How to Design efficient Street lighting-(Part-5)

(2) Surround Ratio (SR):

• Road lighting should be illuminate not only the road, but also the adjacent areas so motorists can see objects in the periphery and anticipate potential road obstructions (e.g., a pedestrian about to step onto the road).
• The SR is the visibility of the road’s periphery relative to that of the main road itself.
• As per industry standards, SR should be at least 50.
• Figure show how road lighting should illuminate both the main road and its periphery.

(F) Lighting Pollutions

• Light pollution is an unwanted consequence of outdoor lighting and includes such effects as sky glow, light trespass, and glare. 
• 30 to 50% of all light pollution is produced by roadway lighting that shines wasted light up and off target.

(1) Glare:

• Glare is the condition of vision in which there is discomfort or a reduction in the ability to see significant objects. Glare affects human vision and it is subdivided into four components, Disability Glare, Discomfort Glare, Direct Glare and Indirect Glare.
• By origin

1. Direct Glare
2. Indirect (reflected) Glare

• By effect on people

1. Disability Glare
2. Discomfort Glare

• Disability glare:
• Disability glare is the glare that results in reduced visual performance and visibility.
• Since disability glare reduces the ability to perceive small contrasts.
• It can impair important visual tasks in traffic such as detecting critical objects, controlling headlights, and evaluating critical encounters, making glare a potential danger for road users.
• LED light sources can provide very high luminance lev­els which may cause glare. For this reason, LED lamps are commonly equipped with diffusers to reduce this luminance.
• Disability glare may vary for dif­ferent individuals and it can be calculated objectively.
• In a particu­lar illuminated environment, the human eye will be able to detect differences in luminance down to a certain threshold. This threshold can be compared for a situa­tion in the same environment when a source of glare is added. By comparing these thresholds, the threshold increment can be derived.
• Discomfort glare:
• Discomfort glare is the glare producing discomfort. It does not necessarily interfere with visual performance or visibility.
• As vertical light angles increase, discomforting glare also increases
• Discomfort glare, on the other hand, is a subjective phenomenon and there is no method for its Rating.
• Although the 9-point De Boer scale (ranging from “1” for “unbearable” to “9” for “unno­ticeable”) is the most widely used in the field of auto­motive and public lighting.
• Direct Glare:
• Direct glare is caused by excessive light entering the eye from a bright light source. The potential for direct glare exists anytime one can see a light source. With direct glare, the eye has a harder time seeing contrast and details.
• A system designed solely on lighting levels, tends to aim more light at higher viewing angles, thus producing more potential for glare.
• Exposed bright light source, for example a dropped lens cobra head or floodlight causes of direct glare.
• Direct glare can be minimized with careful equipment selection as well as placement.

• Figure illustrates two examples of exterior lighting that results in glare.

• Fig shows how full cutoff luminaries (Shielded Luminaires) can minimize this direct glare. In exterior applications, use fully shielded luminaires that directs light downwards towards the ground.
• Indirect Glare: Indirect glare is caused by light that is reflected to the eye from surfaces that are in the field of view – often in the task area.
• Indirect Glare can be minimized with the type and layout of lighting equipment. Direct the light away from the observer with the use of low glare, fully shielded luminaries.
• As the uniformity ratio increases (poor uniformity), object details become harder to see.
• For roadway lighting, good uniformity shows evenly lighted pavement. However, to meet small target visibility criteria, a non uniform roadway surface may be better.
• There should be a balance between uniform perception and detecting objects on the road. Also, emphasis is put on horizontal surface uniformity. In reality, vertical surfaces may require more lighting in order to improve guidance.

How to Reduce Glare:

• Glare and light trespass are more concern when installing floodlights.
• Use shielded Light should be use to reduce Glare.
• Higher mounting heights can more effective in controlling spill light, because floodlights with a more controlled light distribution (i.e., narrower beam) may be used, and the floodlights may be aimed in a more downward direction, making it easier to confine the light to the design area.
• Lower mounting heights increase the spill light beyond the property boundaries. To illuminate the space satisfactorily, it is often necessary to use floodlights with a broader beam and to aim the floodlights in directions closer to the horizontal than would occur when using higher mounting heights.
• Lower mounting heights make bright parts of the floodlights more visible from positions outside the property boundary, which can increase glare.

(2) Sky glow:

• Sky Glow is brightening of the night sky caused by outdoor lighting.
• Light that is emitted directly upward by luminaries or reflected from the ground is scattered by dust and gas molecules in the atmosphere, producing a luminous background. It has the effect of reducing one’s ability to view the stars in Night.

How to Reduce Sky Glow

• While it is difficult to accurately model sky glow, at this point it is presumed that the most important factors are light output and lamp spectral characteristics, light distribution from the luminaire, reflected light from the ground, and aerosol particle distribution in the atmosphere.
• If the quantity of light going into the sky is reduced, then sky glow is reduced. Thus, to reduce sky glow by
• By using full cutoff luminaires to minimize the amount of light emitted upward directly from the luminaire.
• Reduce Lighting Level.
• Make practice to Turn off unneeded lights
• Limited Lighting hours in outdoor sales areas, parking areas, and signages
• Installing Low-Pressure Sodium light sources, which allow astronomers to filter the line spectra from telescopic images.