How to Design Efficient Street Lighting-Part-2
(C) Lighting Fixture:
(1) Fixture’s Mounting Height:
- Higher mounting heights used in conjunction with higher wattage luminaries enhances lighting uniformity and typically reduces the number of light poles needed to produce the same illumination level.
- In general, higher mounting heights tend to produce a more cost-effective design. For practical and aesthetic reasons, the mounting height should remain constant throughout the system.
- The manufacturer’s photometric data is required to determine an appropriate mounting height.
- Typical mounting heights for highway lighting purposes range from 30 ft to 55 ft (9.1 meter to 16.8 meter).
- Mounting heights for light towers or High mast is typically 80 ft (24 m) or greater.
- The installation height is too low, the glare of the lamp increases.
- As the installation high increase, glare decreases, but the lighting utilization rate decreases.
(2) Fixtures Classification:
- The Illuminating Engineering Society of North America (IESNA, IES or BIS1981) provides classifications for luminaires according to their glare control and high-angle brightness.
(a) Full Cutoff (F):
- A luminaire light distribution is designated as full cutoff (F) when Zero intensity at or above horizontal (90° above nadir) and Less than 10% of lamp lumens at or above 80°.
- Full-cutoff fixtures reduce glare dramatically and eliminate direct up light by sending all their light toward the ground .This efficiency should translate into lower bulb wattages if the existing poles are used. However, some lighting engineers believe that to achieve the same illumination uniformity as their semi-cutoff counterparts, full-cutoff fixtures need to be mounted either on taller poles or closer together
- Benefits:
- Limits spill light on to adjacent property, reduces glare. No light is emitted directly from the luminaries into the sky.
- Reduce Lighting Pollution.
- Limitations:
- May reduce pole spacing to maintain uniformity and increase pole and luminaire quantities.
- Application:
- Use for roadway, parking, and other vehicular lighting applications. Minimizes glare and light pollution and light trespass.
(b) Cutoff (C):
- A luminaries light distribution is designated as cutoff (C) when Less than 2.5% Intensity at or above horizontal (90° above nadir) and Less than 10% of lamp lumens at or above 80°.
- The direction of maximum intensity may vary but should be below 65º.
- Benefits:
- Small increase in high-angle light allows increased pole spacing.
- Cutoff system is the reduction of glare.
- Limitations:
- May allow some up light (Sky Light) from luminaries. Typically a small overall impact on sky glow.
- Application:
- Interchange lighting and rural intersections due to the ability to reduce glare.
- Use in applications where pedestrians are present. Provides more vertical illuminance than Full Cutoff luminaires.
- Lamp rating should be less than 3200 lumens.
- The cutoff design is where the luminaire light distribution is less than 25,000 lm at an angle of 90° above nadir (vertical axis) and 100,000 lm at a vertical angle of 80° above nadir.
(c) Semi Cutoff (S) (Medium Beam Angle):
- A luminaire light distribution is designated as Semi cutoff (S) when Less than 5% Intensity at or above horizontal (90° above nadir) and Less than 20% of lamp lumens at or above 80°.
- The direction of maximum intensity may vary but should be below 75º.
- Benefits:
- High-angle light accents taller vertical surfaces such as buildings. Most light is still directed downward.
- Limitations:
- Little control of light at property line.
- Potential for increased glare when using high wattage luminaries. Typically directs more light into the sky than cutoff.
- Application:
- Used for standard road lighting. Adequate glare control is obtained with reasonable spacing.
- The principal advantage of the semi-cutoff system is a greater flexibility in sitting.
- Use in pedestrian areas. If using in residential areas, provide with house side shields to minimize light trespass. Lamp rating should be less than 3200 lumens.
- For the semi-cutoff design, the luminous flux numbers become 50,000 lm for 90° above nadir and 200,000 lm at a vertical angle of 80° above nadir.
- Semi-cutoff fixtures create broad cones of light that permit wide spacing between poles. But such fixtures create harsh glare and send some light directly into the sky.
(d) Non Cutoff (N) (Higher Beam Angle):
- A luminaries light distribution is designated as Non Cutoff (N) when Emit light into all directions.
- No limitations on light distribution at any angle.
- There is considerable output near the horizontal plane.
- Benefits:
- Uniform luminous surfaces such as internally illuminated signs or globes. Wattage should be limited. Suitable for sports lighting, facade, landscape or other applications where luminaires are tilted due to limitations in pole or fixture locations
- Limitations:
- Location and aiming are critical. Most likely of all categories to produce offensive brightness and sky glow.
- Application:
- Used in areas with a lot of background light. Non-cutoff luminaries shall not be used at lower mounting heights because of glare.
- Use for decorative applications only. Lamp rating should be less than 3200 lumens.
- “Full cut off” fixtures must be installed properly, so that the bottom of the fixture is level with the ground.
- “Fully Shielded” fixtures do not allow any light to be emitted above the lowest light emitting
- part, but do not restrict light output in the “glare” zone, 90-80 degrees below horizontal.
(3) Fixtures Distributions (Optical System):
- The Illuminating Engineering Society classified series of Fixture distribution patterns as Types I, II, III, IV, and V.
(a) Type I (Two-way):
- The lateral distribution having a preferred lateral width of 15 degrees in the cone of maximum Lumen.
- Illumination Pattern: Narrow, symmetric luminance pattern.
- Fixture Location: This type is generally applicable to a luminaire location near the center of a roadway where the mounting height is approximately equal to the roadway width.
- Type of Road: The luminaire is placed on the side of the street or edge of the area to be lighted. Most 1or 2 Lane Road
- Application:
(b) Type II (Two Way) :
- Light distributions have a preferred lateral width of 25 degrees.
- Illumination Pattern: Slightly wider illuminance pattern than Type I.
- Fixture Location: They are generally applicable to luminaires located at or near the side of relatively narrow roadways, where the width of the roadway does not exceed 1.75 times the designed mounting height.
- Type of Road: The luminaire is placed on the side of the street or edge of the area to be lighted. It produces a long, narrow, oval-shaped lighted area which is usually applicable to narrower streets.
(c) Type III (Bat Wing) :
- Type III light distributions have a preferred lateral width of 40 degrees.
- Illumination Pattern: It produces an oval-shaped lighted
- Fixture Location: This distribution is intended for luminaires mounted at or near the side of medium width roadways, where the width of the roadway does not exceed 2.75 times the mounting height.
- Type of Road: The luminaire is placed on the side of the street or edge of area to medium width streets.
(d) Type IV (Forward throw “Asymmetric”):
- Type IV light distributions have a preferred lateral width of 60 degrees.
- Illumination Pattern: Widest luminance pattern.
- Fixture Location: This distribution is intended for side-of-road mounting and is generally used on wide roadways where the roadway width does not exceed 3.7 times the mounting height.
- Type of Road: very wide roadway (4 to 6 Lane)
- Applications: Type IV often use at perimeters where Spill Light is required and there is no place to add Pole.
(e) Type V:
- Type V light distributions have a circular symmetry of candlepower that is essentially the same at all lateral angles.
- Illumination Pattern: It produces a circular, wider lighted area and is usually applicable to wide streets.
- Fixture Location: The luminaries are mounting at or near center of roadways, center islands of parkway, and intersections.
- Type of Road: very wide roadway (4 to 6 Lane)
- Applications: Type V often applies to high-mast lighting.
GUIDE FOR LUMINAIRE LATERAL LIGHT TYPE AND PLACEMENT | ||
| Pole Arrangement | Road Width | Type of Distribution |
| One Side or Staggered | up to 1.5 x Mounting Height | Types II-III-IV |
| Staggered or Opposite | Beyond 1.5 x Mounting Height | Types III & IV |
| Center of the Roadway Mounting | up to 2 x Mounting Height | Type I |
Type of Classification | |
| AREA CLASSIFICATION | CUTOFF TYPE |
| Commercial | Full Cutoff or Semi Cutoff |
| Intermediate | Full Cutoff or Semi Cutoff |
| Residential | Full Cutoff |
THE SELECTION OF LUMINAIRE MOUNTING HEIGHTS | |
| Lamp Lumens | Mounting Height |
| ≤20,000 Lumen | ≤35 Foot |
| 20,000 To 45,000 Lumen | 35 To 45 Foot |
| 45,000 To 90,000 Lumen | 45 To 60 Foot |
Fixture Mounting Height | ||||
| Type of LED Luminaries | Type of Road | Lamp mounting height from the floor level (Meters) | Minimum Illumination Level (Lux) at centre of road | Color of Illumination |
| 250-260W | Above 18 | (20 To 22) | 5000K-6500K | |
| 190W | A1 | Between 11 To 15 | (20 To 22) | 5000K-6500K |
| 140-170W | A1 | Between 9 To 15 | (18 To 20) | 5000K-6500K |
| 90-120W | A2/B1 | 07 To 11 | (15 To 18) | 4300K-5600K |
| 70-120W | A2/B1 | 07- To 11 | (15 To 18) | 4300K-5600K |
| 70-120W | B1/B2 | 06 To 09 | (15 To 18) | 4300K-5600K |
| 70-50W | B1/B2/C1 | 7 To 9 | (12 To 15) | 4300K-5600K |
| 45-50W | B1/B2/C1 | 5 To 7 | (12 To 15) | 4300K-5600K |
| 25-30W | B1/B2/C1 | 5 To 7 | (10 To 12) | 4300K-5600K |
Relationship between mounting height and spacing | |||||||
| Mounting Height | Width of road | 6 Meter to 7 Meter | 9 Meter to 10.5 Meter | 12 Meter to 14 Meter | |||
| Pole arrangement | Cut-off Type | Semi Cutoff Type | Cut-off Type | Semi Cutoff Type | Cut-off Type | Semi Cutoff Type | |
| 8 Meter | Single side | 24 | 28 | – | – | – | – |
| Staggered | 24 | 28 | – | – | – | – | |
| Opposite | – | – | 28 | 28 | – | – | |
| 10 Meter | Single side | 30 | 30 | – | – | – | – |
| Staggered | 35 | 35 | 30 | 35 | – | – | |
| Opposite | – | – | 35 | 40 | 30 | 35 | |
| 12 Meter | Single side | 42 | 48 | 36 | 42 | – | – |
| Staggered | – | – | 36 | 42 | 36 | 42 | |
| Opposite | – | – | 42 | 48 | 42 | 48 | |
GUIDE FOR LUMINAIRE LATERAL LIGHT TYPE AND PLACEMENT | |||||
| SIDE OF THE ROADWAY MOUNTING | CENTER OF THE ROADWAY MOUNTING | ||||
| One Side or Staggered | Staggered or Opposite | Local Street Intersection | Single Roadway | Twin Roadways (Median Mounting) | Local Street Intersections |
| Road Width up to 1.5 x Mounting Height | Road Width beyond1.5 x Mounting Height | Road Width up to 1.5x Mounting Height | Road Width up to 2x Mounting Height | Road Width up to 1.5x Mounting Height (each pavement) | Width up to 2.0x Mounting Height |
| Types | Types | Type | Type | Types | Types |
| II, III, IV | III & IV | II (4-way) | I | II & III | I (4-way) & V |