Abstract The phase-contrast technique offers an attractive alternative to small-angle Thomson scattering for the investigation of plasma density fluctuations. This imaging technique converts small relative phase-shifts into detectable intensity variations. A wave-optics treatment is given and test results of a system built for the TCA tokamak are presented. The device is suitable for the study of density fluctuations in the wavelength range from 2 to 200 mm. It uses a low-power continuous CO 2 laser probe beam of 15 cm width and liquid-N 2-cooled HgCdTe detectors. For a 1 MHz bandwidth it allows detection of phase-shifts as low as 3.10 −6rad at the shot-noise limit, corresponding to relative density variations Δn e n e ≅ 10 −4 for typical conditions in TCA.