n In two directions. (1) The term may be applied to illumination angles in three dimensional space. Thus, the incident angle may be 45° relative to the material surface, but this designation does not describe the portion of the circumferential 45° angle possible. The term “bidirectional illumination” generally implies two incident beams separated circumferentially by 180°. (2) The term may also be applied to goniophotometric measurements, to the illumination and to the viewing angle. In either case, the angular distance is expressed as the degrees (or radians) from the perpendicular (normal) to the surface as 0°. Thus, the grazing angle approaches 90°.