Isophote curves on spacelike surfaces in Lorentz-Minkowski space E31
  Isophote curve consists of a locus of surface points whose normal vectors make a constant angle with a fixed vector (the axis). In this paper, we define an isophote curve on a spacelike surface in Lorentz-Minkowski space and then find its axis as timelike and spacelike vectors via the Darboux frame. Besides, we give some characterizations concerning isophote curve and its axis. 
Fatih Dogan and Yusuf Yayli
math.DG