Lighting accounts for a quarter of the energy costs in commercial and institutional buildings. The first step towards environmentally conscious lighting is the careful consideration of opportunities for natural daylighting. Daylilghting should be incorporated wherever possible as it will increase the quality of indoor environment and reduce lighting loads. Deprived of it, the occupants are unaware of outside weather conditions or time of day and subsequently become disoriented, possibly developing negative physical and emotional side effects. Relying on sunlight to illuminate interior spaces can therefore save a large amount of energy where users can control lighting levels. Another technique of utilising natural daylight includes the use of light reflecting panels, which track the movement of the sun and direct it into the building interior. Photosensor systems are also an energy saving option, where the sensors feed information to the control unit, which then dims or increases the level of artificial lighting depending upon the level of daylight in the interior space. Atriums are also a great means of achieving daylighting in deeper buildings. Windows or atrium glazing in Ďlow-eí glass can let in light at the same time blocking out unwanted heat reducing cooling costs. Other advanced films such as electro-chromic films and holographic films can also be used for controlling the entry of light and heat into the building. Paper-fine electro- chromic films, whose electric properties change with a brief pulse of electric current, act as an electric shutter to control the amount of light and heat entering the building. Their heat transmission can vary from 80% when clear and 10% when opaque, thus reducing heat gain. Holographic films can project outside light onto a particular part of the ceiling deep within the building. Coupled with a reflective ceiling, they can illuminate office or living space, which would otherwise receive little or no natural light. These can be made to track the position of the sun and illuminate the same part of the room irrespective of the position of the sun. Daylight systems combining sunscreen and anti-glare layers can provide effective protection from excessive heat gain and at he same time, ensure adequate daylight levels within the building. Such a system consists of two layers of internal blinds, one a sun-screen layer while the second is the anti-glare layer. Set at a constant angle of 90 degrees to the sun, the transparent prismatic, sun-screening louvers reflect the direct, dazzling daylight (which also produces a large amount of heat) but allows diffused sunlight to pass through it. This diffused light is then deflected to the depths of the room by the sun-screening blind. This consists of robust, parabolic, partly perforated, light deflecting aluminium louvers with reflecting top surfaces and matt painted undersides to avoid dazzle near the window.
Light is isomorphic and malleable. The science of Optics has enabled us to shape, bend, compress, expand and infinitely subdivided. Optical daylighting involves the use of sophisticated Freznel lens material to manipulate existing natural energy, daylight, and combined with architectural design, deliver reliable, even daylight deep into the building interiors. The optical lenses can be produced economically on rigid or flexible plastic sheets of varying thicknessí. It can be transparent, translucent or reflective and can assume a large variety of physical and optical shapes, lending a great deal of flexibility to the system. It does not require any special skill or sophisticated knowledge to install nor does it demand much maintenance. The initial cost adds little to the cost of the building and it requires no outside electricity and so has no operating cost. It thus saves on a great deal of energy by eliminating or reducing the need for artificial lighting during the daytime. An even more advanced approach would be to produce energy for electric lighting during the night by the collection and storage of photovoltaic current directly from the sun during the day and linking this to the optical daylighting system, resulting in the development of a totally decentralised lighting system requiring no grid power. However, efficient electric storage technology is still in its infancy and further development to increase the efficiency of such systems is required to achieve the goal of totally self sufficient daylighting system.
Active Solar Optics relies on tracking the sunís movements mechanically, by heliostats, mirrors and lenses. It has the advantage of being uniform, steady and precise and is only marginally dependent on geometry and orientation of the building but it requires a greater degree of precision and maintenance. It is also a pure sunlighting system and does not operate with a cloudy sky.
Passive Solar Optics is the projection of sunlight into interior spaces by passively tracking the sun using paired lenses and mirrors, with no moving parts. The first lens projects light from the source to the second lens, while the second lens directs light into the target area. The system employs a series of fixed linear lenses, each one dedicated to a portion of the sunís position in the sky, arrayed vertically roughly in the East - West axis. It has the advantage of being inexpensive and reliable, not requiring much specialised skill to install and maintain and operating effectively in both sunny and cloudy skies. The disadvantage, however, is that it requires specific building orientation and geometry which may conflict with the design requirements and require adjustment in the building usersí behaviour.
Optical lighting does not address the production of light or the energy to produce light but rather concentrates on the manipulation and efficient of already available light; sunlight, diffused sunlight from cloudy skies and existing electric light, to deliver a specific intensity of light to the interior spaces. Optical lighting is at least 50% more energy efficient in converting a unit of sunlight into interior lighting than electric lighting. It replaces the brute force of industrial technology with physical simplicity thus creating a bond between manís built environment and nature.
Appropriate application is the most important aspect of energy efficient artificial lighting. Typically, good lighting design will include ambient lighting for general background definition, task lighting for individual work and accent lighting to feature certain areas or objects. Because of continuously changing conditions and individual needs of occupancy, lighting must be flexible. Comfortable lighting conditions should be achieved with minimum energy consumption.
Numerous technical features can enhance energy savings in lighting. Separate circuiting and switching should be provided for different tasks and zones. Light controls should be easily accessible so that individuals can make lighting adjustments conveniently. Occupancy sensors, dimming, stepped switching, programmable controls and energy efficient lamps can ensure substantial savings in energy costs. Central control over the lighting system through a Building Automation System, will result in timely and more efficient adjustment of lighting levels in the different zones resulting in comfortable as well as efficient lighting.