The world around us appears as diverse and beautiful as it does owing to the fact that light is scattered. Scattering plays two roles in the intricate process of image formation. First, it is the means by which we perceive an object. An object which does not scatter light cannot be seen. However, scattering also distorts the image observed. Even in relatively pure air, visibility is limited to a range of a few tens of kilometers owing to aerosol and molecular light scattering. It is reduced to tens of meters in conditions of mist or fog. What occurs when we look at an object through a scattering medium? For example, when a point diffuse source is being observed, the radiation undergoes multiple scattering along the path to the photodetector. Therefore, the image produced by a source of this kind (i.e., the irradiance distribution in the image plane) appears as a more or less blurred speck. What we have said so far serves as an introduction to the concept of image transfer theory, or the point spread function ..