International projectionist (Jan-Dec 1946)

PROJECTION lamps, like most electric lamps for lighting purposes, are filled with gas which belps to prevent the evaporation of the filament and allows the filament to burn at a higher temperature and attain the same lifetime. Due to the gas filling, however, it is impossible to decrease the distance between the parts of the spiral indefinitely because of the necessity of avoiding breakdown between adjacent parts. In spite of this limitation, various means are employed to arrive as far as possible at a complete filling of the prescribed area. First, it is obvious that the lamp should be made for a low voltage. The voltage between the parts of the filament, and thus the necessary distance between those parts, then automatically become small. In addition there are other, more important arguments for the choice of a low voltage. At a given power the current is inversely proportional to the voltage. A larger current corresponds to a shorter and thicker wire. A thicker wire may, for a given lifetime, burn at a higher temperature, since it offers a relatively smaller surface for evaporation of the tungsten. The lamps for low voltage therefore have a greater brightness for the same lifetime than those for higher voltage. Filament-Current Requisites In order to obtain an impression of the difference, let us compare a 110-volt/ 100-watt lamp and a 30-volt/100-watt lamp, both for a lifetime of 100 hours. The first has a filament 0.65 mm thick, the temperature of incandescence is 3000° K, the brightness obtained 315 candlepower per square centimeter (cp/sc) ; the second has a filament .155 mm thick, temperature 3100° K, brightness 855 cp/sc. Moreover, the shorter, thicker wire has the advantage that a shorter spiral is Incandescent Lamps for Film Projection By J. J. A. MANDERS Research Laboratory, Philips Incandescent Lamp Company, Eindhoven, Holland The second of a series of articles which cover comprehensively the requirements which have to be met in order to attain maximum efficiency with incandescent film projection lamps. Invaluable data for every professional projectionist. sufficient, thus fewer sections of spiral and consequently less intermediate space between them, and in addition, due to its greater strength, it can be spiralized around a thicker mandrel, so that the width of the sections of the spiral is more advantageous compared with the spaces between. It is even possible here, using a thick mandrel, to fill up the whole filament area very satisfactorily with a single spiral without subdivision. Apart from the heat losses through the leads, which are more important in the case of a short wire and high current [thick lead wires] and which prevent the voltage from ever being lower than about 15, a low voltage has only the disadvantage that for connection with the mains an intermediate apparatus is necessary, either a transformer or, if the FIGURE 3. Filaments of a 200 V narrow film lamp (left), and a 30 V narrow film lamp (right). Top row (a): The image of the filament without auxiliary mirror; bottom row (b): with auxiliary mirror. In the case of the filament for the high voltage, the filling of the area is unsatisfactory, even with the employment of the auxiliary mirror. puri base price be importanl than the efficiencj "I the projector, a resistance. Since, because "I this disadvantage, many manufacturers of projectors preferred i" continue using high \ohages, still other mean of filling the filament area more uniformly an employed. A method which is used in all kind of projection lamp [including those foi low voltage] i the introduction "f a spherical mirror behind the filament. This auxiliary mirror casts a real image of the filament in such a way that the images of the spiral sections fall exactl) in the spaces between the actual spiral sections. The advantage in average brightness on the film aperture to be gained in this way may amount to 50 percent. The best uniformity will obviously be obtained when the intermediate spaces between the sections of spiral are no: wider than the sections themselves, as is the case with the filament of Fig. 2 [shown in the Nov. issue]. If, however, the filament is one for a high voltage, the spiral must, on the one hand, be made thin in order for it to be strong enough with the thin weak wire which must be used; while on the other hand, the spaces between the spiral sections must be large in order to avoid breakdown. Compare such a filament for 220 volts with one for 30 volts [Fig. 3]. With such a rarefied filling of the area, as the figure shows, even the auxiliary mirror can only ensure a moderate uniformity. Double-Spiralized Filament A greater effect is obtained by doubly spiralizing the filament. This principle [sufficiently familiar from the incandescent lamps for ordinary lighting purpose] improves the efficiency by decreasing the transfer of heat to the gas, permits the employment of a somewhat higher temperature for the same lifetime, due to a slight decrease in the evaporation; reinforces the already mentioned black-body effect, and in our case also offers the possibility of filling the filament area about as well as if we were dealing with a low-voltage wire of the now spiralized single spiral. In this way it is possible, for example, to obtain the same average brightness with a 110-volt double spiral as with a 60-volt single spiral. Such a doubly spiralized filament is shown in Fig. 4. Finally, the two-plane assembly is also much employed: the sections of spiral are not assembled in a single plane, but in two parallel planes in such a way that the spirals of one plane lie behind the intermediate spaces of the other [Fig. 5]. It is clear that in this wTay very good uniformity and a very high average brightness can be obtained, es JNTERNATIONAL PROJECTIONIST • December 1946 19