Measurement of dew droplets in initial deposition at dew point by using a phase-shift interference microscope

Abstract

In order to measure the total mass per unit area of dew droplets deposited on a metal plate in the dew-point hygrometer, the shape of a dew droplet deposited on a copper plate was measured accurately by using an interference microscope that employed a phase-shift technique. The microscope was constructed by adding a piezoelectric transducer to an usual interference microscope. A simple method that uses a conventional speaker horn and an optical fiber cable was introduced to depress speckle noise. The shape of a dew droplet deposited on the copper plate surface with 0.1 μm in average roughness was measured with an accuracy of ±3nm. The mass of a dew droplet could be calculated numerically from the volume of its shape and was of the order of 10^-9 g. The total mass of dew droplets deposited per unit area and the deposition velocity were obtained under a gentle wind. The total mass was the order of 10^-5 g/cm² at the beginning of deposition and the deposition velocity was ranged from 2x10^-6 to 6x10^-5 g/cm².min.