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Unravelling Some Lighting Myths
Specifying lighting is becoming more and more complex with LED, Halogen, Metal Halide, Fluorescent and CFL to choose from. Many claims are made as to which is best but ultimately, nowadays, they need to be capable of producing the required amount of light with the minimum waste CO2 being created. This tends to lead one to either LED or CFL.
Efficiency
Many claims are made about different forms of lighting regarding how efficient they are, how much light they produce and how much energy they use. The advent of LED lighting has made this rather more confusing due to the way in which LEDs produce light. The major difference being that it is highly directional. Typically, several figures need to be taken into account to ensure an accurate comparison. All lamps quote an efficacy in Lm/W. However, this does not usually include the ballast/driver losses or the losses in the luminaire itself. All in all this makes it very difficult to get an accurate comparison leading to misleading claims and a high level of confusion for the user.
Output
Another problem faced is that these comparisons are usually done with new lamps. Once they start to age the light output can, and does, drop quite dramatically. At 4000 hours, for example, a metal halide has lost almost 50% of its output. This means that to ensure the required lumen output for health and safety a 250Watt lamp would need to be used when a 125Watt is all that was required initially.
This has huge implications for running costs and CO2 emissions.
Life (Lumen maintenance)
Life is another area where claims can be very misleading and confusing. The average metal halide will last 10000 hours, fluorescent can last up to 20000 hours whilst the lowly halogen is lucky to reach 2000 hours. That, however, is not the end of the story. Most lighting really dislikes being turned on and off. Take metal halide, as an example. If a typical duty cycle means the lamp is turned on for periods of 2hr30 and then turned off, the life will be reduced by up to 45% so rather than the anticipated 10000 hours in reality life is only 5500 hours. Fluorescent and halogen also have their lives shortened by switching off. It does save electricity but causes major maintenance issues. LEDs are quoted anything up to 100000 hours. Apart from the obvious fact that the technology hasn’t been around long enough to confirm this, making it rather meaningless, it is rare that anyone says what happens at 100000 hours. Typically, the LED simply becomes very dark since they don’t tend to suffer from catastrophic failure in the same way as lamps with filaments. A rather more helpful figure to quote would be the useful life. 30000 or 50000 hours to 70% or 50% reduction in output respectively is much more informative. So what that they may last 100000 hours if the output is so dim you cant see!
Maintenance
Having established a lamp can very quickly lose output as it gets older, consider, now, the cost of maintenance. Metal halide life is typically 10000 hours at which point it is often necessary to bring in expensive equipment to reach the fixture. Taking a realistic view of LED life as being 30000 hours then a metal halide will need to be replaced at least twice during this period and a simple halogen a minimum of 15 times. Add the direct cost of the lamp to maintenance and the overall cost of using traditional lighting can be seen to be considerably more than LED.
Other factors
There are several other factors which should be considered when specifying.
Disposal - most lighting contains a small amount of mercury and requires specialist disposal and has, therefore, a cost, albeit included in the initial price.
Instant turn-on – metal halide and fluorescent can take several minutes to reach maximum brightness whereas LED is instant. Slow turn-on has implications for health and safety where a specific light level needs to be maintained.
Heat and UV – All except LED produce unwanted and damaging UV. All are capable of producing heat, including LED products using high wattage LEDs. A hidden benefit from selecting low temperature LED lighting is air conditioning systems. They work less hard saving energy and reducing maintenance costs.
Weight – This is not usually an issue except for some of the high power LED systems using high wattage individual LEDs. Large heat sinks are required to remove unwanted heat which may be an issue for the mounting structure.
Focus & Lux – Another factor often misunderstood is focus, or beam angle. So far we have compared light output and efficiency which matters from a cost point of view. What matters from a use & safety view point is how much light reaches the working surface. Most light sources emit light in all directions, apart from connections, which is then reflected towards the working area. LEDs emit light in a specific direction typically from 40o to 130o for area lighting. Halogen spotlights (the most inefficient light source at 20Lm/W) typically emit from 20o to 40o and produce a very high lux level at a small, specific point by using a narrow beam but will not light an area. Care needs to be taken to ensure like for like is being compared and legislation for minimum light levels is met.
Glare & Flicker– This depends on the fixture. Being more focused, LEDs tend to produce minimal glare. Other sources, where the lamp is visible, or the bowl reflects, produce unwanted glare. Depending on the ballast, some light sources create unpleasant flicker in peripheral vision. LEDs use DC and generate no flicker.
The graph takes into account both ballast/driver losses and luminaire losses and shows
LED
Fluorescent
Metal Halide
Halogen | the clear drop in light level compared with time.
Note: scale change at 10000 hours
Basis
Losses have been averaged to provide typical values for the purposes of the graph.
Ballast
Metal Halide 18%
Fluorescent 10% (electronic)
Fitting/reflectors
Both 30%
Halogen is undimmed
Sources
Osram
Venture Lighting
Alliance for Lighting
Singbee
Thorn
HSE
Summary
The graph shows the four main light sources compared. It is clear that LED is more than capable of holding its own. When the whole life costs of energy use and high maintenance charges are included the argument for LED is compelling.
September 2009
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