Journal of the Society of Motion Picture Engineers (1930-1949)

1948 VARIABLE-AREA LIGHT VALVE 523 the assembly of the light valve of the spacing between the ribbons and of the tension to which they are stretched. The carriages are shown assembled on the light-valve pole piece in Fig. 1. The magnetomotive force is supplied by an Alnico V magnet which forms the case of the valve. The magnetic flux is concentrated at the ribbon gap by means of permandur pole pieces which also constitute the ends of the light valve as shown in the cross-sectional view of Fig. 2. It is possible with such a structure to achieve flux densities in the ribbon gap of the order of 30,000 gauss. The mechanical force developed between the two pole pieces after the valve is magnetized is adequate Fig. 1 — Area light-valve pole piece showing arrangement of ribbon carriages. Fig. 3— Assembled area light valve. to hold the poles and the magnet together without resort to conventional fastening devices. The ribbon carriages and the magnetic circuit employed in the variable-area light valves are identical to those developed for use with the new Western Electric variable-density light valves and have been fully described elsewhere.3 The aperture between the lightvalve ribbons is projected on the film at a magnification of ten times by the light-valve objective lens. The necessity always present in variable-area recording for obtaining sharp photographic boundaries between the exposed and unexposed portions of the sound track imposes a rather severe requirement on the degree of resolution expected of this lens system. Fortunately it has been possible to arrange the optical working distances so that a high-quality standard microscope objective lens can be employed