The MRC function is the visible (VIS) imaging system equivalent to MRTD for infrared (IR) imaging systems. It describes the sensitivity of an optical system in the presence of noise—a difference in contrast between a target bar and a background. The measurement is made by observing the minimum required contrast for each special frequency.

The USAF 1951 target bar, which conforms to the MIL-STD-150A standard (see Fig below), is used as a standard target for MRC measurements.

The resolution bars are arranged in groups and elements (horizontal and vertical directions) of different sizes for different frequencies. The variation of contrast is achieved by illuminating the background from the front, the smallest resolvable group can be selected by a trained observer, and a chart of contrast levels against the spatial frequency at a given illumination level will be provided. Different kind of targets may be used according to the optical system’s needs.