Small dew droplets, which deposited on the mirror surface of a copper plate, were measured by using an interference microscope to evaluate the quantity of dew deposited on the mirror surface of the dew-point hygrometer. A He-Ne laser of 10mW and an optical fiber cable of 3mm in diameter and 120cm in length were used as a light source and an optical guide to the microscope. The fiber cable was shaken slightly with an acoustic speaker to reduce speckle noises in the interference images. A shape of dew droplet deposited on the mirror surface of the copper plate was obtained from the interference fringes, and the mass thickness of dew droplets was also obtained by numerical calculation of the volumes of each dew droplet deposited and was of the order of 10-5 g/cm2. The deposition velocities of dew on the surface under slow wind velocity were also measured. © 1996 SPIE. All rights reserved.