In temperature-controlled printing plate exposers, it is possible for condensation of dampening solution or moisture to occur in a region of printing plates or laser apparatus. In this way, imaging of printing plates can be impaired and/or the laser apparatus can suffer damage resulting from corrosion. An apparatus detects the quantity of dampening solution on a measuring spot in an exposer for printing forms. The intention is for the ingress of condensation within a printing plate exposer to be detected more accurately. A measuring beam having a first intensity is reflected from the measuring spot and a measured signal is generated at a photoelectric conductor. Following evaporation of the dampening layer on the measuring spot, a reference signal is generated at a photoelectric converter by reflected radiation from the measuring spot. By comparing a measured signal and a reference signal, the dampening solution or moisture can be detected.