Street lighting design is the design of street lighting such that people can safely continue their travels on the road. Street lighting schemes never brings the same appearance of daylight, but provide sufficient light for people to see important objects required for traversing the road. Street lighting plays an important role in:
- Reducing the risk of night-time accidents
- Assisting in the protection of buildings/property (discouraging vandalism)
- Discouraging crime
- Creating a secure environment for habitation
Basic Features of Street Light Luminaires
The basic features of a street lighting luminaires are:
- Roadway luminaires are mounted horizontally and thus have fixed vertical aiming.
- Roadway lighting luminaires have particular intensity distributions which are desired to light long narrow horizontal stripes on one side of the luminaire, while minimizing the intensities on the other side of the luminaire.
- The intensity distributions up and down the narrow strip are generally the same.
- Any fixed aimed luminaire which does not have this type of intensity distribution is called an area luminaire.
Main Objectives of Street Lighting Design Scheme
The main objectives of street lighting design scheme are given below:
- Perfect visual sensation for safety
- Illuminated environment for quick movement of the vehicles
- Clear view of objects for comfortable movement of the road users.
Which Lamps are Used in Street Lighting?
Various types of lamps are used in street lighting luminaires. They are
- High pressure sodium lamp
- Metal Halide Lamps
- Low pressure sodium lamps
- Incandescent Lamp (not recommended)
- CFL (used in Lanes or streets only not widely)
Main Factors in the Street Lighting Design Scheme
- Luminance Level Should be Proper
Luminance always influences the contrast sensitivity of the obstructions with respect to the back ground. If the street is brighter, then darker surroundings makes the car driver adapted, unless the driver will be unable to perceive the objects in the surroundings. As per CIE, 5m away from the road on both sides will be lit by Illuminance level at least 50% of that on the road.
- Luminance Uniformity must be Achieved
To provide visual comfort to the viewer’s eyes, enough luminous uniformity is needed. Luminous uniformity means the ratio between minimum luminance level to average luminance level, i.e.
It is termed as longitudinal uniformity ratio as it is measured along the line passing through the viewers position in the middle of the traffic facing the traffic flow.
- Degree of Glare Llimitation is always taken into Design Scheme
Glare means visual discomfort due to high luminance. There are two types of glare created by the street light luminaires, first type is disability glare and second type is discomfort glare. Disability glare is not a strong factor, rather discomfort glare is a common factor due to unplanned street lighting scheme.
- Lamp Spectra for Visual Sharpness depends on the Proper Luminaries
It is very much essential to make an object as per its size and dimension.
- Effectiveness of Visual Guidance is also an important factor
It helps a viewer to guess how far another object is from his position.
Types of Road to Implement Various Street Lighting Design Schemes
As per CIE 12 roads are broadly classified into five types.
Type A of Street Lighting Design
- Heavy and high speed traffic.
- The roads are separated with the separators.
- No crossing is allowed.
- Controlled access
- As the example: express ways.
Type B of Street Lighting Design
- Heavy and high speed traffic.
- Separate road for slow traffic movement or pedestrians.
- As the example: Trunk road.
Type C of Street Lighting Design
- Heavy mixed traffic with moderate speed.
- Rural and urban roads.
- As the examples: Ring Road or Radial Road.
Type D of Street Lighting Design
- Slow traffic and pedestrians’ purpose.
- Road in the city or shopping center.
- As the example: Shopping streets.
Type E of Street Lighting Design
- Mixed Traffic with limited speed.
- Connector road between residential areas.
- As the example: Local street.
Street Light Luminaire
Intensity distribution of the street light luminaire is measured with mirror Goniophotometer. And it is graphically represented by polar intensity diagram.
But Intensity distribution of the road light is measured following C-ɣ photometric convention. In C-ɣ photometry, C is the angle on the road surface plane and ɣ is the angle created between vertical axis of the luminaire and lumen throwing direction, or in other word, ɣ is the angle of incidence.
Initially on the surface of the road, point specific Illuminance values are collected.
Then intensity I is calculated from the equation of the Illuminance,
Where, EP is the Illuminance at point P on the road and h is the vertical height from the point P to the luminaire. After calculation of the intensity, we put all the intensity values making a C-ɣ table as per their angular position.
The format of C-ɣ table is shown above. In this above chart C’ is the position of maximum intensity on the table.
Three basic planes of intensity are considered on the road surface with respect to one luminaire:
- Plane 1: C-0o to C-180o along the road.
- Plane 2: C-90o to C-270o across the road.
- Plane 3: Principle Plane, through the point of maximum intensity of the light, i.e. C’ to C’ + 180o
To obtain C’ we have to prepare intensity distribution chart of the road light luminaire on the road. Where intensity will meet at maximum value this is the degree value of C’. To draw the principle plane axis we have to add 180o with C’.
Spread and Throw Angle of Street Light Luminaire
The two main terms related to the street light luminaire are:
- Spread angle: it is the angle of the luminaire to direct the luminous flux across the road.
- Throw angle: it is the angle of the luminaire to direct the luminous flux along the road.
It is denoted by:
Pole Arrangement Schemes in Street Lighting Design
When the width (W) of the road is nearly equal to the pole height (H), i.e. W = H then the poles are arranged in one side only. Generally pole height is available of 10 meter.
The span between two poles is equal to the road width.
When the width (W) of the road is nearly double the pole height (H), i.e. W = 2H then the poles are arranged along both sides in opposite to each other manner.
The span between two poles may not be equal to the road width.
Staggered Sided or Zigzag Pattern
When the width (W) of the road is nearly 1.5 times of the pole height (H), i.e. W = 1.5 H then the poles are arranged in both sides in zigzag manner.
The span between two poles may not be equal to the road width.
Central Verge Position
When the width (W) of the road is much greater than the pole height (H), i.e. W>>H then the poles are arranged in the central verge of the road. The luminaires are made to face towards both the road surfaces from the central verge.
The span between two poles may not equal to the road width.
What are the Street Light Design Parameters?
Street light design parameter is measured or evaluated or simulated over the span of the road.
- Average maintained luminance level in Lux
- Over all uniformity (U0)of Illuminance for entire area (span × width) of the road
- Longitudinal Uniformity is measured along the length of the road (centre length by default)
- Transverse uniformity is measured across the road along a line passing through the nadir point.
- Disability glare is expressed in threshold increment.
- Discomfort Glare is expressed in glare control mark.
- Unit Power Density is measured for unit length.
where no. of the luminaire (n) = 1 for single sided pole arrangement
= 2 for double sided pole arrangement
= 2 for staggered sides pole arrangement.
How to Compute Average Illuminance on the Road Surface?
Average Illuminance is calculated by Lumen method, where maintenance factor (MF) and coefficient of utilization (COU) are taken into account.
ΦL = Lumen of the Luminaire,
Aeff = effective road surface area under Illumination = Span × Width = S × W
N = Number of luminaire
N = 1 for single sided street lighting design and
N = 2 for double and staggered sided street lighting design,
n = number of lamps used in single luminaire = 1 for street lighting.
Coefficient of Utilization (COU) is the ratio of utilized lumen to the installed lumen. And it is obtained from the COU graph recommended by CIE.
Point Specific Luminance (L) on the Road Surface
It is related to the point specific Illuminance (E).
It is expressed as, L = q × E,
Where, q is the luminance coefficient in and it is dependent on two angles β and ɣ.
β is the angle between plane of light incidence (plane 1) and plane observation (plane 2).
Ɣ is the angle of incidence in plane 1.
As r and q both are the function of two angles β and ɣ, we should write the equation as
How to Compute Point Specific Illuminance from Iso-Lux Diagram of the Street Light?
Iso-Lux diagram is the Illuminance distribution of the street light luminaire on the street or road surface. The point of maximum Illuminance is called Nadir Point. The Illuminance level of other points are given in percentage value with respect to the Emax of Nadir point. Suppose, Emax at nadir is 100 Lux, and at other point Illuminance is 73 Lux, then this point is marked as 73% of Emax. Thus all points with 73% of Emax are joint together to get Iso-Lux diagram for 73% of Emax. All Iso-Lux curves are drawn in this way. Making the nadir point center, two axes along and across the road is drawn.
Suppose, we have the Iso-Lux diagram of the street light.
As per the above sample diagram of Iso-Lux, we have to divide dimension of two axes to term with respect to the luminaire height (h).
Suppose at point P, we have to calculate Illuminance, and we have already the luminaire Iso-Lux diagram.
Now we find out the co-ordinate of this point P with respect to the luminaire position. Suppose this point P is at h distance from luminaire 1 and at 2h distance from luminaire 2 and at 0.8h distance from the road side of luminaires 1 and 2.
Now we have to calculate Illuminance at point P for each luminaire one by one from the Iso-Lux diagram.
Let, Illuminance contribution of luminaire 1 at point P is EP,1 = x1%,
Illuminance contribution of luminaire 2 is at point P is EP,2 = x2%,
Illuminance contribution of luminaire 3 is at point P is EP,3 = x3%,
So, ultimate Illuminance at the point P is
Again Emax is calculated from CIE recommended equation, i.e.
The value of Φ is already provided by the luminaire manufacturer. So we can get the value of Emax and hence EP at the point P.
Glare in Street Lighting
Glare is the visual dis-comfortability of human eyes due to improper level of luminance distribution of the luminaire to view an object. Glare can be classified into two types,
- Disability Glare
- Discomfort Glare
Disability glare makes human eyes disable to see any object for a little while. For an example, when we look at any bright source for a few seconds and then we look at any object with low brightness, we become unable to see this object properly, rather we see black spot for some times. This is one type of momentarily blindness.
Disability glare is measured at threshold increment value. With the aid of the luminous distribution of a luminaire and using the nomogram the threshold increment of a luminaire installation can be determined.
The nomogram is a graphical representation of the formula for the equivalent veiling luminance; threshold increment value is represented by TI and calculated in percentage.
It is defined as
Where, LV is veiling luminance and Lavg is the average luminance of the object or road surface. Where, LV is veiling luminance and Lavg is the average luminance of the object or road surface.
Discomfort glare is not the cause of momentarily blindness like disability glare, but it is responsible to affect the visibility of the human eyes to an extent for long time. This kind of glare depends on the luminaire installation. If the luminance is in higher value, the human eyes cannot observe the object with lower luminance properly beyond this higher luminance. Discomfort glare is calculated in logarithmic term. If we have the specific luminaire index (SLI) of a luminaire, we can easily calculate this discomfort glare of that luminaire. SLI is the luminaire light distribution characteristics.
It is denoted by glare control mark (G). For street lighting, the discomfort glare control mark is given by:
SLI = Specific Luminaire Index,
Lavg = average road surface luminance (cd/m2)
h’ = reduced mounting height (m).
p = number of luminaires per kilometer.
SLI is calculated in terms of logarithmic value.
I80 and I88 are the luminous intensity (cd) along the downward vertical directions in the vertical parallel to the road axis correspond to angle 80 degree and 88 degree respectively.
F is the apparent light emitting area (m2) of the luminaire as seen at an angle ɣ = 76 degree to the downward vertical.
C is the color factor according to the SPD of the electric lamp used. For low pressure sodium lamp C = 0.4 and C = 0 for all other white lamps.
When SLI < 2, glare control is limited. When 2 ≤ SLI ≤ 4, glare control is moderate. When SLI > 4, glare control is high.
Higher value of SLI means lower chance to create discomfort glare.