Selection for Street Light Luminar-(PART-4)

(3) Low Pressure Sodium Lamp (LPS):

Low Pressure Sodium (LPS) lamp is by far the most efficient light source used in street lighting.
LOW Pressure Sodium is not an HID source.
IT is a gaseous discharge type lamp, similar in operations to fluorescent lamps.
While very efficient (160 lumens/watt), LPS lamps are monochromatic light source.
They produce only one light color, a dirty yellow color. That is CRI for LPS is negative.
When this type of lamp is first switched on, a small current passes through the gas giving off a faint red discharge. After several minutes the sodium inside evaporates.
This makes colour perception very difficult which means that it is almost solely used for street lighting.
Light Color:Bright yellow color light
Advantage:
The Low Pressure Sodium lamp has the highest lamp efficacy of all sources
Disadvantages:
Lamps require special ballasts and increase material size as the wattage increases.
Large size makes it difficult to obtain good light control in a reasonably sized fixture.
For a long time the poor color rendition, when the lamp is on, everything around it looks either orange-yellow, black or shades in between them so LPS lamp made it unpopular for use in other than industrial or security applications.
The wattage (energy used) increases as time passes(Age of Lamp increased).
Application:Outdoor lighting i.e. street lighting, security lighting, Parking Light

These are the latest and most energy efficient options for street lighting.
Their brightness is much more uniform and can give up to 50% savings over Sodium Vapour lamps.
Advantages:
Produce less glare and can reduce visual fatigue for drivers and pedestrians.
Long and predictable lifetime
Reduced maintenance costs
Increased road safety
Low power consumption
Dimming can possible. adjusting to specific light levels
Reducing energy consumption and light pollution
Flexible, flat and compact lamp design
High color rendering (CRI)
LED lights are better at focusing light in the downward direction so less light is lost in the air and surrounding environment
Disadvantages:
Very expensive to buy with longer paybacks.
They also LEDs offer the following advantages when used as light sources in street lighting applications.
Adequate heat-sinking is required to ensure • long life with high-powered LED.
Light Color: LED Produce more natural white / yellow light.
Warm up Time: Quick turn on / off .No problem with hot ignition. Turn on / off without time delay

Advantage & Disadvantage of Luminar
Type of Lamp	Advantage	Disadvantage
High Pressure Sodium Vapor Lamp (HPSV)	Long lamp life,Highestlamp output.	High initial cost. Poorcolor rendering, cycles on and off at end of life, not dimmable, cannot use electronic ballast
High Pressure Metal Halide Lamp (HPMH)	Moderately long lamp life. High light output.Makes colors look close to natural.	High initial cost.
High Pressure Mercury Vapor Lamp (HPMV)	Long lamp life, High light output.	High initial cost.
Low Pressure Sodium Vapor Lamp (LPSV)		Completely monochromatic,lends no color perception, shorter life than HPS, optical control difficult
Florescent	Long lamp life, High light output. Low brightness.	High initial cost.Frequent switching cuts life, needs ballast, Runs poorly in cold temperatures
LED	Long life, very efficient, can be dimmable,can offer excellent color quality (w/ less efficiency)	Very high initial cost,very sensitive to overheating, requires large heat sinks, variable color and quality

As the vertical light angle increases than disability and discomfort glare also increase. To distinguish the glare effects on the driver created by the light source, IES has defined the vertical control of light distribution as follows:
The amount of light emitted upward or lower side of laminar and at high or low angle is called shielding of Lights (“Cut off”). It is classified on how much of light is dispersed above the horizontal line of luminaries.
The Cutoff means amounts of light above 90 degrees, but it is generally agreed that the light should be no more than the value at 90 degrees, and should be decreasing as the angle increases. In fact, there could be some measurable light emitted at 180 degrees (Zenith
There are Four Type of arrangement of Luminaries (1) non cutoff, (2) semi cutoff,(3)cutoff, (4)full cutoff.

Fixture Arrangement:
The non-cutoff fixtures usually include the globe-shaped lamps that are mounted on top of lampposts.
These lamps distribute their light in all directions.
Disadvantages:
A major problem is created by the light pollution and glare, as they shoot their light upwards into trees and towards the sky rather than down towards the ground.
Non-cutoff fixtures are rarely found on roadways because they tend to blind the driver.

Fixture Arrangement:
Most of the light can be emitted below 90 degrees but 5% of the light can emitted above 90 degrees of Fixtures and 20 % or less emitted at the 80 degree angle of nadir
Advantage:
These fixtures do a very good job of spreading the light towards the ground but some up light is possible, though not as serious as non-cutoff fixtures.
Semi cutoff fixtures are often mounted on tall poles.
This is the most popular street lighting, lighting distribution arrangement. The semi cutoff fixtures usually refer to the cobra heads, but they can also apply to some lamppost-mounted fixtures that do not emit their light upwards.
Disadvantages:
Little control of light at property line. Potential for increased glare when using high wattage luminaries.
Typically directs more light into the sky than cut-off.

Fixture Arrangement:
Less than 2.5% of the light can leave the fixture above 90 degrees and 10 % or less emitted at the 80 degree angle of nadir
Advantage:
This type light gives more light control than semi cutoffs.
The cutoff lights have a wider spread of light than full-cut offs, and they generate less glare than semi cutoffs. The cutoff lenses consist of a shallow curved glass (also called a sag lens) that is visible just below the lighting area on the fixture
Cutoff fixtures have gained popularity in recent years.
Small increase in high angle light allows increased pole spacing.
Disadvantages:
Allows some up light from luminaries. Small overall impact on sky glow.

Fixture Arrangement:
These lights do not allow any of the light to escape the fixture above 90 degrees (90 degrees above nadir).
Zero light emitted above a horizontal plane drawn through the lowest part of the luminaries, no more than 10% of light emitted at the 80 degree angle above nadir. Also known as “fully shielded.”
Advantage:
Full-cut offs distribute light in a defined pattern, potentially providing more light on the ground at lower power consumption.
Full cutoff luminaries are totally environmentally friendly (causing no light pollution).
Limits spill light onto adjacent property, reduces glare.
Disadvantages:
May reduce pole spacing to maintain uniformity and increase pole and luminaries quantities