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 During
the Spring Convention at Hollywood, Calif., May 20-24, 1935, a symposium
on new motion picture apparatus was held, in which various manufacturers
of equipment described and demonstrated their new products and developments.
Some of this equipment is described in the following pages; the remainder
will be published in subsequent issues of the Journal.
In the
motion picture studios there are a number of lamps that may be classified
under the general term of "spot lamp." For the purpose of this paper,
this classification may be divided into two groups, i. e., the condenser
type and the reflector type. The condenser type embodies a source of illumination,
the light from which is collected by means of a single condensing lens,
usually of the planoconvex form, and means are provided for focusing the
light-source in relation to the lens in order to vary the divergence of
the projected beam. The ratings of these lamps range from 250 to 2000
watts, utilizing filament globes; and, in carbon arc equipment, from 35
to 115 amperes.
In the
group of lamps designated as the reflector type will be found the lamps
embodying light-sources in combination with glass or metal reflectors,
usually of the paraboloid form. It is the present practice of the motion
picture studios to use lamps of the reflector type provided with incandescent
globes, with reflectors ranging in diameter from 18 to 36 inches. Carbon
arc equipment of the reflector type includes the Sun arcs, the majority
of which have reflectors 24 or 36 inches in diameter, although one major
studio employs several Sun arcs using 60-inch reflectors.
The characteristic
common to both the previously mentioned groups is that they may be used
to project a beam of light, the divergence of which may be varied from
a narrow angle, for the "spot," to an angle sufficiently wide to "flood"
a considerable area. By altering the angle of divergence of the projected
lightbeam, the area covered by the beam and the intensity within the beam
may be increased or decreased according to requirements.
In attempting
to improve any product three considerations come to one's attention: first,
the incapacity of the existing product to meet the demands imposed upon
it; second, the extension of the usefulness of the product into new fields
of use; and, third, increasing the efficiency of the product itself.
FIG. 1. MR Type 210, Junior Solarspot. |
The
lamp under consideration in this paper-the MR Type 210 junior Solarspot,
shown in Fig. 1-has been designed to function primarily as a spot
lamp for use in photographing motion pictures. It is not an adaptation
of equipment used in another field of illumination, but embodies in
its design characteristics for overcoming the inability of existing
equipment to fulfill the demands imposed upon it and provides a control
of the lightbeam that widens the utility of the lamp as a tool of
the cinematographer. The advantages achieved in this design have been
largely effected by more efficiently utilizing the light from the
2000-watt G48 C13 bipost type of filament globe used in this equipment
as the light-source.
Spot lamps of the condenser type have the advantage of good control
over the projected beam. Using a 2000-watt lamp as the source, the
beam can be converged to an angle of 8 degrees and flooded out to
an angle as great as 45 degrees, although at such wide divergence
the intensity of illumination is low. The disadvantage of spot lamps
using the tungsten filament globes is their inefficient utilization
of the light.
The
power radiated by the 2000-watt globe is nearly 3 hp., a considerable
portion of which is radiated at wavelengths lying below the visible
range. That is to say, in other words, that the 2000-watt lamp radiates
a lot of heat. |
FIG. 2. Construction of typical 2000-watt condenser
type studio spot. |
The amount of heat radiated is such that even though plano-convex lenses
are made of heat-resisting glass, their size in practical application,
and to prevent excessive breakage, seems to be limited to a diameter of
8 inches and a focal length of 15 inches.
FIG. 3. Intensity distribution of condenser type spot lamp, with 8-inch
diameter, 15-inch focus condenser; source: 120-volt, 2000-watt, G-48 bipost
incandescent lamp.
Fig.
2 illustrates the layout of a typical 2000-watt condenser type studio
spot. Behind the globe is a spherical mirror which is used to collect
the light that would otherwise be unprojected and to reflect it so as
to form an image between the coils of the filament grid. Tests reveal
FIG. 4. Reflector type of lamp equipped with parabolic
mirror, showing angles of collection for spot and flood positions.
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that when such a mirror is used in the combination shown in Fig. 2, good
adjustment will increase the intensity of the beam by approximately 60-75
per cent above that afforded by a globe without such a reflector. With
a beam divergence of 8 degrees in the spot lamp illustrated, it is possible
to effect such a collection of direct and reflected light upon the condenser
lens of only 32 degrees. When such a combination is used for flooding,
with a beam divergence of 45 degrees, the angle of the collected light
is increased to 71 degrees, but the intensity of the beam is so low that
it is not of great photographic use. Fig. 3 shows the angular distribution
of candle-power from a 2000-watt studio spot for beam divergences of 8,
18, 30, and 44 degrees.
The
inherent fault of the condenser type of spot lamp for use with high-wattage
globes is its incapacity to collect a large proportion of the light emitted
by the source. Short-focus, wide-aperture condenser lenses would correct
the difficulty; but for the plano-convex type of condenser, lenses of
suitable focal length would be so thick as to cause great losses in transmission,
and the breakage hazard, which is now rather objectionable, would be greatly
increased due to the thickness of the lenses.
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| FIG. 5. Intensity distribution of reflector type
of spot lamp, with 18-inch parabolic reflector and spill ring; source:
120-volt, 2000-watt, G48 bipost incandescent lamp. |
The
reflector types of lamp have the advantage over the previously described
condenser spot lamps of collecting from the source a larger angle of light.
A schematic drawing of a lamp equipped with a parabolic mirror 18 inches
in diameter and having a focal length of 7 7/8
inches is shown in Fig. 4. The layout shows the lamp adjusted for a narrower
beam of 8 degrees, in which case light within an
angle of 121
degrees is collected from the bulb. The dotted lines show the position
when the light is flooded to an angle of 24 degrees, in which case
the angle of collection of the mirror is 116 degrees. All the light
from the front of the globe is lost, since, with the super-speed film
in present use, it is necessary to apply spill rings to prevent any
unprojected light from falling upon the set that may cause overexposure.
This optical combination is most effective for narrow beam divergences
in the lamps using 2000-watt, G48 C13 globes as the source.
Lamps of this type will spot down to a divergence of 8 degrees without
projecting filament images that are seriously objectionable. |
FIG. 6. Showing angles of collection of light in the 18-inch Sunspot
lamp equipped with mirror. |
FIG. 7. Intensity distribution of reflector type of spot lamp with
18-inch aplanatic metal reflector and spill ring; source: 120-volt,
2000-watt, C13 bipost incandescent lamp. |
When such narrow divergences are required, lamps of this type are most
effective; but the effectiveness is lost when they are flooded due to
the characteristics of the parabolic reflectors. When the source is placed
inside the focus of the reflector the intensity at and near the center
of the beam drops much more rapidly than at the edges of the beam. This
condition begins as soon as the globe is moved in from the focal point,
and becomes more and more pronounced as the divergence increases; until,
when the divergence is great enough, the projected light forms a "doughnut,"
which has no illuminating value in motion picture photography. Diffusing
mediums can correct the bad distribution somewhat, but at the expense
of much loss of illumination. Fig. 5 shows the intensity distribution
of this type of equipment for divergences of 8, 16, and 20 degrees.
To overcome
this fault of the parabolic mirror when used for projecting other than
narrow beams, there are in use in the motion picture industry stamped
metal mirrors, the curvature of which is primarily parabolic, but having
a plurality of facets.
FIG. 8. MR 210 Junior Solarspot lamp, equipped
with concentric plano-convex Fresnel lens.
This type of mirror design injects an element of diffusion which improves
the distribution of intensity in the projected beam. Fig. 6 shows an 18
inch Sun spot in which such a mirror is installed, and the angle of collection
of the light. For the 14-degree divergence the angle of collection is
130 degrees, and for the 24-degree flood position it is 124 degrees. While
mirrors of this design may be constructed from a number of small pieces
of glass, a form of reflector frequently used in Europe, such construction
is not, in our opinion, satisfactory. The amount of handwork involved
in producing such a reflector in our country. Would make its cost prohibitive.
Such reflectors have always tended to deteriorate rapidly, the silver
peeling at the edges of the facets. The faceted metal mirrors used in
the Hollywood studios are finished to a high degree, and are chromium
plated. Their reflectivity is, of course, limited by the reflectivity
of the chromium-plated surface. Their particular virtue is the smoothness
of distribution, for divergences from 14 to 24 degrees. The angular distribution
of an 18 inch Sun spot employing a faceted metal mirror and a 2000-watt
G48 C13 Mazda globe is shown in Fig. 7 for angles of 14 (the narrowest
divergence), 18, 24, and 30 degrees.
In the
motion picture industry it is seldom necessary to project a spot beam
narrower than 10 degrees, which provides a spot of light about eight feet
in diameter at a distance of fifty feet. It is, however, desirable to
be able to flood a lamp to a divergence as great as 40 degrees, provided
that the projected beam at this wide angle is of sufficient intensity
to be of at this wide angle is of sufficient intensity to be of photographic
use.
FIG. 9. Intensity distribution of junior Solarspot;source: 120-volt,
2000-watt, G48 C13 bipost incandescent lamp. |
For the conditions under which spot lamps are used, it is desirable that
the beam have its highest intensity at the center and that the edges be
soft so as to permit overlapping the beams of several such lamps without
building up high intensities in the areas overlapped.
The MR
Type 210 junior Solarspot lamp, illustrated in Fig. 8, is supplied with
a lens of the type known as the concentric plano-convex Fresnel. A lens
of this type can be made quite large in diameter, of short focal length,
and of relatively thin section. This lens was designed particularly to
fulfill the requirements of the junior Solarspot; and when used in combination
with a 2000-watt G48 C13 Mazda globe will project a spot beam having a
divergence of 8 degrees, and a flood beam of 44 degrees. The lens is manufactured
of a superior, heat-resisting glass of high mechanical strength. Referring
to Fig. 8, at the rear of the globe is provided a spherical mirror of
the proper radius and aperture diameter, provided with two simple adjustments.
This lamp utilizes a 2000-watt G48 C13 bipost Mazda globe. Such globes
are, by their nature, virtually prefocusing; and when once the adjustments
in the lamp are set, globes may be mounted or dismounted, and only slight
readjustments of the spherical mirror are required to attain high efficiency
of projection. The wide-aperture, short-focus lenses permit combined collection
of the radiation from the globe and the spherical mirror within an angle
of 74 degrees in the position for an 8-degree divergence, and of 104 degrees
when the lamp is used in its maximum flood position for a divergence of
44 degrees. The short-focus Fresnel lens contributes to the over-all efficiency
of the unit, but only careful attention to the design of the lens has
made possible the excellent distribution provided by the equipment over
a wide range of beam divergence.
Fig.
9 shows the angular distribution of the junior Solarspot for beam divergences
of 8, 18, 24, 30, and 40 degrees. It will be noted that the wide range
of distribution and the degree of intensity attained by this new equipment
adapts it to a wide range of use in motion picture photography. For instance,
with this lamp a person may be covered from head to foot at a distance
of ten feet. A spot that can be flooded to this degree and to such an
intensity makes a very useful lamp for general lighting. The fact that
the projected spot at all times has soft diffusing edges permitting areas
to be overlapped without showing rings or bands of lights, especially
adapts it to back-lighting; and the wide range of intensity within the
various beams is particularly advantageous for such purposes. Much experimentation
has been done with an iris shutter applied to the lamp. By closing the
iris and adjusting the focus of the lamp, a wide range of intensity may
be attained for a given beam divergence for any type of photography demanding
that the spectral composition be maintained constant, as for color photography.
Control of intensity by an iris is most desirable, in avoiding the use
of diffusing screens which have the characteristic of absorbing certain
wavelengths and otherwise causing spectral imbalance.
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