Information retrievable from an image in natural light

A most common way to store information is to encode it in the optical properties of an object and to retrieve it by viewing the object by reflected and transmitted natural (thermalized) light - or even by light emitted by the object itself - for a specified time interval. The discreteness of the radiation degrees of freedom and the statistical properties of thermal (incoherent) radiation impose limitations on the amount of the retrieved information. We derive the maximum 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.