Building Design and Construction Handbook - Merritt - Ventech!

ELECTRICAL SYSTEMS 15.59
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.
FIGURE 15.13 Examples of recessed fixtures: (a) for direct widespread lighting; (b) for direct,
narrow-beam lighting; (c) for asymmetrical direct lighting; (d) for direct diffuse lighting.