\[ (l,m,n) = \left( {\frac{a}{{\sqrt {{a^2} + {b^2} + {c^2}} }},\frac{b}{{\sqrt {{a^2} + {b^2} + {c^2}} }},\frac{c}{{\sqrt {{a^2} + {b^2} + {c^2}} }}} \right)\]
Where :
\[ \frac{a}{{a'}} = \frac{b}{{b'}} = \frac{c}{{c'}}\]
Where :
\[ aa' + bb' = 0\]
Where :