The geometric albedo of an astronomical body is the ratio of its actual brightness at zero phase angle ( i.e. as seen from the light source ) to that of an idealized flat , fully reflecting , diffusively scattering ( Lambertian ) disk with the same cross-section . The geometric albedo of an astronomical body is the ratio of its actual brightness at zero phase angle ( i.e. , as seen from the light source ) to that of an idealized flat , fully reflecting , diffusively scattering ( Lambertian ) disk with the same cross-section .