On focal surfaces formed by timelike normal rectilinear congruence

In this paper, we investigate the focal surfaces obtained by the timelike normal rectilinear congruence whose straight lines are normal to spacelike surface, in the Minkowski 3-Space. Then, the relation λ1KZ1+λ2KZ2 = 0 between Gaussian curvatures of these focal surfaces are examined and finally the relation which concern with area preserving correspondence between the focal surfaces are given.