  We consider the physical limitations imposed on the information content of an
image by the wave and quantum nature of light, when the image is obtained by
illuminating a reflecting or transmitting planar object by natural---i.e.,
fully thermalized---light, or by observation of an object emitting incoherent
(thermal) radiation. The discreteness of the degrees of freedom and the
statistical properties of thermal radiation are taken into account. We derive
the maximum amount of information that can be retrieved from the object. This
amount is always finite and is proportional to the area of the object, the
solid angle under which the entrance pupil of the receiver is seen from the
object, and the time of observation.
  An explicit expression for the information in the case where the information
recorded by the receiver obeys Planck's spectral distribution is obtained. The
amount of information per photon of recorded radiation is a universal numerical
constant, independent of the parameters of observation.
