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164 HANDBOOK OF PROJECTION FOR
from a lens when lens charts are used. It is partly because of the fact that when Moving Picture World lens charts are applied this minimum distance is automatically established that we earnestly recommend all projectionists whose amperage is within the limits of the charts to use them. It is hardly a practical thing for the projectionist to determine by experiment exactly what the minimum distance is, and then figure out the lens combination necessary to give the proper size spot at the proper distance.
OPTICS OF THE CONDENSER.— The condenser may be composed of either two or three lenses, but the universal practice both in the United States of America and Canada is to use only two, though to some extent in England the threelens combination is used, and in Germany we understand that it is used almost exclusively. In the two-lens combination the lens next the light source is known as the "collector" lens, and the front lens is known as the "converging" lens. Where there is a three-lens condenser the lens located between the collector and converging lenses is known as the "centre" lens. There are great possibilities for light loss inherent in the condenser itself.
DISCOLORED LENSES.— When light passes through perfectly clear, colorless glass of good quality, only a very small percentage of its energy is absorbed. In this we refer only to the actual passage of light through the glass — not the loss incident to reflection as the light enters and leaves the lens. Competent authorities place this absorption loss at about 1 per cent, per each inch of glass, for high grade glass. Other competent authorities place it at about .5 of 1 per cent, per centimeter, which is a little higher than 1 per cent, per inch.
When a lens is discolored experiments have proven that the actual absorption loss is not greatly increased. The following figures are from measurements of discolored condenser lenses and give some idea of what it amounts to :
Kind of DiscolorationFocal Length Percentage of light passed by
PianoConvex discolored lens as compared to
Lens. light strength without any lens
at all.
Slight green
7 7/16 inch
88.8 per cent.
Pressed lens, yellow
7 1/16 "
89.2 "
Pressed lens, yellow
7 1/16 "
88.8 "
Dark green
6 11/16 "
87.2 "
Many other lenses were measured, but the results were all within a range of 89.2 high and 87.2 low. When it is con