22 August 2006
Efficacy refers to the amount of light (lumens) that is produced by a light
source for every watt of energy. Different light sources produce light at different
efficacies. Incandescent lamps have the lowest efficacy while fluorescent, induction,
and metal halide sources have highest efficacies. Efficacy must be considered along
with the application to select the most efficacious source that will light the surface or
Efficacy is often the focus of energy efficiency in lighting systems. While this
is important, it is not the only strategy for reducing energy consumption. As described
in the Surface Brightness, Task / Ambient, and Controls sections of Lighting Design
Considerations, what the lighting design illuminates, how it is layered into separate
systems, and how it is controlled (in response to daylight and occupancy) all affect the
energy consumption. Increasing surface brightness can reduce the amount of energy
necessary to light a space. Dividing the lighting system into task and ambient
components allows the ambient system to use less lighting energy and an increase in
light levels is provided only where it is required: at the task, not throughout the entire
space. By controlling these lighting components separately, only the energy that is
required at any given time is consumed.
Mercury Content. Fluorescent, metal halide, and high-pressure sodium lamps
contain liquid mercury to produce the mercury vapor necessary for lamp operation.
When lamps are broken or incinerated the mercury may be released into the soil or the
atmosphere. Mercury has been linked to potential health risks. Some lamp
manufacturers offer product series that feature reduced mercury content.
Recycling. All lamp types except incandescent sources contain some level of
mercury. These lamps should be recycled to avoid release of any mercury into landfills.
The cost of recycling lamps should be included in any life-cycle cost analysis.
Lamp Life. The life expectancy data given by lamp manufacturers refers to
the approximate time at which 50% of the lamps in a group are no longer operating.
The life of standard incandescent and tungsten halogen sources can be extended by
dimming them 5% - 10%. Frequent switching of fluorescent sources can reduce the
lamp life. However, the use of rapid start or programmed ballasts reduces the impact of
frequent starting on the lamp life. Recent developments in lamp technology have
introduced long life lamps that have four to five times the life of standard incandescent
lamps. Examples include LED and induction lamps with useable lives of 50,000-70,000
LIGHT POLLUTION. Light pollution
or sky glow
by light aimed
directly up into the sky and by light reflected off the ground or objects. Sky glow
prevents the general public and astronomers from seeing the stars.
Floodlights, wall packs and other un-shielded luminaires are the major
contributors to sky glow. Overlighting, even with shielded luminaires, reflects
unnecessary light back into the atmosphere and adds to the sky glow. This often