Interior Lighting for Designers

other; an optional center position will turn
both “off.”
A double-pole, single-throw switch is
able to direct the current to two paths at
once. It is used to control two devices simul-
taneously, such as a luminaire and an
exhaust fan; it functions as if two separate
toggle switches were operated by the same
handle.
Athree-wayswitch controls an electrical
load from two locations. This allows the cir-
cuit to use one of two alternate paths to
complete itself. (Several explanations exist
for why a switch that provides control from
twolocations is called “three-way.” Although
these explanations are hypothetical and
flawed, the term is still customary.)
Afour-wayswitch controls a circuit from
three locations, afive-wayswitch controls a
circuit from four locations, and so forth. For
control from many different locations, a low-
voltage switching system is used.

Timers
Atimerautomatically turns on electric light-
ing when it is needed and turns it off when it
is not needed. Timers range in complexity
from simple integral (spring-wound) timers
to microprocessors that can program a
sequence of events for years at a time. With
a simple integral timer, the load is switched
on and held energized for a preset period of
time, usually within a range between a few
minutes and twelve hours.
Anelectromechanical time clockis driven
by an electric motor, with contacts actuated
by mechanical stops or arms affixed to the
clock face.Electronic time clocksprovide pro-
grammable selection of many switching oper-
ations and typically provide control over a
seven-day period. Electromechanical and
electronic time clocks have periods from
twenty-four hours to seven days and often
include astronomical correction to compen-
sate for seasonal changes.

Occupancy Sensors

Occupancy sensors(also calledmotion sen-

sors) automatically switch luminaires on and

off to reduce energy use. They operate in

response to the presence or absence of

occupants in a space. Electrical consump-

tion is reduced by limiting the number of

hours that luminaires remain in use.

Occupancy is sensed by one of four

methods: audio, ultrasonic, passive infrared,

or optical. Occupancy sensors can be

mounted in several ways: they can be

recessed or surface-mounted on ceilings,

corners, or walls; they can replace wall

switches; and they can plug into recepta-

cles. The floor area covered by individual

sensors can range from 150 sq ft in individ-

ual rooms, offices, or workstations to 2,000

sq ft in large spaces. Larger areas are con-

trolled by adding more sensors.

Occupancy sensors can be used in com-

bination with manual switches, timers, day-

light sensors, dimmers, and central lighting

control systems. Careful product selection

and proper sensor location are critical to

avoid the annoying inconvenience of false

responses to movement by inanimate

objects inside the room or people outside

the entrance to the room.

Photosensors

Photosensors(also calleddaylight sensors)

use electronic components that transform

visible radiation from daylight into an electri-

cal signal, which is then used to control elec-

tric lighting. The photosensor comprises

different elements that form a complete

system. The word “photocell” (short for

“photoelectric cell”) refers only to the light-

sensitive component inside the photo-

sensor. The term “photosensor” is used to

describe the entire product, including the

housing, optics, electronics, and photocell.

The photosensor output is a control

signal that is sent to a device that controls

INTERIOR LIGHTING FOR DESIGNERS