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Accuracy

An important issue to consider when using any optical design tool is accuracy. As a designer, you need to trust that your design tool is accurately predicting the distribution of light within your model. Time and money is lost when software tools fail to produce accurate results. We at LTI understand this issue first-hand. We use Photopia on a daily basis to design products for our clients. Based on our experience, and the experience of other Photopia users, we've compiled a few case studies to illustrate how accurately Photopia predicts the distribution of light.

  • Fluorescent Troffer
  • Roadway Luminaire

    • Preface

    Photopia’s accuracy is well proven over its many years of use. Both LTI and our customers have made comparisons between measured and predicted photometric distributions and as long as the physical luminaire matches the Photopia model in all respects, the results are very accurate. Note however, that Photopia makes no attempt to model changes in lamp lumen output as a result of thermal effects. Therefore designs that employ temperature sensitive lamps may show significant differences between the predicted and the measured total luminaire efficiency (LOR) since Photopia is only predicting the “optical efficiency” not the “thermal efficiency” of the design.

    When analyzing a design in Photopia, it is important that the Photopia model match the physical model in all respects, including:

    • Optical Component Geometry – make sure reflector and lens shapes are accurately manufactured, which they often are not especially for sheet metal parts.
    • Optical Surface Properties – make sure the materials/finishes used in Photopia match those used in the physical parts.
    • Lamp Type and Modeled Orientation – in the case of MH lamps, extra care must be taken because the arc shape and position can shift as the lamp is aimed in different orientations.

    Obviously, if the physical model does not match the Photopia model, the accuracy suffers.

    If you plan to distribute Photopia generated photometric files to your customers, please take a look at our recommendations.

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  • Example: Paracube Troffer

      paracube drawing

      Description: 2' x 4' Paracube Troffer, White Painted Reflector, 3 T8 Lamps. Photopia model used 87% white paint, 2,000,000 rays with 10 reflections.

      Efficiency: Photopia = 65.0% vs. Measured = 63.1%

      All measured data supplied by Lighting Sciences, Canada.

      paracube plot

      paracube plot

      paracube plot

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  • Example: Roadway Luminaire

      roadway drawing

      Description:Roadway luminaire with a hydroformed reflector using a 150W SON-T HPS lamp

      BRDF data for the specific reflector finish was measured at LTI's lab and used in the analysis.

      Measured Distribution (shaded Plot showing illuminance at a plane)

      roadway iso lines

      Photopia Predicted Distribution (same plot settings as above)

      roadway iso lines

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    Additional Information

    For additional information regarding Photopia's accuracy and the factors that affect both the measured and predicted values, please review Appendix C from our User’s Guide below. For comments on Photopia and its accuracy from some of our customers, see the Photopia User Reviews link below.

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