Highly sensitive measurement of surface charge distribution using the Pockels effect and an image lock-in amplifier

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Abstract
A visualization and a quantification technique for a latent electric charge image on a dielectric material has been developed by using the Pockels effect and a computer imaging technique. An optical phase modulation technique and a computer image lock-in amplifier technique were used to improve the signal-to-noise ratio. The measurement system has several improved performances; sensitivity of surface change of 1 nC cm-2, spatial resolution of 200 mu m and discrimination of charge polarity. The basic principle of advanced electro-optic surface charge measurement is described and the typical surface charge distribution of positive discharge streamer produced by a high impulse voltage (5 kVpeak) is demonstrated. Also, comparison is made between the modulation method and the non-modulation method.