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    Accurately model LED sources for optimal SSL product designs (MAGAZINE)

    Dec. 18, 2012
    Mark Nicholson of Radiant Zemax describes how creating a virtual prototype with accurate LED source models is a vital step in delivering SSL products that meet design goals.
    Content Dam Leds En Articles Print Volume 9 Issue 12 Features Accurately Model Led Sources For Optimal Ssl Product Designs Magazine Leftcolumn Article Thumbnailimage File
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    This article was published in the November/December 2012 issue of LEDs Magazine.

    View the Table of Contents and download the PDF file of the complete November/December 2012 issue, or view the E-zine version in your browser.

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    In last month’s LEDs Magazine, Thomas Davenport of Synopsys wrote about some of the complexities of designing with LEDs in solid-state lighting (SSL) products. Optical modeling can help translate that theory into real-word SSL product designs. Indeed we will consider how modeling and the creation of virtual prototypes can ensure that a development project delivers on design goals established at the beginning of the project.

    Zemaxdesignfig1

    Fig. 1. By modeling light as rays, which are thin lines that follow the direction of light, and tracing these lines through an optical system, we can produce a virtual prototyping environment for optical and illumination systems. LED sources are especially complex in the rays they produce, and the virtual prototype is the best way to deliver optimal SSL products. Optical system design software uses the ray tracing technique to simulate, optimize and tolerance optical systems.

    For example, in NASA’s recent Curiosity mission to Mars, all the cameras and other optical equipment on the Rover were designed and tested virtually on computers extensively before prototypes were made and the final system assembled. And while not everyone has NASA’s budget, all optical and illumination system engineers can benefit from virtually prototyping their systems before committing to prototyping costs and production tooling.

    The LED challenge

    LEDs provide a challenge because they have the previously-mentioned very complex optical properties. LED brightness changes depending on where on the LED you look, what angle you look at, and what colors you want. While it is tempting to think of white LEDs as little hemispheres that glow uniformly white, doing so can be a very misleading, and costly assumption.

    Zemaxdesignfig2
    Fig. 2. To understand the LED optical complexity, let’s look at some examples. Figs. 1 and 2 show commercially available LEDs in their unlit and lit states. The LED in Fig.1 is a phosphor-converted white LED, while the LED in Fig. 2 mixes color emitters to deliver white or color light. For each LED, the lit state is surprisingly different from what you might think they would look like, based on their unlit appearance.

    In the first case, you might be surprised to discover that the round LED actually lights up as four squares, with almost zero energy firing straight out of the center. The lack of energy in the center is due to the fact that there are four separate emitters packaged as an array in the white LED. You can also see that the individual emitters aren’t precisely uniform in light output or color.

    Zemaxdesignfig3

    Fig. 3. The spatial variation of color in the LED in Fig. 2 might also surprise you. The color mix delivers white light to the human eye and you might expect the source to be a simple white LED. But there is little simple in LED lighting!

    Model creation

    These images were produced by a Scanning Imaging Goniometer (SIG) which rotates a highly calibrated, linear camera around a hemisphere centered on the source under test. These sources are typically LEDs, but can also be halogen sources, arc lamps or virtually any kind of source.

    By capturing pictures of the lit appearance of the source at all angles — and by including a spectrometer, at all wavelengths — we build up a model of the real radiance of the source, that we call a Radiant Source Model (RSM). The RSM accurately describes the light output and spectral distribution of the source.

    The RSM is vitally useful to both LED manufacturers and their customers, SSL product designers. For manufacturers, who make the bare LED die and must then design the encapsulant lens around it, RSMs provide a unique, comprehensive way of understanding the light distribution from both the bare die and the encapsulated device. For SSL designers, the RSM of the finished LED provides complete information on the spatial, angular and color properties of the LED without requiring an understanding of the complex semiconductor physics of the device.

    Zemaxdesignfig4

    Fig. 4. RSMs are used in optical system design programs like Zemax12 to produce sets of rays that exactly model the spatial, angular and color output of the LED. Zemax contains measured data on over 700 LEDs and other light sources, and that library of data is available to other ray tracing codes also. The ray tracing program then traces these rays through whatever the optical system is to provide the desired final output.

    Light pipe example

    For example, if you were designing a light pipe to project an LED beam that behaves like a flashlight, you could start with a simple cylinder of plastic (Fig. 3). But were the design goal to deliver ten times the brightness in the forward direction, you need to change the geometry. Fig. 4 shows a deformed cylinder achieved with freeform surface modeling than can deliver the desired brightness gain.

    Note how some of the high-angle rays are lost where the source and pipe are joined. But the remaining rays are guided by the deformed light pipe shape to result in the brightness gain in the forward direction. The computer program designs the shape given the starting distribution of light represented by the RSM and the desired output based on the specification for the product design.

    Zemaxdesignfig5
    Fig. 5. In another example, a design team might set a goal of developing a collimator optic. Such an optic is intended to reflect a majority of the rays directly forward in the direction that’s perpendicular to the surface upon which the source is mounted, producing a controlled column of light for directional-lighting applications. The collimator optic depicted in Fig. 5 captures all the light from the LED to produce a brighter output over the desired throw.

    By using measured data and virtual prototyping software, optical and illumination designers can include all the details they need to design with speed and confidence. The result is shorter SSL product design cycles, less money spent on prototypes, and optimally-performing production lamps and luminaires.