  This paper describes the application of a laser diffraction technique to the
study of electroconvection in nematic liquid crystal cells. It allows a
real-time quantitative access to pattern wave lengths and amplitudes. The
diffraction profile of the spatial periodic pattern is calculated and compared
quantitatively to experimental intensity profiles. For small director tilt
amplitudes $\phi$, the phase grating generated in normally incident undeflected
light and the first order term correction from light deflection is derived
analytically. It yields an $I\propto\phi^4$ dependence of the diffracted
intensity $I$ on the amplitude of director deflections. For larger director
tilt amplitudes, phase and amplitude modulations of deflection of light in the
inhomogeneous director field are calculated numerically. We apply the
calculations to the determination of the director deflection and measure growth
and decay rates of the dissipative patterns under periodic excitation. Real
time analysis of pattern amplitudes under stochastic excitation is
demonstrated.
