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 PEOPLE
interested in effecting economies in set lighting costs have asked why
studio carbon lamps cannot be operated in series without the use of grid
resistance ballasts. The answer is that they can be operated in series
without ballasts and used in that manner for making motion pictures provided
the user is willing to tolerate certain disadvantages.
For example,
if a single gas set capable of supplying power for ten Type 170 lamps
was available on a location and, for a given shot, more lamps were needed,
by eliminating the ballast from all lamps and operating twenty, in series
of two, the same generator would supply sufficient power.
The advantage
of series operation without ballast are: twice the number of lamps from
a given studio power source, elimination of the weight of the ballasts
and reduced initial cost. The disadvantages are: less arc stability, more
manual operation and less light from each unit.
Table
I shows what is to be expected of Type 90 or Type 170 lamps operated in
series without ballast, and with 12 volts of ballast, as compared to operation
with normal ballast.
With
the series operation tests the light from one lamp was trained on the
photocell of the light meter. Both lamps were carefully controlled to
maintain equal current and arc voltage conditions. If the arc gap in one
lamp were allowed to become shorter than the other that lamp would consume
less than its share of the total wattage and the light output of the two
lamps would be unequal.
When
a high intensity carbon arc lamp is operated singly, with its associated
ballast, only periodic manual adjustment is required to maintain the proper
arc position and continuous steady light output after the initial adjustment.
When
two arcs are operated in series, the performance of each lamp is directly
affected by the operation of the other. Thus, one lamp in good condition
and perfectly adjusted will operate erratically if the operation of its
mate should for some reason become abnormal. When two arcs are so connected
a considerable amount of attention is required to maintain a balance of
inter-dependent arc conditions necessary for reasonably steady light output
from each lamp.
The
addition of resistance ballast between two lamps in series improves the
steadiness of operation and lessens the frequency of manual adjustment
necessary to maintain a given steadiness of light. The greater this resistance,
the less manual adjustment required. As the resistance is increased the
light level decreases as indicated in Table I. In the tests shown in this
table it was determined that the optimum condition for steadiness without
extreme sacrifice of light was with a ballast resistance of 12 volts between
the two lamps. The M-R Duarc broadside lamp is an example of successful
series operation utilizing sufficient grid ballast for arc stability.
This
lamp, however, is of the flame arc type having carbons in co-axial alignment
and the problems of series arc operation are few as compared to those
encountered with the more critical high-intensity arcs having carbons
in angular alignment.
Under
proper conditions of power supply it is practical to use a high intensity
carbon arc lamp singly and without ballast. During the late war we were
called upon to manufacture a high intensity carbon arc searchlight element
for use in tanks. By using an individual power source with the proper
regulation characteristic for each unit it was possible to maintain stable
operating conditions.
Some
projection arc lamps in theatres are operated on low voltages from individual
generators or rectifiers with very little grid ballast. Rotating high
intensity lamps similar to the Type 90 or Type 170 burners are also operated,
two or more lamps, from a single generator of as low as 80 volts output.
The 115-120
volts used in present motion picture studio practice is not necessary
for stable carbon arc operation. With proper grid ballast adjustment and
certain changes in feeding mechanisms all rotating high intensity carbon
arc lamps in the studios could be operated from power sources of much
lower voltages, or approximately 20 percent to 25 percent grid ballast
voltage drop. However, the problems involved in the changing of studio
power supply are obvious.
We realize
that, when compared to the other strictly mechanical costs, the set lighting
costs for a motion picture of great production value are considerable;
we also realize that it is light which makes this production value possible.
We do not believe that economies which could reduce these mechanical costs
are justified if they actually increase the total production costs, or
if they affect the final quality of the picture.
Our approach
to the question of economies in the lighting of large sets has been in
the development of more powerful single sources, such as the M-R Type
450 "Brute" lamp. This lamp delivers twice the light of the Type 170 and
with certain beam spreads the gain is threefold. Some directors of photography
have found that on large sets they are able to replace several Type 170
lamps by the use of one Type 450. Economies of this nature not only reduce
the overall cost of production, but give the directors of photography
better tools to work with.
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