Conventionally, image contrast in the microscopical study of biological cells is generated via Zernike or Differential Interference Contrast (DIC) methods, or via the incorporation into the cells of exogenous fluorescent labels or other dyes. The contrast enhancement created by DIC results in pseudo-3D images, while Zernike contrast suffers from halo artefacts at cell edges, and neither method is quantitative.
Fluorescent labels can be cytotoxic, potentially causing changes in cell physiology. The photo-bleaching frequently associated with fluorescence studies can also cause cell damage over time. These problems are especially acute in assays of sensitive primary cells and stem-cells.