Interior Lighting for Designers

The earliest fluorescent lamps were of
thepreheatkind (figure 7.4). Preheat lamps
have cathodes that must be heated electri-
cally in order to make them emit electrons
and thus ionize the gas in the tube, making it
more conductive and raising the voltage nec-
essary to strike the arc. The current heats the
cathodes; because this occurs before the arc
strikes, it is said to preheat them.
The preheating process takes a few sec-
onds. It is usually controlled by an automatic
starter, which applies current to the cath-
odes of the lamp for a sufficient length of
time to heat them; it then automatically
shuts off, causing the voltage to be applied
between the cathodes and striking the arc.
The preheating is sometimes accom-
plished by holding down a manual start

button, as with some fluorescent desk

luminaires. The button is held down for a few

seconds while the cathodes heat; when the

button is released, the arc strikes. Whether

started manually or automatically, once the

lamp is in operation, the arc maintains the

cathode temperature.

Instant-start lamps are designed to

operate without a starter (figure 7.5). This

simplifies the lighting system and its mainte-

nance. The ballast provides sufficient voltage

Figure 7.4Preheat fluorescent lamp diameters and
bases at one-half actual size.

Figure 7.5Instant-start lamp diameters and bases at

one-half actual size.