An interferometric scattering microscope 1 suitable for medical use with biological samples, such as proteins, viruses or nanoparticles, has a sample holder 2, an illumination source 4, a detector 5, an optical system which directs illuminating light onto the sample and directs both light reflected and scattered from the sample to the detector and a spatial filter 20 which produces a greater reduction in intensity of light within a predetermined numerical aperture than at larger apertures. The predetermined numerical aperture may be the numerical aperture of the illuminating light reflected from the sample location. The spatial filter may reduce intensity within said numerical aperture to 10-2 of the incident intensity or less. The illuminating light may be spatially and temporally coherent. The optical system may comprise a beam splitter arranged to split the optical paths for the illuminating light and the output light, the spatial filter being part of the beam splitter. A spatial filter as used in the microscope and method of adapting an interferometric scattering microscope to produce the microscope above are also claimed.
