A method of direct on-line measurement of thickness of fouling deposits created on the tube walls in a pulverized-coal firing furnace is described, as well as an apparatus suitable for implementation of the method. The method is based on imaging of a light spot, generated on the deposit surface by the apparatus. A position-sensitive image detector is used to track the spot as deposits accumulate, and the image signal is processed in real time, which allows for monitoring the creation of fouling deposits during operation of the furnace. The system simultaneously determines the reflectivity of the deposit surface. The apparatus can be used as part of an automatic soot blowing system.