Find the mean illuminance of the irradiated part of an opaque
sphere receiving
(a) a parallel luminous flux resulting in illuminance E0 at the
point of normal incidence;
(b) light from a point isotropic source located at a distance 1 =
= 100 cm from the centre of the sphere; the radius of the sphere is
R = 60 cm and the luminous intensity is I = 36 cd.