How to well design led street lighting

Good visibility under day or night conditions is one of the fundamental requirements enabling motorists to move along roadways in a safe and coordinated manner. Properly designed and maintained street lighting should produce uniform lighting levels conforming to industry standards. Those levels should facilitate the visibility of motorists, pedestrians, and other objects at night or in situations in which light levels are diminished (i.e., a tunnel).

next we will discuss the subject of Visibility, including quality of lighting.
Photometry, including a discussion of the types of light distributions provided by roadway luminaires.
The types of lighting analysis.
Light levels.
Lumen depreciation.
Lighting analysis computer programs



Good visibility enables the motorist (and pedestrian) to quickly discern significant details of the roadway. Some factors that directly influence visibility are:
• The brightness of an object on or near the roadway.
• Ambient light.
• The size of objects and identifying details.
• The contrast between an object and its surroundings.
• The contrast between pavement and its surroundings as seen by the observer.
• The time available for seeing the object.
• Glare (both disability glare and discomfort glare).
• The quality of the driver’s vision.


The quality of lighting refers to the relative ability of the light to provide the contrast difference in the visual scene so that drivers and pedestrians may recognize the cues required for the seeing task. The following issues are involved in producing a lighting system and care should be taken in balancing them to obtain maximum quality:
• Reduction in disability glare will improve visibility, and reduction in discomfort glare should improve driver performance.
• Reflected glare will conceal some contrast differences and should be reduced.
• A change in pavement luminance (for example, changing the roadway surface from concrete to asphalt) will change contrast and uniformity

• Changes to other background areas (for example, adding noise walls along the shoulder) will also effect lighting quality.
Changes made in some of these areas may adversely affect others. Care must be taken to obtain the proper compromise by adjusting luminaire type, mounting height, uniformity, and luminaire locations.


The term Photometry is used to define any test data that describes the characteristics of a luminaire's light output. The most common types of photometry includes:
• Iso-footcandle performance charts
• Coefficient of utilization (CU) curves
• Vertical and lateral light distribution data
• Lumen maintenance curves and dirt depreciation curves
The following is a review of the more frequently used types of photometry.

Coefficient of Utilization and Iso-footcandle Chart

A coefficient of utilization (CU) refers to the ratio of lumens which ultimately reach the work plane to the total lumens generated by the lamp. A coefficient of utilization curve, as shown in figure 2-1, is provided for luminaires intended for outdoor use. Two curves are shown in the graphic, one for the street side (normally the desired area to be lit) and one for the house side (or the direction away from the primary lighted direction). The street curve represents the utilization of the bare lamp, in percent, as the ratio of lateral distance to mounting height.
An iso-footcandle chart, as shown in Figure 2-1, is used to describe the light pattern a luminaire produces. These charts show exact plots or lines of equal footcandle levels on the work plane with the fixture at a designated mounting height.

Vertical Light Distributions

Vertical light distributions are characteristics of the luminaire and should be considered early in the design process. Vertical light distributions are divided into three groups. Classification is based on the distance from the luminaire to where the beam of maximum candlepower strikes the roadway surface.
• Short distribution - The maximum candlepower beam strikes the roadway surface between 1.0 and 2.25 mounting heights from the luminaire.
• Medium distribution - The maximum candlepower beam strikes the roadway at some point between 2.25 and 3.75 mounting heights from the luminaire.
• Long distribution - The maximum candlepower beam strikes the roadway at a point between 3.75 and 6.0 mounting heights from the luminaire.

On the basis of the vertical light distribution, the theoretical maximum spacing is such that the maximum candlepower beams from adjacent luminaires are joined on the roadway surface.

Lateral Light Distributions

As with vertical light distributions, lateral light distributions are characteristics of the luminaire and should be considered early in the design process. The Illuminating Engineering Society of North America (IESNA) has established a series of lateral distribution patterns designated as Types I, II, III, IV, and V, as illustrated in Figure 2-2.
• Types I and V are classes of luminaires mounted over the center of the area to be lighted. Type I applies to rectangular patterns on narrow streets. Type V applies to areas where light is to be distributed evenly in all directions. Type V and a modified Type I are, generally, the class of luminaires applied in high mast lighting systems.
• Types II, III, and IV are classes of luminaires to be mounted near the edge of the area to be lighted.
• Type II applies to narrow streets.
• Type III applies to streets of medium width.
• Type IV applies to wide street applications

Luminaire Cutoff Classification, Spill Light, and Sky Glow

The IESNA classifications of cutoff relate to the luminaire’s light intensity near the horizontal. (For a more detailed discussion of luminaire cutoff classification refer to IESNA RP-8.) The classifications were developed to help recognize the level of glare associated with a luminaire. Community associations and astronomers, however, have adapted the classifications, in an effort to reduce sky glow. Sky glow produces a luminous haze and limits our ability to see the stars. Astronomers are concerned with sky glow, as they must filter it from their observations. The growth in popularity of amateur astronomy is producing an increased awareness of sky glow. A few communities around Virginia have declared the night sky to be a natural resource to be protected.
Some efforts to reduce sky glow are rather straightforward – use shields and visors on floodlights to limit up-light. Other attempts are not as simple as they seem. For example, eliminating high angle light (above horizontal) from street lighting fixtures so that they cast all light downward will dictate that fixtures are placed closer together, increasing energy demand and increasing light reflected from the ground, thus sky glow may actually intensify. When lighting is installed near a major astronomical observatory or an intrinsically dark area (National Park), sky glow must be limited to low levels. For areas where observatories are not a concern, addressing the two other objectionable issues, glare and spill light, will often reduce sky glow to acceptable levels.

In the development of a roadway lighting plan, and in accordance with the Virginia State Law, the design engineer should make every effort to:
1. Reduce glare to provide a safe driving experience.
2. Reduce spill light (light trespass) outside the VDOT right-of-way.
3. Reduce sky glow and light pollution that effect the quality of our lives.
In order to meet all of these goals, the lighting designer should first develop a conceptual lighting plan based on “Full Cutoff” fixtures readily available from several manufacturers. This fixture restricts any light from being emitted above the horizontal, but may produce unacceptable levels of glare and light trespass.
Alternative lighting concepts should be developed using “Cutoff” fixtures and “Semi-Cutoff” fixtures. These fixtures limit the light intensity near the horizontal, but do not restrict the total amount of light emitted above the horizontal.
The final choice of luminaire optical distribution will be based on functionality: •

Which lighting system provides the highest quality lighting at the lowest cost? Specifically, the most effective illumination, at the lowest installation, maintenance, and energy cost.
Which luminaire restricts the light to areas inside the VDOT right of way?
Which lighting system provides the most effective reduction of sky glow? This issue can not be simply addressed by limiting the choice of luminaire to “Full Cutoff”. Due to the effects of light reflectance from pavement and grass, reducing pole spacing, and in turn, increasing the energy demand (sometimes referred to as: “watts-per-mile”) has been shown to increase up-light and sky glow.


VDOT requires all roadway lighting designs to meet the lighting criteria as discussed in the current IESNA publication, Recommended Practices for Roadway Lighting (RP-8). The illuminance criteria must be met for all sections of a roadway project. This guideline also includes meeting the criteria for the Veiling Luminance Ratio (Lv/Lave).

The lighting designer should also attempt to meet the luminance and STV criteria. Meeting all three criteria along a straight section of roadway is typically not feasible. It may be possible to meet only the illuminance criteria in more complex roadway geometry. Specifically, the algorithm for Veiling Luminance Ratio is based on a straight roadway section and cannot be adapted to a roadway with even a gentle curve.
The lighting designer should attempt to design the roadway lighting system based on illuminance and luminance lighting criteria and should coordinate with the TE/L&D Manager to determine the lighting design that will provide the best visibility.

The Illuminance Method

The roadway optimizer found in the AGI-32 lighting design software calculates the minimum illumination and provides the designer with the average-to-minimum uniformity. However, the designer must understand where the minimum illumination levels lie on the roadway and adjust the design parameters to provide a high quality design.

The luminaire spacing calculated by the roadway optimizer is based on the average level of illumination and/or the minimum illumination on the area of roadway. This establishes the quantity of illumination. The uniformity ratio, relating the average illumination level to the value of minimum illumination, is used as one way of specifying the quality of lighting. Because the design process already defines the average level of illumination, the next step involves finding the minimum point of illumination.

The Iso-footcandle template

Referring to the iso-footcandle diagram in Figure 2-3, the lines defining the illumination levels will occur at various mounting heights from the luminaire. These lines form what is known as a template.

The AGI-32 lighting design software can produce a set of iso-footcandle lines based on the luminaire mounting height. The lines should be defined as the minimum and half-minimum illumination levels required for the final roadway light plan. The example template shown in Figure 2-3 displays the 0.2 fc iso-footcandle line (minimum) and 0.1 fc iso-footcandle line (half-minimum).
The templates shown in Figure 2-3 represent two luminaires. The IESNA illumination criteria for a particular roadway will set the average illumination and uniformity levels. The minimum illumination is calculated from these two values.

For example:
• Average maintained illumination: 0.6 fc
• Uniformity (Average/Minimum): 3:1
• Minimum illumination = (0.6 / 3.0) = 0.2 fc
• ½ Minimum illumination = 0.1 fc
In the overlay region the total illumination will be the sum of the light from both luminaires.

Once it is understood how the iso-footcandle values are defined, the next step is to locate the point of minimum illumination expected to occur on the roadway. Depending on the roadway width, luminaire mounting height, type of luminaire, and mounting configuration, the minimum point will usually occur at one of several typical locations as identified in Figure 2-4 as areas A, B, C, or D. After checking the illumination at each of the anticipated low points, the design parameters can be adjusted to improve the uniformity ratio:
• In most cases, the luminaire offset (which can be altered by increasing or decreasing the length of the luminaire arm), the mounting height, or the pole spacing must be altered.
• Other situations will require choosing a different optical distribution.
• In some cases, the luminaire wattage will need to be adjusted to improve the uniformity. Typically, however, this effort will only increase (or decrease) the average light levels without affecting the uniformity.
The illuminance method for lighting a roadway also requires meeting the Veiling Luminance ratio criteria (Lv/Lave). The roadway optimizer will calculate this value based on the reflectance value (R) assumed for the roadway. Decreasing the luminaire spacing can reduce the veiling luminance ratio and reduce the resulting glare. Consequently, this action will increase the average illuminance on the roadway.
The reflectance value (R) should be set to match the type of material used in constructing the roadway. Lacking a clear understanding of the type of roadway surface (i.e., asphalt or concrete), the reflectance value should be set to R3.