  Mach-Zehnder and conoscopic interferometry are used to explore photoelastic
properties of anisotropic crystal materials. In a number of cases an
application of both techniques significantly improves an accuracy of piezooptic
and photoelastic measurements. The performance of such combined approach is
demonstrated on tetragonal lithium tetraborate (LTB) single crystals, as an
example. Special attention is paid to methodological and metrological aspects,
such as measurement accuracy and the quantitative error analysis of the
resulting measurements. Performing the interferometric measurements for
different geometries of piezooptic coupling the full sets of piezooptic and
photoelastic tensor constants of LTB crystals have been determined. The
acoustooptic efficiency, on the other hand, has been evaluated using the
magnitudes of photoelastic constants derived from the piezooptical
measurements. For the geometries with strong photoelastic coupling LTB
demonstrates quite large acoustooptic performance with figure of merit value ,
$M_2$ achieving 2.12 $\times$ 10$^{-15}$ s$^3$/kg. It is several times larger
than that of strontium borate crystals, nowadays the best acoustooptic material
in deep ultraviolet spectral region.
