Abstract: A method for determining the presence of knots and/or voids in lumber surfaces by uniformly heating the wood and then submitting the wood to an infrared camera, providing a thermal image of the surface of the wood. Knots and voids heat differently from the remaining wood areas, the differential heating being observed by the inspection of the thermal image, or more preferably by conversion of the image to a digital signal which is communicated to a frame grabber and data processor, permitting frame-by-frame inspection of the heat distribution on the surface of the wood. Locations of differential heating corresponding to knots and voids are detected, and can be communicated to subsequent processing devices, to either exclude all or some of the defect areas, or control the use to which the defect-bearing wood is put.