Real time imaging of two-dimensional charge on dielectric surfaces

A technique has been developed to measure two-dimensional (2D) charge distributions on dielectric surfaces in real time. A thin layer of polymer-dispersed liquid crystal (PDLC) placed between a dielectric substrate and a conducting plane undergoes a transition from an opaque to a transparent state due to the electric field of the surface charge. The cell is uniformly illuminated from the front and the transmitted light, which is proportional to the local charge density, is imaged using a charge coupled device camera. Due to the finite resistivity of the liquid crystal, however, the static field from the charges must be modulated in time in order to produce a sustained transition inside the PDLC. The modulation is produced by placing a rotating metal chopper near the surface. The magnitude and sign of the charge are measured by calibrating the response of the PDLC to a series of bias voltages. The technique has been used to image surfaces as large as 3 mmX5 mm with 30 μm spatial resolution in 333 ms.