In optical microscopy, Hoffman modulation contrast (HMC) is a technique for enhancing the contrast in unstained transparent biological specimens, invented by Dr Robert Hoffman in 1975. This is achieved by using components in the light path which convert phase gradients into variations in light intensity. Specimens thus illuminated have a 3D appearance which renders them more easily visible. In some specimens, the 3d appearance may be misleading, as a feature which appears to cast a shadow may not necessarily have distinct physical geometry.

An example of the use of HMC illumination is in in-vitro fertilisation, where under brightfield illumination the near-transparent oocyte is hard to see clearly.

HMC systems typically consist of a condenser with a slit aperture, an objective

Objective (optics)

In an optical instrument, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. Objectives can be single lenses or mirrors, or combinations of several optical elements. They are used in microscopes, telescopes,...

 with a slit aperture, and a polariser which is fitted between the condenser and the illumination source and is used to control the degree of contrast. The principle of HMC is used by a number of microscope manufacturers who have introduced their own variants of the technique, for example, Nikon advanced modulation contrast (NAMC), Olympus relief contrast (RC) and Leica's integrated hoffman modulation contrast (IMC).