A survey is given on the principle, the theory and the practical performance of an instrument that measures transmittance modulation over an extremely wide range of spatial frequencies. In a conventional microdensitometer, a microscope lens magnifies a small section of the sample onto a narrow single slit in front of the light detector. The Multiple-Sine-Slit Microdensitorrteter MSSM uses laser two-beam interference to project a sinusoidal intensity distribution of a great number of periods directly onto the sample. The total light flux transmitted while interference fringes scan across the transmittance pattern is collected by an integrating sphere in contact with the sample. The absence of any imaging elements gives the MSSM a constant instrument MTF over its entire working range, which at present spans from 4 to 1500 cycles/mm. By the large scanning area and the great number of periods in the scanning function a narrowband characteristic is achieved that efficiently suppresses grain noise and improves signal-to-noise ratio. Due to these features, MTF's could be determined up to 300 cycles/mm for coarse-grained emulsions and up to 1500 cycles/mm for high resolution emulsions as they are used for holography, optical data storage and integrated circuit manufacture.