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Understanding LED lighting

by Guy Harding
New LED products are launched seemingly every day and there is an abundance of advice, efficiency claims, dire warnings and baffling techno-speak to filter. If you are not a lighting professional, it can be daunting to try and understand the opportunities and pitfalls. The following should help.

Photo ©: Woodhouse

The quality of LED light
There have been some widely reported cases in the press recently of residents complaining when LED lighting has been installed in their street. A couple of local authorities have been forced to suspend the introduction of LED lighting due to complaints about ‘glare’, ‘lanterns looking like UFOs’ and ‘they are not bright enough’; one local paper has even reported claims of ‘LEDs damaging brains’.

Naturally this is an area that demands validated investigation since one piece of recent research by Celia Sánchez-Ramos at Complutense University of Madrid (http://bit.ly/14nB98i) presented a possible link between long-term exposure to blue light and retina damage. However, analysis shows that the potential risk to the retina was demonstrated in conditions that equate to staring at a light source equivalent to a 100W lamp for 12 hours. 

Blue light is also known to stimulate the brain and exposure late at night has been linked with disruption of sleep patterns.  But is it the ½ hour walk, cycle or drive home under LED street lighting or the subsequent two to three hours spent staring at a tablet computer or LED TV that could cause harm? 

For decades we have been used to our streets being bathed in the monochromatic orange glow of low pressure sodium lamps or the warm white light emitted by the more recent metal halide lamps. Most LEDs emit much more white/blue light, providing some streets with a cooler feel but also the benefit of better colour rendering.

Another factor that affects the perception of the ‘brightness’ of LED lighting is that the optics are so much more focused so light is now being precisely targeted at the roads and footways where it is needed rather than escaping to illuminate house frontages and gardens. This should also minimise the effects of light spill on sleep patterns.

Analysing savings and the importance of light on the ground
What are the most important factors when assessing a lighting scheme for energy efficiency? It essentially boils down to energy in and the desired output i.e. light on the ground. These two criteria are easily measured and give us a real indication of true efficiency. Firstly we should consider the energy input into the scheme. This is not necessarily simply the rated wattage of the lamps or the nominal wattage of the LED luminaire. It is the actual power that is consumed by the luminaire, including its control gear and/or power supplies. Secondly, and very importantly, how good are the optics at putting the light where we want it, on the area to be lit on the ground?  This can only be assessed by measuring the actual average illuminance (or luminance in the case of main roads.) Illuminance is a measure of light hitting any surface and is relevant for minor roads and pedestrian areas; luminance is the light reflected off the road surface back to the driver’s eyes, which is relevant for driving on higher speed roads.

These two criteria have led to the development of SLEEC (Street Lighting Energy Efficiency Criteria) ratios.
The SLEEC is calculated as follows:

SLEEC = (W/Lux)/m²
where

W (total energy used) = actual power used by (each) lamp in watts x number of luminaires

Lux = average illuminance

m² (total area coverage) = area to be lit, e.g. road used in example below is 500x9m = 4500m²



The best way to demonstrate this is to examine a typical lighting scheme for an urban residential road, lit to the appropriate British Standard for illuminance. Let us assume that it is a 500m long x 9m wide road that needs to be lit; we will look at three lighting solutions based upon common light sources


Table 1
  Luminaire power Actual power used Number of luminaires Spacing Average illuminance SLEEC
High Pressure sodium (SON) 70W 83W 13 40m 7.62 Lux 0.031 W/Lux/m
CosmoPolls (metal halide) 60W 67W 13 39m 7.52 Lux 0.026 W/Lux/m
LED luminaire 54.7W 54.7W 12 45m 7.74 Lux 0.019 W/Lux/m


The SLEEC figures in Table 1 offer a realistic means of obtaining a true and easily comparable measure of efficiencies for lighting so that we can determine whether we really are making savings. This methodology allows lighting designers to examine the actual savings and investment cost benefits across tens, hundreds or thousands of kilometres of road.


Table 2
  Energy use for a typical lighting year (4200 hours) Potential energy cost per year based on £0.12p/kWh Energy cost over 20 years (not allowing for energy price rises) Energy cost equated to 100km of similar road over 20 years
High pressure sodium (SON) 4531 kWh £544 £10,879 £2,175,800
CosmoPolls (metal halide) 3658 kWh £439 £8,779 £1,755,800
LED luminaire 2756 kWh £331 £6,616 £1,323,200


Table 2 shows that significant savings can be made by specifying more efficient light sources. 

In summary, there is a multiplicity of benefits to specifying a high quality LED luminaire when designing a lighting scheme. 

More precise direction of the light to where it is needed will reduce light pollution and have less impact on our wildlife; our streets will feel different at night as we become accustomed to a more natural colour rendering of our surroundings; tangible financial benefits exist in evident energy savings, coupled with the reduction of maintenance costs (compared to discharge lamps), as well as the opportunity for CO2 reductions which may also be reflected in the bottom-line. Not least, exciting new luminaire designs will surprise and delight, and the availability of these new luminaires will enhance the whole landscape.

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