The quality of the communication fibres depends directly on the quality of the preform. There exists a large number of various destructive and non-destructive methods for testing of the preforms. The methods are objective or visual. The visual methods are very useful for quick qualitative testing especially at the beginning phase of the development of the preform making method. This paper describes a visual method of preform testing which is based on examination of the shadow pattern arising by the transmission of the light beam across the preform. To obtain sufficient intensity of the picture the beam is scanned along the preform axis. Rotationally non-symmetric optical system and a vibrating mirror serve for the scanning. As a light source the He-Ne laser is used. The interpretation of the shadow pattern is given. The pattern contents dark and bright fringes. From their number, position and width one can evaluate the number, width and refractive index of the films deposited by the MCVD method.