Since they are nested, we can apply [6] and define the mapping [F.sub.[beta], [gamma]] ([alpha], t) for isoptics of pair [[GAMMA].sub.-[beta]] and [[GAMMA].sub.[gamma]].
and [omega] = [pi] - [alpha], then we can write the Crofton integral formula for isoptics [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] in exterior of C that is for [alpha] [member of] ([beta] + [gamma], [pi]).
The geometric condition for convexity of isoptics is given in [5].
We can, similarly as it was done for isoptics, give a condition which implies that all secantoptics of an oval C are convex.
[1] Cieslak, W.; Miernowski, A.; Mozgawa, W., Isoptics of a closed strictly convex curve, Global differential geometry and global analysis (Berlin, 1990), 28-35, Lecture Notes in Math., 1481, Springer, Berlin, 1991.
