The task of creating a streetlamp optical system has changed over the last 20 years and due to this change, so has optical design software. Let’s compare the task of designing optical street lamp components 20 years ago with the process today.
Twenty years ago, the optical lighting engineer would design either a high-pressure sodium (HPS) or high-intensity discharge lamp, using one bulb to do the entire streetlamp illumination system. Ten years ago there was a switch to LEDs paired with total internal reflecting lens components that fit over the LED to create structured arrays producing the asymmetrical angular pattern to illuminate a specific spatial area. These designs consisted of multiple LEDs/lenses of 11-13 components in a row and then arrayed in the opposite direction to create a 2D array to meet the specified streetlamp regulation.
A good example of these two types of systems is shown in Figure 1, where the HPS streetlamps in Alameda California were replaced by Leotek’s Green Cobra streetlamps providing much better night time visibility. Today’s challenge is to meet the same requirements as 10 years ago but with just a few peanut shaped free-form lenses to fit over high brightness LEDS and work with additional stringent requirements limiting glare, street trespass, and upward lighting criteria with specific angular and spatial criteria.
The current roadway lighting levels specification for the United States is the National Standard Practice for Roadway Lighting RP-8-00 by IESNA and CIE 140 in Europe. CIE changed their regulations from the older CIE 30.2 to the new revamped CIE 140 due to requests for an updated standard that decreased the spacing between poles for better visual clarity and better luminance criteria from the observed driver perspective.
The roadway light levels for different types of roadways in the United States for both illumance and luminance-based criteria must follow the IESNA RP-8 standard which is shown in Table 1. Streetlamp designs must meet the lux per pavement classification, uniformity and veiling luminance ratios for each roadway type. Other design criteria of importance in current streetlamp design are the Backlight, Uplight and Glare (BUG) ratings mentioned in the addendum of IES TM-15-11. For instance, the current specification for sky glow should be less than 5% of the total output of the fixture.
Ten-Year-Old Design Possibilities
Ten years ago, the typical lens system used lenses placed on top of LED arrays to produce the angular output needed for the specified uniformity on the street or roadway. Usually TIR lenses with conventional optical features in the center of the lens are used to place light where needed. A typical individual streetlamp lens with rays traced is shown in Figure 2. The LED and lens arrays can be placed in either a planar or rotated array around an axis to send light into zones that overlap to create the best total uniformity on the roadway. Diffusers are also used in this scenario below the LED and lens arrays to create better uniformity and reduce the harsh, direct light from the LED/lens source.
The hybrid scenario can be the most costly since it uses both lenses and reflectors. These systems use reflectors to shape the large angular LED emission pattern and lenses with diffusers on the front of the lenses to create better uniformity, without the harsh LED point source emission. Angled reflector structures provide angular output cutoff to keep the light in the targeted roadway or street area, as shown in the Carclo Technical Plastics design shown in Figure 3.
The next challenge is to get enough illumination output with acceptable uniformity across the roadway. A singular LED does not provide sufficient illumination, so an array strategy is used. Two possible array options are a linear array where the LEDs and lenses are in a planar pattern and a rotational array where rows of LEDs and lenses are rotated about a central axis. In most cases worldwide, a 3:1 uniformity ratio is best, but many city specifications allow for a 5:1 ratio.
Finally, the glare cutoff pattern and up lighting standards must be taken into account in the streetlamp design. This is where baffle design must be used to stop light exiting into undesirable roadway regions and also out of motorist and pedestrian sight lines. Incorporating baffles where light is redirected back into the streetlamp and re-reflected can enhance the overall light output, create a softer effect, and when used judicially, lower the number of LEDs needed to meet light output specifications.
Ten year old LED systems with just 30 or so LEDs would look and perform as shown in Figure 4.
The same optical lighting engineer now creates designs using as little as four high brightness LEDs and four hybrid free-form lenses to create the same output pattern but also having to take into account many new requirements. These new requirements include limiting upward light, reducing glare to oncoming drivers, increasing total efficiency, meeting a certain CIE color specification and creating a pleasing lit appearance to any viewer. This change in reducing optical components, meeting new stringent requirements and simulating lit appearance changes the workflow of the lighting optical engineer and pushes optical software manufacturers to increase the complexity and capability of their software. Let’s look in detail to see how optical software has become more sophisticated to meet these demands.
First, why all of the interest in better standards? It has become obvious that the better street lighting becomes, life also benefits. Exterior lighting of this type is needed to create a visually pleasing environment to support nighttime activities, deter crime, provide sufficient lighting for roadways and surface streets to prevent accidents, and for pedestrians to walk safely on sidewalks and walkways. Interesting fact – since the advent of street lighting there has been a reduction in pedestrian crashes by 50% and fatalities are 3 to 6.75 more likely in the dark than in daylight.[3,4]
It is important that street lamp lighting not be intrusive; creating glare into oncoming automotive and commercial driver vision is dangerous, light trespass is annoying and insufficient light levels welcomes crime and causes crashes and injuries with motorized vehicles and pedestrians. Upward lighting can also be an annoyance, especially in areas where ordinances are specific to minimize upward propagating light. This is particularly true in areas that have observatories. A balance of just the right amount of light in the right areas is the best design solution.