In the present paper, an attempt has been made to visualize local flow structures inside the upper dilute zone of a pilot-scale circulating fluidized bed (CFB) under realistic flow conditions, i.e. with cross-sectional average solids volume concentrations ranging from 0.1 to 1 vol.%. The high-speed video technique in combination with the laser sheet technique has been used. Since it is not possible to penetrate from outside with a laser sheet into the flow, an endoscope has been used to guide the laser light into the fluidized bed. Particles or loosely packed agglomerates passing the illuminated light sheet area reflect the laser light. This provides the possibility for an undisturbed observation of the local flow pattern via the camera endoscope. To understand the governing optical effects, a model has been developed which is able to estimate the influences of the solids properties and the operating conditions on the ratio of the received light intensity to the emitted one. Digital image processing was successfully applied to pre-process the obtained image sequences and to gain a deeper insight into the observed flow structures. Using specific image processing methods, shapes and velocities of the flow structure were calculated.