We have measured the critical values of electric E(C) and magnetic H-C fields in different types of Freedericksz transitions for 4-trans-n-hexyl-cyclohexyl- isothiocyanatobenzene (6CHBT). Capacitive and optical methods have been employed to detect these transitions. In the case of twist-type deformation we have proposed a new interference method for threshold fields determination. On the basis of these measurements diamagnetic anisotropy Delta(chi) splay K-11, twist K-22 and bend K-33 elastic constants have been calculated for cells of various thickness, coated with different polyimides.