A point on a perfectly Lambertian surface is observed twice from the same position. The first time it is illuminated by a point light source with direction s$1\backslash$epsi $R^3$ and the second time $itis$ illuminated $by$ a point light source with direction $s2\backslash$epsi $R^{3.}$ The light sources are unit strength, $i.e.||s1||=||s2||=1.$

The intensity $of$ the two observed reflections are $i1$ and $i2$ respectively. The two observations provide a partial constraint when estimating the surface normal direction.

$a$