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siderably with the installation of spill
rings, since a considerable portion of the mirror was blocked out. However, their use was found necessary since the uncontrolled front light made it extremely difficult to secure proper lighting effects.
A spot lamp of this design has a rather objectionable characteristic in that light reflected from a considerable portion of the mirror is blocked off by the lamp house as the beam is widened, which means that a smaller angle of light is picked up by the mirror, and, therefore, less total light emitted from the equipment in the flood position than in the spot position.
Fig. 1 shows the characteristics of light emitted from the glass parabolic mirror.
Faceted Mirrors
Unless large reflecting surfaces are used, the intensity in the flood position is so low as to be ineffective. The dark centre of this type of lamp was so objectionable in lighting the motion picture set that several types of faceted mirrors, first in glass and then in metal, were used. This aplanatic mirror which, although it greatly decreased the efficiency of this unit in the spot position, helped to correct the dark centre, therefore came into fairly general use.
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Figure 2
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5
Vig. 2 shows the characteristics of light emitted from the aplanatic mirror.
The Use of Condensing Lenses
The plano-convex condensing lens in spot position overcame the difficulties of dark centres, hot rings, and uncontrolled front light. However, the efficiency of such a unit was exceedingly low, and, therefore, it was impractical to use this type of equipment except for special effect lighting. Although in equipment using the plano-convex lens a mirror was usually inserted behind the light source to pick up the back light and re-direct it through the lens, with the type of lamps then available, it was found so difficult to keep these mirrors properly focussed that they were generally discarded in actual practice. Fig. 3 shows the efficiency of a plano-convex unit.
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Figure 3
Characteristics Needed for Lighting Equipment
In discussing the needs of the Cameramen it soon became evident that lighting equipment which would more nearly meet their requirements should have the following basic characteristics :—
The light beam should be completely controlled, that is there should be no un
controlled front light emitted from the