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16-MM SOUND-FILM PROJECTION 389
lens is capable of producing a reduced image of the lamp filament at the plane of the film. Because of this the filament must be of the single horizontal coil variety and must not tend to sag when heated. Any movement away from the optical centre would displace the beam of light passing from the film to the photo-cell. If the film is then caused to move past this scanning beam at a constant rate the beam of light passing to the photo-cell will be modified at a frequency dependent upon the variations in the photographic image.
Sound Scanning Apertures
Many sound projectors employ a mechanical slit at the scanning point instead of the reduced image of the exciter lamp filament as shown in Diagram 'A', Figure 166. One such arrangment is shown in Diagram 'B' in which it is possible to employ a mechanical slit ten times larger than the actual height which is scanned on the film. In this design the objective lens throws an enlarged image of the sound track onto a correspondingly large slit. Such an arrangement has two main advantages; firstly that the actual position of the exciter lamp filament is no longer important since the beam of light from this lamp is only used to evenly illuminate the required scanning region and, secondly, a mechanical slit ten times larger than the scanned area may be more easily manufactured than one which is equal in size to this area.
The actual dimensions of any mechanical slit may therefore vary considerably from one machine to the next although, by the use of suitable optical systems, all scanning light beams should have an effective size equal to the following standard recommendations: (1) The scanning slit height measured along the film length -shall be between 0-0005 and 0-0007-inch (0-0125 and 0-0175-mm) and (2) the scanning slit width measured across the film -shall be between 0-070 and 0-072-inch (1-778 and 1-829-mm).
With such very small areas of illumination, and the size of any one cycle of a high frequency note, it is very necessary that the optical units used in the sound head may be critically focused onto either surface of the film, that is, on either the surface nearest to or furthest from the exciter lamp when the film is in the plane indicated in Figure 166. This provision is necessary since, although the emulsion surface may in most cases be towards the light source, for special processes it may be in the reverse position and, naturally, critical focusing will only be obtained when the optical system is sharply focused on the emulsion itself.