Software Advances

Figure 5. Today's peanut-shaped lens with PCB board, shown with a few rays emitted.

To keep up with the regulations, software programs have added new tables and figures to help designers determine, through virtual prototyping, if their designs meet or fail the new TM and CIE regulations.

Instead of presenting a design that meets these standards, let’s look at the case where it doesn’t and how software can be used to show non-compliance. The new design type of 4 LEDs is shown in Figure 5 with rays traced to show light emittance through the peanut shaped free-form lens array.

The first new software advance critical for a streetlamp design is the BUG luminaire classification figure. This figure is used to categorize a streetlamp to see if it passes the IES TM-15-11 standard. The BUG rating figure quickly gives designers the knowledge of how much light is going in the forward, backward, upward, and trapped direction in the fixture for their design.

Figure 6. Luminaire Classification System Plot

Figure 6 shows the BUG report for the 4-HB LED street-lamp. The normal BUG figure shows more light projecting forward onto the street than backward light, which illuminates sidewalks and pathways. Upward light is considered either glare or light pollution, and finally there is the light trapped in the luminaire itself. The designer always hopes that both the upward and trapped light is 0, but this is rarely the case.

Notice in the figure that 18.2 percent of the light is trapped, 35.2 percent propagates in the forward direction onto the street, and 16.8 percent emits backward onto the sidewalk. We still need another analysis tool to see if light trespass is going to be a problem and determine the pattern on the walkways and roadways for problem areas. The trapped light in this design is mostly due to the free-form lens shape creating a lot of total internal reflection in the LED/lens assembly and could easily be redesigned to reduce this waste of energy by using differently shaped profiles of the peanut shaped lenses over the LEDs.

Additional utilities include the classification of the roadway type. Roadway classification states the type, length and distribution of the lamp. This particular design falls into the Type 1, short classification as shown in Figure 7 which is mostly for lighting walkways, bike paths and sidewalks, and sometimes roadways. Type I is a two-way lateral direction distribution having a preferred lateral width of 15 degrees in the cone of the maximum candlepower. Generally, this type of luminaire is located near the center of a roadway where the mounting height is approximately equal to the roadway width, which may not be sufficient for automotive transportation.

Figure 7. Road luminance plot of 3 sets of opposing streetlamps on a two-lane road.

Finally, the road luminance plot gives the final determination of whether the new streetlamp design will meet the RP-8 standard. Figure 8 shows the luminance plot for a set of 3 opposing street-lamps on a two-lane road. It is easily seen that this lamp does not project a symmetrical beam side-to-side along the road, which will cause a problem with oncoming traffic if used. This is the figure that designers should use to determine if the light trespass is a problem and if the streetlamp design meets the uniformity and spatial requirements of IES RP-8.

Conclusion

As new, more stringent requirements for streetlamps are demanded, future software will have to evolve to help meet these design challenges. Newer utilities that software designers will have to create include:

  • Utilities to create photorealistic scenes of oncoming drivers to the streetlamp setup

  • Road luminance analysis taking into account the different types of scatter from road surfaces

  • Better dark sky evaluation including the use of buildings and scene generation to provide the full streetlamp experience to both drivers and pedestrian.

This article was written by Michael Gauvin, VP Sales and Marketing, Lambda Research Corporation (Littleton, MA). For more information, contact Mr. Gauvin at This email address is being protected from spambots. You need JavaScript enabled to view it. or visit here.