Optical imaging and characterization of weakly scattering phase objects, such as isolated cells, bacteria and thin tissue sections frequently used in biological research and medical applications, have been of significant interest for decades. Due to their optical properties, when these 'phase objects' are illuminated with a light source, the amount of scattered light is usually much less than the light directly passing through the specimen, resulting in a poor image contrast using traditional imaging methods. This low image contrast can be overcome using, for example, chemical stains or fluorescent tags. However, these external labeling or staining methods are often tedious, costly and involve toxic chemicals.
