International projectionist (Jan-Dec 1947)

crease in the breakdown voltage in the gas. Finally, it may be mentioned that for seme 8 or 16-mm film projectors, with a very compact construction, lamps have been made in which the filament is brought especially close to the wall by excentric placing in the envelope. In the case of the 400-watt lamp shown in Fig. 12, for example, the distance between filament and wall is only 7 mm. This is made possible by the employment of the hanging construction and by an arrangement for cooling in the projector housing in such a way that the part of the envelope lying opposite the filament is the most strongly cooled. For the rest, it is striking that in this lamp the spiral sections are horizontal. The vertical position is usually chosen in order to prevent short circuit between the sections in case of any slight sagging of the spiral during use. The horizontal position has the advantage that the light emitted in the axes of the envelope can easily be used for the scanning of the sound track of the film, so that a separate lamp is not needed for that purpos.e. In the case of the lamp in Fig. 12, use is made of this possibility, which exists indeed only with the hanging construction, since in the ordinary, standing construction the light along the axis direction would have to pass through the upper parts of the envelope, which are most subject to blackening. Although in projectors for 35-mm film the dimensions of the apparatus are much less critical than in the case of narrower film, here also for other reasons the diameter of the envelope must be kept as small as possible. Since in general these projectors work with a mirror condenser, the auxiliary mirror is placed in front of the lamp and thus cuts FIGURE 11. Gas current in the lamps of Figure 10. off a part of the beam of light which the condenser mirror throws on the film. The auxiliary mirror should therefore be small, and since the mirror is placed on or outside the envelope, the envelope itself must be small. Attempts have indeed been made to avoid this necessity by placing a very small mirror inside the envelope very close to the filament. This mirror had to be made of molybdenum because of the intense heat. The relatively low coefficient of reflection of molybdenum (60 per cent in the cold state, about 75 per cent at white heat, compared with 95 per cent for a mirror of aluminum deposited on the wall) and the practically unavoidable loss of adjustment upon becoming hot, however, make the advantage of this method more or less illusory, especially since the hanging construction has made it possible to reduce the diameter of the envelope very much at the position of the auxiliary mirror (in the lamp of Fig. 10, left, it amounts to only 60 mm.) Operating Characteristics We have already pointed out that the lamp must be resistant to the mechanical vibrations to which it is exposed when the projector is in use. These vibrations are composed of many components of different frequencies and are different for each type of projector. It is therefore impossible to indicate a lamp construction which could be used in all cases. Attempts are rather made empirically during the development of the lamps to render a lamp insensitive to the special vibrations of the type of projector for which it is intended. For this purpose the lifetime is determined of test models of the lamp and a normally working model of the projector, and the construction of the leads and hooks are altered until the desired lifetime in operation is fully achieved. This work is made more difficult by the requirement that the leads and hooks must not stand in the way of the light beams which the filament emits .towards the condenser and auxiliary mirror. The nominal lifetime can only be attained if the lamp is burned precisely at the prescribed voltage or current. If the lamp is calculated for a certain voltage, an increase in voltage of 1 per cent decreases the lifetime by 14 per cent; while on the other hand a decrease in voltage of 1 per cent causes a decrease in brightness of 4 per cent. With a lamp for a certain current the corresponding variations upon change in current are still larger, namely 25 per cent in lifetime and 8 per cent in brightness. The difference between the two kinds of lamps consists in the fact that, in INTERNATIONAL PROJECTIONIST • January 1947 FIGURE 12. Philips 16-mm film lamp 30 V, 400 W, with hanging construction and with excentrically placed filament, whereby the distance between filament and wall of the envelope is reduced to 7-mm. The spiral sections are horizontal in this case, the light emitted downward is used for scanning the sound track of the film. The sections of the spiral do not lie in a flat plane but in a cylindrically bent plane, whereby a similar effect is obtained as in the two-plane assembly. choosing the dimensions of the former, account is taken of a gradually decreasing current strength during the life of the lamp, due to the fact that the filament is becoming thinner by evaporation, and a consequently increasing resistance; in the case of the second lamp a gradually increasing voltage is discounted in the dimensions for the same reason. Since, as a result of this, the brightness will decrease slightly in the first lamp and increase in the second lamp, the use of lamps for constant current is always considered desirable for projectors in order to compensate for a decrease in light due to blackening of the walls and deformation of the filament. Since, however, according to the foregoing in the Philips lamps this decrease in light yield has been reduced to practically zero, the burning of the lamp at constant voltage would be more recommendable in connection with the aforementioned slighter sensitivity to variations in the operating condition, and also because the "gain in light upon the ageing of the lamps for constant current is of course only obtained at the expense of a lower temperature of incandescence and brightness in the new lamp. In any case the voltage or current stamped on each lamp should be exactly maintained during its use if full profit is tj be had with respect to brightness and 15