Subsequent processing of pieces of wood following the bark peeling device require the wood to have a certain quality. For this purpose the pieces of wood are examined as to the degree of unbarking, dimensions, shape, possibly insufficient disbranching and wood quality (impaired e.g. by rotting, pests, damage done by game). Depending on the design of the installation, the inspected logs are at present divided into two material streams (wood - bark) or three material streams (wood - bark - discard). Mechanical sorting is complicated, since determining and measuring bark content in a mass flow of wood is extremely difficult. Mechanical recognition of different wood qualities is further complicated by the similar appearance of dark wood, insufficiently peeled wood and bad wood (rotting, pests, etc.). Mechanical recognition of the quality of wood is also difficult since the pieces of wood are usually arranged on a constantly moving conveyor and the pieces of wood are often overlying each other or stacked upon each other within one observation zone.
All the above circumstances render mechanical separation of the pieces of wood into different qualities difficult, and therefore sorting out has ordinarily continued to be done manually. Capacity increases are achieved by having the pieces of wood visually examined by an operator and accordingly selectively actuating gates, for example, electromagnetically operated gates, to divert the wood to an appropriate station or work zone. Because of human shortcomings and because of subjective decisions of the operating personnel, it may thus happen that insufficiently peeled pieces of wood or unfit pieces of wood are not eliminated from the processing system which may adversely affect the quality or operation of the resulting product.
Methods attempting to separate pieces of wood according to their bark content are known. This is typically done by heating the pieces of wood and measuring the infrared radiation of the material. This method has the disadvantage in that apart from a relatively high energy consumption, the intake of heat by the pieces of wood is also influenced by many other factors as, e.g., moisture, type of wood, etc., thus making it impossible to draw exact conclusions as to the degree of unbarking. Moreover, it is only possible to distinguish at most two different categories (wood - bark). It is not generally possible to determine the type of wood, physical defects in the wood, shape or dimension.