Tremendous advances have been made in the last twenty years in extending the usefulness of the light microscope in visualizing living biological specimens, and it is now possible to visualize most thin tissues in an unstained state at – or near – the limit of optical resolution. in general, however, these techniques have been used successfully on biological specimens in the transmitted light mode; unstained biological specimens generally do not have sufficient albedo or sufficient difference in refractive index from their surroundings to be efficiently visualized in epi-illumination. A number of applications, including studies of the growth of bacteria on metallic substrates, and studies of the studies of the formation of interfaces between living neurons and silicon chips, would benefit from being able to resolve living, unstained biological specimens growing on opaque substrates

We found that by imaging biological specimens on a reflective substrate in the reflection mode of the confocal microscope, we could get images that resemble transmission mode differential interference contrast images.