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We do not see footcandles, we see brightness, either primary sources or reflected surfaces. And then the brain scans the visual image and perceives and interprets the scene.
Brightness and visibility are the most important perceptions in evaluating the exterior lighting. The ability to see faces of people and what they are carrying are more important than being able to detect color of clothing and hairstyles. Preventing break-ins, being able to see people at night, and being able to see to avoid accidents are also important performance requirements for exterior lighting.
Traditionally, street lighting has been the basic component of public outdoor lighting. In urban settings, it is the street lighting, along with traffic signals and signs, that organizes and defines the visual environment at night. The quality of this visual information is critical for both traffic safety and a pedestrian's sense of security.
Public lighting systems can help define urban character and image. These lighting systems may illuminate for streets, roadways, sidewalks, pedestrian malls, pathways, bikeways, parks, monuments, buildings, structures, statues, fountains, and landscapes. A hierarchy of public lighting connotes the relative importance and character of cityscapes and enhances their information-giving value. The height and location of poles and the size and shape of equipment all contribute to the lighting hierarchy.
Special features and amenities of urban environments should be lighted to reveal their importance. Buildings and monuments can serve as markers or reference points to provide visual orientation. Urban landscape elements can also act as visual anchors and serve as points of arrival for neighborhood residents. Consistency and coordination applied to lighting special features strengthens a public lighting design and can improve the sense of community. Also, the streetscape or pedestrian spaces should appear consistent with the community theme, both day and night so the lighting equipment should be integrated into the daytime scenes.
There are seven steps that comprise an effective community-responsive design process for implementing a public lighting system.
1. Determine community lighting goals. List and prioritize community goals such as aesthetics, community identity, safety, security, crime prevention, light pollution, light trespass, equipment location, equipment appearance, energy effectiveness, and economics.
2. Determine a community theme. Establish the style of lighting desired, such as historical, modern, active, or subdued. The theme should relate to the architectural style of the area.
3 Develop a family of luminaires. Select luminaires that complement the community theme. Establish pole styles and mounting heights. Develop a hierarchy of luminaires matching the application.
5. Determine how luminaire luminances affect perceptions of the environment. High luminances projected directly from luminaires and excessive luminance differences between surfaces and areas within the field of view can be distracting, uncomfortable, and even disabling.
6. Provide design guidelines. Design guidelines that establish the steps for planning public and private lighting (residential and commercial) for communities, owners, and developers. These guidelines should explain community themes and goals, including the family of luminaires or related families for different districts selected to provide visual consistency and set the standard for construction and glare control.
7. Educate developers of lighting ordinances. Consideration might be given to developing a lighting ordinance in areas where light trespass or light pollution are important to the community.
Getting the right amount of light depends on understanding what is the nature of the space; you need to know what will go on there, what has brought the people to this space and what are their expectations? Think about the light being reflected from the things that matter to the people who will use the space. People viewing the lit area should consider it to be bright and people and objects within it to be clearly visible.
A recent study tested the belief that people experience greater sense of security as light increases. This showed that although ratings for security lighting were higher for the sites with higher illuminance (above 3 fc) the improvement was slight.
A frequent problem is glare. It prevents the collection of visual information. By creating veiling illumination over the retina of the eye, low contrast objects become invisible. Unfortunately, it is difficult to measure glare.
Another study showed that both color rendering and illuminance were important. Metal halide lamps were better than high pressure sodium lamps for color naming at low illuminance. The difference between the metal halide and high pressure sodium lamps became greater as illuminance increased.
Perceived brightness is critical in night driving. With all the people who are driving with some form of impairment—some of it visual and some in their blood streams—it is important to remember that sometimes the brain will not allow us to perceive what our eyes are “seeing.”
• The Amount of Light is the quantity of lumens released into, or admitted into, the design space. • The Spatial Distribution of Light comprising the composition of two- and three-dimensional lighting patterns. • The Spectral Distribution of Light, is the color attributes of the lighting. • The Temporal Distribution of Light, which includes not only sequential changing of the spatial and spectral distributions of lighting within a space, but also the sequence of visual effects as people redirect their view or move through the space.
Safety & Security Lighting Safety can be defined as freedom from danger, whereas security is freedom from worry. Security is often considered the psychological version of safety. Features of the environment that compromise safety can be identified and illuminated, but because security involves psychology as well as vision, it is a more difficult design criterion for exterior lighting. In general, comfortable, well-defined exterior environments with clear zones of recognition are perceived as secure. This design approach provides a feeling that there is enough response time to avoid or escape potential threats. People often associate higher illuminance or greater luminance with safer surrounds, but poorly directed light can reduce visibility and thereby reduce both safety and security.
Light Pollution Dust, water vapor, and other particles reflect and scatter light that is emitted into the atmosphere. The result is the sky glow found over all urban areas, sometimes called atmospheric or astronomical light pollution. Although this sky glow is not injurious, it does deprive urban residents of the opportunity to stargaze and can hamper astronomers' attempts to view the night sky through telescopes.
Limit flux above horizontal. Street and area-lighting systems, including lighting for sports activities, parking lots, and vehicle sales lots, should be designed to minimize or eliminate direct upward flux emission. A full cutoff luminaire does not emit light above the horizontal plane.
Minimize non-target illumination. Lighting systems that project light upward, such as architectural and sign lighting, should be designed to minimize light that does not illuminate the target area.
Turn off outdoor lighting during times of low use. Designers should discuss with owners turning off some or all of the outdoor lighting, including advertising sign lighting, building elevation lighting, and interior high-rise office building lighting, after normal hours of operation or before midnight unless needed for safety and security.
Light Trespass The topic of light trespass is somewhat subjective, because it often relates to unmeasurable and undefinable factors. A typical example is the "light shining in my window" complaint. A simple solution to this problem is to shield the offending luminaire so that its luminance is not directed toward the complainant. Light trespass usually falls into one of two categories: unwanted light received in adjacent properties (high illuminance levels), and excessive brightness occurring in the normal field of vision (nuisance glare). Efforts have been made in numerous jurisdictions to write ordinances or laws controlling light trespass.
The purpose of performing a calculation is to reduce uncertainty, or to express this point more positively and put it into a lighting design context; it is to increase one’s confidence that an envisioned effect will be realized. Far more important than the ability to operate a calculator or a computer is the ability to relate one’s vision of an end result to quantifiable aspects of lighting. No calculation can perform the act of design for you, but it can enable you to propose a novel solution with reasonable confidence that the envisioned effect will be achieved .
A creative lighting designer envisions an end result. To achieve that goal, the designer must provide a technical specification of lamps, luminaires, locations, aiming angles, and controls.
Successful exterior lighting design employs layers of light. Layered lighting provides minimal ambient illumination with accents on hazards, destinations, and architectural features. Sidewalks, trees, and building facades can be used as reference points and backdrops for such important features as crosswalks and intersections on roadways, or stairs and changes in elevation on pathways. Highlights can also be provided on gathering places and on interesting features such as bridges, statues, or plantings. Layered lighting defines the spatial characteristics of the environment and helps minimize dark areas that would otherwise serve as areas of concealment.
Landscape and Site Illumination Let There be Light!
REFERENCES 1. IESNA. Committee on Sports and Recreational Lighting. 1988. Current recommended practice for sports and recreational area lighting, IES RP-6-1988. New York: Illuminating Engineering Society of North America. 2. IES. Roadway Lighting Committee. 1983. American national standard practice for roadway lighting, ANSI/IES RP-8-1983. New York: Illuminating Engineering Society. 3. IES Roadway Committee. Subcommittee on Off-Roadway Facilities. 1998. Lighting for parking facilities, IES RP-20-1998. New York: Illuminating Engineering Society of North America. 4. IESNA. Roadway Standard practice Subcommittee of the IESNA Roadway Lighting Committee. A discussion of Appendix E--Classification of luminaire light distributions from the American National Standard for Roadway lighting RP-8-1983 Roadway Lighting, TM-3-1995. New York: New York: Illuminating Engineering Society of North America. 5. The IESNA Recommended Practice for Lighting Merchandising Areas, IES RP-2-1985 is under revision. Expected publication date is Spring 2000. 6. Rea, M. S. 1996. Essay by invitation. Light. Des. Appl. 26(10): 15-16.
7. He, Y., M. Rea, A. Bierman, and J. Bullough. 1997. Evaluating light source efficacy under mesopic conditions using reaction times. Journal of the Illuminating Engineering Society of North America 26(1):125-138. 8. IESNA. Outdoor Environment Lighting Committee. 1999. Lighting for exterior environments: An IESNA recommended practice, RP-33-1999. New York: Illuminating Engineering Society of North America. 9. IESNA. Ballast Task Force. 1996. Ballasts and the generation of light, DG-8-96. New York: Illuminating Engineering Society of North America. 10. Moyer, J. L. 1992. The landscape lighting book. New York: John Wiley. 11. National Fire Protection Association. 1999. National electrical code, NFPA 70. Quincy, MA: National Fire Protection Association.
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