The refractive indices of rubidium hydrogen selenate are measured for several wavelengths at room temperature, and the transmission spectrum is measured from the UV (240 nm) to the near IR (2000 nm). The orientation of the optical indicatrix with respect to the crystal axes is also determined at several wavelengths in the visible range. Using a new method based on the deflection o f l ight b y t he f erroelastic d omain structure, we also determine refined values for birefringence at several w avelengths. Finally, the dispersion of the three birefringences in the range 450-900 nm is deduced from polarimetric measurements. This set of results yields complete knowledge of the linear optical characteristics required for interpretation of the electro-optical and nonlinear optical properties of this compound.

The deflection of light in ferroelastic crystals results from refraction and reflection at domain wa lls. When the tilt angle of the principal axes in neighboring domains is small, simple relationships between the crystal birefringences and the angles of the deflected beams can be deduced from Snell's law of r efraction. As a rule, this condition is satisfied a t W -domain w alls i n f erroelastic s pecies t hat h ave a b iaxial p rototype p hase. In this case, measurement of the deflection a ngles p ermits o ne t o d etermine t he b irefringences e asily. This method has as its main advantages independence of the sample thickness and the need for only rough sample preparation. It is absolutely insensitive to temperature fluctuations. We have applied the method to crystals of rubidium hydrogen selenate and dihydrated barium chloride as illustrative examples.