A method and an apparatus which defines the amount of wood material present in a bark flow and controls a debarking process on the basis of the data to reduce wood losses in the debarking process. In accordance with the invention, the bark flow is imaged by a camera and an image processing unit processes the image by using different whitenesses of the picture elements or pixels of the image as a basis for defining programmatically the amount of wood material in the bark flow. The image processing unit is adapted to produce an output signal for controlling the debarking process. In addition, the invention also provides a method and an apparatus which defines the amount of wood material in a bark flow being moved to a combustion process and controls the combustion process on the basis of the data.