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 The
high-intensity carbon arc affords certain advantages as a light-source
for photography that are not possessed by other illuminants. Within the
restricted area of its positive carbon is concentrated an intrinsic brilliancy
greater than that afforded by any other artificial light-source. Fortunately,
the distribution of radiant energy throughout the spectrum of a high-intensity
carbon arc coordinates well with the spectral sensitivity of photographic
emulsions and the transmission factors of camera lenses.
For the
purpose of more effectively utilizing high-intensity are sources in motion
picture photography, two lamps have been recently developed. The M-R
Type 90 lamp (Fig. 1) operates at 120 amperes. The M-R Type 170
lamp (Fig. 2) has a capacity of 150 amperes. The designs of these two
lamps, which are in general quite similar, embody many new factors that
greatly enhance their utility and add to the convenience of operating
them. Fig. 3 shows the mechanism of the Type 90 high-intensity
arc lamp, in which the following vital improvements have been incorporated:
(1) increased rotational speed of positive carbon; (2) continuous non-intermittent
feeding of both positive and negative electrodes; (3) rapid-action positive
and negative manual adjustments.
When
these new high-intensity arc lamps were being designed it was known that
they would find extensive use in color motion picture photography. The
requirements of illuminants for color cinematography are very rigid as
to uniformity of spectral distribution and intensity.
FIG. 1. (Left) MR Type 90 high-intensity arc spotlamp
(rear view).
FIG. 2. (Right) MR Type 170 high-intensity arc spotlamp (front view).
Considerable experimenting was done with various rotational speeds of
positive electrode. It was found that at approximately 14 rpm. an optimal
condition was established and that the crater was very symmetrical. It
was noted that at that speed crater rims that had been chipped, either
by careless striking or other causes, were quickly restored to symmetry.
The maintenance
of a properly shaped, symmetrical crater is one of the most important
requirements for stability of the arc. In the arcs under discussion, the
angle between the positive and the negative electrodes is 127 degrees.
It has been found that at that angle practically any carbon suitable for
use under high-intensity conditions performs reasonably well. The 127-degree
angle results in better performance than the much flatter angle generally
used in the design of searchlights and Sun-arc lamp mechanisms. The objection
to this angle is that in striking the negative to the positive, the contact
between the two electrodes comes at the rim of the positive crater, which
tends to cause chipping. That was one of the reasons why the lamp was
designed to be struck manually rather than automatically. The other reason
was that automatic striking would have increased the cost of the mechanism
to a very considerable extent.
FIG. 3. Mechanism of MR Type 90 high-intensity arc spotlamp.
The
feeding arrangements of these new high-intensity arc mechanisms differ
from other designs in combining continuous, non-intermittent feeding of
the positive and negative electrodes with rapid positive electrode rotation.
So far as the writer is aware, all high-intensity arc mechanisms used
in motion picture photography, having rotational speeds sufficiently rapid
to maintain good crater conditions, have had feeding arrangements of the
"stop and go" type. Only by uniform, non-intermittent feeding and positive
electrode rotation faster than 10 rpm. can the constant stability of performance
be attained in high-intensity arcs used for illumination in color photography.
The mechanics of the feeding arrangements in these lamps permitted a rapid
hand control: one revolution of the negative hand-feeding crank advances
the negative electrode 0.1 inch. A similar movement of the positive hand
control moves the positive electrode 0.08 inch.
Quietness
of operation is, of course, essential in all motion picture equipment
designed for use in conjunction with sound-recording apparatus. The motors
of these new high-intensity arc lamps have grease-packed reduction gears,
and no shafts or parts other than the motor reduction gearing have rotational
speeds greater than 46 rpm. All shafts and rotating members, other than
those rotating at the slowest speeds, are mounted upon either oil-less
or ban-bearings. Again and again, in recent productions, these lamps have
been operated satisfactorily within six or eight feet of the microphones.
Provision has been made for completely stopping the driving motor in the
few situations that arise when the microphone has to be placed within
six feet of the lamp.
FIG. 4. Typical distribution curves of Type 90 high intensity arc.
Aside
from improvements in the mechanism, probably the most outstanding other
improvement in these new equipments is the application of "Morinc" flat
corrugated lenses as a means of collecting and projecting the light of
the arc. The carbons used in the Type 90 are a 13.6-mm. positive
and a 3/8 X 9-inch coppercoated negative. The Type 170 uses a 16-mm.
positive carbon and a 7/16-inch copper-coated negative carbon. It is characteristic
of these high-intensity arc combinations that the most effective portion
of their radiation falls within an angle of 60 degrees each side of the
axis, and principally within a total angle of 80 degrees. Within 80 degrees
the intensity is not less than 10 to 17 per cent of the intensity at the
axis. Heretofore, in order to project the light from a high-intensity
motion picture arc spotlamp effectively, the principal optical means employed
have been parabolic reflectors and spherical lenses.
FIG. 5. Typical distribution curves of Type 170
high intensity arc.
The high temperature developed directly in front of the positive crater
in arcs operating at currents greater than 100 amperes is such that lenses,
for practical use, have been limited in diameter to 8 inches, and to focal
lengths such that the arc would not be closer to the lens than 6 inches
when the desired flooding angle was attained. Experience has demonstrated
that if lenses greater than 8 inches in diameter were used, or if the
arc were brought closer than 6 inches, the glass elements were subjected
to pitting by the copper coating of the negative electrode and the hazard
of breakage was greatly increased, regardless of the efforts of lens manufacturers
to make use of the advantages of heat-resisting glass having low coefficients
of expansion.
The 24-
and 36-inch parabolic mirrors, which have given the best performance,
have both been limited to the 18 3/4-inch focus. The mechanical limitation
has been imposed by the fact that there must be room for interposing a
negative carbon of suitable length between the crater and the mirror surface
when the lamp is adjusted for flooding. This prevents the positive crater
from being placed nearer than ll 3/4 inches from the center of the mirror
surface. Even though the lamp operators on the stages exercise the utmost
care to protect their sun arcs from wind and sudden changes of temperature,
there has always been a great deal of dissatisfaction regarding the breakage
of parabolic mirrors in studio use. The "Morinc" lens, designed for these
new high-intensity arc lamps affords ideal distribution of illumination
for photographic purposes. Figs. 4 and 5 illustrate the distribution attained
at various angles of divergence when the lamps are used for flooding,
and show the wide range of divergence attainable. Variations from spot
beams of 8 degrees to floods of 44 degrees are produced having smooth
fields of photographically useful light. The edges of the various beams
vignette and make it practicable to overlap fields of illumination, as
is often necessary in motion picture set lighting, without creating high-intensity
areas in the overlaps or distinct markings defining the circumference
of the field.
In both
lamps every effort has been made to facilitate operating them in their
many applications in the studios. Each has a resistance grid, so designed
that it may be removed from the pedestal, permitting the grid and lamp
head to be taken as a complete operating unit to the overhead cat-walks
and parallels.
The Type
90 lamp weighs 224 pounds and is tending to replace the 24-inch Sun
arc, which as a rule weighs more than twice as much. The Type 170
lamp, which weighs 311 pounds, is, in most cases, replacing the 36-inch
Sun arc, many of which weigh considerably more than 600 pounds each.
In recognition
of the value of reflector arcs of the type generally classed as Sun arcs,
it should be noted that for divergences of less than 15 degrees they show
definite superiority, and it is not anticipated that projectors of the
lens type will displace them for uses requiring such narrow beams.
Lamps
of the types described have carried the major burden in all artificial
lighting used in the Technicolor productions Trail of the Lonesome
Pine, Dancing Pirates, Garden of Allah, Ramona,
and God's Country and the Woman. Very considerable numbers of lamps
are now in use at the Warner Brothers, Metro-Goldwyn-Mayer, Paramount,
United Artists, and Columbia studios in Hollywood, and at London Films
Denham Studio in England for both black-and-white and color photography.
DISCUSSION
Mr. Palmer:
Is the motor connected directly across the line, so that the speed does
not vary with arc voltage?
Mr. Richardson:
It is a shunt motor, connected across the arc. We experimented with placing
the fields across, or ahead of, the resistance, but adopted the present
arrangement because we have to separate the rheostat from the lamp head
on account of the heat. The motor changes speed somewhat with the slight
volt age fluctuations resulting from changes of arc length, so we have
some degree of regulation.
The mechanism
will work the positive carbon down to about 3 5/8 inches, when consumed
to the limit. There have been many attempts to devise ways of saving carbons;
but using such heavy currents as we do in the high-intensity lamps, perfect
conductors are required to get the current to the tip of the positive
carbon.
Mr. Misener:
Are the motors sealed?
Mr. Richardson:
They are entirely enclosed, and require very little servicing. In fact,
I do not believe that, of 250 lamps we have in operation, more than 10
have ever required servicing. Eventually brushes will have to be replaced;
but if the motor is used, say, two hours a day, the brush life should
be quite long.
Mr. Crabtree:
We read in the newspapers about the high temperatures existing in the
Hollywood studios. What is being done in the way of air conditioning?
Mr. Richardson:
That question has been asked a number of times. I have not heard of the
terrific heat, except that the temperature does get rather high at the
studios in the San Fernando Valley. The outdoor summer temperature there
is occasionally 100 or over. As far as I know the stages are not refrigerated,
although many of them pass the air through water sprays, which have some
cooling effect.
Mr. Tasker:
Many stages are now equipped with blowers for changing the air during
takes. There are three conditions under which the heat becomes uncomfortable:
when one is working in a small closed room, particularly for photographic
reasons; when the actors are wearing arctic or winter costumes on the
stages; and when working with color.
Mr. Richardson:
A large cold-storage company in Hollywood had an enormous ice storage
building that had become obsolete due to the activities of the electric
refrigerator manufacturers. They conceived the idea of turning it into
a cold stage, and installed refrigerating equipment. When you see Columbia's
Lost Horizon you will see the actors walking about with vapors coming
from their nostrils-a very convincing arctic scene, because the temperature
may be as low as 20 degrees, even on a summer's day.
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