Interior lighting may be accomplished with natural or artificial illumination, or both.
Natural illumination is provided by daylight. Artificial illumination usually is produced by consumption of electric power in various types of lamps and sometimes by burning candles or oil or gas in lamps. Usually, electric lighting for building spaces is produced by lighting devices called luminaires, which consist of one or more lamps, a fixture in which the lamps are held, lenses for distributing the light, and parts for supplying electricity. Fixtures may be portable or permanently set in or on ceilings or walls.
To meet specific lighting objectives, the following lighting methods may be used alone or in combination:
General Lighting. This provides uniform and, often, diffuse illumination throughout a space. This type of lighting is useful for performing ordinary activities and for reducing the relative luminance of surroundings when local lighting is applied to a work area.
Local or Functional Lighting. This provides a high level of illumination on the relatively small area in which a task is to be performed, such as reading, writing, or operation of tools.
Accent Lighting. This actually is a form of local lighting, but it has the objective of creating focal points for observers, to emphasize objects on display.
Decorative Lighting. This employs color or patterns of light and shadow to attract attention, hold interest, produce visual excitement or a restful atmosphere, or create esthetic effects.
Illumination may be classified as indirect, semiindirect, diffuse or direct-indirect, semidirect, or direct.
For indirect lighting, about 90 to 100% of the illumination provided in a space is directed at the ceiling and upper walls, and nearly all of the light reaches the task by reflection from them. The resulting illumination is, therefore, diffuse and uniform, with little or no glare.
For semiindirect lighting, about 60 to 90% of the illumination is directed at the ceiling and upper walls, the remaining percentage in generally downward directions.
When overhead luminaires are used, the downward components should be dispersed by passage through a diffusing or diffracting lens to reduce direct glare.
The resultant illumination on a task is diffuse and nearly glarefree.
General diffuse or direct-indirect lighting is designed to provide nearly equal distribution of light upward and downward. General-diffuse luminaires enclose the light source in a translucent material to diffuse the light and produce light in all directions. Direct-indirect luminaires give little light near the horizontal. Quality of the resulting illumination from either type depends on the type of task and the layout of the luminaires.
For semidirect lighting, about 60 to 90% of the illumination is directed downward, the remaining percentage upward. Depending on the eye adaptation level, as determined by overall room luminance, the upward component may reduce glare.
Diffuseness of the lighting depends on reflectance of room enclosures and furnishings.
For direct lighting, almost all the illumination is directed downward. If such luminaires are spread out, reflections from room enclosures and furnishings may diffuse the light sufficiently that it can be used for general lighting, for example, in large offices. A concentrated layout of these luminaires is suitable for accent, decorative, or local lighting. Because direct lighting provides little illumination on vertical surfaces, provision of supplementary perimeter lighting often is desirable.
Lighting Distribution. Luminaires are designed for a specific type of lamp to distribute light in a way that will meet design objectives. For this purpose, luminaires incorporate various shapes of reflectors and various types of lenses (Fig. 15.13). Also, size and shape of openings through which light is emitted is controlled.
A luminaire may provide symmetrically or asymmetrically distributed light.With symmetrical distribution (Fig. 15.11), the level of illumination, fc, on a work plane is nearly the same at the same angle with the vertical and at equal distances from the light source. With asymmetrical distribution (Fig. 15.13c), the luminaire concentrates light in a specific direction. Symmetrical distribution is appropriate for general lighting. Asymmetrical distribution is advantageous for accent lighting.