Drive Electronics Overview

In its unpowered state, an SBG is optically active, providing maximum diffraction efficiency. In order to cause the SBG to become an optically inactive transparent cell (i.e. for its diffraction efficiency to fall to zero) an electric field must be applied to it. This field is generated by applying a voltage between the transparent ITO electrodes applied to the inner surfaces of the cell walls. The applied voltage must be AC in order to maintain a DC balance within the cell. Typically, the AC signal comprises sinusoidal or square waves in the region of 100Hz - 2kHz.

Diffraction efficiency is roughly inversely proportional to the voltage applied, but the minimum amplitude required to reduce the efficiency to precisely zero depends on the type of DigiLens™ and the "prescription" of the holograms recorded into any given SBG layer.

In electrical terms, a single layer of an DigiLens™ can be roughly equated to a parallel plate capacitor in parallel with a shunt resistor to emulate the DC leakage current. A capacitance value of around 8nF per square inch for gap spacings of 5-8microns is typical, and leakage current is < 1mA at maximum voltage. This makes the drive requirements very similar to those of electroluminescent (EL) panels, used as backlights in mobile phones, PDA's and even wristwatches. A variety of switching regulator Ics, which require a minimum of external components, have been developed for such applications.