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164
BRITfSH KINEMATOGRAPHY
Vo1. 17, No. 5
to give the total output in lumens. The unit a lumen is essentially a measure of light as seen by the eye. Another method would be to use a photo-electric photometer fitted with a filter which gives accurate colour correction to visual sensitivity, but particular care must be taken to ensure its correct calibration to arc light (most photometers are calibrated to tungsten light, and a correction is applied for arc light, but this may easily lead to error). Only an approximate figure can be obtained by using a photo-electric instrument without a filter, for such meters are sensitive to ultra-violet radiation ; a considerable amount of such radiation is emitted by an arc2 but much of it may be absorbed by glasses in the optical system, especially flint glass, and the amount on the screen may be very variable between projectors.
Available Light Output.
The distribution and output of this projector is affected by the position of No. 3 lens. Its normal position is chosen to give good uniformity, without significant fall-off in the corners, for the whole range of objective lenses. The light output, at 300 amps., and distribution through a 3 x 2.2 ins. gate is given in Table 3. At lower arc currents the light output is reduced proportionately.
These variations in distribution are so small that the screen appears to be
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very evenly illuminated. It is only a trained eye which can see the fall-off in the corners with the 6.4-in. lens, and this can be corrected by moving No. 3 lens a little. The reduced performance with the 22-in. lens is immaterial, because this is used only for small pictures.
As an example in practical terms, the light output shown in this Table is found to give good results on pictures 30 ft. x 22 ft., photographed at f/3.2 on Plus X stock, the subjects being daylight exteriors. If the area of the picture be reduced, the camera may be stopped down, e.z. 21 ft. x 154 ft. at //4.5, or 15 ft. x 11 ft. at //6.3.
The effect of moving No. 3 lens has been investigated further. By " spotting " the gate until the fall-off in the corners just begins to be apparent, the distribution over the screen is as shown in Fig. 3, using the 10-in. objective. The rectangle represents the 3 x 2.2 ins. gate. At 300 amps, the total light output is 63,000 lumens.
When No. 3 lens is moved to " flood " the gate, the distribution of Fig. 4 is obtained, and in this case it is possible to illuminate satisfactorily the 3x3 ins. gate aperture shown. Although, of course, the brightness has been reduced, the increase in permissible area of gate results in a net increase in light output, giving 68,000 lumens at 300 amps.