Mesopic Street Lighting System Conserves Energy

The human vision system has two types of receptors in the retina, cones and rods, to transmit visual signals to the brain. The current system of photometry is based on how the eye’s cones respond to different wavelengths. Cones are the dominant visual receptor under photopic (daylight) lighting conditions. Rods function primarily under very dim (scotopic) conditions. With the current system of photometry, it remains unclear which luminous efficacy function should be used for nighttime applications where electric lighting is used and both rods and cones contribute to vision (mesopic conditions).

The LRC’s proposed Unified system of Photometry was designed to characterize light at any level including mesopic levels, bridging the photopic and scotopic luminous efficacy functions. LRC researchers developed a mesopic street lighting system designed to reduce energy use while maintaining or improving perceptions of visibility, safety, and security. The LRC team demonstrated the new mesopic system in the City of Groton, Connecticut.

Field test evaluation

High-pressure sodium (HPS) lights at two locations were replaced with white light sources (induction and ceramic metal halide), which better optimize human vision under mesopic conditions.

The new mesopic street lighting system:

• Met all utility requirements;
• Operated in both cold and hot climates without dramatic degradation of light output;
• Used 30-50% less energy than the HPS systems;
• Demonstrated that using less wattage, thereby lowering illuminance levels, reduces the light reflected from the road surface, a major contributor to light pollution (sky glow).

The overall results verified that the mesopic street lighting system can conserve energy in rural and suburban areas.

There are approximately 13 million streetlights in America. LRC experts estimate that about half of these have the opportunity to take advantage of mesopic lighting strategies, this would translate to an annual savings of 1 billion kWh, and a reduction in annual savings of 1 billion kWh, and a reduction in power plant CO₂ emissions of 546,000 tons per year.

Survey results

Responses to surveys conducted before and after the installation of the new light sources revealed that area residents perceived higher levels of visibility, safety, security, brightness, and color rendering as both drivers and pedestrians with the new lighting systems than with the standard HPS systems.

The findings in Groton concur with similar research conducted by the LRC in Easthampton, Massachusetts and in Austin, Texas.

Transportation Lighting Group, Lighting Research Center

Rensselaer Polytechnic Institute

Outline

• Properties of the eye
• Scattering theory
• Induction
• Action spectra for glare-disability and discomfort

The human eye Light travels through the cornea, aqueous, lens and vitreous bef ore reacting the retina.
Transmission of ocular media Media are not transparent Transmission reduces with age: half by age 50, one-third by age 60
Scattering Holladay (1926) noted that a glare source in the field of view had the same effect on foveal (central) vision as a uniform luminous veil Upon adjusting the light to the blind spot the effect remained, and he postulated that scattered light in the eye actually created this veil Stiles (1929) and colleagues (Stiles and Crawford, 1937) refined the theory including peripheral vision Fry (1954) further refined the work of Holladay (1926), Stiles (1929) and others to derive the familiar equation promulgated by the IESNA Lighting Handbook (2000):
It’s not just optics… Edges and contrasts are detected through receptive fields at the retinal level

Induction

• Fry’s (1954) formula breaks down when the glare is near the line of sight
• Induction is a neural interaction:
• Very large differences in field of view affect their visibility
• The way the visual system ‘enhances” edges through receptive fields, contrast might be a hindrance in a “hypercontrast” situation exacerbating effects of scatter

Action spectrum?

• Holladay (1926) tested glare sources of differing colors (white, blue, red) and found a small (but not significant) difference between red and blue/white
• Holladay (1926) and Stiles (1929) assumed scatter in the eye to follow Rayleigh scattering ½⁴ (“blue” scatters more than “red”)
• Moon et al. (1943) demonstrated that is not the case – no wavelength dependence

Spectral response: Disability glare No wavelength sensitivity, so the response mirrors the spectral sensitivity of the part of the retina in question, almost always the fovea (photopic)
Spectral response: Discomfort glare If discomfort glare also had a photopic response, glare sources of differing spectral content would be rated equally uncomfortable
Discomfortable glare Scotopic? No-HID and halogen headlamps have similar scotopic (rod-stimulating0 output but HIDs are consistently rated more uncomfortable (Flannagan, 1999) Short-wavelength-cone? Maybe (Fotios and Levermore, 1998) based on excess “brightness” Color channel? Maybe (Flannagan et al., 1989) Maybe both?
The nighttime driving environment Potential glare sources abound Oncoming headlights Street lights Traffic lights One’s own dashboard Knowing physiological responses will help in improved design of the roadway visibility system.