Opto-electronic humidity sensor: A review

•Detailed study of various nanomaterials as opto-electronic humidity sensor.•Focused on the techniques used in such type of optical sensors.•The sensing materials were deposited either on the base of the prism or on the U-shaped fibre.•The mode of propagation of light may be transmission, reflection or partially refraction.•Being optical, such types of sensors are very useful for the detection of moisture level at remote places. The present paper reports the detailed study of various metal oxide nanomaterials as opto-electronic humidity sensor. The sensor elements were fabricated and characterized as thin or thick film based on the semi-conducting metal oxide. As semi-conducting metal oxides are known for their n-type conduction because of the presence of oxygen vacancies and change in refractive indices with the exposure of moisture therefore they were proven to be very good sensors for humidity. Depending upon the need, the sensing materials were deposited either on the base of the prism or on the U-shaped borosilicate glass rods or on some other transparent substrates for opto-electronic sensing. Light from LED or He–Ne laser was launched into the sensing element from one side and collected into the other side by optical power meter. The mode of propagation of light was transmission, reflection or partially refraction. Modulations in the intensity of light with changes in humidity were recorded. Being optical in nature, such types of sensors are very useful for detection of moisture level at remote places or unmanned stations. The primary purpose of this paper is to focus on the techniques used in these sensors.