This article discusses how to use the Roadway Lighting Analysis in OpticStudio and specific features that can make the Roadway Lighting tool more efficient and productive.
Authored By Akash Arora
OpticStudio has a few tools that can aid the designer of roadway lights in analyzing and optimizing luminaires. The Roadway Lighting Analysis will compute specifications based upon the CIE 140-2000 standard and quickly indicate whether the luminaire is passing or failing each specification. The NSC Roadway Lighting Wizard takes this one step further and provides these data as optimization targets with the full power of OpticStudio’s optimization engine available. In this article, we look at the details of these features and how they can make designing roadway lighting more efficient and productive.
As stated previously, OpticStudio’s Roadway Lighting Analysis calculates specifications according to the CIE 140-2000 standard. The values computed pertain to the distribution of light from a luminaire on the roadway and the glare seen by a driver approaching the lit section of roadway. The computed values include average luminance, overall uniformity of luminance, longitudinal uniformity of luminance, threshold increment, surround ratio, average illuminance, minimum illuminance, and uniformity of illuminance. Luminance values depend on the composition of the roadway (surface classification) and on the position of the oncoming driver. The value provided is the worst among all lanes of the roadway as dictated by the CIE standard. Illuminance values simply compute the distribution of lamp flux on and adjacent to the roadway.
All of these values require several parameters to be specified that define the roadway lighting configuration. As shown in the settings window below, the analysis allows the user to specify mounting height, spacing, arrangement, curb offset, number of lanes, lane width, surface class and road class.
The parameters with units of length are always specified in lens units. However, units of meters are typically most reasonable in roadway lighting applications. Also note that the computed data is always in units of meters and lumens, as this is what the CIE 140-2000 standard dictates. The output of the analysis is a text listing of specified settings, calculated values, targets (based upon the road class), and an indication of pass/fail status.
This analysis operates in one of two ways: with or without ray tracing. Frequently, a user may possess a far-field data file of a roadway luminaire in IESNA (.IES) or EULUMDAT (.LDT) format. These types of data files are provided on manufacturer websites. If this file is loaded into the current system via the Source IESNA or Source EULUMDAT objects, the analysis will compute the roadway lighting data based upon the far-field data they contain, with no need for ray tracing. The user simply needs to define the origin object in the settings as the source they wish to analyze.
Alternately, it may be the case that the user is designing the roadway luminaire and doesn’t have the far-field data file. In this case, the origin setting should be set to an object that represents the local coordinate system of the lamp; it should be as close as possible to the center of the luminaire. This could be the main reflector or a source that is part of the lamp. In this analysis scenario, the only objects and sources that should exist in the system should be part of the luminaire. OpticStudio will trace rays from all sources as part of the analysis and compute the far-field luminous flux distribution. The computed roadway specifications will then be based upon this data. This setup method is also used for optimization.
With the luminaire defined as discussed previously, the Roadway Lighting Wizard allows the user to optimize based upon any of the computed roadway lighting data. The same roadway settings must be defined to generate a merit function, in addition to more common ray trace and merit function settings.
Once the settings have been specified, OpticStudio will populate the MF with the appropriate targets. OpticStudio first needs to compute the far-field intensity distribution of the system. The NSTW optimization operand will trace rays using the settings specified in the dialog and save the far-field intensity data. The NSRW operands compute the data of interest. The merit function tool will add all eight specification values to the merit function, even though only five must meet specific targets according to the standard. The other three are provided for reference with zero weighting. Note that the standard calls for threshold targets, so the tool will add OPGT and OPLT operands as weighted targets.
Optimization then proceeds as it does for any other non-sequential system: variables are defined and the local and global optimizers can be invoked.
As stated previously, if the origin specified in the Roadway Lighting analysis is an IES or LDT file, the analysis will be computed based upon the far-field data in the file. Additionally, the tilt of the luminaire will be accounted for. No other parameters (e.g. position, user-defined power, etc.) will be taken into account. The IES file attached to this article was obtained from a street light manufacturer’s website and contains the measured, far-field flux distribution of an LED luminaire. Save this file to "<data>\Objects\Sources\IESNA" and open OpticStudio.
In pure NSC, load this file by adding a Source IESNA object and choosing this file. Make sure to change the system units (System Explorer...Units) to meters and lumens. The intensity distribution can be seen in the Source Polar Viewer (Libraries tab...Source Viewers...Source Polar Plot) and Source Directivity Viewer (Libraries tab...Source Viewers...Source Directivity Plot) windows shown below. The polar plot has been set to show data out to a polar angle of 90 degrees. The roadway in this coordinate frame would be oriented along the y-axis. Note the relatively sharp cutoff at approximately 80 degrees. This is intentionally done to limit light emitted into the sky, known as light pollution. Also note the intensity peaks are not directly in front of the luminaire (zero azimuthal angle), likely to illuminate the road for drivers approaching the lit section of roadway.
Before we see how this luminaire compares to the roadway standard, make sure to rotate it 90 degrees about the Z-axis. The Roadway Lighting Analysis assumes the roadway is oriented parallel to the X-axis. The IES file, as we have seen, has the luminaire oriented with the roadway parallel to the Y-axis. While the polar and directivity plots ignore the system setup, the roadway analysis will take this into account. Once done, go to the Analyze tab...Applications...Roadway Lighting.
Try changing the various settings and see what effect they have on the computed values. For example, as the mounting height is increased, the average illuminance drops, but the uniformity of illuminance increases. These are the types of tradeoffs that roadway lighting designers must make. With a quick indication as to whether the luminaire is passing or failing each specification, one can determine whether optimization may be necessary.
The shaded model images below illustrate the luminaire output in both false color and true color. These graphics use a technique whereby a detector is placed below a transparent slide (the road) to aid in visualization. The mechanical lamp structure was simulated with an imported CAD object, which is solely used for aesthetic purposes. Its effect on the analysis computations is negligible, and ignored in this scenario.