instant start lamps of comparable length.
Rapid-start ballasts are less expensive,
smaller, and have lower power loss than
instant-start ballasts.
Rapid-start lamps are sometimes oper-
ated with instant-start ballasts. The instant-
start ballasts provide higher voltage to start
the lamps, but they do not supply current to
the cathodes during lamp operation. The
savings is approximately 2 to 3 watts per
lamp.
Because the cathodes of rapid-start
lamps are heated continuously during opera-
tion, these are the only fluorescent lamps
that can be dimmed or flashed.
Trigger-startballasts permit the opera-
tion of preheat fluorescent lamps up to 32 W
without the use of starters. This circuit was
developed prior to the rapid-start circuit and
is similar in operation: it provides continuous
heating of the cathodes, and starters are
unnecessary. The lamps are made with bipin
bases to permit the flow of current through
the cathode filaments before the lamp starts.
For reliable starting, rapid-start and trig-
ger-start lamps must be mounted within 1 in
of a 1-in-wide grounded strip of metal, or
within ½ in of a ½-in-wide grounded strip of
metal, running the full length of the lamp.
This is usually provided by a wiring channel
or reflector in the luminaire housing.
T8 and T5 lamps
Good color-rendering fluorescent lamps
require the use of rare-earth phosphors,
which are more expensive than standard
phosphors. For this reason, smaller-diame-
terT8andT5lamps are produced. (The T8
[1-in-diameter] bulb uses only two-thirds of
the phosphor quantity required by theT12
[1½-in-diameter] bulb, for example; it is
therefore less expensive to produce.) The
smaller diameter also gets the phosphors
closer to the arc, increasing the efficiency of
light generation.
In addition to better color rendering, the
rare-earth phosphors provide a substantial
increase in lighting efficiency. System
efficacies up to 80 lumens per watt for T8
lamps on magnetic ballasts and up to 105
lumens per watt for T8 lamps on electronic
ballasts compare with 65 to 75 lumens per
watt with T12 lamps and ordinary phos-
phors. Due to their higher efficacies, T8
lamp-ballast systems have replaced conven-
tional T12 lamps in many applications.
(Lamp manufacturers reportlamp effi-
cacy:lumens per watt when the lamp is
operated under reference conditions.
System efficacyis lumens generated by the
lamp when operated by a given ballast
divided by the input watts to the ballast.)
As tube diameter decreases in size,
luminance is increased, requiring better
methods of shielding the source. The smaller
bulb diameter also increases the luminaire
optical efficiency, however, yielding smaller
luminaires with the potential for improved
light-distribution patterns.
For T8 lamps, three kinds of rare-earth
phosphors are available: RE-70, RE-80, and
RE-90. Color temperature is varied accord-
ing to the relative balance among the phos-
phors. RE-70 and RE-80 lamps have three
narrow-emission phosphors that produce
three “peaks” of visible energy: a blue, a
green, and a red (see color plates 19, 20,
and 21). RE-70 lamps contain a coat of con-
ventional phosphors and a thin coat of the
rare-earth triphosphors to produce a CRI of
70 to 79. RE-80 lamps contain a thick coat
of the rare-earth triphosphors, increasing
CRI to 80 to 89 with full light output and
lumen maintenance.
RE-90 lamps do not use the three
narrow-emission phosphors of the other two
rare-earth lamps. These quad-phosphor
lamps contain four wider-emission phosphors
that produce CRIs of 95 at 3000 K and 98 at
5000 K (see color plates 22 and 23).INTERIOR LIGHTING FOR DESIGNERS