isophote

300px|thumb|ellipsoid with isophotes (red)

In geometry, an isophote is a curve on an illuminated surface that connects points of equal brightness. One supposes that the illumination is done by parallel light and the brightness is measured by the following scalar product: b(P)= \vec n(P)\cdot \vec v=\cos\varphi where is the unit normal vector of the surface at point and the unit vector of the light's direction. If , i.e. the light is perpendicular to the surface normal, then point is a point of the surface silhouette observed in direction Brightness 1 means that the light vector is perpendicular to the surface. A plane has no isophotes, because every point has the same brightness.