Part II - The Complete Guide to LED StreetLights

If you have been searching for information on LED lights on the net you would have come across many confusing claims about the light output of LED devices. Some sites would have told you that LEDs are poor at producing light while the others would have claimed that they are among the most efficient lighting technologies. The reason behind this anomaly lies in the rapid strides made in LED lighting technology. In fact, efficiency of LED bulbs has gone up during the few months that it took us to collect data for this series of articles.

R Raghavan and N Narendran, of the Lighting Research Center summarized the improvements in LED lighting technology in a recent paper.

Lighting Technology Light Output (lumens) Efficiency (Lumens / Watt) Energy Use in KW per hour Savings
Fluorescent Tube (32 W) 2200 68 0.76 Base Case
LED 38 Watts (2002 vintage) 1200 32 0.91 -18%
LED 12 Watts (2010 vintage)

The improvements in LED lighting technology and the low profile nature of LEDs have made this the most energy efficient choice for almost all lighting applications.

Light Distribution Requirements for Street Lights

The amount of light emitted by a streetlight beyond 75 degrees from the Nadir should be minimized. Light beyond this angle can cause direct glare to motorists. LED streetlights that have far better control over the light that they produce are ideal for reducing this glare. HPS lamps that are more frequently used have an angle of distribution beyond 150 degrees and can cause direct glare.

The picture above shows the distribution of light when wide-angle HPS lights are used. A considerable amount of light is wasted. Residents of houses near the road suffer from light trespass and must keep their curtains drawn to tackle excessive and unneeded illumination of their windows. The consequent light pollution makes life difficult for astronomers and nature lovers too.

The light from an LED outdoor light is better focused in the task area. People living in houses adjacent to the outdoor light need not worry about drawing their curtains as the light is incident on the task area and is not distributed in a very wide angle. Obviously when the light is not being used to illuminate the sky and adjacent houses, far fewer lumens are needed.

Uniformity of light distribution

Uniformity of light distribution by a street light can be measured after installation. Uniformity depends on optical properties of the light source, quality of the luminaries’ reflectors, overlapping areas between luminaries among others. Uniformity of light distribution can be determined by three ratios –

  • Average to minimum light intensity ratio
  • Maximum to minimum light intensity ratio and
  • Maximum to average light intensity ratio.

Of these, the first two are easier to understand and are therefore used more often.

The uniformity of light distribution is vital to reduce glare. Glare control is particularly important in street lighting applications. Glare causes the pupil to shrink thereby reducing the amount of light entering the eye. Squinting, blinking, using devices like a peak cap (to cut the brightest portion of the visual field), using sunglasses (To lower the luminance of the entire field of vision) are the commonest methods of dealing with glare. None of these is a suitable method for motorists and other road users at night. Similarly, disability glare caused by light emitted in an angle beyond 75 degrees to the nadir is usually due to light scattering within the eye thus reducing the contrast of the image formed on the retina.

A practical example of the impact of uniformity of light produced by LED StreetLights

Thus, when LED Street Lights were installed in the parking facility of the Prairie School in Wisconsin, LED lights producing barely 8040 lumens replaced HPS lights producing mammoth 19,000 lumens. This was made possible by

  • Better light spread – The maximum: minimum light intensity ratio was 8:1 for Led lights while for HPS lamps it was 27.25: 1.
  • Better Scotopic performance – The mean light intensity level was 1.01 photopic lumens which is equivalent to roughly 1.7 pupil lumens or 2.45 photopic lumens produced by an HPS lamps.
  • Better optical control of light - All the light is directed downward. There is therefore no need to use reflectors to direct light downward. The consequent loss in light output due to limitations on reflector efficiency is avoided.

Both theory and practice demonstrate the benefits of LED StreetLights. They also underline the importance of system thinking, smart technologies and reducing wastage. Instead of waging a war of ‘Lumens’ on darkness the current crop of managers and lighting designers are opting for precise lighting with LED technology.

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