Electro-Optic Cell With Transverse Electric Field

This invention relates to electro-optic shutters employing a suspension of dipole rod-like particles oriented for light transmission or rejection by an electric field. The term electro-optic shutter is intended to include more generally electro-optic cells useful for variable density filters, light choppers, displays, controllable polarizers and the like. Electro-optic fluid shutters have found a place in the art and representative are various Marks U.S Pat. Nos. 3,848,964, 3,527,525, RE 28,211, 3,512,876, 3,341,274, 3,257,903. Often discussed in these references is the problem of response time: although the dipole particles respond rapidly to an applied field and are oriented to provide a transparent window, the opaquing of the window tends to be slow by comparison since the dipole rods must move into random orientations by Brownian motion. Several of the references, for example, U.S. Pat. No. 3,848,964, describe forced disorientation by multi-electrode structure in the cell to which voltage pulses are applied generating a transverse electric field to provide a uniform positive disorientation of the dipoles. A limitation of this approach is that the transmittance of the panel depends on the proportion of the area taken up by the electrode lines as compared to the space between the lines. If the lines are far apart, the applied voltage to the conductors must be high to attain the necessary transverse field strength to move the dipoles. As the lines are moved closer together, the transmittance decreases.

In the present invention, the transverse electric field in the fluid shutter is provided by optically transparent piezoelectric transformer elements. A relatively low voltage such as 100 volts can be used to energize the piezoelectric voltage step-up transformer which then provides a combination of transverse electric field and motion to rapidly reduce the longitudinal alignment of the dipoles and the second transmitter of the cell.

FIG. 1 shows an electro-optic cell using a pair of PLZT optically transparent piezoelectric transformers located in the fluid suspension of dipole particles.

FIG. 2 shows an exploded view of the electro-optic cell of FIG. 1.

Referring now to the drawings wherein like numerals are used in each of the Figures, there is shown generally at 10 an electro-optic cell having upper and lower transparent plates 12 and 14, the transparent plates comprising substrates 16 and 18, respectively, such as glass, and transparent electrodes 20 and 22. Edge member 24 is sealed to the plates 12 and 14 to complete the cell chamber. Within the chamber is a fluid 26, such as VARAD fluid, having in suspension dipole particles, and also two piezoelectric transformer elements 28 and 30.

Element 28, and also element 30, is a single- or polycrystalline ferroelectric piezoelectric and optically transparent material such as quartz, barium titanate, lithium niobate, Rochelle salt, ammonium dihydrogen phosphate (ADP) or lanthanum modified lead-zirconate titanate (PLZT) of various compositions. It is operated as a piezoelectric voltage step-up transformer. The operation of a piezoelectric transformer of this general type using PZT is taught, for example, in the Berlincourt U.S. Pat. No. 3,562,792. In one successful embodiment of my invention involving PLZT, the optically transparent element 28 is approximately 4 cm long by 2 cm wide and has a thickness of one millimeter. The resonant frequency is .perspectiveto. 40 kilohertz. The piezoelectric body has different portions polarized differently. A portion of the body at both ends in the area of the electrodes may be polarized transversely while the extended central portion of the body is polarized longitudinally. The longitudinally polarized portion is the high voltage secondary of the transformer and may provide a voltage gain of 10 to 200 times. In the sample described, it was about 100 times. Thus, if the primary electrodes are energized with about 100 volts, the voltage in the central area of the body with respect to the ends will be about 10,000 volts. The field generated by this high voltage and the associated transformer motion (40 kHz vibration) are sufficient to quickly eliminate the longitudinal alignment of the dipoles which are nearby the transformer element. Electrical contact is made to the transformer at metal film electrodes 32, 34, 36, and 38. To energize the transformer an alternating voltage V is connected across electrodes 32 and 34 and across 36 and 38. The voltage step-up transformer action results in a high voltage being generated in the central section of the element. Piezoelectric transformer element 30 is mounted at right angles to element 28, the two elements operating as polarizers when energized.

The mounting means (not shown in FIGS. 1 and 2) of the piezoelectric transformers in the chamber may be a foam rubber mounting and the foam rubber mounting may or may not be located at a displacement node of the transformer depending on whether the transformer is driven at a fundamental or higher harmonic.

When the dipole particles are free to move within the chamber, they respond to Brownian motion and become randomized so that the shutter is opaque. When a voltage V.sub.c is applied to electrodes 20 and 22 imposing an electric field in the cell, the dipole particles become aligned with their long direction parallel to the electric field (the Z direction) and normal to the plates 12 and 14. The shutter then becomes transparent and transmits light from a suitable light source 40 through the shutter to a light detecting means 42. When the voltage V.sub.c is removed, the orientation of the dipole particles will again become randomized but as has been discussed above, the time interval for this to occur is longer than desired. In the present invention, when V.sub.c is removed, V is applied to energize the piezoelectric transformers 28 and 30. The resulting high voltages generated are effective to align dipoles in X and Y directions. As soon as the PLZT element 28 begins to vibrate, piezoelectric effects generate an electric field, E.sub.y, and the particles of the fluid adjacent element 28 will align themselves in the y-direction. Similarly, element 30 begins to vibrate and the piezoelectric effects generate an electric field E.sub.x, and the dipole particles in the fluid adjacent element 30 will align themselves in the x direction. This provides a rapid opaquing of the shutter.