Building Design and Construction Handbook - Merritt - Ventech!

15.70 SECTION FIFTEEN
Light distribution from fixtures, to summarize, may be accomplished by means
of transmission, reflection, refraction, absorption, and diffusion. Reflectors play an
important role. Their reflectance, consequently, should be high—at least 85%. The
shape of a reflector—spherical, parabolic, elliptical, hyperbolic—should be selected
to meet design objectives; for example, to spot or spread light in a building space
or to spread light over a fixture lens that controls light distribution. (The need for
a curved reflector, which affects the size of the fixture, can be avoided by use of a
Fresnel lens, which performs the same function as a reflector. With this type of
lens, therefore, a smaller fixture is possible.) Light control also is affected by shielding,
baffles, and louvers that are positioned on fixtures to prevent light from being
emitted in undesirable directions.
A wide range of light control can be achieved with lenses. Flat or contoured
lenses may be used to diffuse, diffract, polarize, or color light, as required. Lenses
composed of prisms, cones, or spherical shapes may serve as refractors, producing
uniform dispersion of light or concentration in specific directions.
Types of Installations. Luminaires may be classified in accordance with type and
location of mountings, as well as with type of lighting distribution: flush or recessed
(Fig. 15.13), ceiling mounted (Fig. 15.14), pendent (Fig. 15.15), wall mounted (Fig.
15.16) or structural.
Structural lighting is the term applied to lighting fixtures built into the structure
of the building or built to use structural elements, such as the spaces between joists,
as parts of fixtures. Structural lighting offers the advantage of a lighting system
conforming closely to the architecture or interior decoration of a room. Some types
of structural lighting are widely used in residences and executive offices. For the
purposes of accent or decorative lighting, for example, cornices, valences, coves,
or brackets are built on walls to conceal fluorescent lamps. For task lighting, fixtures
may be built into soffits or canopies. For general lighting, large, low-brightness,
luminous panels may be set flush with or recessed in the ceiling.
Lighting objectives can be partly or completely met with portable fixtures in
some types of building occupancies. For the purpose, a wide variety of table and
floor lamps are commercially available. Because the light sources in such fixtures
are usually mounted at a relatively low height above the floor, care should be taken
to prevent glare, by appropriate placement of fixtures and by selection of suitable
lamp shades.
Number and Arrangement of Luminaires. With the type of lamp and fixture and
the required level of illumination known, the number of luminaires needed to produce
that lighting may be calculated and an appropriate arrangement selected. The
lumen method of calculation, which yields the average illumination in a space, is
generally used for this purpose.
The method is based on the definition of footcandle (Art. 15.10.4), in accordance
with which the level of illumination on a horizontal work plane is given by
lumens output
fc � (15.35)
2
area of work plane, ft
Lamp manufacturers provide data on initial lumen output of lamps, but these values
cannot be substituted directly in Eq. (15.35), because of light losses in fixtures and
building spaces and the effects of reflection.