The present work describes the design of a purely optical sensor for the detection of surface dew. The operation of the device is based on the frustration of total internal reflection. Constructions with conventional optical elements and with a holographic optical element (HOE) are discussed. The HOE is recorded by He-Ne laser and the diffraction efficiency is measured for s and p polarizations of the reconstructing beam. Also, the sensor characteristic for various dew densities of the sensitive surface is obtained theoretically and is proved experimentally. The sensor may be useful for isolation control of high-voltage devices.