The field of optical imaging is focussed on techniques to extract useful information about a physical system from the spatial structure of light. There are two main themes to research in this field, the resolving of previously unknown structures ranging in size from microns in microscopy to distant galaxies in astronomical telescopy; or if the structure is selected from an a priori known set, then its unique discrimination, such as in data read-out from a CD or DVD. In general, both types of imaging involve the collection and focusing of light after interaction with the object. However, the process of information extraction can be quite different. In resolving an unknown structure, a full two dimensional image is usually desired. Here, the metric of success is generally the resolution of the final image. In most cases diffraction is the key concern, presenting the diffraction limit to the resolution of the final image as approximated by Abbe (Born & Wolf, 1999). There are ways to overcome this limit, such as by utilising non-linearities (Hell et al., 2009), or using metamaterials (Pendry, 2000) to form so called superlenses, and this is a vibrant and growing area of research. The focus of this Chapter, however, is on the second theme of imaging, discrimination between a set of known structures.