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 Summary.--During
the evolution of cinematography there has been a constant demand for increased
light from a single source. Early attempts to meet this demand were made
by improvements in lamp projection optics and by increasing both size
and power input of the light source.1 In
recent years a great deal of work has been done toward increasing the
intrinsic brilliancy of the high-intensity carbon-arc source. This has
resulted in a series of carbons known as super-high intensity carbons.2,3
This paper will describe the requirements of the motion picture industry
which brought about the production of these super-high-intensity carbons
and will cover details of the development and design of carbon-arc lamps
to burn them. The use of super-high-intensity carbon-arc units in motion
picture studios may properly be divided into (a) Process background projection;4
(b) Set lighting.1
Process
Background Projection.-Process background projection is a means whereby
a stereopticon slide or a motion picture may be projected onto a translucent
screen to form the background for a set which has been constructed on
the opposite side of the screen. Photographing both the projected image
and the set results in a composite picture.5
Inasmuch as the set on the camera side of the screen is illuminated for
proper photographic exposure, it is quite evident that the screen light
must be of a much higher level than that in a motion picture theater.
When this process first came into use, standard motion picture projection
lamps were the only available equipment. In attempts to increase screen
light the studios tried carbons from every available source and were requesting
carbons of higher current-carrying capacity than the design of the lamps
permitted. At that time, carbons of higher current capacity were also
of larger diameter and not of increased intrinsic brilliancy. Because
the optical systems in use were filled by the smaller diameter, lower
current-capacity carbons, the light gain was negligible and the increased
heat often resulted in unsatisfactory operation.
Higher
levels of the proper intensity and quality of screen light called for
coordinated effort between the studios and the various suppliers of equipment
and materials. This demand resulted in activity on the part of the Research
Council of the Academy of Motion Picture Arts and Sciences which led to
a co-operative investigation of the entire subject by all of the studios
and manufacturers involved.6 Subsequent
to the investigation by the Research Council Process Projection Committee,
a report was issued covering recommendations on process projection equipment.6
Fig 1. The Mole-Richardson test lamp. Close-up view through housing door
opening showing details of the carbon-burning mechanism.
The
Mole-Richardson Company agreed to design and build a process projection
lamphouse which would meet the requirements outlined in the report. Inasmuch
as a carbon-arc lamp is designed to feed and control carbons, this work
was carried on in close co-operation with the National Carbon Company,
Inc.
A
laboratory test lamp was designed and several of them were built for use
in Mole-Richardson Company's and the National Carbon Company's research
and development laboratories. This unit (Fig. 1) will accommodate any
size positive carbon from 11 to 18 mm in diameter and may be adapted to
burn other sizes. Separate motors control positive feed, negative feed,
and positive rotation, so any desired variable of those three factors
may be quickly obtained. The positive head may be adjusted for various
lengths of carbon protrusion, and different types of air- and water-cooled
positive carbon current input contacts may be used. The negative carbon
head is mounted on arms which allow it to be moved to carbon trim burning
angles from coaxial alignment to 90 deg. Adjustments are provided for
alignment of the carbons in both the horizontal and vertical directions.
Fig 2. Curves showing intrinsic brilliancy across the crater face of
the "national" 16-mm super-high-intensity studio positive at 225 amp
and the 16-mm high-intensity studio positive at 147 amp.
The
negative head will accommodate various sizes of air-cooled negative carbons,
and can also be equipped with water-cooled negative carbon-current input
contacts. This lamp makes it possible to obtain any set of operating conditions
which may be desired for experimental work under conditions of very close
control.
As
a result of a co-operative testing program, the National Carbon Company
supplied a carbon trim consisting of a 16-mm X 22-in. super-high-intensity
positive and a 17/32- X 9-in. heavy-duty copper-coated negative to burn
at a maximum current of 225 amp and 75 arc v (Fig. 2). This trim was chosen
over others tested because of high intrinsic brilliancy, uniform distribution
across the crater face, and steadiness of burning. For comparison, Fig.
2 also shows the standard 16-mm X 20-in. set-lighting carbon which is
used in the M-R Type 170 lamp, and which delivers about one half the horizontal
candle power.
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| FIG. 3. M.-R. Type 250 process background projection lamp. Oblique
view showing front and operator's sides. (The plate shown assembled
to the front of lamphouse and the rheostat knob in lower left corner
of control panel are parts of an associated process background projection
equipment and are not furnished with the lamp.) |
FIG. 4. M.-R. Type 250 process background projection lamp. View
of operator's side with lamphouse door and control-panel door open.
(The bank of three rheostats shown in the lower left corner of the
control box is part of an associated process background projection
equipment and not furnished with the lamp.) |
The
Mole-Richardson Type 250 process projection lamp (Figs. 3 and 4), which
burns this carbon trim, and its associated Type 251 grid (Fig. 5) have
been designed and produced.
The
major features of the lamp design are briefly described as follows:
(1)
General construction.-The outline dimensions of the lamphouse are
such that it can be conveniently assembled with the associated process
projection apparatus. The front portion of the lamphouse is arranged to
accommodate the light-collecting optical system together with its supports
and adjustments. Large hinged access doors are provided in the lamphouse
and control box for ease of maintenance.
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| FIG 5. M.-R. Type 251 grid for Type 250 process background projection
lamp. Oblique view showing side and contactor-panel end. |
Aluminum construction is used wherever possible so that the weight is
kept to a minimum. The housing and its doors are double-walled construction
with the inner wall fabricated of asbestos material. This type of construction
results in a low transmission of heat and sound from the arc to the surrounding
area. Extra space is provided in the control box for mounting instruments,
switches, etc., which are used with the associated process projection
apparatus. The control box and lamphouse can be conveniently separated
for ease of handling and shipment.
(2)
Positive carbon control.-The positive carbon is rotated continuously
so that an even crater is maintained. A photronic cell control device
causes the positive carbon to be fed forward as it burns so that the source
of light is maintained within very close limits at the focal point of
the light-collecting optical system.
(3)
Negative carbon control.-The desired arc length is continuously
and closely maintained by a control circuit which positions the negative
carbon. When the arc switch is turned on, the negative carbon is caused
to be fed forward until it contacts the positive carbon, thus establishing
the arc. The control mechanism then immediately retracts the negative
carbon to a position corresponding to the proper arc length, and maintains
this arc length by continually feeding the carbon forward as it burns.
(4)
Cooling. The lamp is designed for satisfactory operation without
forced ventilation so that the objectionable noise of a ventilating fan
is absent. Openings in the lamphouse are provided for natural draft, and
are arranged in such a manner that the resulting air currents do not interfere
with the stability of the arc.
The
water-cooled positive head encloses carbon-contact brushes which are cooled
by their contact with a water-cooled casting. The cooling water is circulated
through the casting between the arc and the brushes, so that the operating
temperature of the brushes is considerably lower than in conventional
designs, and it is expected that little or no brush maintenance will be
required.
A
water-flow indicator is located on the rear of the lamphouse.
(5)
"Douser". A "douser" in the form of a metallic plate is provided,
which can be swung into position between the positive carbon and the light-collecting
optical system. Its motion is mechanically interlocked with the motion
of the operator's lamphouse access door. Closing or opening the door causes
the douser to assume its position between the positive carbon and the
optical system. Hence after the door is closed, the douser will protect
the optical lens from heat shock and hot particles caused by striking
of the arc. When the door is opened, the douser protects the lens from
thermal shock which might result from cool air entering the housing from
the outside. Manual positioning handles are located external to the lamphouse,
so the operator can "turn on" or "douse" the light through the optic system
while the arc continues to burn.
(6)
Control panel.-The control panel is equipped with instruments for
indicating the line voltage, arc voltage, arc current, and length unburned
positive carbon. An arc-image screen provides the operator with a calibrated
visual indication of the positions of carbons. Knobs are provided for
setting the arc-length regulating circuit, and for manual adjustment of
the positive and negative carbon positions. The lamp operation is entirely
automatic, and is controlled by a small "off-on" toggle switch located
on the control panel.
The
M-R Type 251 grid which is supplied with the process projection
lamp is designed specifically for the application. Adequately ventilated
grid resistor units, which carry the arc current and produce the required
voltage drop, are positioned in the center of the unit. Selector switches
are mounted on a switch panel on one end of the unit, with connections
made to various taps on the grid resistors. By manipulation of these switches,
satisfactory arc operation can be attained with are currents of 150, 180,
200, or 225 amp with any line voltage of 110 to 130 v in 5-v steps.
Two
line contactors, a starting contactor, a time-delay relay, all auxiliary
relay, and a selenium rectifier are mounted on the contactor panel on
the end of the unit opposite to the switches. The coils of the main-line
contactors are connected across the supply through the arc switch on the
lamp-control panel, in series with the selenium rectifier. The rectifier
prevents the main-line contractors from being energized if the supply
to the system is not of the correct polarity.
A
"starting resistance" is provided in the grid circuit to limit the current
on arc strike, and hence prevent the positive carbon crater from being
damaged by the initial thermal shock. The starting resistance is automatically
cut out of the circuit when negative carbon has retracted to its approximate
operating position. This operation is accomplished by time delay auxiliary
relay and starting contactor.
Two
bus bars are provided on the grid for connection to the direct current
supply. The grid is equipped with three cables for connections to the
lamp, two heavy single-conductor cables for conducting the arc current,
and one small three-conductor cable for the control circuits. The unit
is mounted on sturdy rubber-tired casters of large diameter for portability.
The
above-described process projection lamp and grid combination is representative
of the present-day knowledge in the art of producing this particular type
of projection equipment. However, research and developmental work is continually
being conducted with efforts directed toward more and steadier light.
Experiments are being made in connection with the possible use of a small-diameter
water-cooled graphite negative carbon, which may produce a steadier light
than is produced with the larger air-cooled negative carbons, and with
no loss of light.
Tests
are being made on a brushless water-cooled positive carbon contact unit
without moving parts, which also promises to contribute toward the steadiness
and increase in light output. Positive carbons are being considered which
have brilliancies of as high as 1400 candle power per mm.2
and which may be operated up to 400 amp.
Set
Lighting. -- A careful study of cinematographic technique indicates
that the present-day cinematographer often strives for an illusion of
a "one-source" lighting, particularly in medium and long shots. While
he must use a large number of units for balance, modeling, back light,
and other effects which indicate his individuality, he works for an over-all
result suggesting that the illumination is coming from one source of tremendous
brilliancy such as is found in nature when the sun is just at the right
position. This effect may only be obtained with a high-brilliancy source
sufficiently small in area to cast well-defined shadows. The shadows cast
by the other units are either covered by the main source or are eliminated
with fill light, and while there may be a hundred lamps on the set, all
noticeable shadows are cast by the main source, creating the illusion
of a one-source lighting.
Previous
to the advent of the super-high-intensity studio-type
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Fig 6. M-R Type 450 super-high-intensity arc spot
lamp. Oblique front view showing 24-in. diameter Fresnel lens and
operator's access door.
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carbon there were three general types of carbon arcs available for the
motion picture studios:7 (1) the low intensity
carbon arc where the principal light source is incandescent solid carbon
at or near its sublimation temperature; (2) the flame arc where the light
source is the entire arc stream made luminescent by the addition of flame
materials; (3) the high-intensity carbon arc where, in addition to the
light from the incandescent crater surface, there is a significant amount
of light originating in the gaseous region immediately in front of the
carbons as the result of the combination of high current density and an
atmosphere rich in flame materials.
The
low-intensity carbon arc has no present use in motion picture studio set
lighting. The flame carbon arc is used in general lighting units for front
light, fill light, and to illuminate backings. The high-intensity carbon
arc is used in spotlamps.
The
M-R Type 170, operating at 140 to 150 amp and 64 to 67 arc v, has
been the most popular carbon arc lamp for use in creating a one-source
lighting effect and for boosting daylight on exteriors.1
However, the rather high light levels used on color pictures indicated
a need
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| Fig 7. M-R Type 450 super-high-intensity arc spot lamp. Oblique
rear view showing operator's control panel on rear of control-mechanism
housing. |
for a unit of still greater volume and penetrating power.
A
demand on the part of directors of cinematography for higher-powered sources
resulted in some
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Fig 8. Arc element and control mechanism subassembly for M-R Type
450 super-high-intensity arc spotlamp. Front oblique view showing
unit removed from lamphouse.
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attempts by the studios to adapt the 16-mm X 22-in. super-high-intensity
carbon to the M-R Type 170 lamp. The same troubles were encountered
that had plagued the process projection departments when they attempted
to increase current in standard projection lamps beyond the design characteristics.
When
a carbon trim is burned at 225 amp in a Type 170 lamphouse, the interior
of the unit becomes overheated, endangering the carbon-feed motor-current
leads, and positive carbon brushes. A carbon trim which will burn steadily
under conditions of proper lamphouse ventilation may become erratic and
unsteady if the control mechanism is overheated.
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| Fig 9. Chart indicating relative illumination characteristics of
M-R Type 450 lamp burning the 16 mm super-high-intensity studio positive
at 225 amp and M-R Type 170 lamp burning the 16 mm high intensity
studio positive at 150 amp. |
The gear ratios controlling the carbon-feed rates do not correspond to
the burning rates or the burning-rate ratio of the higher-current carbons.
To
meet the demand for a higher-powered unit, the M-R Type 450 lamp was designed
(Figs. 6 and 7). This unit is equipped with a 24-in. diameter Fresnel-type
condenser lens. The drum is of sufficient diameter to ensure proper ventilation,
the unit is wired for the increased current, and the feed.1motor, feed-motor
rheostat, arc switch, and pin-plugs are located in a separate compartment
on the
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| Fig 10. Typical curves showing illumination at center of 20-ft diameter
spot at various distances from M-R Type 450 lamp burning the 16 mm
super high intensity studio positive at 225 amp and M-R Type 170 lamp
burning the 16 mm high intensity studio positive at 150 amp. ( The
diameter of the spot is defined as the diameter at which the illumination
is 10 per cent of the maximum illumination present at the center of
the spot.) |
back of the lamp in order to limit their operating temperature
rise. This separate compartment and the lamphead mechanism can be removed
from the lamphouse as a unit (Fig. 8). Hence, a subassembly of the working
parts can be set up on a bench for convenient servicing. The carbon trim
consists of a 16-mm X 22-in. super-high-intensity MP studio positive and
a 17/32- X 9-in. HD cored Qrotip negative burning at 225 amp and 75 arc
v.
The
chart in Fig. 9 gives a comparison of the illumination characteristics
of the Type 450 and Type 170 lamps. In the maximum flood condition, the
amount of luminous flux in the Type 450 beam is approximately double the
amount in the Type 170 beam, and the apparent horizontal beam candle power
is almost tripled. In the minimum spot position, both the flux and candle
power values for the Type 450 lamp are approximately twice those for the
Type 170.
A
comparison of the illumination of a 20-ft diameter spot as produced by
the Type 450 and Type 170 lamps is shown in Fig. 10. It is apparent that
the Type 450 lamp represents a considerable increase in the "penetrating
power" of lamps for studio-set lighting.
Another
unit in the advanced stages of design is a super-high-intensity spot projector
which will be similar to the Type 450, but which will be equipped with
an integral optical system for throwing a well defined and closely controlled
spot for use in follow shots such as would be made in a skating picture.
Experience
gained in the manufacture of specialized searchlight equipment during
the war will be of considerable value in increasing further the light
output of motion picture studio lamps using super-high-intensity carbons.
We
wish to acknowledge the co-operation of the Transparency Department and
Mr. Farciot Edouart of Paramount Studios in the design and production
of the special process lamp; the splendid cooperation of numerous cinematographers
and the Electrical Department members in the work which was done on the
"Brute" Type 450 amp; and the National Carbon Company, Inc.
REFERENCES
1.
LINDERMAN, R. G., HANDLEY, C. W., AND RODGERS, A.: "Illumination in Motion
Picture Production", J. Soc. Mot. Pict. Eng., XL, 6 (June 1943), p. 333.
2.
JONES, M. T., LOZIER, W. W., and Joy, D. B.: "New 13.6-Mm Carbons for
Increased Screen Light", J. Soc. Mot. Pict. Eng., XXXVIII, 3 (March 1942)
p.229.
3
JONES, M. T., ZAVESKY, R. J., AND LOZIER, W. W.: "A New Carbon for Increased
Light in Studio and Theater Projection", J. Soc. Mot. Pict. Eng., 45,
6 Dec. 1945), p. 449.
4.
JOY, D. B., LOZIER, W. W., AND NULL, M. R.: "Carbons for Transparency
Process Projection in Motion Picture Studios", J. Soc. Mot. Pict. Eng.,
XXXIII, 4 Oct. 1939), p. 353.
5
EDOUART, F.: "The Paramount Transparency Process Projection Equipment",
J. Soc. Mot. Pict. Eng., XL, 6 (June 1943), p. 368.
6
Research Council of the Academy of Motion Picture Arts and Sciences: "Recommendations
on Process Projection Equipment", J. Soc. Mot. Pict. Eng, XXXII 6 (June
1939), p. 589.
7.
MACPHERSON, H. G.: "A Suggested Clarification of Carbon Arc Terminology
as Applied to the Motion Picture Industry", J. Soc. Mot. Pict. Eng., XXXVII
5 Nov. 1941), p. 480.
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