Theatre Design & Technology - Summer 1982 - 22

Base: Light Intensity
In regulating light density to only 0.3% of the rated value:
Coiled Filament Temperature: 1 AGO°C
Bulb Temperature:
320°C
Lamp Output:

130 W

Lamp Voltage:

55 V

In this instance the prerequisite for the cyclic process is
close to the limit.
Minimum Coiled Filament Temperature = 1,400°C
The bulb temperature is still above the permissable value
limit.
In transferring these values to lamps with other output levels some differences must be expected (Fig. 5).
The lamps shown in Figure 6 were developed especially for
stage and studio lighting. Their rated service life is 200 hours.
The first column on the left in Figure 6 shows a standard
100 W lamp which has a burned out filament after having surpassed its rated service life. The center column in Figure 6
shows a lamp which as been operated at half of its power consumption for 600 hours. Underload in this instance has been
many times that of the service life of the lamp. Tungsten deposits can be clearly seen on the bottom support wires. Recrystallization can also be seen to have occurred on the filament. Yet despite all of this, the lamp is still ready for use after
600 hours of operation. Finally, the column on the extreme
right of Figure 6 shows a RH 1000 W halogen incandescent
lamp after 600 hours of operation at a 250 W load. Almost no
signs of wear can be seen. The service life of such a underloaded lamp is certainly considerably greater than 600 hours.
In short, it can be said that halogen incandescent lamps for
studio use can generally be severely underloaded without
worrying about damaging effects upon service life. Care, however, should be taken in monitoring variations in color temperature.
It has often been held that underloading actually increases
the service life of lamps. This is not so with respect to discharge lamps. Unfortunately, there are at present no general
measurement values available since discharge lamps can vary
widely in their design and individual characteristics. It is thus
very difficult to make general statements.
The primary cause for reduced service life is insufficient
heating of electrodes as described in the discussion of Figure
1. Years of experience has shown that a reduction in performance of up to 50% is possible without the risk of damage to
service life. It may even be possible to expect an increase in
service life up to this limit.
Reductions in power consumption of from 10 to 5% result
in severe blackening and reduction in service life. External
heating, as in the case of fluorescent lamps, is in the case of
most discharge lamps not possible.

Figure 6: Operation of a RH 1000W Halogen Incandescent
Lamp at Underload

lw
°C

PL lk
°C

v w

2400 220 000
2200 200

!XX)

.-11
W

2000 180 !OJ
1800 160 m 700

1600 140 600 600
1400 120 :m :m

._-Hw

1200 100 IlYJ IlYJ
1000

80 3J()

800
600

60·200 200
40 100 100

400

---

--r-tk-

...-

---VV
.......r---

..........-.
~-/

l----r~~

t..

~ f--

0.3

0.6

Light Intecsliy (

COiled flla';'ent"
temperature
= Bulb

temperature
UL = Lamp voltage
PL = La'i'P out~ut

/- ,

./ .~,

100

'f.) -

Figure 5: Control of a 1000W Halogen Incandescent Lamp

Figure 7 shows the negative effect of extreme underloading
upon the premature blackening of mercury-vapor high-pressure lamps.
A similar effect can be seen with respect to sodium highpressure lamps. The values shown in Figure 8 were determined with respect to a large number of lamps. Individual
models from other series or other manufacturers can generally show considerable variation in characteristics. These deviations between manufacturers and production series, however, are kept at a minimum with respect to rated output.
With underloading of approximately 50% a reduction in
light current as shown by Curve A (100%) can be expected.
(Note: the variation in lamp characteristics such as the uniformity of power consumption and of light current are considerably greater than with standard operation).
When lamps are operated at underload external operating
conditions have an increasing effect upon lamp characteristics such as power consumption and light current. In the
case of high-pressure lamps (Hg and Na) the mercury is to
some extent still in a condensed state. Slight variations in the
ambient temperature cause a more or less severe vaporization
of the mercury and, in turn, considerable variation in power
consumption and light current. Furthermore, care must be
taken to insure that manufacturing tolerances which could
normally be discounted to not have an appreciable effect as
well. Figure 9 provides some answers to this problem. The
data is based on the evaluation of a large number of lamp test
values. Given natural manufacturer tolerances with respect to
light current were eliminated in the evaluation. Figure 9 shows
that, for example, mercury-vapor high-pressure lamps having
Figure 7: Underloading of Discharge Lamps

I":

Hg - Brenner 250 IN - 500

S~d

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