A method of determining the orientation of a grain structure of a turbine blade or vane using an optical technique, illuminates a surface with a beam from a light source and then manipulates the relative orientation of the surface, a light beam and detector to determine the relative orientation which provides the peak reflectance of the beam from the surface to the detector. Further surfaces of the component are similarly manipulated and illuminated and the orientation of the grain structure calculated from the determined relative peak reflectance orientations of the surface, first further surface and second further surface. Determination of the grain orientation allows the value of high angle boundaries to be determined. Also disclosed is apparatus suitable for performing the method.