LED Street Lighting:
A Design Based Approach For
Maximum Light Efficiency And Energy Saving
As opposed to traditional approach wherein
a customer demands specific wattage from manufacturers, in a design
based approach, customer demands light, i.e. Lux levels. Eventually,
from a lighting product, what matters is light and not watts.
Public Lighting amounts to over 1% of total
energy consumption and roughly 4,400 MW of connected load in India.
With India’s urban population set to increase from 400 million in
2011 to about 600 million or more by 2030, energy consumption for
street lighting is set to shoot up significantly as well. New
technologies like Light Emitting Diodes (LEDs) present an excellent
option to reduce energy consumption in street lighting by as much as
50-60%, as demonstrated in many parts of the world. An LED is a
semi-conductor device that emits visible light of a certain colour.
High power LEDs are capable of producing higher light output using
lesser electricity as compared to conventional light sources.
In addition, the co-benefits of LED include longer life, better
quality lighting, lower maintenance costs, enhanced public safety
and improved user satisfaction. LEDs have a relatively long useful
life, when compared to conventional sodium vapour or metal halide
lamps, i.e. 50,000 – 100,000 hours versus 15,000 hours.
LED Lighting is changing the landscape of
outdoor lighting and today, outdoor lighting plays a crucial role in
the safety and security of our streets and public places. Thus,
Street lighting has to be smarter to comply with new legislation,
environmental challenges and requirements to reduce energy usage.
The LED market in India has been growing at
a rate of over 50% for the last five years (2009-10 to 2013-14) and
is expected to sustain this growth rate in the next five years
(2014-15 to 2018-19).
The Government of India has announced a lot
of incentives and measures in order to prioritise the use of LEDs in
the country. It is clear that the Street lighting application leads
India’s LED lighting market, due to various state governments’
initiatives to replace conventional sodium lights with LED lights.
This article on LED Street Lighting
endeavours to demonstrate how a design based approach can be adopted
to maximise both light and energy efficiency and save on the total
project cost for the customer.
What is a design-based approach?
As opposed to the traditional approach,
wherein a customer demands specific wattages from manufacturers, in
a design-based approach, the customer demands light i.e., Lux
Levels, which may be as per the National Lighting Code or customer
specific and does not specify watts at all. Eventually, from a
lighting product, what matters is light and not watts.
The customer provides the site details like
the Pole Height, Pole Spacing, Road Width, Boom Length etc. to the
manufacturer – and based on these details, manufacturer simulates
designs in special lighting softwares by using their own IES files
to achieve average lux level and uniformity ratio as per National
Lighting Code or IS 1944 or as demanded by the customer.
The National Lighting Code or IS1944
classifies the type of roads as given under:
Objective of the customer:
To achieve the required illumination level
and uniformity ratio with least wattages (Power Consumption) and
eliminating dark patches...
Thus, taking into consideration the above
table and choosing the appropriate type of road, the customer
floats, an RFQ based on illumination level and uniformity ratio and
not on power consumption. He/she also takes into consideration the
energy cost over the lifetime of LEDs and to minimise that.
Design-based approach is the most practical
approach for LED Street Lighting as it takes into consideration not
only the initial investment but also the energy cost during the
lifetime of LED Street Light.
Case study: Traditional approach
Let us understand the difference between two
In the old traditional approach, the
customer demands specific wattages i.e. 80W LED Street Light to
replace the existing 150W HPSV. The customer asks for Quotation of
80W LED Street Light from three different sellers. He/ she has not
specified any lighting levels, and so there are lot of differences
in specifications of all different manufacturers as under:
If we analyse the table above, Manufacturer
C offers his 80W LED Street Light at Rs. 9000/-. The customer would
opt to buy the Street Light from Manufacturer C considering lowest
price. However, post installation, the illumination levels and
uniformity would definitely disappoint the customer.
Manufacturer A has a price of Rs. 11000/-
for his 80W LED Street Light and he is offering an Avg. Lux of 15,
which is almost double that of C. The system efficacy is also
100 lm/W and uniformity ratio is also > 0.5, which is excellent. Had
Manufacturer A to offer the same specifications as offered by
Manufacturer C, he would have offered just 40W instead of 80W. This
way, the customer would not only save on the initial capital
investment, but also save additional energy of 40W per street light
during its lifetime.
By just specifying watts, the customer is
losing on an excellent quality product.
Why should you adopt a design- based
As seen in the case study above, there are
differences in specifications in luminaires of different
manufacturers. The major differences may be in:
Luminaire Design - Thermal Management
Type & Efficiency of LEDs used
Quality & efficiency of optics employed with
reference to the type of load Driver’s efficiency and quality.
Thus, these differences may lead to adoption
or procurement of poor quality street lights, which would not
deliver light as the customer would have originally thought of. This
would disturb the very purpose of LED Street Lighting.
For the customer, what matters eventually
are lux levels and the uniformity ratio obtained from least
wattages. Thus, inferring from the above table, price of a street
light does not depend on watts but on lux levels and uniformity
Design-based approach in LED street lighting –
a case study
In this case, the customer demands an
average lux of 15, Uniformity Ratio > 0.4 and provides the design
details to the manufacturer: The Pole height is 9 m, Road width is 8
m on two sides with a 1 m median in the centre, Boom Length is 1 m
and the degree of tilt is 5°. The manufacturer has to design
Let us see and analyse the designs provided
by three manufacturers to achieve the required illumination &
Looking at the table below, Manufacturer A
provides the desired illumination and uniformity with 70W,
Manufacturer B with 90W & Manufacturer C with 110W.
Manufacturer C offers highest wattages i.e.,
110W. At the same time, for the customer, the initial capex if
he/she opts for Manufacturer C is the lowest i.e., Rs. 10000. But if
we look at the overall cost which the customer will incur over the
lifetime of the Street Light, i.e., Product Cost + Energy Cost,
Manufacturer A’s LED Street Lighting system would be the most
economical. The total savings by opting for Manufacturer A as
compared to B & C is substantial and therefore the intial capex can
If we consider 100 Street Lights in the
project, the customer would incur a total cost of Rs. 34.5 lacs by
installing Manufacturer A’s lights, Rs. 42 lacs for Manufacturer B &
Rs. 49.5 lacs for Manufacturer C. Now whose lights would be the most
ideal and beneficial to install? Obviously Manufacturer A’s.
This is just a hypothetical example to show
that even if the initial investment incurred by the customer is
high, the total cost by opting for Manufacturer A’s product would be
substantially low. It may also be the case that the customer would
incur same capex for 70W & 110W.
The difference in wattages of all three
manufacturers is on account of the type of LEDs used, adoption of
optics/lenses, efficiency of driver and thermal management.
Similarly, for a completely new installation
with new poles, the customer can save a lot on capital costs and
energy costs by providing road dimensions to manufacturers and
asking them to design the Wattages as well as Pole Spacing. With a
longer pole span, the no. of poles and luminaires can be drastically
Physical verification of average lux and
uniformity ratios - 9 point method measurement
Average Lux and uniformity ratio can be
easily verified by installing 3 LED Street Light samples on 3 Poles
in case of single side installation or 6 LED Street Light samples on
3 poles in case of a median.
Light is to be measured at 9 points as shown
in the diagram with help of a simple lux meter.
Average Lux= (P1+P3+P9+P7) / 16 +
(P2+P4+P8+P6) / 8 + P5 / 4
Uniformity Ratio = Min Lux / Avg Lux.
(Generally, the minimum lux would be at P8 since it is the farthest
point from both the poles.)
Thus, the lighting quality can be ensured by
a mock sample installation.
Benefits of design-based approach
By adopting a design-based approach, the
customer is taking into consideration the total project cost over
the lifetime of the product, which is very practical.
At the same time, the lighting levels and
the uniformity ratios achieved are as per IS 1944 or as per
customer’s requirement. There is no compromise on the quality of
Importantly, the customer is saving maximum
energy by going for least wattages from the available LED Street
Lights in the market.
There may be a drastic decrease in the
overall capital cost when the number of poles and luminaires are
reduced as a result of longer pole span in case of new installation.
In case of replacements too, saving in energy costs will be
Source: Lighting India,
Climate Change Association