely than the rayset (Type 1) model on the left. The image on the right shows that the predicted and measured candela plots trace each other more closely, especially at the higher vertical angles, which are critical in a roadway application. If the simulations are underpredicting these values, then the optimization of the optic will be misdirected. The rayset model on the left shows very significant deviations between the high angle light, the angle at which the peak intensity occurs, and the nadir intensity, all of which are main design criteria in a roadway optic.

Case 2: Roadway Type 5 Lens without Index Matching Gel

The differences between the rayset and Photopia model performance are not as great as in Case 1, but the left images below do show an upward shift in the beam angle and significantly more light directly below the luminaire. Accurately predicting the peak vertical angle in the intensity distribution is another critical issue in this type of lens. The right images below show a more accurate overall beam shape and peak beam angles.

Case 3: Medium Beam Lens with Index Matching Gel

This case illustrates how the differences between the 2 model types remains significant when a gel is used between the LED and lens even in a much narrower beam distribution. The higher intensities seen in the central part of the beam using the Type 1 model on the left result from the extra lumens that were not directed to the higher angles in the distribution where they belonged. The Type 3 model results shown on the right reveal a much closer beam shape at the full range of angles in this distribution.

Summary

The 3 cases presented illustrate that there are significant differences in simulated results depending on the source modeling method used. These results show that a Type 3 model more closely matches the measured performance than the Type 1 model for both wide and medium beam lenses. The differences are greatest when an index matching gel is used between the LED and lens. The main reasons for this are that in addition to the challenge Type 1 models have in creating accurate 3D ray emanation points, all of their digital images showing the luminous view of the source are measured in air. When a gel is used between the LED and the lens, light never exits from the LED primary lens into air so the measurements are inappropriate. Since Type 3 models include the lens geometry, the material can simply be changed to account for the glass / gel interface instead of glass / air.

The 2nd set of data shows that the Type 1 model fairs better when there is no gel, yet it does not outperform the Type 3 model. Wider beam optics are more sensitive than narrower beam optics to exactly how much light is directed onto each part of the lens. As the beam gets narrower, more light is directed to the same angles in the beam and differences in the amount of light sent to each part of the lens between the simulation and physical reality become less important. It should also be noted that other 3D ray emanation point geometry mapping options were tested such as mapping the points to a sphere and the results did not vary significantly from those presented here.

Given a choice between Type 1 and Type 3 source models for the same LED, a Type 3 model will likely produce more accurate results, especially as the beam gets wider. If gels are used between the LED and lens, then Type 1 models should not be used since the measurements on which they are based is not appropriate for this situation.

The simulation data in these case studies was provided by Kurt Wilcox and Chris Strom at Ruud Lighting. ITL in Boulder, Colorado provided the physical measured data for comparison.

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