Inclusions in a transparent panel (5) are detected by placing a light transmissive interface (3) in contact with the panel (5), and transmitting a beam of light (1) through interface (3) into panel (5). Within the panel (5), the light beam (7) propagates along a path including total internal reflections at surfaces of panel (5). When the light beam (1) intercepts inclusions (10) or other defects at least some of it is scattered, and leaves the panel (5). This scattered light is then observed. Thus, a large zone of the panel (5) can be inspected, with light only being detected in the case that it arises from scattering by inclusions or other defects.