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The life of a projection lamp is of some importance to the user. It is instructive to calculate the lamp cost per hour, based on the lamp price divided by the makers' rated life in hours. The following table shows the lamp cost per hour for 100 watt, 300 watt and 750 watt lamps.
Cost of Projection Lamps
Wattage
Lamp Replacement Cost per Hour
Lamp Replacement . Cost per 2,000 Lumen/Hours
100 300 750
2.8 pence
6.2 pence
9.3 pence
3.5 pence
2 pence 1 penny
The author has found that makers' estimates of lamp life are usually on the low side, and in practice these highly stressed projector lamps stand up to their job remarkably well. Lamp costs, however, are a factor in sub-standard projection costs.
It will be seen that the lamp cost per hour increases rapidly as the wattage increases, but when a light output factor is introduced it is found that the cost per lumen hour is steadily reduced with lamps of higher wattage.
These figures, however, relate to the lamp outside the lamphouse. Whefl the lamp is placed in the projector, the high wattage lamp fails to show a proportional increase of light compared with the lamp of lower wattage.
Optical Efficiency
The optical efficiency of the average projector can now be considered. The amount of light collected by the condenser is only a small portion of the total emission from the filament. It is reasonable to assume that little more than 10% of the total light is picked up by the condenser system, and even after allowance is made for the slight addition which can be expected if the mirror behind the lamp is in good condition and accurately adjusted, there is still a considerable loss to be faced.
In lamphouse design we have advanced only a little from the days of the magic lantern with its multiple wick oil lamp. We no longer suffer from the smell that was characteristic of an old time " magic lantern " display, but as much light as ever is still wasted inside the lamphouse.
There have been attempts to increase the light entering the condenser by placing the lamp filament a little in front of the mid position of the bulb, but the offset filament is not a general feature of projection lamps.
The use of a reflector behind the lamp is normal practice, and this reflector is expected to return some of the light emitted from the back of the filament. This may happen in some cases, but it is the author's opinion that the reflector has generally lost its reflecting power, or if any remains, the fitting is so badly adjusted as to be useless.
In practice it is very difficult to adjust a lamphouse reflector accurately, as it is far too hot to handle and the glare makes it hard to see the effect of any adjustment.
The aperture of the projecting lens affects the light on the screen, and with a lamp and condenser properly adjusted to the projection lens a large lens aperture will provide better screen illumination. This improvement, how]