On isophote curve and its characterizations
  Isophote comprises a locus of the surface points whose normal vectors make a constant angle with a fixed vector. Main objective of this paper is to find the axis of an isophote curve via its Darboux frame and afterwards to give some characterizations about isophote and its axis. Particularly, for isophotes lying on a canal surface will be obtained other characterizations again. 
Fatih Dogan and Yusuf Yayli
math.DG