Modes of vibration in rectangular, orthotropic, free-free plates are determined optically, using an electronic speckle pattern interferometer (an instrument called the VibraVision) as real time instrument and a double pulsed ruby laser to record the corresponding holographic interferograms. It is found that the first three modes of vibration has a strong dependence only of one of the main material parameters at a time, namely in plane shear modulus and the two Young's moduli, respectively. It is therefore a simple task to determine these material parameters with this nondestructive, noncontact optical method. Poisson contraction ratios are harder to determine. One method is to tune the sides of the plate so that the second and third modes are turned into a ring-shaped and a cross-chaped mode. The Poisson contraction number has a stronger influence on these modes and can be determined. Experimental results and theory are presented.