Theatre Design & Technology - May 1968 - 18

Charaderistics of Theatrical Lighting Equipment
Using Tungsten-Halogen Lamps
T.M. LEMONS and R.E. LEVIN
Sylvania Lighting Products Inc.

INTRODUCTION Development of the tungsten-halogen lamp
has created an interest in improving the performance of
equipment used in theatre, television, and film lighting.
New light sources and new optical designs can improve the
performance of these instruments without changing their basic
characteristics. However, the incorporation of new light
sources into existing luminaire types will be accomplished
with varying degrees of success. The results can be disappointing when the characteristics and specific performances
of the equipment are not understood.

The operation and performance of theatrical lighting instruments are closely related to the light source used. As new
sources are developed and become available, luminaire designs must be modified and new luminaires must be developed to utilize fully the potential of these sources. Due
to the aura of magic presently associated with the tungstenhalogen lamp (formerly known as the quartz-iodine lamp), it
often is used even when the true advantages of this lamp
are not obtained. Occasionally, it is necessary to discard
present concepts of how the equipment or the optical system should appear; resistance to change in the outward appearance, lens, reflector, lamp type, lamp shape, etc. may
impede progress toward better lighting equipment.
When an instrument produces twice as much light at the
center of its beam as another instrument, it will be of less
value if it has one quarter the light near the edge of the
beam. In fact, many instruments can be adjusted over such
a range through lamp and optical system manipulation.
Equipment which can provide increased performance but
must be precisely adjusted each time a lamp is replaced will
be of little value when there is no one willing or available
to make such adjustments. Instruments which deteriorate
rapidly or introduce operating restrictions also are of questionable value. Certain performance characteristics are expected for each type of instrument. If these characteristics
are changed, it is important to determine if the changes are
desirable.
MAINTENANCE The light output of an instrument will decrease during the life of the instrument. One major factor
contributing to this effect has been the reduction in light
output of a standard incandescent lamp during the life of
the lamp. The typical decrease for these lamps ranges from
25 to 40 per cent. The tungsten-halogen lamp overcomes
this factor since it has a nearly constant light output during
its life.

Another contribution to equipment maintenance is the deterioration of the reflector and/ or lens. Often, a reflector is
smaller and closer to the lamp in an instrument designed for
tungsten-halogen lamps than in an instrument designed for
standard incandescent lamps, and the increased heat may result in reflector deterioration. Lenses often become chipped,
scratched, or cloudy. Dirt contributes to light loss by collecting on the lamp, reflector, and lens. Cleaning of these
elements will restore some of the lost light, but scratching
of the lens or reflector surfaces results in a permanent light
loss. Further, the optical alignment may be disturbed during
a cleaning operation.
When all of these maintenance factors are added together,
the light loss of luminaires can easily reach 60 to 75 per
cent. The use of tungsten-halogen lamps in properly designed equipment can reduce this loss to 25 to 40 per
cent. A good maintenance program will provide further improvements through proper cleaning and adjustment of lighting equipment.
LAMP CHARACTERISTICS Rated values for incandescent
lamp types, including light output (lumens), power consumption (watts), life, and color temperature, are averages obtained from large sample groups. Normally it is accepted
that any individual incandescent lamp may vary from its
rated value by about 8 per cent in lumens, 5 per cent in
watts, and 50 0 K in color temperature. At the rated life, approximately 50 per cent of the lamps in a large group will
have failed. Some lamps fail early; most lamps fail between
80 and 120 per cent of rated life; and a few lamps last as
much as three times the rated life.
Tungsten-halogen lamps vary from their rated values in a
similar manner. They differ from standard incandescent lamps
in that the color temperature and lumen output remain essentially constant throughout life. Detailed discussions of
tungsten-halogen lamp characteristics in relation to theatrical
applications are available. 2
LUMINAIRE CHARACTERISTICS: A complete characterization
of the light output for a theatrical luminaire requires presentation of intensity distribution curves as a function of the
direction about the instrument's long axis. The light from
most instruments is nominally symmetrical about their centerlines. In this case, a plot of intensity (candlepowerl
versus angle from the centerline is sufficient; the plotted
intensity would be an average for several planes through the
light beam. A briefer, although less complete, representation

THEATRE DESIGN AND TECHNOLOGY

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