Building Design and Construction Handbook - Merritt - Ventech!

ELECTRICAL SYSTEMS 15.67
FIGURE 15.16 Examples of wall-mounted fixtures: (a) small diffuser type;
(b) linear type, for example, four-lamp incandescent or a fluorescent lamp; (c)
bullet-type, directional fixture, for accent lighting; (d) fixture for directional
night lighting; (e) exposed-lamp fixture for direct lighting.
thrown to close the electrical circuit. The preheat type requires a separate starter,
which allows current to flow for several seconds through the cathodes, to preheat
them. For a rapid-start lamp, the cathodes are electrically preheated much more
rapidly without a starter. For an instant-start lamp, high voltage from a transformer
forms the arc, without the necessity of preheating the cathodes.
Instant-start lamps may be of the hot-cathode or cold-cathode type, depending
on cathode shape and voltage used. Efficacy of cold-cathode lamps is lower than
that of hot-cathode lamps. Both types are more expensive than rapid-start lamps
and are less efficient in lumen output. Instant-start lamps, however, come in sizes
that are not available for rapid-start lamps and can operate at currents that are not
feasible for rapid-start lamps. Also, instant-start lamps can start at lower temperatures,
for instance, below 50�F.
The life of most types of fluorescent lamps is adversely affected by the number
of lamp starts. Cold-cathode lamps, however, have a long life, which is not greatly
affected by the number of starts.
Fluorescent lamps last longer and have a higher efficacy than incandescent
lamps. (See Tables 15.8 and 15.9.) Hence, fluorescent lamps cost less to operate.
Initial cost of fixtures, however, may be higher. Fluorescent lamps require larger
fixtures, because of tube length, and special equipment, such as ballasts and transformers
(Figs. 15.13 to 15.16). Also associated with such lamps is ballast hum and
possible interference with radio reception. Pattern control of light is better with
incandescent lamps, but lamp brightness is low with fluorescent lamps, so that there
is less likelihood of glare, even when the lamps are not shielded.
Color rendering of light emitted by a fluorescent lamp depends on the phosphors
used in the tube. The best color rendering for general use may be obtained with
the deluxe cool white (CWX) type. Check with manufacturers for color characteristics
of lamps currently available, because new lamps designed with color rendering
appropriate for specific purposes are periodically introduced.
High-Intensity-Discharge (HID) Lamps. These lamps generate light by passage
of an electric arc through a metallic vapor, which is contained in a sealed glass or
ceramic tube. The lamps operate at pressures and electrical current densities sufficient
to produce desired quantities of light within the arc. Three types of HID lamps
are generally available: mercury vapor, metal-halide, and high-pressure sodium.
Major differences between them include the material and type of construction used
for the tube and the type of metallic vapor. In performance, the lamps differ in
efficacy, starting characteristics, color rendering, lumen depreciation, price, and life.
(See Tables 15.8 and 15.9.) HID lamps are available with lumen outputs consid-