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Incandescent .Lamps for Film Projection

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Incandescent .Lamps for Film Projection 72 PHILIPS TECHNICAL REVIEW VOL. 8, No. 3 INCANDESCENT .LAMPS FOR FILM PROJECTION by Th. J. J. A. MANDERS. 621.326.73 Because of the characteristics of the optical system in which film projection lamps are used, the following requirements must be made of such lamps: The filament must have great brightness and must fill a given small area as completely as possible. the diameter of the bulb must he small and the lamp must be able to be adjusted automntically. On the basis of examples from: among the numerous types of cinema and 8 and 16 mm film lamps manufactured by Philips, it is explained how these and other requirements are satisfied. Particularly the construction of the filament and the part played by the gas filling of the lamp are discussed from different points of view. Cinem~ lamps and 16 mm :filni lamps brightness, the are lamp continued in use. During the last few..years before the war the super high- When in addition to thenormal film 35 mmwide, pressure mercury lamp appeared as a new serious 1 used in cinemas,the narrower film (originally 9 /2 rival of the are lamp 1), and the intrusive narrow mm, later 8 and 16 mm' also) began to he used, it film has in the meantime also taken possession of seemed at first as if not only the field of appli- this aid in order to win new territory. The field of cation but also the projection apparatus for the projection .in auditoria, which are not too large, two types would differ in principle. The main remàins in the undisputed possession of the inca~- problem in projection technique is that of passing descentlamp, either with normal or with 8 or 16mm the large' light flux, which must be thrown on the fil~ . screen, through the small film window, whosearea In discussing incandescent lamps for film pro- in the case of 16 mm film is about one fifth, and in jection, we shall not, according to the above, be that of 8 mm film about onetwentieth of that for able to draw any sharp boundary between lamps nor~al film. Narrow film was therefore predestined for normal film - calledcinema lamps ~ and lamps for use in the home or in small auditoria with a for narrower film. There is indeed a difference in small projection screen, whereIesslight was needed. lifetime: according to universally acceptedstandards And while normal film was projected with the help the lifetime of cinema lamps must he 100 hours, of are lamps and.mirror condenser,the narrow film. while in the case of narrow film lamps 50, 25 or projector for the small audience worked exclusively .even only 10 hours is considered sufficient. This with mcandescent lamps and lens condensers. Only differenceis, however,based more on tradition than in that way could they be made sufficiently'small upon logical grounds. In principle the requirements and cheap,' safe in use, easily transportable and for cinema and for narrow film lamps are the same, fairly foolproof. and the fact, that in practice each type of projec- At the present time, however, the former sharp tion lamp is stated to belong to one or the other boundary. hetween normal and narrow film pro- category is' mainly due to the fact that in general jection is becoming vaguer. This is due mainly to each type of projection lamp can only be used in a the fact, that the advantages of the narrow fihI;. certain type of projector, and that projector will (cheaper installation, lower film costs, possibility naturally be intended either for normal film or for of the use of non-inflam.m~blefilm material) have narrower film. gained moreterritory for it whichbelonged originally only.to normalfilm: the halls and auditoria' in which The optical system Band 16 mm film is shown are growing larger and In the process of projection the filmpicture occu- larger and approaching the size of ordinary cinema pying the film window is thrown on the screen as theaters, and non-portable apparatus is now being an enlarged image by the objective; see the right- made for that size Dim. Another reason for this hand part ~ffig. 1. development was that the noiseless and quietly In the case of normal film the area of the film burning incandescentlamp, whichwas being manu- window is 3.17 cm2, the area of the projection factured for the growing 8 and 16 mm film pro- screen in a medium large cinema may he for in- jectors in types of steadily increasing power, was stance 20 m2• Thus even if all.the light which falls succeeding in displacing the unsteady sputtering on the film reaches the screen, the intensity of carbon are, evenin normal film projectors. Thiswas illumination of the screenwill be more than 60 000 possible to a certain extent in smaller cinema, but not in large theaters, where, because of its' greater 1) See Philips techno Rev. 4, 2, 1939. ,MARCH l!M,(J INCANDESCEN'I' I,AMPS FOH FILM PIWJEC'l'lON , 73 , ' times as small as that on the film window'. If for . as possible. In addition the Ioss factor k' must be example it is desired' to have 50 lux on the screen kept as small as possible. In our case the losses at (75 or 100 lux is usually demanded), the intensity the film window constitute a large part of these of illumination on the film, taking into account the Lsses. Due to the fact that the' filament of an losses by reflection and absorption in the objective incandescent lamp is not a uniformly luminous (about 50 percent), must amount to about 6 mil- ' plane, it may not be focussed exactly on the film lion lux. window. If it were, the structure of that filament image would also he projected by the objeetive on the screen. The filament is therefore focussed farther away, in the objective or at à spot between obj ective and film window. In other words, th~ film window is not situated at the narrowest part of the beam of light produced by the 'condenser, and a larger or smaller part (70 percent for' example) , of this beam is cut off by the edges of the :6.im 46/65 / window. 'The more uniform the brightness of the Fig. 1. The optical system used for film projection. The light filament, the closer its' image may be focussed to' _ from the filament of the lamp L is collected over a wide the film window and the smaller die 'Ioss at 'thàt angle by the eendenserlens C and focussed on or near the film window F. The film picture passing F is projected by the point. The greatest possible uriiformity of bright- objective 0 on the screen S, ' ness of the filament is thus a second requirement. Further requirements follo~ from a consider- This enormously intense illumination is' obtained ation of the condenser part of the projector. 'The by projecting the image of a very bright light source, light from the projection lamp is' 'obviously used in ,OUl'case the filament' of the projection lamp, on more economically the larger the angle qJ compared or near the film wi~dow by means of à condeneer to' 1fJ = a: If it is desired to 'limit the distance he- system; see the left-hand part of fig.1.As may be tween source of light and film window'to a certain' .seen in the figure the condenser has the' function length, in order' to prevent the 'whole' projector of collecting the light emitted by the source over a from' becoming too large; a large angle qJ means very wide angle qJ and concentrating it on the film. that the' filament of the lamp must' be .able to be within a small~r angle 1fJ. The angle ~jl should be placed very close to the condenser, thus that the equal to the angle a within which the objective diameter of the 'envelope of the lamp must be made can take up light from the film window. If ljl < 11, as small as possible. Furthermore wi1:hgiven values the obj ective is not' entirely' fil1ed and if ~p >.a, of qJ and 1fJ a certain enlargement of the filament' is unused light passes the obje~tive. , obtained. Since the size of the image of the filament Ordinari.y the performance of a proj ector is charac- i~ fixed by the size of the objectivo to be filled, the terized by the totallight flux (j) which the objective minimum size of the filament is thus also prescribed. throws upon the screen. The following formula can ' In general ii is found that the filainent must he derived,2) in a simple way: occupy a rectangle of between 5 and 10 mm Width. Thé filament 'may be made larger, but there is in (j) = k· F· B . 11: sin2a. .... the first approximation no profit'in that . F is the area of the film window, B the (average) With the dimensions the position of the filament , brightness of the light SOUIceand k a factor which is also exactly prescribed. It is therefore desirable takes all the losses into account, the chief of which in constructing the lamp to take care that in inser- are the losses in the condenser and the objective, ting the lamp in theprojector the filament automat- the losses at the film window and the losses by the ically occupies the correct position in the optical .; .' rotating sector which interrupts the light in the system. periods when the film is moved. Finally the requirement about lifetime.
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