Journal of the Society of Motion Picture and Television Engineers (1950-1954)

With ADP, the electric field is placed parallel to the light beam, necessitating transparent electrodes. Nesa glass or a thin coating of chromium or gold makes excellent electrodes. Nesa glass (stannous chloride sprayed on glass), the most lightefficient, is the electrode used in the present modulators. An interesting property results when the mode of operation is such that the light beam and electric field are parallel ; namely, the light retardation depends only on the potential across the crystal. This is important since it makes the operating voltage independent of the aperture of the optical system (Fig. 2). With a transverse electric field, the wider the aperture, the smaller the electric field in the crystal for a given applied voltage and therefore less retardation. If the electric field is parallel to the light passing through the crystal,, the electric field is constant and independent of the aperture. This would seem to indicate that the voltage would depend on the thickness of the crystal. This is not the case, however, since the electric field is halved by doubling the crystal thickness, but the time, or length of path, the light beam is in the crystal is doubled and the two effects cancel. Therefore, the total retardation depends only on the potential placed across the crystal. Apertures of any size can be used and closed with a given potential, in contrast to the extremely high voltage which would be necessary for a Kerr cell with large apertures. If the crystal thus described is placed between crossed polarizers and the zero potential condition examined, the resulting transmission field, instead of being uniform, shows ellipses of alternate dark and bright sections. This configuration is the last phenomenon that needs investigation in connection with ADP crystals. It is caused by the angular field of the beam of light entering the crystal. ADP not only retards the light beam when placed in an electric field, but it has a complex retardation function with incident light angle. That is to say, beams entering at different angles, in general, are retarded by different amounts. Consider the zero potential condition and refer to Fig. 3. The center black portion indicates retardation for light that is normally incident to the crystal. Notice that for zero potential applied, there is no birefringence; therefore, there is a dark spot in the center since only a single ray is passed and the polarizers are crossed. The next light ring on Fig. 3 indicates the locus of rays of all light entering at an angle away from normal incidence. The next dark section indicates a retardation of one ray with respect to the other of 360°. So, while the light areas look alike and dark areas look alike, they nonetheless represent different amounts of relative retardation. It is for this reason that only the region near normal incidence is utilized in the modulator. This is accomplished by placing the crystal structure in a collimated beam. However, the conventional sound track has finite dimensions and it will be impossible therefore to collimate the beam exactly. The effect of having to use a finite angular field will be discussed later. Transfer Characteristics With the device thus outlined, the first point of interest is that of the transfer characteristics, that is, a statement on voltage input versus light output. Figure 4 is a summary of the important relationships associated with analytically expressing the transfer characteristics. The function is not linear but it does have a reasonably linear portion. By expanding the transfer function about an arbitrary bias point into a Bessel expansion, it is seen that harmonic distortions will occur due to the nonlinearity of the transducer as the percent modulation is increased. By choosing a proper bias point, the function can be made symmetrical about the bias point, thus causing all even harmonics to vanish. Figure Dressier and Chesnes: Electrooptic Recording 207