Mesopic Lighting for Street Lighting
Street Lighting: Mesopic LED vs. HPS
This technology allows street lights to be tuned to how humans see under moderately low light (mesopic) conditions, which can be used to reduce the luminance of the road surface while providing the same or better visibility.
Item ID: 25
Technical Advisory Group: 2009 Lighting TAG (#1)
It is important that street lighting provide the best visibility and safety for drivers while also saving energy. This can be accomplished by selecting lights in the spectral range visible at the low light level provided by streetlights on residential and lower traffic roadways. About 50% of street lighting provides light levels in this range, referred to as the mesopic range. Studies have shown that white light, rich in the blue and green spectrum, enhances peripheral visibility in these conditions compared to the most common yellowish light provided by High-Pressure Sodium systems. Selecting a white light allows lower wattage lamps to be used. The white light provided by Light Emitting Diodes (LEDs) comes in a variety of wavelengths and could be selected for optimized mesopic lighting applications. LED technology is already showing some economic and energy-saving advantages over traditional light sources, and promises to provide even more savings in the future.
The 10th edition of the Lighting Handbook from IES now includes a multiplier table for correcting photopic light levels to mesopic levels, allowing for lower light output, as measured by light meters. However, each IES committee is responsible for the proper application of these in their respective practices. In the latest edition of IES RP-8-14, the IES Roadway Lighting Committee recommends limitations on use of Mesopic multipliers for roadway lighting design. The committee is recommending that these Mesopic multipliers only be used to assess the luminances of off-road locations in applications for street lighting where the posted speed limit is 25 mph or less. While these are recommended practices, not code, and municipalities are not obligated to follow RP-8, it is considered standard practice.
Baseline Description: 100 Watt High Pressure Sodium
Baseline Energy Use: 525 kWh per year per unit
Most low speed (under 40 mph) roadways are illuminated from dusk to dawn utilizing photocell controls on, 70-150W High Pressure Sodium (HPS) yellow lights due to their relatively high efficacy and long life. While providing basic visibility, the yellow light does not render colors as identifiable and the illuminance provided is with a hot spot below the fixture fading either to the next fixtures influence, or in rural areas to the dark zone. Though full cutoff fixtures are installed more frequently in new construction to reduce light pollution and glare, dropped cobra heads are the most common fixture and provide a view of the lamp and glare from a distance. Maintenance of HPS lights can be at least as costly an expense as the energy use. Using a 100 watt (120 input watts) HPS fixture as a baseline, energy use for 12 hours per night consumes 525 kWh per year. Basic fixtures are in the $100 range.
Manufacturer's Energy Savings Claims:
"Typical" Savings: 40%
Savings Range: From 30% to 50%
Lighting manufacturer's cite various studies about improved visibility with white light at night and claim the energy savings of the more efficient technologies as a basis for savings. Some specify the scotopic/photopic (S/P) ratio. By using a light source with higher S/P ratios, visibility is improved in mesopic conditions while the photopic light measurement (what a light meter reads) can be the same as a lamp with lower S/P ratios.
One factor rarely mentioned is the studies assume that only the light from the streetlights is affecting vision, while at night there are also oncoming headlights, neon signage and other lighting in the visual field to consider. Eyes must readjust to a lower light level every time a source of brighter light is encountered. This issue may be made worse with the use of dimmer lights from a mesopic design.
Best Estimate of Energy Savings:
"Typical" Savings: 40%
Low and High Energy Savings: 30% to 63%
Energy Savings Reliability: 3 - Limited Assessment
Energy savings are dependent on the illuminance levels of the current street lighting or street lighting illuminance recommendations. The higher the illuminance level (footcandles), the less the energy savings possible by tuning a light source to mesopic vision. As light levels increase, the eye shifts toward photopic vision and the value of tuning the light source to mesopic vision diminishes.
The Gateway Demonstration project at Lija Loop (Myer, 2009) in Portland, Oregon used 53W LED units to replace 100W HPS units and saved 31% of the energy use. Since that time, LEDs have continued to improve in efficiency. This study was prior to the mesopic multiplier being approved, and it was thought the new LED light levels might fall too far below the standards required for Portland. If the city adopts the mesopic standard, this may not be an issue.
Most current LED street lighting installations are exceeding predicted energy savings.
Energy Use of Emerging Technology:
315 kWh per unit per year
Energy Use of an Emerging Technology is based upon the following algorithm.
Baseline Energy Use - (Baseline Energy Use * Best Estimate of Energy Savings (either Typical savings OR the high range of savings.))
Based on 4200 operating hours per year for streetlights, the life of LED streetlights rated at 70,000 hours would be almost 17 years and if controls reduced output or turned lights off part of the time, even longer, compared to 5 years for HPS lamps.
Actual life will depend on operating conditions, remaining below the design limits is critical, thermal management, and life of components other than the LEDs (Navigant, 2013 Pg 35-37). Many LED outdoor lighting luminaires list life at 50,000 hours but many products are higher, to 100,000 hours and more as products in the field are showing good performance and more tests and standards are helping to predict life.
Currently no data available.
Installed first cost per: unit
Emerging Technology Unit Cost (Equipment Only): $224.00
Emerging Technology Installation Cost (Labor, Disposal, Etc.): $100.00
Baseline Technology Unit Cost (Equipment Only): $150.00
Since utilities tend to amortize street lighting capital costs over long periods of time (27 years), the added capital cost of long-life street lighting equipment such as LEDs or induction is offset by reduced costs for energy and maintenance. Capital costs for LEDs are in the $200 - $300 range compared to $150 for HPS. Another benefit for investor-owned utilities is that profits for long-life street light equipment are three times higher than current HPS street lighting, (although there are now 80,000-hour HPS lamps available). While current LED costs may run two to four times as much as HPS technology, the prices continue to drop and the product continues to improve.
Simple payback, new construction (years): 3.9
Simple payback, retrofit (years): 17.1
Cost Effectiveness is calculated using baseline energy use, best estimate of typical energy savings, and first cost. It does not account for factors such as impacts on O&M costs (which could be significant if product life is greatly extended) or savings of non-electric fuels such as natural gas. Actual overall cost effectiveness could be significantly different based on these other factors.
Different parts of the eye respond differently to different light levels, which affects how well we see. Our eyes react to moonlit darkness with scotopic vision (black and white) and daylight/bright light with photopic vision (full color). Mesopic vision is experienced with low artificial light levels at night, between the scotopic and photopic light levels, such as light from an average streetlight( generally High Pressure Sodium (HPS) product using 100 watts or less). In this range, the brighter the light gets, the more some color comes into play. The focus of a person’s view is centrally located (on axis) in front of them. The dimmer the light gets, the more scotopic vision takes over and peripheral vision (off-axis) becomes more important. Peripheral vision is what helps with depth perception and warns us of approaching targets from the side. Different light sources provide different light spectrums. Because the eye is only able to perceive certain wavelengths at certain light levels, mesopic lighting strives to provide a spectrum the eye can make use of at this low light level. Current lighting standards only address the photopic light levels, even though they may not be contributing to good vision at the lower levels. A new way of measuring the usefulness of light at various levels of intensity proposed by the Lighting Research Center, called the unified system of photometry, is being adopted by the Commission Internationale de l’Eclairage (CIE). This will enable the scotopic lumens of a light source to be used in calculating the effectiveness of lamps at various light levels. Studies have shown that in low light conditions, the ability of the peripheral vision to detect moving objects is enhanced with green and blue light. In addition, it takes less light in this spectrum to see those targets well than when light is higher in the red and yellow spectrums. White light is high in green and blue light, making fluorescent (includes induction), metal halide, and LED systems good choices. Since it takes less light to see the intended target with the white light, a lower-powered lamp may be used, which saves energy. LED technology promises to deliver white light (and tunable shades of white) at lower wattage than traditional lighting sources and could be a major provider of mesopic lighting. Roadways are lit to provide safety for motorists as well as pedestrians, cyclists, and animals along and on the roadway. Providing light of a quality and quantity that enhances this vision can improve vision and reduce energy use by producing the wavelengths that are helpful. Headlights provide light most intensely straight ahead and are quite bright. Roadway lighting lights the road and usually the shoulder area and even further off to the side where cyclists, pedestrians, and animals may be active and can intersect traffic.
The 10th edition of the Lighting Handbook from IES (IESMC, 2012) now includes a multiplier table for correcting photopic light levels to mesopic levels, allowing for lower light output, as measured by light meters. However, each IES committee is responsible for the proper application of these in their respective practices. In the latest edition of IES RP-8-14, the IES Roadway Lighting Committee recommends limitations on use of Mesopic multipliers for roadway lighting design. The committee is recommending that these Mesopic multipliers only be used to assess the luminances of off-road locations in applications for street lighting where the posted speed limit is 25 mph or less. While these are recommended practices, not code, and municipalities are not obligated to follow RP-8, it is considered standard practice.
Most street lights use HPS technology. Current IESNA practice for outdoor lighting now provides a multiplier to account for the mesopic effect to apply to photopic light levels. (IESMC, 2012) However, each IES committee is responsible for the proper application of these in their respective practices. In the latest edition of IES RP-8-14, the IES Roadway Lighting Committee recommends limiting use of mesopic multipliers for roadway lighting design to off road, low speed locations.
The unified photometry system needed to evaluate different light sources for their value under all outdoor lighting conditions, including mesopic has been developed and proven effective, and is included by the IES Handbook for calculating outdoor lighting levels in the mesopic visual zone.
The improved visibility of peripheral moving objects contributes to safer driving and potentially fewer accidents.
End User Drawbacks:
Staff at utilities and municipalities should be provided with training to enhance their understanding of how and why mesopically tuned street lighting work and how they can utilize the unified photometry system.
When transitioning to a mesopic design, the pole spacing may need to be changed.
Operations and Maintenance Costs:
Maintenance savings based on the much longer life of LED technology is expected to exceed the cost of energy savings in many cases. While LEDs have a long lamp life, if they are in a dirty environment they may require multiple cleanings through their life to maintain the light output. Selecting products less likely to accumulate dirt can help. Drivers, lenses, seals, sensors, controls and internal construction quality can all contribute to maintenance requirements. Service contracts including maintenance and energy use are common for street lighting systems and new rates may need to be renegotiated to realize the savings a new LED system provides.
Anticipated Lifespan of Emerging Technology: 17 years
Based on 4200 operating hours per year for street lights, the life of LED street lights rated at 70,000 hours would be almost 17 years compared to 5 years for HPS lamps. If controls reduced output or turned lights off part of the time, LED street lights could last even longer.
Actual life will depend on operating conditions, remaining below the design limits is critical, thermal management, and life of components other than the LEDs (Bardsley, et. al., 2014 Pg 35-37). Many LED outdoor lighting luminaires list life at 50,000 hours but many products are higher, up to 100,000 hours and more. Products in the field are showing good performance and more tests and standards are helping to predict life. Note that on some LED models, the lamp drivers may fail prior to the end of lamp life, shortening effective life of the unit below that expected.
In populated areas, white light is becoming increasingly popular so low wattage lamps using metal halide or fluorescent, including induction, might also be used.
Reference and Citations:
Parking Lot Luminaire Calculator
Lighting Research Center
Outdoor Lighting: A Short Guide to Applications, Objectives and Considerations
Assist recommends... (Alliance for Solid-State Illumination Systems and Technologies)
Recommendations for Evaluating Parking Lot Luminaires
ASSIST recommends... (Alliance for Solid-State Illumination and Technologies)
Outdoor Lighting: Visual Efficacy
ASSIST Recommends... (Alliance for Solid-State Illumination and Technologies)
Mesopic Street Lighting Demonstration and Evaluation: Final Report
Lighting Research Center for Groton Utilities
International Commission on Illumination (CIE)
Eloholma, et. al.,
Mesopic Vision and Photometry
Balkan Light 2008, Ljubljana, Slovenia, Lighting Engineering Society of Slovenia
The Lighting Handbook: 10th Edition
Illuminating Engineering Society
Spectral Effects of Lighting on Visual Performance at Mesopic Lighting Levels
Illuminating Engineering Society - Mesopic Committee
Extensive table of effective luminance values
Halonen, et. al.,
CIE and Mesopic Photometry
International Commission on Illumination (CIE)
Provides information about the CIE Technical report 191:2010, basis for a ISO/CIE Standard and guidance for mesopic lighting
Demonstration Assessment of Light-Emitting Diode (LED) Street Lighting - Host Site: Lija Loop, Portland, Oregon
U.S. Department of Energy
Bardsley, et. al.,
Solid-State Lighting Research and Development: Multi-Year Program Plan
U. S. Department of Energy, Solid-State Lighting Program