Gradients of acoustic pressure in nematic liquid crystal, known as acoustic streaming, are responsible for the acousto-optic effect, according to researchers from Chung Yuan Christian University in Chung-Li and National Chao Tung University in Hsinchu, both in Taiwan. A better understanding of the phenomenon may lead to improved liquid crystal light valves and spatial light modulators. The team, which reported the results of its quantitative studies in the April 1 issue of Applied Optics, measured the power of a HeNe laser beam passing through a liquid crystal cell as a function of the repetition rate and pulse voltage of the ultrasound pulses applied to the cell. The work suggests that there is no threshold for the effect and that a power law governs the dependence of the optical transmission power on the intensity of the ultrasound pulses.